U.S. patent application number 13/669284 was filed with the patent office on 2013-05-23 for communication apparatus and method.
This patent application is currently assigned to The University Court of the University of Edinburgh. The applicant listed for this patent is The University Court of the University of Edinburgh. Invention is credited to Mostafa Afgani, Harald Haas, Wasiu Popoola, Gordon Povey, Sinan Sinanovic, Dobroslav Tsonev, Ian Underwood.
Application Number | 20130126713 13/669284 |
Document ID | / |
Family ID | 45421274 |
Filed Date | 2013-05-23 |
United States Patent
Application |
20130126713 |
Kind Code |
A1 |
Haas; Harald ; et
al. |
May 23, 2013 |
COMMUNICATION APPARATUS AND METHOD
Abstract
A detection system for use with a communications system and an
associated communication systems, methods, portable electronics
devices and geolocation and/or reporting devices, the detection
system having at least one radiation detector for receiving a
radiation signal, wherein the at least one radiation detector
includes a plurality of sensing elements, and the detection system
is configured to detect the radiation signal using differing
subsets of sensing elements at differing times and determine data
encoded in the radiation signal based on the radiation detected by
the different subsets of sensing elements.
Inventors: |
Haas; Harald; (Edinburgh,
GB) ; Povey; Gordon; (Edinburgh, GB) ; Afgani;
Mostafa; (Edinburgh, GB) ; Sinanovic; Sinan;
(Edinburgh, GB) ; Tsonev; Dobroslav; (Edinburgh,
GB) ; Underwood; Ian; (Edinburgh, GB) ;
Popoola; Wasiu; (Edinburgh, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
the University of Edinburgh; The University Court of |
Edinburgh |
|
GB |
|
|
Assignee: |
The University Court of the
University of Edinburgh
Edinburgh
GB
|
Family ID: |
45421274 |
Appl. No.: |
13/669284 |
Filed: |
November 5, 2012 |
Current U.S.
Class: |
250/208.2 |
Current CPC
Class: |
H04B 10/691 20130101;
H04B 10/60 20130101; H04B 10/116 20130101 |
Class at
Publication: |
250/208.2 |
International
Class: |
H04B 10/60 20060101
H04B010/60 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 2011 |
GB |
1119063.4 |
Claims
1. A detection system for use with a communications system, the
detection system comprising at least one radiation detector for
receiving a radiation signal, wherein the at least one radiation
detector comprises a plurality of sensing elements, and wherein the
detection system is configured to detect the radiation signal using
differing subsets of sensing elements at differing times and
determine data encoded in the radiation signal based on the
radiation detected by the different subsets of sensing elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to United Kingdom
Application No. 1119063.4, filed Nov. 4, 2011, which is hereby
incorporated herein in its entirety by reference.
[0002] The present invention relates to a communication method and
apparatus and an associated receiver, particularly relating to
receiving a communications signal using an optical sensor.
BACKGROUND
[0003] Data may be encoded using radiation by time modulating a
radiation source. For example, in visible light communications, the
intensity of light produced from a light source, such as an LED,
may be modulated over time in order to encode data in a light
signal. A photo-detector can then be used to receive the
time-modulated signal which is decoded to reveal the data that was
transmitted by the light source.
[0004] A digital camera can be conveniently used to receive the
signal, which is then processed to extract the encoded data. In
order to achieve an acceptably high transmission rate and achieve
communication without obvious light flickering by the transmitting
light source(s), the transmitting light source must be switchable
between intensity levels at a suitably high rate. Conversely, the
photo-detector must meet certain requirements, for example, by
having an image capture rate fast enough to distinguish between the
intensity transitions. However, at the same time, it would be
beneficial if such communications methods could be used with common
or off-the shelf apparatus.
[0005] In certain communications methods, particularly in visible
light communications, it is desirable to change the power of the
radiation emitted by the transmitter, i.e. to use dimming of the
radiation source. However, these changes can affect the data
transmission capabilities.
[0006] Various techniques are employed in the art in an attempt to
address this problem. For example, in the IEEE 802.15.7 standard, a
method referred to as variable pulse position modulation (VPPM) is
used. This involves changing the pulse duration depending on the
dimming level required. In VPPM, the data rate is independent of
the dimming level, but the bandwidth efficiency is poor. Other
techniques employed to address this problem involve changes of the
light intensity level. Problems associated with at least some of
these techniques include degradation of the data rate performance
for high dimming (low optical power).
[0007] Orthogonal frequency division multiplexing (OFDM) methods
are popular for modulating signals in order to transmit data over
dispersive channels. However, it is desirable to reduce the power
consumption of communication systems. For example, this may be to
maximise battery life for portable devices, or simply to save
operating costs or reduce energy usage.
[0008] A variation on the OFDM modulation scheme, called SIM-OFDM,
has been proposed in order to reduce the power required by
communications devices relative to those that use traditional OFDM.
The SIM-OFDM technique is described in "Subcarrier Index Modulation
OFDM" by R. Abualhiga and H. Haas, in Proc. of the International
Symposium on Personal, Indoor and Mobile Radio Communications
(PIMRC), Tokyo, Japan, Sep. 13-16, 2009.
[0009] SIM-OFDM introduces an additional dimension alongside
conventional OFDM encoding, the additional dimension coming from
the state, i.e. active or inactive, of each frequency carrier
available. In this way, frequency carrier states (i.e. used or
unused) are used to encode data according to an on-off keying
modulation scheme. As in OFDM, each active carrier transmits a
signal that is modulated using a conventional modulation scheme
such as but not limited to M-QAM. Each inactive carrier is set to a
zero state. Hence, the power used to convey each M-QAM signal can
also be used to encode further data by simply being present or not
in a particular frequency carrier band. The SIM-OFDM concept is
illustrated in FIG. 1.
[0010] In this case, the incoming bit stream is divided into blocks
of bits, each having a length of N(0.5*log 2(M)+1), where N is the
number of frequency carriers, and M is the constellation size of
the respective M-QAM modulation scheme that is used. Each of these
blocks is divided into two parts. The first N bits of the block
form a first sub-block (B.sub.OOK). The remaining 0.5*Nlog.sub.2(M)
bits form a second sub-block (B.sub.QAM). The first sub-block
(B.sub.OOK) is inspected and the majority bit type is determined by
checking which bit value, 1 or 0, has most occurrences. The
frequency carriers that have the same position inside the OFDM
frame as the bits from the majority bit type in B.sub.OOK are
classified as "active", and the rest of the frequency carriers
(i.e. those that correspond to the minority bit type) are
classified as "inactive". Inactive carriers are given the amplitude
value 0+0j, where j= -1. The first 0.5*N active frequency carriers
are given amplitude values corresponding to the M-QAM constellation
symbols necessary to encode the second sub-block (B.sub.QAM). The
remaining active carriers can be used to signal the majority bit
type of B.sub.OOK to the destination receiver and they will be
assigned a signal whose power is equal to the average power for the
given M-QAM scheme. Afterwards, an N-point IFFT transformation is
performed in order to obtain the time-domain signal, which is
transmitted.
[0011] In this way, for example, if the binary sequence [0 1 0 0 0
1 1 1 0 1 0 1] is to be transmitted using 4-QAM and 6 carriers,
then the sequence is divided into a first sub-block [1 1 0 1 0 1]
and a second sub-block [0 1 0 0 0 1]. The second sub-block is
modulated into frequency carriers using 4-QAM modulation. Since the
majority bit in the first sub-block is 1, then an active carrier is
chosen to represent 1. In this case, the 4-QAM modulated signals
are transmitted on the first, third and fifth frequency carrier
channels. The sixth carrier, which is also active, can be used to
convey to the destination what the majority bit type in B.sub.OOK
is. It will be allocated power equal to the average power of the
respective M-QAM scheme. Its positive amplitude will represent the
majority bit type--in this case 1. This carrier channel allocation
effectively encodes the first sub-block as [1 1 0 1 0 1].
[0012] A slight modification of SIM-OFDM involves signalling the
majority bit type either through secure communication channels, or
by reserving one particular frequency carrier and transmitting the
desired value with a sufficiently high signal to noise ratio. It
should also be noted that this modulation scheme saves power from
all inactive carriers at the expense of spectral efficiency. The
described configuration has been referred to as Power Saving Policy
(PSP). In an alternative embodiment, for each single OFDM frame,
the unused power from the inactive carriers can be reallocated to
the active ones, which could lead to a performance enhancement.
[0013] Once a signal has been received by the receiver at the
destination, it is transformed into the frequency domain with a
fast Fourier transform operation. Then all the frequency carriers
are inspected. Those carriers whose power is above a predetermined
threshold are marked as active, and the rest of the carriers are
marked as inactive. At least half of the total number of carriers
are active. Hence, in case that less than 0.5*N active carriers are
detected, the threshold value is decreased by a small step and the
inspection is performed again. This procedure is done iteratively
until at least 0.5*N active carriers are detected. Then the first
sub-block (B.sub.OOK) is reconstructed from the detected states of
the carriers and the known majority bit type. Afterwards, the first
0.5*N active carriers are demodulated according to the respective
M-QAM scheme in order to reconstruct the second sub-block
(B.sub.QAM) in the conventional manner. The spectral efficiency of
this scheme is:
log 2 ( M ) 2 + 1 bits carrier ##EQU00001##
[0014] It is an object of at least one embodiment of the present
invention to improve the performance of the SIM-OFDM scheme. The
bit error rate (BER) performance of SIM-OFDM in an Additive White
Gaussian Noise (AWGN) channel is illustrated in FIG. 2.
[0015] It is at least one object of at least one embodiment of the
present invention to provide an improved or alternative
communication system and detector and/or to at least partially
address at least one problem with the prior art.
STATEMENTS OF INVENTION
[0016] According to a first aspect of the present invention is a
detection system for use with a communications system, the
detection system comprising at least one radiation detector for
receiving a radiation signal, wherein the at least one radiation
detector comprises a plurality of sensing elements, and wherein the
detection system is configured to detect the radiation signal using
differing subsets of sensing elements at differing times and
determine data encoded in the radiation signal based on the
radiation detected by the different subsets of sensing
elements.
[0017] The radiation signal may comprise a modulated radiation
signal, such as an intensity modulated radiation signal. The
detection system may be configured to determine data encoded in the
radiation signal by determining the radiation intensity and
preferably the average radiation intensity detected by each subset
of sensing elements.
[0018] The radiation detector may be configured to generate one or
more images from the received radiation signal. Each image may be
generated within an image detection period.
[0019] The radiation detector may be configured to capture
different portions of each image at different times within the
associated image period. Each portion of the image may be detected
by a differing subset of sensing elements. The detection system may
be configured to sample a plurality and preferably all of the
subsets of sensing elements within each image detection period. The
detection system may be configured to generate each image by
combining the portion of the image generated by each subset of
sensing elements within the associated image period in order to
form the image.
[0020] Each subset may comprise different sensing elements to at
least one and preferably each of the other subsets. Each subset may
be sampled for a predetermined sample time.
[0021] The at least one radiation detector may comprise an image
sensor. The at least one radiation detector may comprise a CMOS
photodetector an active pixel sensor or a charge coupled device
(CCD) or the like. Each sensing element may comprise a photodiode
or a reverse bias p-n junction or the like. The radiation detector
may comprise an infra-red detector, such as a passive infra-red
detector or an ambient light sensor or a photo-voltaic element.
[0022] The at least one radiation detector may be a pixelated
detector. Each sensing element may comprise or be comprised in an
associated pixel of the detector. The sensing elements may be
formed in an array, grid or matrix. The sensing elements may be
arranged in rows and/or columns.
[0023] The detector may comprise or be comprised in a digital
camera.
[0024] At least one and preferably each subset of sensing elements
comprises a block of sensing elements. Each block of sensing
elements may comprise one or more sensing elements.
[0025] Preferably each subset of sensing elements may comprise one
or more rows or columns of sensing elements. The detection system
may be configured to sample the radiation detector on a row by row
or column by column or pixel group by pixel group basis.
[0026] The detection system may be operable using a rolling shutter
and/or a line scan technique. The rolling shutter may comprise
using the radiation detector to collect an image by sampling the
sensing elements on a row by row or column by column or pixel group
by pixel group basis and generating an image by combining the
output of each sampled row or column or pixel group of sensing
elements.
[0027] The detection system may be configured to determine elements
of the data, such as bits, each data element being determined from
the radiation detected by an associated subset of sensing elements,
for example, by using the average intensity of radiation detected
by the associated subset of sensing elements.
[0028] The radiation signal may be modulated or encoded using
on-off keying.
[0029] The detection system may be configured to normalise the
average intensity of radiation detected by each subset of sensing
elements, for example, the signal or average intensity may be
normalised between 0% and 100%.
[0030] The radiation may comprise light such as visible light,
infra-red, near infra-red or ultra-violet light or the like.
[0031] The detection system may be configured to remove at least a
portion of a background image from the portion of the image
generated by each subset of sensing elements.
[0032] The system may be configured to illuminate the background at
a predetermined intensity, such as an intensity corresponding to
one or more intensity levels that are used to modulate/encode data
in the radiation signal for a time sufficient for at least one and
preferably each subset of sensing elements to be sampled. The
predetermined intensity may be an unmodulated intensity. The
detection system may generate a background image using the
radiation detected from at least one and preferably each subset of
sensing elements when illuminated at the predetermined intensity.
The processor may be configured to subtract a corresponding portion
of the background image from the portion of the image generated by
each subset of sensing elements.
[0033] The detection system may be configured to average several
image portions generated by each subset at varying times in order
to generate a corresponding portion of the background image for
each subset. The detection system may be configured to filter the
signal received by adjacent subsets of sensing elements in order to
normalise the intensities and generate corresponding portions of
the background image from the normalised intensities.
[0034] The detection system may be configured to perform brightness
and/or contrast normalisation over a detected image (i.e. over one
or more subsets), for example, if an image is bright in one area
and dark in another.
[0035] The detection system may be configured to perform clock
recovery, for example, by determining a frequency and/or phase of
the transmitted signal by analysing a plurality of data bits (e.g.
each data bit being determined from a corresponding subset of
sensing elements) and determining the time of changes of intensity
of the radiation signal relative to the sample time of each
subset.
[0036] The detection system may be configured to perform error
correction, for example using forward error correction
techniques.
[0037] The sensing elements may be configured to detect colour. The
data may be encoded into the radiation signals using intensity
modulation and/or colour keying.
[0038] The detection system may be configured to apply interference
cancellation and/or multi-user detection methods such as successive
interference cancellation, joint detection, joint transmission,
precoding techniques (dirty paper coding), superposition coding or
minimum mean square error (MMSE) estimation.
[0039] The radiation detector may comprise or be comprised in a
camera of a portable electronics device, such as a mobile phone or
other personal communications device, a tablet computer, a palm top
computer, a notebook computer, a digital camera, a PDA, a laptop
computer, a personal computer or the like. Processing of any
signals generated by the sensing elements and/or control of the
sensing elements and/or detection system may be at least partially
carried out by one or more processors of the portable electronics
device, for example when suitably programmed and/or configured.
[0040] The portable electronics device may comprise an ambient
light sensor. The ambient light sensor may be used as an
alternative or additional radiation detector to the digital
camera.
[0041] The processor may be configured to use radiation intensity
detected by the ambient light sensor to normalise the image and/or
construct the background image.
[0042] The radiation signal may be generated by a light source. The
light source may comprise an LED. The light source may comprise a
screen or monitor or projector or projection screen. The average
intensity of light output by the display may be controlled to
modulate the light it emits and thereby encode the data. For
example, the light emitted by the monitor or screen may be
modulated by intensity modulating the light output by the backlight
of the display, or by shifting the intensity levels output by the
LEDs of an LED display or OLEDs of an OLED display or pixels of a
pixelated display, or by displaying an image that results in a
required light intensity output.
[0043] The light source may be a light source comprised in a
portable device such as a mobile phone/smartphone, a tablet
computer or the like. For example, the light source may comprise an
LED light source provided in a portable device for illumination,
for example, when taking a picture with an in-build camera. In this
way, using the LED or screen of the portable device as a
transmitter and a camera of a portable device as a receiver may
permit device to device optical communications.
[0044] The light source may comprise a screen or monitor on which
an image is produced by scanning and modulating a spot or a line.
Temporal modulation of the intensity of the spot or line may be
applied in a manner that has little effect on the perceived image
content yet encodes data temporally or spatially within the image
for later detection.
[0045] The detection system may be directly and/or indirectly in
communication with a processing resource and/or a secondary data
store, the processing resource accessing secondary data from the
data store dependent on the data determined by the detection
system. For example, the secondary data may comprise location data
and/or data associated with a product and/or data associated with
the transmission system used to generate the signal or a further
system associated with the transmission system.
[0046] According to a second aspect of the present invention is
method for detecting a radiation signal in a communications system,
the method comprising: [0047] receiving the radiation signal using
at least one radiation detector, wherein the at least one radiation
detector comprises a plurality of sensing elements; [0048] sampling
differing subsets of sensing elements at differing times; and
[0049] determining data encoded in the radiation signal based on
the radiation detected by the different subsets of sensing
elements.
[0050] The method may comprise one or more method steps
corresponding to one or more features described in relation to or
involving use of the apparatus described in connection with the
first embodiment.
[0051] According to a third aspect of the present invention is a
system comprising a transmission system and a detection system
according to the first aspect, the transmission system comprising a
light source and being configured to modulate the output of the
light source in order to encode data for transmission to the
detection system.
[0052] The transmission system may be configured to encode data by
modulating the intensity of light emitted by the light source in
order to encode the data.
[0053] The light source may comprise one or more light emitters,
such as LEDs or OLEDs (organic light emitting diodes).
[0054] The light source may be a light source comprised in a
portable device such as a mobile phone/smartphone, a tablet
computer or the like.
[0055] The light source may comprise a screen or monitor on which
an image is produced by scanning and modulating a spot or a line.
Temporal modulation of the intensity of the spot or line may be
applied in a manner that has little effect on the perceived image
content yet encodes data temporally or spatially within the image
for later detection
[0056] The light source may comprise a display such as a computer
monitor or television screen or projection system or the like. The
transmission system may be configured to modulate the light
produced by at least a portion and preferably the whole of the
display so as to encode the data.
[0057] The light source may be configured to produce a plurality of
colours. The light source may comprise a plurality of light
emitters, at least one light emitter being configured to produce a
different colour to at least one other light emitter. The light
source may be configured to selectively produce a plurality of
colours by varying or controlling the colour temperature of at
least one and optionally each light emitter, which may comprise
individually controlling the colour temperature of light emitters.
The transmission system may be configured to encode the data by
encoding data elements by colour, for example, by selectively
activating an appropriately coloured light emitter to encode a data
element. The transmission system may be configured to encode the
data using colour shift keying, for example, by using mixes of
different colours in certain combinations to convey different bit
sequences.
[0058] The light source may comprise a plurality of spatially
separated light emitters. The transmission system may be configured
to encode the data by spatially encoding data elements, for
example, by selectively activating an appropriately positioned
light emitter to encode a data element.
[0059] The transmission system may be configured to encode data at
a rate in the order of and/or corresponding to the sample time
required for the detection system to sample each subset, for
example such that each individual data element (e.g. a bit) is
encoded by operating the light source for a period that is
substantially equivalent to, the same as or of the same order as
the sample time that each subset of sensing elements of the
detector system is sampled for.
[0060] According to the fourth aspect of the invention is a method
of communicating data, using the communication system of the third
aspect, the method comprising encoding data by modulating a
radiation signal and transmitting it using the transmission system
and receiving the radiation signal using the detection system and
sampling differing subsets of sensing elements of the detection
system at differing times and determining data encoded in the
radiation signal based on differences in the radiation detected by
the different subsets of sensing elements.
[0061] The method may comprise using the detection system described
above in relation to the first aspect.
[0062] According to a fifth aspect of the present invention is a
geolocation or navigation device comprising a detection device
according to the first aspect, wherein the detection device is
adapted to receive radiation signals that encode location data and
wherein the detection system is configured to sample differing
subsets of sensing elements of the detection system at differing
times and determine the location data encoded in the radiation
signal based on differences in the radiation detected by the
different subsets of sensing elements.
[0063] The location data may comprise a location identifier and/or
coordinates. The navigation device may be configured to access a
data store and access data associated with the determined location,
such as a location name, location coordinates, or commercial or
user interest data such as items for sale near the location,
advertisements, activities available near the location and the
like. The data associated with a determined location may comprise
information or a presentation associated with a display or exhibit
or other feature associated with the determined location, such as
information or a presentation associated with a display at a
determined location in a museum or with a painting at a determined
location in a gallery or with a location in a historical building
and so on.
[0064] The geolocation or navigation device may be comprised in or
configured for use with a geolocation and navigation system, the
geolocation or navigation system comprising at least one and
preferably a plurality of spatially separated transmission systems
according to the third aspect, each transmission system being
associated or associatable with a location and configured to encode
and transmit location data associated with the location and/or
transmission system.
[0065] According to a sixth aspect of the present invention is a
geolocation or navigation system comprising at least one and
preferably a plurality of spatially separated transmission systems,
the at least one transmission system comprising a light source and
being configured to modulate the output of the light source in
order to encode data for transmission to the detection system of
the first aspect, each transmission system being associated or
associatable with a location and configured to encode and transmit
location data associated with the location and/or transmission
system.
[0066] The geolocation and/or navigation system may be configured
for use with the one or more geolocation and/or navigation devices
according to the fifth aspect.
[0067] According to a seventh aspect of the present invention is an
apparatus reporting system comprising an apparatus configured to
monitor at least one parameter, the apparatus comprising at least
one transmission system, the at least one transmission system
comprising a light source and being configured to modulate the
output of the light source in order to encode data associated with
the at least one parameter for transmission to the detection system
of the first aspect.
[0068] The apparatus may comprise a light source such as a light
bulb. The apparatus may comprise a piece of plant or machinery. The
at least one parameter may comprise a parameter of the
apparatus.
[0069] According to an eighth aspect of the present invention is a
method of preventing copying of an image, the method comprising
displaying or encoding an image to be displayed such that the
intensity of the image is varied or modulated.
[0070] The intensity may be modulated or varied by switching
between one or more intensity levels. Varying or modulating the
intensity of the image may comprise lowering the intensity of at
least some and preferably each pixel of an image. The method may
comprise switching between intensity levels of the image at a
switching rate that is equal to or of the same order of magnitude
as a rolling shutter scan frequency used in a camera system, for
example, more than 30 times per second, preferably more than 150
times per second and most preferable more than 300 times per
second.
[0071] The image may comprise a moving image such as a movie or
television clip or one or more still images.
[0072] In this way, if someone tries to take an unauthorised
digital photograph or movie clip of the image using a camera that
employs a rolling shutter, then the resulting digital photograph or
movie clip would possess bands or stripes of varying intensity,
thereby spoiling the image. However, by providing this at a fast
modulation rate, the changes would be imperceptible to the eye,
which would only register the average intensity.
[0073] According to a ninth aspect of the present invention is
image carrier medium, comprising an image, and comprising means for
implementing the method of the eighth aspect.
[0074] The image may comprise a moving image, such as a movie,
film, television program, video clip or the like. The image may
comprise one or more still images. The image may comprise a digital
image.
[0075] According to a tenth aspect of the present invention is an
image display apparatus configured to display an image and adapted
to implement the method of the eighth aspect.
[0076] According to an eleventh aspect of the present invention is
a transmission system, the transmission system comprising at least
one transmitter element, the transmission system being configured
to encode at least one data symbol or element by providing one or
more signals in selected transmission carriers or channels, the
selection of transmission carriers or channels being representative
of the at least one data symbol or element, the transmission system
being configured to vary the output power of the transmission
system by varying the number of carriers or channels used to encode
at least some and optionally each data symbol or element.
[0077] The transmission system may be configured to perform dimming
and/or vary the level of dimming, which may be used to encode at
least some and optionally each data symbol or element.
[0078] Each data symbol or element may comprise one or more bits of
binary data.
[0079] The transmission system may be configured to transmit
communications signals in a communications system.
[0080] The transmission system being configured to encode data
symbols or elements by providing one or more signals in selected
transmission carriers or channels and leaving at least one
unselected channel inactive or having no or reduced signal.
[0081] The transmission system may be configured to encode one or
more of the data symbols or elements using spatial modulation or
space shift keying. The transmission may comprise a plurality of
spatially separated transmitter elements, such as radiation
emitters. Each transmission carrier or channel may comprise a
spatially separated transmission carrier or channel, wherein each
transmission carrier or channel may be associated with at least one
different and/or corresponding spatially separated transmitter
element.
[0082] The transmission system may be configured to encode data
symbols or elements by selectively activating and/or encoding data
symbols or elements using one or more transmitter elements and/or
selectively deactivating and/or not encoding data symbols or
elements using one or more transmitter elements. The transmission
system may be configured to selectively activate transmitter
elements, such as LEDs, so as to emulate a signal waveform, which
may be used to encode data symbols or elements.
[0083] The transmission system may be configured to encode at least
some of the units of data using temporal modulation. Each
transmission carrier or channel may comprise a different time
slot.
[0084] Each transmitter element may comprise an optical
transmitter, preferably one or more LEDs. The communications system
may comprise an optical communications system.
[0085] Data transmitted may comprise at least a first portion of
data comprising the at least one data symbol or element and at
least a second portion of data comprising at least one further data
symbol or element.
[0086] The transmission system may be configured to encode the
first portion of data using spatial or temporal modulation, for
example as spatial or temporal symbols. The transmission system may
be configured to encode the further data symbols or elements as
signal symbols, for example, by using amplitude or phase
modulation.
[0087] The transmission system may be configured to encode the
further data elements by modulating at least one, optionally a
plurality and preferably each transmission carrier or channel, i.e.
by performing intra-channel or intra-carrier modulation. The
intra-channel or intra-carrier modulation may comprise, for
example, M-QAM modulation, on-off keying, binary phase shift keying
or the like.
[0088] The signals may be receivable by a receiver. The receiver
may be configured to determine intensity differences or an
intensity pattern in the received signal, for example, caused by
different locations of the transmitter elements. The detector may
be configured to determine the data symbols or elements of the
first portion of data from the determined intensity differences,
for example, by comparing the determined intensity differences or
pattern with reference patterns associated with reference data
elements.
[0089] In this way, by providing a first portion of data encoded
using spatial or temporal encoding by selection of the transmission
channels used to provide signals and providing a further portion of
the data by conventionally encoding the data in each signal, the
bandwidth efficiency is increased relative to conventional signal
modulation techniques such as 4-QAM alone. Note that the order in
which encoding/decoding techniques applied is entirely
interchangeable, i.e. the first portion may be encoded using
conventional signal modulation, whilst the second portion may be
modulated using spatial and/or temporal modulation.
[0090] The signal power output by the transmission system may be
varied using the above method by varying the number of channels
used to encode each data element. For example, if spatial
modulation is used, a transmission system may be provided that
comprises a plurality (for example four) spatially separated LEDs
as transmitter elements. Each of the transmitter elements may be
assigned a binary combination, e.g. 00, 01, 10 or 11. In this way a
selected data element 00, 01, 10 or 11 can be transmitted at any
time by activating the associated LED. The further data elements
can be encoded by modulation of the output of the selected LED in
the usual manner, for example, by using 4-QAM. In this way, each
data element is transmitted using one selected transmitter element
in the form of an LED from the four LEDs at a time and the dimming
level is 25%. However, the data elements may also be encoded based
on which LED is switched off rather than which LED is switched on.
For example, instead of emitting light on a second LED from the
four LEDs to encode the data element 01, the first, third and
fourth LED may be turned on and the second LED may be turned off.
In this case, the dimming level of the transmitter is 75%. The data
elements may be similarly encoded using combinations of two LEDs
switched on and two switched off and assigning combinations to
represent particular data elements. In this way 50% dimming may be
achieved.
[0091] Similar effects may be achieved using time slots or
frequency slots (sub carriers) rather than spatial separation. In
this case, the dimming is achieved by varying the proportion of a
time slot or frequency slot (sub carrier) that the transmitter
elements are active for in order to encode the data elements.
[0092] Using the transmission system, variation in the dimming
level of the transmission system can be achieved without reducing
the transmission rate, which may be improved over conventional
signal modulation methods.
[0093] The transmission system may be configured to encode data
elements by varying the dimming and/or power and/or intensity
output by the transmission system. For example, each dimming level
of the transmission system may be associated with a value of a data
element and the dimming level may be controlled/switched in order
to encode the data elements. The dimming level may be used to
encode third data elements of a third portion of the data. In this
way, controlled dimming may be used to implement an M-PAM
(multilevel pulse amplitude modulation) encoding scheme without the
need for a digital to analogue converter at the transmitter to
generate the different intensity levels. This may be used
alternatively to or additionally with the modulation/encoding
schemes detailed above in order to provide an alternative encoding
scheme or increased bandwidth efficiency.
[0094] According to a twelfth aspect of the present invention is a
method for transmitting signals in a communications system, the
method comprising encoding data elements by providing one or more
signals in selected transmission carriers or channels, the
selection of transmission channels being representative of the data
elements, and varying the output power of the transmission system
by varying the number of carriers or channels used to encode each
data element.
[0095] The method may comprise using a transmission system
according to the previous aspect.
[0096] According to a thirteenth aspect of the present invention is
a communications system, comprising a transmission system as
described above and a receiver, the transmission system being
configured to encode data elements by providing one or more signals
in selected transmission carriers or channels, the selection of
transmission channels being representative of the data elements,
the transmission system being configured to vary the output power
of the transmission system by varying the number of carriers or
channels used to encode each data element, and the receiver being
configured to receive the signals.
[0097] The receiver may be configured to determine intensity
differences or an intensity pattern in the received signal, for
example, caused by different locations of the transmitter elements.
The detector may be configured to determine the data elements of
the first data from the determined intensity differences, for
example, by comparing the determine intensity differences or
pattern with reference patterns associated with data elements.
[0098] According to a fourteenth aspect of the present invention is
a communications method comprising encoding data elements by
providing one or more signals in selected transmission carriers or
channels, the selection of transmission channels being
representative of the data elements, varying the output power of
the transmission system by varying the number of carriers or
channels used to encode each data element, and receiving the
signals.
[0099] The method may further comprise determining intensity
differences or an intensity pattern in the received signal, for
example, caused by different locations of the transmitter elements.
The method may comprise determining the data elements of the first
data from the determined intensity differences, for example, by
comparing the determined intensity differences or pattern with
reference patterns associated with data elements.
[0100] According to a fifteenth aspect of the invention is a
receiver for use with the transmission system and/or the
communication system as described above, the receiver being
configured to determine intensity differences or an intensity
pattern in a received signal and determine data elements from the
determined intensity differences.
[0101] The receiver may be configured to compare the determine
intensity differences or pattern with reference patterns associated
with data elements in order to determine the encoded data
element.
[0102] According to a sixteenth aspect of the invention is a method
for receiving a signal from the transmission system and/or in
communication system as described above, the method comprising
determining intensity differences or an intensity pattern in a
received signal and determining data elements from the determined
intensity differences.
[0103] According to a seventeenth aspect of the invention is a
transmission system for transmitting data as part of a
communications system, the data comprising a plurality of data
symbols or elements, the transmission system being configured to
divide the data into at least a first data portion and a second
data portion, wherein the first data portion is communicated by
transmitting signals in selected carrier channels, wherein the
transmission system is configured to encode at least one data
symbol or element by selecting a relative order of at least one
first carrier channel having a first operational state and at least
one second carrier having a second operational state.
[0104] One of the first or second operational states may comprise a
signal being carried by the associated carrier channel. The other
of the first or second operational states may comprise an inactive
and/or unused and/or zero state carrier channel or transmitting a
signal at a level that is lower or otherwise distinguishable from
the signals of the first state.
[0105] The data symbol or element may comprise at least one bit of
binary data.
[0106] The signals being carried by the carrier channels may
comprise a modulated or encoded signal, such as a M-QAM signal. At
least one of the signals being carried by the carrier channels may
modulate or encode the second data portion.
[0107] The carrier channels may be sequential.
[0108] For example, one of a data bit 0 or 1 may be encoded by
providing a signal on a preceding or first carrier channel of a
pair of carrier channels and leaving a following or second carrier
channel of the pair of carrier channels inactive. The other of data
bits 1 or 0 may be encoded by leaving the preceding or first
carrier channel of the pair of data carrier channels inactive and
providing a signal on the following or second carrier channel.
[0109] At least one and optionally each carrier channel may
comprise a different frequency band or channel. At least one and
optionally each carrier may comprise a different time slot. At
least one and optionally each carrier may comprise a different
spatial position, for example, or a transmitter element such as an
LED.
[0110] The number of first carrier channels may be equal to the
number of second carrier channels.
[0111] The encoding may be based on a predetermined look-up table
or the like. The encoding may be based on an algorithm that matches
blocks of bits to a combination of carrier channels within a
sub-block of the total number of carrier channels.
[0112] The transmitter may be configured to convert at least one
bipolar signal into one or more unipolar signals by transmitting
only the absolute values of a bipolar signal and encoding the signs
separately. The signs may be encoded within the same frame,
preceding frames, or following frames. The signs may be encoded
within the relative order of the carrier channels, which may be
frequency, time, or spatial carrier channels, and may be encoded as
symbols that modulate the carrier channels, or may be encoded in a
separate modulation scheme on a separate part of the transmission
stream. The signs may also be conveyed to the destination on a
separate transmission channel, or a separate part of the
communication system.
[0113] Signs, phase or other information may be transmitted using
spatial and/or spectral modulation. For example, a first
transmitter element, such as a first LED, may be activated when the
sign is positive, and a second transmitter element, such as a
second LED, may be activated when the sign is negative. Similarly,
at least a pair of LEDs having different colours or an LED
configured to produce two or more colours (e.g. by varying it's
temperature) may be provided and the respective differing colours
may be associated with positive or negative signs respectively.
[0114] These techniques used to transmit signs need not be limited
to transmission of signs, e.g., they could be used to transmit
other data such as phase information. For example, phase
information may be encoded in the spatial domain, which may
comprise use of a transmitter with a plurality of transmitter
elements, such as LEDs, wherein use of selected transmitter element
may be indicative of a different phase. For example, the first
transmitter element may be indicative of a first phase, such as
45.degree., use of the second transmitter element may be indicative
of a second phase, such as 90.degree., use of the third transmitter
element may be indicative of a third phase such as 135.degree. and
use of the fourth transmitter element may be indicative of a fourth
phase such as 0.degree.. Whilst it will be appreciated that the
above example uses four phases and transmitter elements for use
with QPSK signals, it will be appreciated that other encoding
schemes and numbers of transmitters/phases may be used.
[0115] In a specific but non-limiting example, the transmitter may
be configured to convert at least one multipolar signal into two or
more unipolar signals. The unipolar signals may comprise, for
example, time resolved signals/signals modulated in the time domain
and/or frequency resolved signals/signals modulated in the
frequency domain and/or spatially resolved signals/signals
modulated in the spatial domain. At least one of the unipolar
signals may be inactive or have zero intensity or at least an
intensity that is distinguishable from any signal intensity used in
at least one other of the unipolar signals. At least one other of
the unipolar signals may have a magnitude that is equal or
equivalent to a magnitude of the multipolar signal. The transmitter
may be configured to encode a sign (e.g. positive or negative) of
the multipolar signal by using a relative order of at least two of
the converted unipolar signals. For example, if the unipolar signal
having the same magnitude as the original signal is provided first
and the inactive signal is provided second, then this may be
representative of a positive signal having a magnitude equal to the
first converted signal and if an inactive or zero converted signal
is provided first and a converted signal having the magnitude of
the original signal is provided second, then this may be
representative of a negative signal having a magnitude that is
equivalent to the magnitude of the second signal. It will be
appreciated that the orders used to represent positive and negative
signals may be reversed if preferred.
[0116] According to an eighteenth aspect of the invention is a
method for transmitting data in a communications system, the data
comprising a plurality of data symbols or elements, the method
comprising dividing the data into at least a first data portion and
a second data portion, communicating the first data portion by
transmitting signals in selected carrier channels, wherein the
relative order of at least one first carrier channel having a first
operational state and at least one second carrier having a second
operational state is representative of each data symbol or element
of the first data portion.
[0117] The method may comprise using a transmitter as described
above.
[0118] According to a nineteenth aspect of the invention is a
communications system comprising a transmission system as described
above and a receiver for receiving a data signal from the
transmission system, wherein the receiver is configured to
determine the relative order of at least one carrier channel having
a first operational state and at least one second carrier channel
having a second operational state in order to determine at least a
portion of the data.
[0119] According to a twentieth aspect of the present invention is
a method of communicating data that comprises a plurality of data
symbols or elements, the method comprising: [0120] dividing the
data into at least a first data portion and a second data portion,
communicating the first data portion by transmitting signals in
selected carrier channels, wherein the relative order of at least
one first carrier channel having a first operational state and at
least one second carrier having a second operational state is
representative of each data element or symbol of the first data
portion; [0121] receiving the signal from the transmission system,
determining the relative order of the at least one carrier channel
having a first operational state and the at least one second
carrier channel in order to determine at least the first portion of
the data.
[0122] The method may comprise a method as described above and/or
comprise use of a transmission system as described above and/or a
communications system as described above.
[0123] According to a twenty first aspect of the invention is a
transmitter and/or encoder for transmitting and/or encoding at
least one bipolar signal, the transmitter and/or encoder being
configured to encode a magnitude or absolute value of the at least
one bipolar signal into at least one unipolar signal and further
configured to encode and/or transmit a sign or phase of at least
one bipolar signal separately and/or differently to the
corresponding magnitude or absolute value of the at least one
bipolar signal.
[0124] The signs or phases of the at least one bipolar signal may
be encoded within the same frame, preceding frames, or following
frames. The signs or phases may be encoded within the relative
order of carriers that carry the unipolar signals, which may be
frequency, time, or spatial carriers, and may be encoded as symbols
that modulate the carriers, or may be encoded in a separate
modulation scheme on a separate part of the transmission stream.
The signs or phases may also be conveyed to the destination on a
separate transmission channel, or a separate part of the
communication system.
[0125] Optionally but not essentially, the transmitter and/or
encoder may be configured to encode each bipolar signal into two or
more corresponding unipolar signals, which may be encoded on first
and second carrier channels. The transmitter may be configured to
encode the sign or phase of the bipolar signal based on the
relative order of the first and second operational states. One of
the first or second operational states may be indicative of the
magnitude or absolute value of the bipolar signal.
[0126] According to a twenty second aspect of the present invention
is a receiver for receiving a signal from a transmission system,
the receiver being configured to receive at least one unipolar
signal from the transmission system, determine a magnitude of at
least one bipolar signal from the at least one unipolar signal and
determine a sign or phase of the at least one bipolar signal,
wherein the sign or phase of the at least one bipolar signal is
encoded and/or transmitted separately and/or differently to the
corresponding magnitude of the at least one bipolar signal.
[0127] The receiver may be configured to reconstruct the bipolar
signal using the determined magnitude and sign or phase of the
bipolar signal.
[0128] The signs or phases of the at least one bipolar signal may
be encoded within the same frame, preceding frames, or following
frames. The signs or phases may be encoded within the relative
order of the carriers, which may be frequency, time, or spatial
carriers, and may be encoded as symbols that modulate the carriers,
or may be encoded in a separate modulation scheme on a separate
part of the transmission stream. The signs may also be conveyed to
the destination on a separate transmission channel, or a separate
part of the communication system.
[0129] Optionally but not essentially, the receiver may be
configured to determine the relative order of at least one carrier
channel having a first operational state and at least one second
carrier channel having a second operational state in order to
determine the sign or phase of the bipolar signal based on the
relative order of the first and second operational states.
[0130] The receiver may be configured to receive a signal from a
transmission system as described above and/or be configured for use
in a communications system as described above.
[0131] According to a twenty third aspect of the present invention
is a method for decoding a signal received from a transmission
system, the method comprising receiving at least one unipolar
signal from the transmission system, determining a magnitude of at
least one bipolar signal from the at least one unipolar signal and
determining a sign or phase of the at least one bipolar signal,
wherein the sign or phase of the at least one bipolar signal is
encoded and/or transmitted separately and/or differently to the
corresponding magnitude of the at least one bipolar signal.
[0132] The method may comprise receiving a signal sent using the
method as described above or from a transmission system as
described above.
[0133] According to a twenty fourth aspect of the present invention
is a method of converting at least one bipolar signal into at least
one unipolar signal, the method comprising determining a sign or
phase of at least one component of the bipolar signal, encoding
and/or transmitting the absolute values of a bipolar signal in the
unipolar signal and encoding and/or transmitting the sign or phase
of the at least one bipolar signal separately and/or differently to
the encoding and/or transmitting the absolute values of a bipolar
signal.
[0134] For example, the method may comprise converting at least one
of the components of the multipolar signal into corresponding first
and second unipolar signal components. The first and second
unipolar signal components may have different amplitudes or
magnitudes. The order of the first and second unipolar signal
components may be dependent on the sign or phase of the
corresponding multipolar signal component.
[0135] At least one of the first or second unipolar signal
components may be indicative of the intensity or magnitude of the
corresponding multipolar signal component. The other of the first
or second multipolar signal components may have an amplitude or
magnitude of zero and/or comprise an inactive or empty carrier
channel.
[0136] The order of the first and second unipolar signal components
over time may be dependent on the sign or phase of the
corresponding multipolar signal component. The first and second
unipolar signal components may be resolved and/or separated in the
time, frequency and/or spatial domains.
[0137] According to a twenty fifth aspect of the present invention
is a computer program product adapted to implement the apparatus or
method of one or more of the preceding aspects.
[0138] According to a twenty sixth aspect of the present invention
is a carrier medium comprising the computer program product of the
twenty fifth aspect or a programmable apparatus when programmed
with the computer program product of the twenty fifth aspect.
[0139] It will be appreciated that features analogous to those
described above in relation to any of the above aspects may be
equally applicable to any of the other aspects.
[0140] Apparatus features analogous to those described above in
relation to a method and method features analogous to those
described above in relation to an apparatus are also intended to
fall within the scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0141] Examples of the present invention will be described in
relation to the following drawings:
[0142] FIG. 1 is a schematic of communication system;
[0143] FIG. 2 is an expanded view of a photodetector of a detection
system for use in the communications system of FIG. 1;
[0144] FIG. 3a is an image generated by a detector of the system of
FIG. 1;
[0145] FIG. 3b is another image generated by the detector of the
system of FIG. 1;
[0146] FIG. 3c is another image generated by the detector of the
system of FIG. 1;
[0147] FIG. 4 is an illustration of a spatial modulation
system;
[0148] FIG. 5 is a plot of bandwidth efficiency and dimming for a
spatial modulation system having thirty two transmitter
elements;
[0149] FIG. 6 is an illustration of a prior art SIM-OFDM
method;
[0150] FIG. 7 is a plot showing the performance differences between
SIM-OFDM and OFDM for differing QAM constellation sizes;
[0151] FIG. 8 illustrates an encoding or modulation method;
[0152] FIG. 9 shows a specific example of the method of FIG. 8,
where the total number of carriers is six and the number of active
carriers is three;
[0153] FIG. 10a shows an OFDM signal in the time domain;
[0154] FIG. 10b shows the OFDM signal of FIG. 10a that has been
subjected to a DC shift;
[0155] FIG. 10c shows the OFDM signal of FIG. 10a transformed using
the method of an embodiment of the present invention;
[0156] FIG. 11 shows the performance of the method illustrated in
FIG. 8 relative to OFDM;
[0157] FIG. 12 shows the performance of the method illustrated
using FIG. 10c relative to ACO and OFDM for bipolar signals as a
function of the electrical signal to noise ratio;
[0158] FIG. 13 shows the performance of the method illustrated
using FIG. 10c relative to ACO and DCO for unipolar signals as a
function of the electrical signal to noise ratio;
[0159] FIG. 14 shows the bit error rate performance of the method
illustrated using FIG. 10c relative to ACO and DCO as a function of
the optical signal to noise ratio;
[0160] FIG. 15 schematically illustrates a method of communicating
information;
[0161] FIG. 16 schematically illustrates a suitable
encoding/transmitter apparatus;
[0162] FIGS. 17A to 17G schematically illustrate an example of how
information (FIG. 17A) is divided into a first information portion
(FIG. 17B) and a second information portion (FIG. 17C), the
allocation of different modulations types to different orthogonal
subcarriers based on the first information portion (FIG. 17D),
which symbols from the second information portion are modulated
onto which subcarriers (FIG. 17E), the recovery of a data word
representing the first information portion (FIG. 17F) and the
recovery of a data word representing the second information portion
(FIG. 17G);
[0163] FIGS. 18A to 18I schematically illustrate an example of how
information (FIG. 18A) is divided into a first information portion
(FIG. 18B), a second information portion (FIG. 18C) and a third
information portion (FIG. 18D), which different modulations types
are allocated to which orthogonal subcarriers based on the first
information portion (FIG. 18E), which symbols from the second
information portion are modulated onto which subcarriers and which
symbols from the third information portion are modulated onto which
subcarriers (FIG. 18F), the recovery of a data word representing
the first information portion (FIG. 18G), the recovery of a data
word representing the second information portion (FIG. 18H) and the
recovery of a data word representing the third information portion
(FIG. 18I);
[0164] FIG. 19 schematically illustrates a method for selecting an
encoding scheme from a plurality of different encoding schemes,
each of which has a different allocation of modulation types to the
first information portion;
[0165] FIG. 20 schematically illustrates a suitable
decoding/receiver apparatus;
[0166] FIG. 21 schematically illustrates a time-frequency
block;
[0167] FIG. 22 illustrates a cyclic delay diversity modulation
scheme;
[0168] FIG. 23 schematically illustrates a time-frequency-spatial
block;
[0169] FIG. 24 schematically illustrates an example of multi-domain
encoding;
[0170] FIG. 25 schematically illustrates a transmitting and
receiving apparatus suitable for applying multi-domain
encoding;
[0171] FIG. 26 is a graph showing the Average Bit Error Probability
(ABEP) when a fading correlation model is considered assuming a
uniform power allocation mechanism among the transmit antennas
according to the prior art;
[0172] FIG. 27 is a graph showing an analysis of the ABEP for
uncorrelated fading channels and for a different power imbalance
among wireless links assuming a uniform power allocation mechanism
among the transmit antennas according to the prior art;
[0173] FIG. 28 is a graph showing an analysis of the ABEP for
correlated fading channels and for a different power imbalance
among the wireless links assuming a uniform power allocation
mechanism among the transmit antennas according to the prior
art;
[0174] FIG. 29 is a graph showing the ABEP for correlated fading
assuming a uniform power allocation mechanism among the transmit
antennas according to the prior art;
[0175] FIG. 30 shows a system setup for SM-MIMO;
[0176] FIG. 31 shows a system setup for SM-MIMO;
[0177] FIG. 32 shows a system setup for SM-MIMO;
[0178] FIG. 33 shows numerical results obtained when employing a
scheme according to the prior art;
[0179] FIG. 34 shows numerical results obtained when employing
another scheme according to the prior art;
[0180] FIG. 35 shows numerical results obtained when employing the
Time-Orthogonal Signal Design (TOSD)-SM scheme;
[0181] FIG. 36 shows a comparison among various Spatial Modulation
proposals; and
[0182] FIG. 37 shows a system setup for SM-MIMO.
DETAILED DESCRIPTION OF THE DRAWINGS
[0183] Embodiments of the present invention relate to systems that
time modulate radiation to encode and transmit data or other
information. In a specific embodiment that relates to light
communications, the intensity of light produced from a light source
is modulated over time in order to encode data or other
information. A photo-detector can then be used to receive the
time-modulated signal which is decoded to reveal the data which was
transmitted by the illumination source.
[0184] A digital camera can be conveniently used to receive the
modulated light signal, which is then processed to extract the
encoded data. In order to achieve an acceptably quick transmission
rate and/or achieve communication without too much noticeable light
flickering by the transmitting light source(s), the photo-detector
must meet certain requirements, for example, by having an image
capture rate (i.e. frames per second) above a certain minimum that
is dependent on the application. However, at the same time, it
would be beneficial if such communications methods could be used
with common or off-the shelf apparatus.
[0185] In particular, it would be beneficial to be able to use a
portable communications device that comprises a digital camera,
such as a smart phone or tablet computer to receive and process the
modulated light signals or to use the apparatus commonly used in
such devices to provide a low cost, readily available receiver.
However the image capture rate (frames per second) of the cameras
provided with such devices is often too low to permit a desired
data transmission rate to be achieved. For example, digital cameras
used in portable devices typically have frame capture rates of less
than and up to 30 frames per second. If a visible light signal was
captured at one sample per frame then the maximum bit rate would
only be 30 bits per second. In addition, data transmission at these
slow rates may result in excessive flickering of the light source,
which can be disconcerting and off putting for users.
[0186] Advantageously, the present inventors have found that a
previously undesirable artefact of camera devices that use rolling
shutter techniques in order to capture images can be advantageously
used to increase the data receiving rate of light communication
systems.
[0187] FIG. 1 shows a communications system 5 that operates using
light modulation to encode and transmit data. The system comprises
a transmitter 10 and a receiver 15.
[0188] The transmitter comprises a light source 20, such as a light
source comprising one or more LEDs 25a, 25b, 25c. The light source
20 is coupled to a transmitter processor 30 that converts data to
be transmitted into a modulation scheme indicative of the data. In
the present embodiment, the modulation used comprises intensity
modulation of the light emitted by the light source 20.
Specifically, use of on-off intensity modulation is described.
However, it will be appreciated that other intensity modulation
schemes, such as multi-level intensity modulation, may also be
used. It will also be appreciated that alternative or additional
modulation schemes such as colour or spatial modulation may be
used.
[0189] The receiver 15 comprises a digital camera 35. In the
present embodiment, the digital camera 35 is conveniently
incorporated into a portable electronics device 40 such as a smart
phone, a tablet computer or a palmtop computer, as shown in FIG. 1.
The digital camera 35 comprises a CMOS photodetector array 45 that
comprises a pixelated grid matrix of photodiodes, as is known in
the art, and shown in FIG. 2. The portable electronics device 40 is
configured to operate the camera 35 using a rolling shutter or a
line scan method. In this way, when an image is to be taken using
the camera 35, the portable electronics device 40 sequentially
samples the CMOS photodetector array 45 one line 50a, 50b, 50c, 50d
(i.e. a column or row) of photodiodes at a time to generate
corresponding columns or rows of an image until all of the lines of
the photodetector 45 have been sampled. Each of the rows or columns
of the image collected using each of the lines of pixels of the
CMOS photodetector 45 are subsequently combined to produce an
entire image. Although such rolling shutters or line scans are
often produced electronically by sequentially sampling lines of
pixels of the detector, it will be appreciated that the rolling
shutter may alternatively be implemented using other techniques,
such as a mechanical shutter.
[0190] By using a rolling shutter, the processing resource required
to collect the image is greatly reduced, thus allowing a lower cost
camera arrangement and reducing the processing overhead required
relative to systems that collect the entire image concurrently
(i.e. a global shutter). Furthermore, it is possible to increase
the sensitivity of the detector. However, in some situations, the
use of this technique can also result in undesirable artefacts in
the image obtained. For example, if an image of a fast moving
object is being taken, the resulting image can appear skewed, since
each line of the image is taken at a different time period and not
simultaneously. Therefore, a fast moving object often moves in the
time between each line of the photodetector being sampled such that
the corresponding lines of the image are not in register, resulting
in the fast moving object appearing smeared or skewed in the
resulting image. Furthermore, if the lighting conditions change
suddenly or rapidly during the collection of the image, then this
can result in a change in brightness and/or contrast of the image
across the image. The change in brightness and/or contrast may be
continuous or result in bands of varying brightness and/or contrast
in the picture depending on the speed of the change in lighting
relative to the scan rate of the detector. This effect may result
from, for example, a camera flash being fired during only part of
an image collection (and thereby affecting some scanned lines but
not others) or as a result of strobe lights (e.g. the flashing
lights of emergency vehicles) or during light bursts having varying
intensity, such as lightning. These artefacts are typically
undesirable, and it is usual for manufacturers and users to wish to
minimise or eliminate these effects.
[0191] However, in at least one embodiment of the present
invention, artefacts of the rolling shutter are advantageously used
to increase the data collection rate of the detector when used as a
receiver 15 for a visible light communications system 5. In this
case, the transmitter processor 30 modulates data that comprises a
header, a payload and a tail, into a light signal 55 by modulating
the light produced by the light source 20. In this embodiment the
modulation is by intensity modulation of the produced light, and
specifically by on-off keying (OOK). In this case, the light source
20 is turned on and off, each of the on-and off states representing
a different binary bit.
[0192] The suitably programmed and configured portable electronics
device 40 is used to collect an image that is illuminated by the
modulated light source 20. In this case, the camera 35 is pointed
at a surface 60 illuminated by the light source 20, or
alternatively can be pointed towards the light source 20 itself. A
processor of the portable device 40 activates the camera 35 in
order to capture one or move images illuminated by the light source
20. The captured images can comprise one or more "still" images or
the captured images can be comprised in a movie, for example as
frames of the movie.
[0193] The light source 20 is modulated at a rate that is
correlated with or in the range of the rolling shutter or line scan
rate of the camera 35. For example, the intensity of the light
source 20 can be modulated at a predetermined rate that is
equivalent to the time taken for a predetermined number of lines of
the photodetector 45 to be scanned or sampled. For example the
predetermined rate could be equivalent to the time taken to scan or
sample one or alternatively two or more columns 50a, 50b, 50c, 50d
or rows of the photodetector 45 in order to minimise
synchronisation effects. It will be appreciated that the modulation
rate need not exactly correspond to a line scan rate (e.g. due to
differences in synchronisation between the transmitter 10 and
receiver 15) and that the technique would still work if the
majority of a particular data collection period corresponded to
receiving the desired light intensity, such that once the signal is
averaged or binarised over all of the photodiodes in the row or
column 50a, 50b, 50c, 50d of the photodetector 45, the various
intensity levels of the modulation scheme can still be
discriminated.
[0194] In this way, the images collected by the digital camera 35
will contain horizontal or vertical stripes 65 of differing
brightness or intensity corresponding to the switching on or off of
the light source 20 that encodes the OOK data. The processor of the
portable device 40 can then decode these stripes 65 to reveal the
encoded data.
[0195] Using this technique, the maximum rate of the data transfer
is determined by the camera 35 itself. For example, in video camera
mode, an Apple iphone 4 has 720 horizontal lines (720 rows of
pixels) and the frame rate is 30 frames per second. In theory, up
to 720.times.30=21,600 samples per second can be obtained due to
the rolling shutter. However, since the transmitter may not be
synchronised to the camera clock, a bit of information (i.e. a
pulse of illumination, or period of light or dark) may lie between
the samples and so the actual data rate may advantageously be made
lower than the theoretical maximum rate to avoid the need for
accurate synchronisation. For example, the data rate used may be 10
kbit/s.
[0196] The image could optimally be taken of a plain, light
coloured surface 60. However, any illuminated surface 60 or even
object can be used, since the "image noise" can be removed by
post-processing. The image does not need to be focussed.
[0197] The received images are decoded by measuring the average
intensity of each row (or column 50a, 50b, 50c, 50d, depending on
which is scanned as part of the rolling shutter) of the image and
the intensities normalised between 100% and 0%. After any filtering
and normalisation the data can be obtained via a binary
threshold.
[0198] As an example, the surface 60 shown in FIG. 3a is
illuminated using an LED based light source 20. The surface is
illuminated by a white LED lamp. FIG. 3a shows the image collected
when no modulation is applied to the light source 20. FIG. 3b shows
the image obtained when the LED 20 has been modulated by a square
wave, i.e. in a 101010101010 sequence. In this case, each of the
dark bands 65a correspond to 0's whilst the light bands 65b
correspond to 1's (although the inverse may be alternatively used).
FIG. 3c shows the image collected when the LED 20 is modulated with
data using OOK and the data sequence shown is decoded as
001011100110. In this case, the variation in intensity detected
within each image collected can be advantageously used to determine
data encoded in a modulated light signal at a higher detection rate
than would be possible if each frame/image was used to detect each
bit of information, by utilising an artefact of rolling shutter or
line scan techniques that was conventionally considered to be
generally undesirable.
[0199] The portable device 40 is configured to apply
post-collection processing to the collected images. For example,
image processing can be used to remove the background from the
collected image(s). This is particularly beneficial if the camera
35 is pointed at an object rather than a blank surface 60 or the
light source 20 itself. A particularly unfavourable background is
one having horizontal stripes which run parallel to the rows of the
photodetector 45. In this case, if background removal is not
performed, then this can cause false readings in the data extracted
from the image. Two examples of suitable methods for reducing the
image noise are detailed below, which may be used alone or in
combination. However, it will be appreciated that a skilled person
could apply other suitable noise reduction and/or background
cancellation techniques.
[0200] As an example of a background removal/noise reduction
method, an entire frame or several frames could be periodically
collected with no modulation of the light produced by the light
source 20 (or the light being provided at a known intensity). In
this way, an image such as that shown in FIG. 3a is obtained. The
image within these un-modulated frames can then be filtered out of
subsequent frames that are subject to data modulated illumination
in order to remove noise due to the background image.
[0201] As another example of a background removal/noise reduction
method, a series of frames/images that comprise illumination using
modulated light can be averaged to obtain the background image,
which can then be subtracted from subsequent frames/images. This
method advantageously generates the background image "in use" and
does not require the system to take a dedicated background image
under specified lighting, thereby increasing the time available for
transmitting data. However, this method may require a good balance
of ones and zeros in the signal and averaging may need to be
carried out over several frames/images in order to obtain an
acceptable background image. This technique is also particularly
suited to slow moving images (when the camera is held still).
However, filtering across adjacent lines of the detector to
normalise the intensities can be used to reduce some of the noise
caused by zero/one imbalances.
[0202] The portable device 40 is optionally configured to apply
brightness and contrast normalisation. If an image is bright in one
area and dark in another area, the image can be normalised. This
variation in brightness/contrast within the background image might
be caused by, for example, less light falling on parts of a surface
or by the differing reflective properties of the surfaces within an
image. Adjusting or normalising the brightness and contrast over
the area of an image can make it easier for the data to be
recovered.
[0203] Another technique that is optionally applied is clock
recovery. To minimise errors in the decoding of the data, the
timing of each bit must be known with a reasonable degree of
accuracy. This can be done with off-line processing. For example,
the transmitter clock can be assumed to be stable across many
frames/images. The clock parameters to be determined are the
frequency and phase and these can be determined from multiple bits
and/or frames/images by detecting the position of data changes
relative to the detected frame/image rows.
[0204] In addition, standard forward error correction methods can
be used with the data to reduce bit errors. Because the data can be
post-processed over many frames there is considerable scope for
interference cancellation and other signal reconstruction
techniques known in the art.
[0205] Various techniques may be used in order to increase the data
transmission rate. For example, it will be appreciated that other
modulation schemes may be used alternatively or additionally with
the intensity modulation scheme described above.
[0206] For example, spatial modulation may be used. In a particular
example of this, the light source comprises a plurality of LEDs
25a, 25b, 25c, the LEDs 25a, 25b, 25c being spatially separated,
for example, in a row or column direction (depending on whether the
photodetector 45 is sampled on a row by row or column by column
basis respectively). In this case, the data may be encoded
temporally by exploiting the rolling shutter or line scan method,
as detailed above, and also spatially in order to increase the data
transmission rate.
[0207] As another additional or alternative modulation example,
colour modulation can be used. In this case, the light source is
provided with a plurality of light emitters 25a, 25b, 25c of
varying colour or is provided with one or more light emitters 25a,
25b, 25c whose colour is changeable and/or selectable, for example
by varying the colour temperature of the required light emitter
25a, 25b, 25c. In this way, further data may be encoded by
modulating and detecting/extracting data by colour, or colour
temperature. In a particularly advantageous example of colour
modulation, colour shift keying can be used where a mix of
different colours is used in certain combinations to convey
different bit sequences, to thereby increase the data transmission
rate.
[0208] Furthermore, varying intensity modulation schemes can be
used to encode the data. For example, although the embodiment of
the invention that is described above uses on-off keying, it will
be appreciated that a multi-level intensity modulation scheme may
be used to encode the data, in which a three or more pre-determined
intensity levels can be assigned to represent given data elements
rather than just on or off, in order to increase the data
transmission rate.
[0209] In addition, techniques known in the art of
telecommunications such as Interference cancellation and multi-user
detection can be used for multiple user access and removal of
co-channel interference such as successive interference
cancellation, joint detection, joint transmission, precoding
techniques (e.g., dirty paper coding), superposition coding or
minimum mean square error (MMSE) estimation. Alternatively,
multiuser interference can be avoided using coordinated multiuser
access using techniques such as TDMA (time division multiple
access), FDMA (frequency division multiple access), SDMA (space
division multiple access), or CDMA (code division multiple
access).
[0210] In an embodiment of the present invention, the portable
device is equipped with an ambient light sensor (not shown). These
may be provided in the device to determine an overall or average
light level experienced by the sensor, the results of which can be
used, for example, in the calculation of exposure times for a
camera 35 or to determine a parameter of an optical sensor. The
ambient light sensor can optionally be utilised to improve
operation of the device as a detector in a communications system 5.
For example, the ambient light sensor can be used additionally or
alternatively to the camera 35 to decode the data if the ambient
light sensor can react and be read fast enough. In cases where both
the ambient light sensor and the camera 35 are used, there is thus
an alternative or secondary source of information to that received
by the camera 35. The ambient light sensor can also be used as an
input to any post processing, e.g. to help normalise the
images.
[0211] In the examples described above, an LED based lamp or array
25a, 25b, 25c is used as a light source 20. However, it will be
appreciated that this need not be the case. For example, the screen
or monitor of another device could be used as the light source.
This may be achieved by way of example through the modulation of a
backlight of an LCD display or the image displayed on the monitor
or screen can be varied in order to modulate the light intensity
emitted by the monitor. In another example, a light source
comprised in a portable device such as a mobile phone/smartphone, a
tablet computer could be used. For example, the light source could
comprise an LED light source provided in a portable device for
illumination, for example, when taking a picture with an in-build
camera. In this way, using the LED or screen of the portable device
as a transmitter and a camera of a portable device as a receiver
may permit device to device optical communications.
[0212] Although visible light is preferably used, it will be
appreciated that non-visible portions of the electromagnetic
spectrum may be used, such as infra-red or near-infra red
radiation. Use of these wavelengths can be advantageous in some
applications in that these wavelengths are detectable by many
readily available photodetectors but are not visible to a user of
the system and do not interfere with or distract their vision. Use
of these wavelengths also allows use of existing systems such as
presence detection systems that operate using near infra red or
infra red sensors to operate as detectors in a communication system
such as that described above.
[0213] It will be appreciated that the system 5 described above
allows communications based on light modulation to be more widely
used and lead to creative ways to communicate. For example, users
may be able to download an "app" or other program that allows their
smart phone or other mobile electronics device to be used to
receive and utilise data communicated using light modulation.
[0214] For example, a supermarket or other store could provide
various light sources 20 such as LED's in the store, each of which
can be modulated to encode and repeatedly transmit an associated
identifier. The store or supermarket could provide an application
that, when run by the user, activates a camera 35 of the user's
smart phone 40 and implements the camera operation scheme described
above in order to collect and extract data encoded in the images
received by the camera 35 and exploiting the properties resulting
from the rolling shutter. In this way, when the camera 35 of the
smart phone 40 collects an image that is illuminated by the light
emitted by a particular light source, the processor of the smart
phone 40 is operable to extract the identifier encoded in the light
emitted by the particular light source. The downloaded application
is then operable to access a look-up table or database that
contains details of where the detected light source (and thereby
the user) is located in the store. This location information can be
used, for example, to guide the user around the store or to
download comments, advertisements or offers relating to stock that
is held near that location. In addition, the determined location
data can also be used by the store, for example, in aggregating
consumer behaviour such as who was shopping, how long they spent in
each area and so on. Other examples include the ability to provide
targeted promotions and/or tie the collected location data with
loyalty scheme data in order to provide cross referenced
information, such matching location and actual purchase data.
[0215] Another example of an application of the above transmission
technique is a smart light bulb. For example, an LED light bulb may
be provided that contains a processor and memory that allows it to
log usage and other data such as a unique ID, lamp type and spec,
the date and time it was first switched on, its on/off operations
or usage since first activation, its total power consumption and
the like. The data can be encoded by modulation of the light
produced by the LED as described above to transmit the information.
The user can then access the usage data for that particular light
source, simply by pointing their smart phone 40 or other camera
containing device at the light and running an application that
implements the camera 35 operation and data extraction scheme
detailed above in order to extract the encoded data.
[0216] Of course, other applications will be apparent to a skilled
person. For example, data may be encoded within a computer program
or television program or advertisement by modulating the intensity
of the computer monitor or TV screen to encode the data. The user
may then simply point the camera 35 of their camera based
electronic device 40 (e.g. smart phone) at the monitor or screen
and activate a previously downloaded application that implements
the above camera operation scheme and associated data extraction.
In this way, the computer program, television program or
advertisement can be used to transmit an activation code to the
phone which is used to determine an associated action contained in
a look-up table in the phone. In this way, a degree of interaction
between the computer program, television program and/or
advertisement is possible using the user's mobile phone 40.
[0217] Further embodiments of the invention are now described.
[0218] FIG. 4 shows a spatial modulation system 1005 for
illustrating an embodiment of the present invention. The system
comprises a transmitter 1010 that, in this embodiment, comprises
four transmitter elements in the form of LEDs (Tx0, Tx1, TX2, Tx3).
The transmitter 1010 comprises or is communicatively linked to
processing apparatus (not shown) for encoding data comprising one
or more data elements (e.g. one or more bits of binary data) into
light signals for transmission to a corresponding receiver (not
shown) by controlling and modulating the operation of the
individual transmitter elements (Tx0, Tx1, TX2, Tx3).
[0219] The receiver comprises optical detection means known in the
art such as one or more CMOS or CCD photodetectors, photodiode
arrays or the like, for detecting the optical signal emitted from
the transmitter. The receiver, for example, may comprise the
receiver 15 shown in FIG. 1. The receiver also comprises or is
communicatively linked to a receiver side processing apparatus for
recovering the encoded data from the received signal. The receiver
is thereby configured to determine intensity differences or an
intensity pattern in a received optical signal and determine data
elements from the determined intensity differences. For example,
the determined intensity differences or pattern is compared with
reference patterns associated with reference data elements in order
to determine the encoded data element.
[0220] Each transmitter element (Tx0, Tx1, TX2, Tx3) is operable to
selectively transmit a 4-QAM (quadrature amplitude modulation)
encoded signal. Each transmitter element (Tx0, Tx1, TX2, Tx3) is
able to transmit the 4-QAM-symbol indicated by the two-bit
sequences shown in bold, which will be referred to as a
signal-symbol. In addition, each transmitter (Tx0, Tx1, TX2, Tx3)
is assigned a unique two-bit sequence (shown in normal type);
referred to this a spatial-symbol. In this way, for example, a four
bit data unit can be encoded by generating a light signal by using
an LED (Tx0, Tx1, TX2, Tx3) associated with two of the bits of the
data unit and encoding the other two bits of the data unit in the
signal using signal modulation schemes known in the art, such as
4-QAM. At every transmission step, four bits (two that constitute a
signal-symbol, and two that constitute a spatial-symbol) are
transmitted within the bandwidth of 4-QAM. The optical receiver is
able to detect the spatial-symbol by the exploitation of intensity
differences at the receiver caused by the different locations of
the LEDs. The signal-symbol is decoded in a traditional way using
an appropriate detector. As a consequence the bandwidth efficiency
is doubled compared to pure 4-QAM.
[0221] In the Example shown in FIG. 4, if the transmitter 1010 is
to send a data [0 1 1 1], then the transmitter element TX selects
the transmitter element Tx1 to transmit a 4-QAM modulated signal
that is representative of the bit sequence [1 1]. Since the emitter
Tx1 is associated with the bit sequence [0 1], when the receiver
detects an intensity distribution pattern indicative of a signal
emitted from the position of Tx1, the receiver determines that the
first bits are [0 1]. The receiver then decodes the modulated
signal to extract the sequence [1 1] from the 4-QAM modulation
scheme in the usual fashion. The original data unit of [0 1 1 1]
can then be reconstructed.
[0222] In the example given above, only one out of four transmitter
elements (Tx0, Tx1, TX2, Tx3) are on at any given time. In this
case, the dimming is 25%. However, the transmitter 1010 is
switchable between differing dimming states, i.e. different output
powers or intensities, in a manner that integrates dimming and data
transmission. It does this by switching between modes in which a
differing number of LEDs are used to send each signal, i.e. to
encode the spatial symbol. This allows for a variation in dimming
levels of the transmitter 1010, and at the same time maintains the
high bandwidth efficiency for digital data transmission.
[0223] For example, 75% dimming can be achieved by an inversion of
the transmission convention for the spatial-symbol. Specifically,
each spatial-symbol is represented by switching all LEDs on, but
one. The spatial-symbol determines which LED is off. For example,
if the sequence [1 0] is to be transmitted in the spatial domain,
Tx2 would be switched off.
[0224] 50% dimming can also be achieved by switching on two LEDs at
any given time instance and having the remaining LEDs switched off.
There are 4!/(2!.times.2!)=6 combinations for selecting two out of
four LEDs, where "!" denotes factorial. With 6 combinations .left
brkt-bot.log.sub.2(6).right brkt-bot.=2 bits can be encoded, where
.left brkt-bot..cndot..right brkt-bot. denotes rounding to the
lower integer.
[0225] In the above dimming scheme, two more combinations are
obtained than would be required to encode 2 bits. This excess of
combinations can be utilised to select the combinations that
minimise the error performance at the receiver, the error
performance at the receiver being determined using methods known in
the art, for example, maximum likelihood (ML) detection, sphere
decoding, maximum receive ratio combining (MRRC), and the like.
[0226] With the above method, the dimming is changed between 25%,
50% and 75% without affecting the transmission rate, which is still
twice as high as can be achieved with conventional 4-QAM.
[0227] Although an example is described above that uses four
transmitter elements (Tx0, Tx1, TX2, Tx3), this concept can be
generalised to other numbers of transmitter elements. If N
transmitter elements in the form of LEDs are provided in an array,
the dimming level can be specified by L=(n/N).times.100%, where n
is the number of simultaneously active LEDs. The bandwidth
efficiency, .eta., is the number of bits transmitted per
transmission step. In conventional on-off-keying (OOK) systems,
.eta.=1. Here, .eta. can be determined as follows:
.eta.=[log.sub.2(N!/(n!(N-n)!))]
[0228] For example, for N=32, the bandwidth efficiency and
corresponding dimming levels are depicted in FIG. 5.
[0229] In this example, the minimum bandwidth efficiency is five
(for dimming of 1/32.times.100%=3.1% and 31/32=96.9%) which, for a
transmitter having thirty two LEDs, is five times higher than in
OOK. The bandwidth efficiency is even further improved when
approaching a dimming of 50%. The maximum is achieved for 50%
dimming, and is a factor of twenty nine times larger than OOK. For
example, if OOK achieves 10 Mbps, with this system and 50% dimming,
a transmission rate of 290 Mbps can theoretically be achieved.
[0230] Although the above example uses the spatial domain to encode
a portion of the data, the same technique is also applicable to the
frequency and/or time domains.
[0231] In the case of time domain encoding, instead of assigning
symbols or data units to corresponding spatially separated LEDs or
combinations of spatially separated LEDs, the transmitter is
configured to associate time slots with particular data units or
symbols. For example, the signal could be transmitted by a single
LED or group of LEDs and the signal is divided into sequential time
windows, each time window having, for example, four sequential time
slots. In this case, the first time slot may be associated with a
symbol of [0 0], the second time slot may be associated with a
symbol of [0 1], the third time slot may be associated with a
symbol of [1 0] and the fourth time slot may be associated with [1
1]. If the data unit [0 1 1 1] is to be sent in any given time
window, then a signal in which [1 1] is encoded using conventional
modulating techniques, such as 4-QAM, is sent in the second time
slot. The receiver that receives the signal determines that a
received signal was transmitted during the second time slot and
determines that the first part of the data corresponds to [0 1].
The 4-QAM modulated signal is then demodulated to extract the data
symbol [1 1] in the conventional fashion. In this way the original
data [0 1 1 1] may be determined.
[0232] When using the frequency domain, e.g. using OFDM, instead of
assigning symbols or data units to corresponding spatially
separated LEDs or time slots, each frequency sub-carrier is
associated with particular data units or symbols. In this way,
switching selected subcarriers on/off can be used to encode the
corresponding symbol or data unit. Therefore, information can be
encoded in the frequency domain similarly to the encoding in the
time and spatial domains described above. At the same time,
switching off subcarriers provides an additional benefit of
reducing the average output signal power.
[0233] Using the dimming method described above, the power or
intensity of the signal may be varied by varying the number of time
slots within each time window used to encode the data, in an
equivalent manner to the variation of the number of LEDs used to
transmit each signal given in the above spatial modulation based
example. In this case, if the duration of the time slots is
suitably small, then the observed, averaged optical power or
intensity output by the transmitter is raised or lowered dependent
on the number of time slots used to encode each symbol.
[0234] In the time domain, the technique becomes a form of pulse
position modulation (PPM). Again using the example with N=32, this
is equivalent to thirty two different time slots into which a pulse
can be provided. For dimming of 1/32.times.100%=3.1%, only one
pulse can be present but in any of the 32 slots (giving the
equivalent of 5 bits of information). For dimming of
16/32.times.100%=50%, there are over six hundred million different
combinations of sixteen pulses in any of the thirty two time slots
(giving the equivalent of 29.2 bits of information).
[0235] The spatial modulation and PPM techniques can be combined to
encode the data in both space and time. For example with four LEDs
and eight time slots it is possible to create a space-time
constellation with N=32. Thereby, the bandwidth efficiency can be
increased.
[0236] The different dimming levels lead to different combination
numbers and so data bits must be mapped to the potential
combinations depending on the dimming level. At the receiver, the
dimming level will be known and so reverse mapping can be used to
decode the data.
[0237] Advantageously, the techniques for varying dimming of the
transmitter described above can be used to implement a multilevel
pulse amplitude modulation (M-PAM) without the need for a digital
to analogue converter.
[0238] In encoding schemes that use M-PAM in conjunction with
intensity modulation, the intensity level of a single optical
transmitter is divided into M different intensity levels (typically
M equally spaced intervals) and each intensity level is associated
with a corresponding symbol or data element. Thereby, each symbol
or data element of a signal can be transmitted by operating the
optical transmitter to produce light at an intensity associated
with that symbol or data unit.
[0239] The number of bits that can be transmitted with this scheme
is log.sub.2(M). This, however, requires a digital to analogue
converter at the transmitter to generate the different power or
intensity levels.
[0240] However, if there are instead M optical transmitter element
(e.g. single LEDs) in an LED array, M different receive power
levels can be generated by switching on one or more, i.e. 1, 2, 3,
. . . , M, LEDs at the same time. It is possible to control the
overall power or intensity in this way because intensity signals
only add incoherently (i.e., there is no fading effect as compared
to radio frequency (RF)). This means, while the transmit power of
each individual LED is the same, i.e., no DAC is required, the
intensity at the receiver is subject to M different levels
depending on how many LEDs are on at the same time. It will be
appreciated that a corresponding approach based on time modulation,
as detailed above, may be used instead of or additionally to
spatial modulation.
[0241] In this way, M-PAM encoding may be used instead of, or
advantageously in addition to, spatial or temporal modulation
techniques and/or the normal amplitude or phase modulation of
signals using methods such as 4-QAM to encode data, giving three
encoding schemes that may be used to encode or modulate portions of
the data to be transmitted and thereby increasing the bandwidth
efficiency.
[0242] Further embodiments of the invention will now be
described.
[0243] At least one embodiment of the present invention comprises a
transmitter, having at least one radiation emitter or transmitter
element for emitting a signal and a processor for modulating the
radiation emitter(s) in order to encode data comprising at least
one data symbol or element. Examples of suitable transmitters
and/or receivers are shown, e.g. in FIGS. 1, 20, 25, and 30 to 32.
Preferably but not essentially, the transmitter is an optical
transmitter and the at least one radiation emitter comprises an
optical transmitter such as an LED.
[0244] The signal from the transmitter is received by a receiver,
comprising at least one corresponding receiving element, such as a
CMOS or CCD detector or a photodiode array, and a processor for
extracting the data from the received signal.
[0245] As discussed above, communications systems can be configured
to transmit data using one or more of various known modulation or
encoding schemes, such as OFDM and SIM-OFDM.
[0246] The authors have found by studying the SIM-OFDM method in
the presence of Additive White Gaussian Noise that the expected
improved system performance compared to conventional OFDM
modulation techniques is not achievable, as can be seen from FIG.
7. Without wishing to be bound to any particular theory, there may
be various possible reasons for this. First, using coherent on off
keying (OOK) detection requires a threshold, whose level should not
be higher than the power of the M-QAM symbol closest to 0.
Otherwise, symbols whose power is lower than the threshold will not
be detectable even under high SNR conditions and a constant BER
floor will be reached above zero. The low threshold level does not
allow the OOK scheme to take full advantage of the high power in
each carrier for higher order M-QAM. Second, in order to correctly
demodulate a given M-QAM symbol, it is not only necessary to
correctly detect the state (i.e. active or inactive) of its
carrier, but also the states of all carriers before it. This is
necessary because incorrect detection of a carrier state causes the
bits in the second sub-block (B.sub.QAM) to be misplaced and become
out of sequence, which completely destroys the M-QAM information in
any subsequent active carriers.
[0247] One possible solution would be to transmit the exact number
of excess carriers, N.sub.ex=N.sub.a-N/2, separately for each
frame, just like the majority bit type is sent to the destination,
where N.sub.a represents the number of active carriers. That way,
instead of using a threshold for on-off keying (OOK) detection, the
number of active carriers N.sub.a with the highest power can be
taken as active for each frame, provided that N.sub.ex is securely
transmitted to the destination. This technique leads to better
performance, but is still insufficient. If all active carriers are
used to transmit M-QAM symbols, the spectral efficiency is slightly
increased to:
E [ N a ] N log 2 ( M ) + 1 bits carrier ##EQU00002##
where E[N.sub.a] stands for the statistical expectation of
N.sub.a.
[0248] FIG. 8 illustrates an encoding or modulation method
according to an embodiment of the present invention. As in
SIM-OFDM, data is split into at least two portions, wherein a first
portion (B.sub.OOK) of the data is encoded by transmitting signals
using selected carrier channels, wherein the remaining (i.e.
non-selected carrier channels) are left inactive and/or at zero or
low intensity. A second portion (B.sub.QAM) of the data is encoded
by modulating the active carrier channels, for example, by using
amplitude modulation techniques known in the art such as M-QAM.
[0249] However, instead of using every carrier state to encode a
bit (or other data element), as is the case in SIM-OFDM, the
present invention uses the states of two or more carriers, in this
case, a carrier pair. The processor of the transmitter is
configured to encode bits in the first data portion B.sub.OOK by
selecting which carrier from the pair is active. In this case, when
a data bit 1 is encountered, the first or preceding carrier is
selected to be active (i.e. a signal is provided/carried on the
first or preceding carrier) and the second or following carrier is
left inactive. When a data bit 0 is encountered, the first or
preceding carrier is left inactive and the second or following
carrier is made active (i.e. a signal is provided on it). The
processor of the transmitter is configured to encode the bits in
the second data portion B.sub.QAM by modulating all of the active
carriers using an modulation scheme such as M-QAM, so that each
active carrier channel encodes a data symbol or data element of the
second data portion B.sub.QAM in the form of an M-QAM symbol.
[0250] At the receiver, once the signal is received, the processor
of the receiver is configured to process the carriers in the
received signal two at a time and the carrier states, active or
inactive, are determined by comparing the relative power levels of
each carrier in the pair. The carrier with more power is considered
active. Based on the determined states of the carrier pairs, the
first data portion B.sub.OOK is determined.
[0251] Afterwards, all of the active carriers are demodulated
according to the associated demodulation scheme (in this case
M-QAM) and the second portion of the data B.sub.QAM is
reconstructed.
[0252] In this approach, it is not necessary to determine and
transmit majority bit type, as the determination of which carrier
is active is based on a comparison of a pair of carriers and not an
individual carrier with a threshold. Using this technique, the
overall spectral efficiency is slightly reduced to:
log 2 ( M ) 2 + 1 2 bits carrier ##EQU00003##
[0253] However, any error in the detection of the carrier states
influences only the M-QAM symbol encoded in the relevant
carrier.
[0254] Optionally, the concept can be extended to using more than
two carriers at a time to represent bits from the first data
portion B.sub.OOK. For example, six carriers can be used, with
three carriers being set as active and the rest of the carriers
being set as inactive. In this example, there are 6!/3!3!=20
possible combinations to represent bits. This means that a total of
four bits (2.sup.4=16<20) can be encoded in 6 carriers' states
when three are active, as depicted in FIG. 9. The encoding can be
based either on a predetermined table or an algorithm that matches
blocks of bits to a combination of L.sub.a active carriers in a
sub-block of L carriers in total. The spectral efficiency is
thereby increased.
[0255] Extending this to a group of L carriers of which L.sub.a of
the carriers are set as active, the spectral efficiency of the
system becomes:
L a log 2 M L + log 2 ( L ! L a ! ( L - L a ) ! L bits carrier
##EQU00004##
[0256] The BER performance can get worse as L increases, since the
negative effects described for the original SIM-OFDM method appear
inside each group of L carriers. As L.sub.a approaches L, the
spectral efficiency of the system gets closer to that of
conventional OFDM. As L approaches N, and L.sub.a approaches 1, the
spectral efficiency of the system starts to resemble that of Pulse
Position Modulation (PPM). As L approaches N, and L.sub.a
approaches N/2, the spectral efficiency of the system gets closer
to that of the former SIM-OFDM scheme. In any case, the present
invention has an advantage over SIM-OFDM because it keeps a
constant number of active carriers and requires no majority bit
type information.
[0257] In cases where inter-symbol interference is not an issue,
OFDM does not provide particular advantages to the system. In this
case, the concept can be realized in the time domain in exactly the
same manner, where the carriers would correspond to time samples
rather than frequency carriers.
[0258] Use of the above method may result in a number of advantages
over the existing SIM-OFDM technique. For example, the number of
active carriers, N.sub.a, is known at each instant, so it need not
be transmitted and the usage of a threshold is not necessary. In
addition, the number of active and inactive samples is the same in
each frame, so majority bit type does not need to be relayed to the
destination. Furthermore, false detection of a carrier state
influences only the M-QAM symbol it encodes and the error does not
propagate in the rest of the frame. Advantageously, the bit error
rate vs.
E b N o ##EQU00005##
performance is improved compared to the former SIM-OFDM scheme and
in certain cases compared to conventional OFDM. Additionally,
peak-to-average power ratio (PAPR) is reduced relative to the
SIM-OFDM and OFDM schemes and a power efficient modulation scheme
for optical wireless communication is introduced.
[0259] A comparison of the performance of a communications system
that operates using the above encoding method relative to a
corresponding system using the conventional OFDM method in the
presence of Additive White Guassian Noise (AWGN) is illustrated in
FIG. 11. As can be clearly seen from this, a system using the
modulation/encoding scheme of the present invention achieves better
bit error ratio results than the prior art systems under the same
conditions.
[0260] Further research of the properties of SIM-OFDM based
techniques by the present inventors have shown that such systems
can achieve better peak-to-average power ratio (PAPR) than
equivalent systems using conventional OFDM. For OFDM with square
constellation M-QAM, the PAPR is calculated as:
N 3 ( M - 1 ) M + 1 ##EQU00006##
[0261] A general formula for PAPR estimation is:
N a 3 ( M - 1 ) M + 1 ##EQU00007##
[0262] The PAPR depends on both the number of active carriers,
expressed by Na, and the way they are modulated, expressed by the
ratio:
3 ( M - 1 ) M + 1 ##EQU00008##
[0263] The best PAPR is achieved using Frequency Shift Keying
(FSK), since Na=1 and
3 ( M - 1 ) M + 1 = 1. ##EQU00009##
The worst is achieved in the case of conventional OFDM when Na=N,
and both N and M are as high as possible. An advantage of the above
encoding method of the present invention over conventional OFDM and
SIM-OFDM comes from the fact that in general it requires less
active carriers to represent the same amount of information.
[0264] For the particular purpose of optical wireless communication
using intensity modulation (IM) at the transmitter and direct
detection (DD) at the receiver, another embodiment of the present
invention can be realized. In optical communication systems, there
is an issue with using optical communications systems to transmit
bipolar data signals 2005, i.e. signals having both positive 2010
and negative 2015 signal components, as an optical transmitter such
as an LED can generally only transmit positive real signal values.
In OFDM, N time domain samples 2020 of a real OFDM frame with N
carriers are obtained after the required modulation steps, as shown
in FIG. 10a. Such a signal is made real for the purposes of IM/DD
communication. Additionally, the OFDM signal can be made positive
by introducing a DC shift as depicted in FIG. 10b. This approach is
known as DCO-OFDM.
[0265] An alternative approach is known as ACO-OFDM in which
properties of Fourier transforms are exploited so that a positive
signal can be obtained in the time domain by simply ignoring
(cutting off) any negative values. However, this approach has half
the spectral efficiency of DCO and half the power efficiency for
bipolar signals.
[0266] An embodiment of the present invention (referred to as
Unipolar orthogonal frequency division multiplexing, U-OFDM, by the
present inventors), provides a more elegant solution, that
outperforms ACO. As shown in FIG. 10c, in the U-OFDM method
according to an embodiment of the present invention, each time
sample 2020 of the bipolar OFDM signal of FIG. 10a is transformed
into two time samples 2020A, 2020B. If the original time sample
2020 was positive 2010, the first one 2010A of the two new time
samples 2020A, 2020B is equal to the amplitude of the original time
sample 2010, so it can be called an "active sample". The second
time sample 2010B is equal to zero, so it can be called an
"inactive sample". If the original time sample 2020 in the bipolar
signal 2005 of FIG. 10a is negative 2015, the first one 2015A of
the two new unipolar samples 2020A, 2020B is set to zero, so it can
be called "inactive sample". The second unipolar time sample 2015B
is made equal to the absolute value of the original bipolar time
sample 2015, so it can be called an "active sample". This way, only
the absolute value of the signal 2005 is transmitted, and the sign
of each sample 2020 is encoded in the position of the "active" and
"inactive" samples in each pair.
[0267] This concept can be easily extended intuitively. The
essential part of the U-OFDM algorithm is in transmitting only the
absolute values of the bipolar signal and the signs separately. The
signs, which are effectively equal to one bit of information each,
can be encoded in a variety of different ways. The case presented
in FIG. 10c shows how the signs can be encoded in the relative
position of the active and inactive samples. Additionally or
alternatively, the signs can be encoded as bits and/or can be
modulated on frequency carriers, time carriers and/or spatial
carriers. They can be part of the current frame, the previous
frame, the next frame, etc. They can also be conveyed to the
destination on a parallel communications channel or as a separate
part of the system. The modulation type can be any existing digital
modulation scheme. Different approaches towards the sign encoding
will lead to different spectral efficiencies and different bit
error rate performances.
[0268] In the specific example given in FIG. 10c, the spectral
efficiency of OFDM is halved since no bits are transmitted in the
inactive sample states. This can be mitigated in a similar manner
to the previously described concept by encoding more than one
sample sign in a group of more than two samples. At the receiver,
the maximum of each sample pair is taken. Its amplitude becomes the
amplitude of the original sample, and the sign or phase is
retrieved from its position in the pair. Afterwards, the
demodulation process can continue as in conventional OFDM.
[0269] By employing the U-OFDM method described above, the
performance of the communication system can be improved over
communication systems that use the existing DCO-OFDM and ACO-OFDM
techniques, as shown in FIGS. 12 to 14. The BER performance of
U-OFDM in the presence of Additive White Gaussian Noise (AWGN)
compared to pure OFDM and ACO-OFDM for bipolar signals is
illustrated in FIG. 12. Performance of U-OFDM compared to DCO and
ACO for unipolar signals is illustrated in FIG. 13. The biasing
levels for DCO are adopted from J. Armstrong and B. J. C. Schmidt,
"Comparison of Asymmetrically Clipped Optical OFDM and DC-Biased
Optical OFDM in AWGN" IEEE Communication Letters 12(5):343-345, May
2008, such that no noticeable distortion is experienced in the BER
curves due to signal clipping. FIG. 14 presents the comparison
between U-OFDM, ACO and DCO for optical SNR introduced in the above
article by Armstrong et. al. for the purpose of comparing optical
efficiency of the modulation schemes.
[0270] As a summary example of an embodiment of the present
invention, two copies of a bipolar signal are made. The bipolar
signal is made up of a plurality of samples/portions. The first
copy is kept in its original form. Samples of the second signal are
switched in polarity (multiplied by -1 so that positive become
negative and negative become positive). Then the negative samples
in both copies are clipped. In this way, the first copy retains the
original positive samples and substitutes the negative samples with
zeros. The second copy retains the original negative samples as
positive samples and substitutes the original positive samples with
zeros. Both copies are now unipolar.
[0271] The copies can be transmitted in two separate time slots,
streams, or divisions of other transmission mechanisms. At the
receiver, the original signal can be reconstructed from the first
and second copy after both are received, for example, by simple
subtraction of the second signal copy from the first one. In this
way, positive samples of the first signal copy will stay unaltered
and positive samples of the second signal copy will shift polarity
again to become negative samples. Of course, the zero samples will
have no influence on any of the reconstructed samples since adding
or subtracting a zero does not introduce a change.
[0272] Alternatively, samples in both signal copies could be
examined in corresponding pairs (e.g. the first sample of the first
copy is compared with the first sample of the second copy, the
second sample of first copy is compared with the second sample of
the second copy, and so on) to determine whether the original
sample is contained in the first or the second copy (e.g. the value
of a sample in one copy will be zero whilst the value in the
corresponding sample in the other copy will be a positive number).
In this way, the value and sign associated with the original sample
can be determined. Since there is noise present at each sample, an
example of a method for determining which copy holds a sample and
which copy holds a zero is to take the higher value of the two as
the sample an to consider the other (lower) one as a zero. In that
way, zero samples can be disregarded instead of added to the
"active" samples, and ideally the noise power could be reduced by
half compared to the other approach.
[0273] Importantly, techniques such as the above comprise the
division of an original sample into negative and positive samples
in two separate unipolar information sequences which can be
recombined later without breaking the original frame structure.
[0274] Some optional features that may be used in or with any of
the methods and apparatus' described above are described below.
[0275] The following features relates to communicating information
by modulating multiple carriers. In particular, some of the
following features relate to communicating information by
modulating orthogonal frequency division multiplexing (OFDM)
subcarriers.
[0276] Modulation is a technique by which a carrier wave is
modified or modulated to encode information.
[0277] As an example, for a fixed frequency carrier wave,
modulation of the carrier wave may be achieved by varying the
amplitude of the carrier wave or the phase of the carrier wave to
encode information. To improve disambiguation between different
encoded information, the variation of the amplitude of the carrier
wave or the phase of the carrier wave to encode information is
typically discrete.
[0278] To increase bandwidth (the amount of information transmitted
per second) the carrier wave may be separated into orthogonal
components (I and Q components) that differ in phase by .pi./2
radians. The orthogonal components are then independently
modulated.
[0279] Examples of current modulation techniques include, for
example, binary phase shift keying (BPSK) and multilevel quadrature
amplitude modulation (M-QAM).
[0280] Modulation increases information transmission rates by
enabling that within a given bandwidth more information is conveyed
(upper bounded by the Shannon equation).
[0281] In orthogonal division multiple multiplexing (OFDM),
multiple carrier waves are defined. These subcarriers are
orthogonal to each other. This enables each subcarrier to be
independently modulated.
[0282] It would be desirable to provide for better data
transmission and reception.
[0283] Described herein is a method of communicating information
comprising dividing the information into at least a first
information portion and a second information portion; modulating a
plurality of domain resources to encode the first information using
an index or grammar; and encoding the second information by
modulation of domain resources.
[0284] Modulating the plurality of domain resources to encode the
first information can comprise allocating at least two different
modulation types to a plurality of sub-carriers. Encoding the first
information portion can comprise selecting which modulation type is
allocated to which domain resource according to the index or
grammar, which applies meaning to which modulation type is
allocated to which domain resource.
[0285] Modulating the plurality of domain resources to encode the
first information can comprise encoding at part of the first
information portion using one or more domain resources of a first
domain and another part of the first information portion is encoded
using one or more domain resources of a second domain.
[0286] Modulating the plurality of domain resources to encode the
first information can comprise encoding at least a further part of
the first information portion using one or more domain resources of
a third domain. The first, second and/or third domains can comprise
the frequency, time and/or spatial domains and/or the domain
resources can comprise sub-channels of a frequency domain, beams
arranged in a spatial domain and/or time slots in a time
domain.
[0287] Values can be associated with domain resources of at least
one domain and wherein at least part of the first information
portion is encoded by providing a signal using a domain resource
indicative of the value of the portion of the data. Values can be
associated with domain resources of at least two domains, and
wherein the first information portion is at least partially encoded
by providing a signal using domain resources from the at least two
domains indicative of the value of parts of the first information
portion.
[0288] Described herein is a method of communicating information
comprising dividing the information into at least a first
information portion and a second information portion; encoding the
first information portion by allocating at least two different
modulation types to a plurality of subcarriers; and encoding the
second information by modulation of subcarriers belonging to a
subset of the plurality of subcarriers.
[0289] Encoding the first information portion can comprise
selecting which modulation type is allocated to which subcarrier
according to a grammar that applies meaning to which modulation
type is allocated to which subcarrier. Each subcarrier can be
associated with an index in a ordered series of indexes, wherein
the modulation type of a subcarrier assigns a value to the
associated index and wherein assigned values, ordered according to
the ordered series of indexes, provide a data word representing the
first information portion.
[0290] Optionally, the method comprises encoding the first
information portion by allocating only two different modulation
types to the subcarriers.
[0291] The first information portion optionally comprises a first
subset of information bits and a second subset of information bits.
The method can further comprise selecting a first information
encoding scheme from a plurality of different first information
encoding schemes, each of which has a different allocation of
modulation types to the first and second subsets.
[0292] The plurality of different first information encoding
schemes can be predetermined and shared with a receiver. Control
information that identifies the selected first information encoding
scheme can be transmitted. The control information is optionally
encoded by modulation of subcarriers.
[0293] A first subset of information bits and a second subset of
information bits are optionally identified within the first
information portion; the majority subset of the first and second
subsets selected; and the majority subset used to allocate a first
modulation type to a first plurality of subcarriers. A first
information encoding scheme, for determining the allocation of the
at least two different modulation types amongst the plurality of
subcarriers, that maximises the available bandwidth for the
remaining information portions can be selected.
[0294] Some of the bandwidth for encoding control information
and/or for degenerate encoding a portion of the second information
portion can be utilised. Optionally, at least two different
modulation types comprise a first modulation type that modulates a
subcarrier with a first fixed amplitude and a second modulation
type that modulates a subcarrier with a second fixed amplitude.
[0295] Optionally, at least two different modulation types enable
on-off keying (OOK), the first amplitude representing suppression
of subcarriers and the second amplitude representing non
suppression of the subcarriers. The method can comprise power
reallocation from the suppressed subcarriers to the non suppressed
subcarriers. At least two different modulation types can comprise a
first modulation type that modulates a subcarrier with a higher
order modulation and a second modulation type that modulates a
subcarrier with a lower order modulation. The plurality of
subcarriers optionally comprise subcarriers of two or more domains,
for example, the frequency domain, the time domain and the spatial
domain.
[0296] Described herein is a transmitter apparatus comprising means
for dividing the information into at least a first information
portion and a second information portion; means for encoding the
first information portion by allocating at least two different
modulation types to a plurality of subcarriers; and means for
encoding the second information by modulation of subcarriers
belonging to a subset of the plurality of subcarriers.
[0297] The transmitter apparatus optionally comprises means for
performing any of the methods described above.
[0298] Also described herein is a transmitter apparatus comprising:
splitter circuitry configured to divide the information into at
least a first information portion and a second information portion;
inter-carrier modulation circuitry configured to encode the first
information portion by allocating at least two different modulation
types to a plurality of subcarriers; and intra-carrier modulation
circuitry configured to encode the second information by modulation
of subcarriers belonging to a subset of the plurality of
subcarriers.
[0299] Described herein is a method of communicating information
comprising: decoding first information portion by determining the
allocation of at least two different modulation types to a
plurality of subcarriers; and decoding second information by
demodulation of subcarriers belonging to a subset of the plurality
of subcarriers; and combining the first information portion and the
second information portion.
[0300] Decoding the first information portion optionally comprises
determining which modulation type is allocated to which subcarrier
and applying meaning according to a grammar that applies meaning to
which modulation type is allocated to which subcarrier.
[0301] Each subcarrier can be associated with an index in a ordered
series of indexes, wherein the modulation type of a subcarrier
assigns a value to the associated index and wherein assigned
values, ordered according to the ordered series of indexes, provide
a data word representing the first information portion.
[0302] The method optionally comprises determining which modulation
type is allocated to which subcarrier; decoding the first
information portion according to different encoding schemes by
applying meaning according to a grammar which applies a meaning to
which modulation types are allocated to which subcarriers, wherein
each encoding scheme has its own grammar; and selecting one of the
first information portions created according to different encoding
schemes. The method optionally comprises identifying the encoding
scheme is use; and decoding the first information portion by
determining which modulation type is allocated to which subcarrier
and applying meaning according to a grammar of the identified
encoding scheme that applies meaning to which modulation type is
allocated to which subcarrier.
[0303] At least two different modulation types can comprise a first
modulation type that modulates a subcarrier with a higher order
modulation and a second modulation type that modulates a subcarrier
with a lower order modulation.
[0304] Described herein is a receiver apparatus comprising means
for decoding first information portion by determining the
allocation of at least two different modulation types to a
plurality of subcarriers; means for decoding second information by
demodulation of subcarriers belonging to a subset of the plurality
of subcarriers; and means for combining the first information
portion and the second information portion;
[0305] Optionally, the receiver apparatus comprises means for
performing the method of the fourth aspect.
[0306] Described herein is a receiver apparatus comprising
inter-carrier modulation detection circuitry configured to decode
first information portion by determining the allocation of at least
two different modulation types to a plurality of subcarriers;
intra-carrier demodulation circuitry configured to decode second
information by demodulation of subcarriers belonging to a subset of
the plurality of subcarriers; and combiner circuitry configured to
combine the first information portion and the second information
portion.
[0307] Described herein is a system comprising a receiver apparatus
as described above and a transmitter apparatus described above.
[0308] Described herein is a method of communicating information
comprising dividing the information into at least a first
information portion and a second information portion; communicating
the first information portion by allocating an operational pattern
to a plurality of wireless channels; and communicating the second
information by wireless communication via one or more of the
wireless channels.
[0309] The wireless channels optionally have a plurality of
potential different states wherein the first information is
communicated by allocating a state for each wireless channel,
wherein the collection of states form the operational pattern.
[0310] The wireless channels can include orthogonal subcarriers
that have different modulation states. The wireless channels
optionally include time slots that have different modulation
states.
[0311] Described herein is a method of communicating information
comprising dividing the information into at least a first
information portion and a second information portion; encoding the
first information portion by allocating at least two different
modulation types to a plurality of orthogonal multiplexing domain
subunits; and encoding the second information by modulation of
orthogonal multiplexing domain subunits belonging to a subset of
the plurality of orthogonal multiplexing domain subunits.
[0312] Described herein is a method of communicating information
comprising: dividing the information into at least a first
information portion and a second information portion; encoding the
first information using domain resource-index modulation; and
encoding the second information by modulation of domain
resources.
[0313] Described herein is a method of communicating information
comprising dividing the information into at least a first
information portion and a second information portion; encoding the
first information using inter-domain resource differentiation; and
encoding the second information by modulation of domain
resources.
[0314] Described herein is a method of communicating information
comprising: dividing the information into at least a first
information portion, a second information portion and a third
information portion; encoding the first information portion by
allocating at least two different modulation types to a plurality
of first orthogonal multiplexing domain subunits; encoding the
second information portion by allocating at least two different
modulation types to a plurality of second orthogonal multiplexing
domain subunits; and encoding the third information by modulation
of orthogonal multiplexing domain subunits belonging to a subset of
the plurality of first orthogonal multiplexing domain subunits and
second orthogonal multiplexing domain subunits.
[0315] Described herein is a method of communicating information
comprising: dividing the information into at least N portions;
encoding the each one of M of the N portions by allocating at least
two different modulation types to a plurality of orthogonal
multiplexing domain subunits; and encoding the N-M portion(s) by
modulation of orthogonal multiplexing domain subunits.
[0316] Optionally, the method comprises encoding at least one of
the M portions by allocating at least 2.sup.M different modulation
types to a plurality of orthogonal multiplexing domain subunits.
The plurality of orthogonal multiplexing domain subunits optionally
comprises domain subunits from at least two domains, for example,
the frequency domain, the time domain and the spatial domain.
[0317] FIG. 15 schematically illustrates a method 3002 of
communicating information. The method comprises a series of
sequential blocks 3004, 3006, 3008.
[0318] At block 3008, information is divided into at least a first
information portion and a second information portion.
[0319] As illustrated in FIG. 17A, the information 3011 may be a
series of information bits, such as, for example the series of
binary bits illustrated.
[0320] As illustrated in FIGS. 17B and 17C, the first information
portion 3013 and the second information portion 3015 are
non-overlapping portions of the information 3011.
[0321] Referring back to FIG. 15, at block 6 the first information
portion 3013 is encoded by allocating at least two different
modulation types to a plurality of orthogonal frequency division
multiplex (OFDM) subcarriers.
[0322] An example of an allocation of different modulations types
M1, M2 to orthogonal subcarriers F1 to F8 is illustrated in FIG.
17D. This Figure illustrates a modulation map 3017 that records
which modulation type is allocated to which subcarrier.
[0323] Encoding the first information portion 3013 comprises
selecting which modulation type is allocated to which subcarrier
according to a grammar that applies meaning to which modulation
type is allocated to which subcarrier.
[0324] The modulation map has an ordered series of indexes 3021. In
this example the indexes increase sequentially from 1 to 8. Each of
the indexes 3021 is associated with a particular information bit of
the first information portion 3013. Each of the subcarriers F1-F8
is associated with a particular index 3021. The bit value of the
first information portion 3013 at a particular index assigns the
modulation type of the subcarrier associated with that index.
[0325] In this example, it can be observed that the grammar assigns
the modulation type M1 to bit values of 1 in the first information
portion 3013 and the modulation type M2 to a bit values of 0 in the
first information portion 3013.
[0326] The grammar is shared with a receiver so that the receiver
by detecting the modulation types of the subcarriers can reproduce
the first information portion 3013 as the data word 3022. At the
receiver, the modulation type of a subcarrier assigns a value to
the associated index. The assigned values, ordered according to the
ordered series of indexes, provide a data word 3022 representing
the first information portion 3011 as illustrated in FIG. 17F.
[0327] Encoding of the first information portion 3013, results in
different modulation types being used with different subcarriers.
In the example of FIG. 17D, two different types of modulation M1,
M2 are used. A first subset of the eight subcarriers (subcarriers
F1, F4, F6, F7) are modulated using the first type of modulation
M1. A second subset of the eight subcarriers (subcarriers F2, F3,
F5, F8) are modulated using the second type of modulation M2.
[0328] Referring back to FIG. 15, next at block 3008 the second
information portion 3015 is encoded by modulation of subcarriers
belonging to a subset of the plurality of subcarriers.
[0329] Referring to FIG. 17E, a symbol map 3020 records which
symbols are modulated onto which subcarriers. The second
information portion 3015 is divided into symbols and each symbol is
modulated, in order, onto a subcarrier. In this example, all the
symbols are modulated onto only the first subset of the eight
subcarriers (subcarriers F1, F4, F6, F7).
[0330] In summary, the block 3006, encodes the first information
portion 3013 using inter-carrier modulation. Information is encoded
by creating differences between subcarriers. The first information
portion 3013 is encoded by allocating modulation types to
subcarriers according to a grammar that applies meaning to which
modulation type is allocated to which subcarrier. The block 3008,
encodes the second information portion using intra-carrier
modulation. A symbol is encoded by creating detectable differences
(e.g. phase and/or amplitude) within a subcarrier.
[0331] FIG. 16 schematically illustrates a suitable encoding
apparatus 3010. The apparatus 3010 comprises a splitter block 3012,
an inter-carrier modulation block 3014 and an intra-carrier
modulation block 3016.
[0332] For example, the splitter block performs block 3004 of the
method 3002, the inter-carrier modulation clock 3014 performs block
3006 of the method 3002 and the intra-carrier modulation block 3016
performs block 3008 of the method 3002.
[0333] The blocks may be implemented using software, hardware or a
combination of software and hardware.
[0334] The splitter block 3012 divides the information 3011 into at
least a first information portion 3013 and a second information
portion 3015.
[0335] The inter-carrier modulation block 3014 encodes the first
information portion 3013 by allocating at least two different
modulation types to a plurality of subcarriers.
[0336] The intra-carrier modulation block 3016, receives data 3017
identifying the allocation of modulation types to subcarriers by
the inter-carrier modulation block 3014, and encodes the second
information portion 3015 by modulation of a subset of the
subcarriers. The modulated subcarriers 3019 are then
transmitted.
[0337] FIG. 20 schematically illustrates a suitable decoding
apparatus 3030. The apparatus 3030 comprises a combiner block 3032,
an inter-carrier modulation detection block 3034 and an
intra-carrier demodulation block 3036.
[0338] The blocks may be implemented using software, hardware or a
combination of software and hardware.
[0339] The inter-carrier detection block 3014 decodes the first
information portion 3013 by detecting the modulation type of each
subcarrier. The detected modulation type assigns a value to the
carriers associated index according to a grammar. The assigned
values, ordered according to the ordered series of indexes, provide
a data word 3022 representing the first information portion 3011 as
illustrated in FIG. 17F.
[0340] The intra-carrier demodulation block, receives data 3017
identifying the allocation of modulation types to subcarriers, and
decodes a data word 3024, representing the second information
portion 3015, by demodulation of a subset of the subcarriers.
[0341] The combiner block 3032 combines the first information
portion 3013 and the second information portion 3015 to recreate
the information 3011.
[0342] In the example illustrated in FIGS. 17A-17G, two different
modulation types are illustrated M1 and M2.
[0343] In one embodiment, the first modulation type M1 may modulate
an allocated subcarrier with a first fixed amplitude and the second
modulation type modulates an allocated subcarrier with a second
fixed amplitude. The fixed amplitudes may be used to enable on-off
keying (OOK) in which only the first modulation type is used to
encode the second information portion 3015. On-Off keying is
enabled by letting the second amplitude represent suppression of
subcarriers and the first amplitude represent non suppression of
the subcarriers.
[0344] Different power control strategies may be used in the event
that subcarriers are suppressed. Suppression of a subcarrier means
that it is not transmitted and therefore has zero associated
power.
[0345] In one power control strategy, there is no reallocation of
power. If a subcarrier is suppressed then that power is saved.
[0346] In a different power control strategy, there is reallocation
of power. The total transmit power is kept constant, with the power
per subcarrier increasing as more subcarriers are suppressed. This
results in an increase in power per subcarrier. There is therefore
a better signal to noise ratio, and a reduced bit-error ratio or
there is an opportunity to select a higher order modulation scheme
that results in even higher transmission rates.
[0347] In other embodiments, both modulation types M1, M2 may be
used to encode the second information portion 3015.
[0348] In other embodiments, both modulation types M1, M2 may be
used to encode the second information portion 3015 and an
additional third information portion 3040. One example of this will
now be described with reference to FIG. 15 and FIGS. 18A to
18I.
[0349] At block 3004, information 3011' is divided by splitter
block 3012 into at least a first information portion 3013, a second
information portion 3015 and a third information portion 3040.
[0350] As illustrated in FIG. 18A, the information 3011' may be a
series of information bits, such as, for example the series of
binary bits illustrated.
[0351] As illustrated in FIGS. 18B, 18C and 18D, the first
information portion 3013, the second information portion 3015 and
the third information portion 3040 are non-overlapping portions of
the information 3011'.
[0352] Referring back to FIG. 15, at block 3006 the first
information portion 30013 is encoded, at the inter-carrier
modulation block 30014, by allocating at least two different
modulation types to a plurality of orthogonal frequency division
multiplex (OFDM) subcarriers.
[0353] An example of an allocation of different modulations types
M1, M2 to orthogonal subcarriers F1 to F8 is illustrated in FIG.
18D. This illustrates a modulation map 3017 that records which
modulation type is allocated to which subcarrier. Encoding of the
first information portion 3013, results in different modulation
types being used with the subcarriers. In this example, a first
subset of the eight subcarriers (subcarriers F1, F4, F6, F7) are
modulated using the first type of modulation M1. A second subset of
the eight subcarriers (subcarriers F2, F3, F5, F8) are modulated
using the second type of modulation M2.
[0354] Referring back to FIG. 15, next at block 3008 the second
information portion 3015 is encoded,
at the intra-carrier modulation block 3016, by modulation of
subcarriers belonging to the first subset of the plurality of
subcarriers.
[0355] Referring to FIG. 18F, a symbol map 3042 records which
symbols are modulated onto which subcarriers.
[0356] The second information portion 3015 is divided into symbols
and each symbol is modulated, in order, onto a subcarrier. In this
example, all the symbols are modulated onto only the first subset
of the eight subcarriers (subcarriers F1, F4, F6, F7).
[0357] Referring back to FIG. 15, next block 3008 is repeated using
the third information portion 3040. At block 3008 the third
information portion 3040 is encoded, at the intra-carrier
modulation block 3016, by modulation of subcarriers belonging to
the second subset of the plurality of subcarriers.
[0358] In this example, the third information portion 3040 is
divided into symbols and each symbol is modulated onto one of the
second subset of the subcarriers (subcarriers F2, F3, F5, F8).
[0359] At the decoder 3030, the inter-carrier detection block 3014
decodes the first information portion 3013 by detecting the
modulation type of each subcarrier. The detected modulation type
assigns a value to the carriers associated index according to a
grammar. The assigned values, ordered according to the ordered
series of indexes, provide a data word 3022 representing the first
information portion 3011 as illustrated in FIG. 18G.
[0360] The intra-carrier demodulation block, receives data 3017
identifying the allocation of modulation types to subcarriers. It
decodes the second information portion 3015 by using the
appropriate demodulation technique (for modulation type M1) on the
subcarriers identified as using the modulation type M1. This
demodulation produces the data word 3024 which reproduces the
second information portion 3015. It decodes the third information
portion 3015 by using the appropriate demodulation technique (for
modulation type M2) on the subcarriers identified as using the
modulation type M2. This demodulation produces the data word 3044
which reproduces the third information portion 3040.
[0361] The combiner block 3032 combines the first information
portion 3013, the second information portion 3015 and the third
information portion 3040 to recreate the information 3011'.
[0362] The first modulation type M1 and the second modulation type
M2 have different orders. In the illustrated example, the first
modulation type M1 has a higher order than the second modulation
type M2. The first modulation type uses symbolic modulation, where
the symbol length L1=2. The second modulation type uses symbolic
modulation, where the symbol length L2 is different to L1. In this
example, the second modulation is of a lower order (L2<L1) as
L2=1.
[0363] M-QAM modulation may, for example, be used as the first
modulation type M1. Binary phase shift keying BPSK may, for
example, be used as the second modulation type M2.
[0364] This implementation results in an improved spectral
efficiency compared to the on-off keying embodiment described
previously but at the cost of increased complexity at the receiver
3030.
[0365] At block 3006 in FIG. 15, the first information portion 3013
is encoded by selecting which modulation type is allocated to which
subcarrier according to a grammar that applies meaning to which
modulation type is allocated to which subcarrier. In the example of
FIG. 17D, it can be observed that the grammar assigns the
modulation type M1 to bit values of 1 in the first information
portion 3013 and the modulation type M2 to a bit values of 0 in the
first information portion 3013. This grammar defines a particular
first information encoding scheme.
[0366] It is possible to define different first information
encoding schemes using different grammars.
[0367] For example, a different grammar could assigns the
modulation type M1 to bit values of 0 in the first information
portion 3013 and the modulation type M2 to a bit values of 1 in the
first information portion 3013. This grammar defines a different
first information encoding scheme.
[0368] FIG. 5 schematically illustrates a method 3050 for selecting
an encoding scheme from a plurality of different encoding schemes,
each of which has a different allocation of modulation types to the
subsets of the first information portion 3013.
[0369] At block 3052, the first information portion 3013 is
analysed.
[0370] There may be a putative allocation of a first encoding
scheme to the first information portion 3013. A quantitative value
indicative of the available bandwidth for encoding the second
information portion 3015 may then be determined.
[0371] There may be a putative allocation of a second encoding
scheme to the first information portion 3013. A quantitative value
indicative of the available bandwidth for encoding the second
information portion 3015 may then be determined.
[0372] At block 3053, the encoding scheme with the greatest
available bandwidth may then be selected. The selected encoding
scheme determines how at least two different modulation types are
allocated amongst the plurality of subcarriers in dependence upon
the first information portion 3013 and maximises the available
bandwidth for the remaining information portions.
[0373] As an example, consider the situation when the first
information portion 3013 has the information bits 11010110. The
first information portion 3013 has a first subset of information
bits {1 1.sub.--1.sub.--1 1_} and a second subset of information
bits {.sub.-- --0.sub.--0.sub.-- --0}.
[0374] According to a first encoding scheme, a bit value of 1 in
the first information portion is associated with modulation type M1
and a bit value of 0 in the first information portion is associated
with modulation type M2. The modulation type M1 corresponds to
symbol encoding and the modulation type M2 corresponds to carrier
suppression (on-off keying). Consequently there will be five
subcarriers allocated to the first modulation type M1, each of
which can communicate a symbol. The bandwidth is therefore five
symbols.
[0375] According to a second encoding scheme, a bit value of 1 in
the first information portion is associated with modulation type M2
and a bit value of 0 in the first information portion is associated
with modulation type M1. The modulation type M1 corresponds to
symbol encoding and the modulation type M2 corresponds to carrier
suppression (on-off keying).
[0376] Consequently there will be three subcarriers allocated the
first modulation type M1, each of which can communicate a symbol.
The bandwidth is therefore three symbols.
[0377] The first encoding scheme is therefore selected and used.
The majority subset of the first subset (1's) and the second subset
(0's) determines the encoding scheme in this example. The majority
subset of the first information portion may be identified by
calculating the Hamming weight of the first information portion
3013 at block 3052.
[0378] The available bandwidth is five symbols but only four
symbols are required to encode the second information portion 3015.
There is therefore an additional symbol for use. In some
embodiments this symbol is used to encode again a symbol of the
second information portion. This degenerate encoding increases the
quality. In another embodiment, this symbol is used to indicate the
selected encoding scheme to the receiver 3030. In other
configurations of the first information portion 3013 there may be
more than one extra symbol available and these symbols may be used
for degenerate encoding and indicating the selected encoding
scheme.
[0379] As another example, consider the situation when the first
portion has the information bits 00010110. The first information
portion 3013 has a first subset of information bits {0
0.sub.--0.sub.--0 0_} and a second subset of information bits
{.sub.-- --1.sub.--1.sub.-- --1}.
[0380] According to a first encoding scheme, a bit value of 0 in
the first information portion is associated with modulation type M1
and a bit value of 1 in the first information portion is associated
with modulation type M2. The modulation type M1 corresponds to
symbol encoding and the modulation type M2 corresponds to carrier
suppression (on-off keying). Consequently there will be five
subcarriers allocated to the first modulation type M1, each of
which can communicate a symbol. The bandwidth is therefore five
symbols.
[0381] According to a second encoding scheme, a bit value of 0 in
the first information portion is associated with modulation type M2
and a bit value of 1 in the first information portion is associated
with modulation type M1. The modulation type M1 corresponds to
symbol encoding and the modulation type M2 corresponds to carrier
suppression (on-off keying). Consequently there will be three
subcarriers allocated the first modulation type M1, each of which
can communicate a symbol. The bandwidth is therefore three
symbols.
[0382] The first encoding scheme is therefore selected and used.
The majority subset (0's) of the first subset (0's) and the second
subset (1's) determines the encoding scheme in this example. The
majority subset of the first information portion 3013 may be
identified by calculating the Hamming weight of the first
information portion 3013 at block 3052.
[0383] The available bandwidth is five symbols but only four
symbols are required to encode the second information portion 3015.
There is therefore an additional symbol for use. In some
embodiments this symbol is used to encode again a symbol of the
second information portion. This degenerate encoding increases the
quality. In another embodiment, this symbol is used to indicate the
selected encoding scheme to the receiver. It other configurations
of the first information portion 3013 there may be more than one
extra symbol available and these symbols may be used for degenerate
encoding and indicating the selected encoding scheme.
[0384] As the encoder can use any one of a plurality of different
encoding schemes for encoding the first information portion 3013,
then the receiver 3030 will need to take this into account.
[0385] According to one embodiment, the receiver makes no
assumption as to the encoding scheme used and performs the decoding
process for each predetermined scheme. Error correction techniques
are then used to determine which of the information 3011 produced
for each scheme is the most accurate. However, this requires
significant processing at the receiver 3030 and may require the
introduction of forward error correcting codes which reduces
bandwidth.
[0386] According to another embodiment, the encoder apparatus 3010
signals the encoding scheme used to the receiver apparatus 3030.
The signalling may be done independently of the subcarriers 3019 or
it may utilize the subcarriers 3019. For example, the signalling
may utilize an extra symbol as described above.
[0387] In the example given above, a frequency domain resource is
divided into domain subunits (subcarriers). Each of the domain
subunits forms one of a plurality of multiplexed wireless channels.
Each of the domain subunits is orthogonal and each is separately
indexed by a domain resource index.
[0388] The encoding of the first information portion by allocating
at least two different modulation types to a plurality of
subcarriers allocates an operational pattern to the plurality of
wireless channels/domain subunits. The channel/subunit has a
plurality of potential different states and the first information
is communicated by allocating a state to each channel/subunit. The
collection of states form the operational pattern.
[0389] In other embodiments, a different domain resource is divided
into domain subunits. Each of the domain subunits forms one of a
plurality of multiplexed wireless channels. Each of the domain
subunits is orthogonal and each is separately indexed by a domain
resource index. The encoding of the first information portion
allocates an operational pattern to the plurality of wireless
channels/domain subunits. The channel/subunit has a plurality of
potential different states and the first information is
communicated by allocating a state to each channel/subunit. The
collection of states form the operational pattern.
[0390] Thus different domain resources may be used in addition to
or as an alternative to the frequency domain resource. Examples of
domains include the time domain where the channels/subunits are
time slots, the spatial domain where the channels/subunits are
antenna locations or beams. Another technique that may be used for
encoding information is cyclic delay diversity. As illustrated in
FIG. 22, in this technique, data streams may be offset by a
variable delay. Information can be encoded by associating data
values to specified delays, for example, by associating a binary
bit zero with no delay and associating a binary bit one with the
presence of a delay.
[0391] The index-modulation concept can be applied to multiple
domains/channels (i.e. spatial, time slot, frequency) by having
different groups of M1 and M2 for different
domain/channel-subunits.
[0392] In the time domain, a transmission frame is subdivided into
smaller time units of equivalent (however, not necessarily) length
(time slots). We can index each time slot, and can assign different
information to each of the time slots--similar to OOK the
subcarriers described above. Thus first information 3010 may be
encoded to two consecutive time slots, by allowing the first slot
to include modulated information (second information)and keeping
the second time slot empty.
[0393] In the spatial domain, in MIMO
(multiple-input-multiple-output) systems such as beam forming
systems, we have multiple beams and can index the beams. Thus first
information 3010 may be encoded to two adjacent beams, by allowing
the first beam to convey modulated information (second information)
and preventing the second beam from conveying information (e.g. by
switching it off).
[0394] It is possible to simultaneously use two or more domains.
The first information is divided into subsections, one for each
domain.
[0395] For example, consider FIGS. 21. This Figure illustrate the
encoding of a first subsection 1101 of the first information using
four channel/subunits in the frequency domain and the encoding of a
second subsection 1001 using four subunits/channels in the time
domain.
[0396] For example, consider a scenario of four subcarriers
associated with the binary word 1101, and four time slots
associated with the binary word 1001.
[0397] For the frequency domain we can define two groups of
modulation levels each of size 2, i.e. G1:{M1:off, M2:8-QAM}, and
G2:{M1:BPSK, M2:4-QAM}.
[0398] Now, the binary word 1101 can be mapped to the frequency
domain as follows G1 G1 G2 G1.
[0399] Similarly, the binary word 1001 can be mapped to the time
domain as follows M1 M2 M2 M1. The resultant time-frequency block
(chunk) can be found in the Figure.
[0400] Although the above example shows encoding of a first
subsection of the first information using the frequency domain and
the encoding of a second subsection using the time domain, i.e. two
dimensional encoding, it will be appreciated that the first
information can be encoded using more than two domains. An example
of this is shown in FIG. 23, which illustrates encoding of the
first information using the frequency, time and spatial domains,
i.e. three dimensional encoding.
[0401] In the example of FIG. 23, a first subsection of the first
information is encoded using the frequency domain, a second
subsection of the first information is encoded using the time
domain and a third subsection of the first information is encoded
using the spatial domain, for example, through the use of a MIMO
system as described above. The example of FIG. 23 illustrates four
sub carriers f1, f2, f3 and f4 in the frequency domain, four time
slots t1, t2, t3 and t4 in the time domain and four spatially
separated beams s1, s2, s3 and s4 in the spatial domain. It will be
appreciated that other amounts of frequency sub carriers, time
slots or beams could be used and that the amount of each need not
be the same.
[0402] For the frequency domain, two groups G3 and G4 of modulation
schemes are defined, each group comprising four modulation types
G3:{A, B, C, D} and G4:{W, X, Y, Z}. As in the previous examples,
the modulation types A, B, C, D, W, X, Y and Z may be selected from
a wide range of suitable distinct modulation types known in the
art, such as OOK, BPSK, 4-QAM, and the like. In this way, using
modulation types A, B, C or D from the first group G3 for a
particular frequency sub carrier indicates a binary value of 1,
whilst use of modulation types W, X, Y or Z from the second group
G4 for a particular frequency sub carrier indicates a binary value
of 0.
[0403] For the time domain, two groups G5 and G6 of modulation
types are defined, each group comprising four modulation types
G5:{A, C, W, Y) and G6: {B, D, X, Z}. Use of modulation types A, C,
W or Y from group G5 for a particular time slot indicates a binary
value of 1, whilst use of modulation types B, D, X or Z from group
G6 for a particular time slot indicates a binary value of 0.
[0404] For the spatial domain, two groups G7 and G8 of modulation
types are defined, each group comprising four modulation types
G7:{A, B, W, X) and G8: {C, D, Y, Z}. Use of modulation types A, B,
W or X from group G7 for a particular beam indicates a binary value
of 1, whilst use of modulation types C, D, Y or Z from group G8 for
a particular beam indicates a binary value of 0.
[0405] Therefore, referring to FIG. 23, it can be seen that,
regardless of time slot or beam, modulation types A, B, C or D from
group G3 are used for each of frequency sub carriers f1, f2 and f4
and modulation types W, X, Y or Z from group G4 are used for
frequency sub carrier f3. Therefore, in this case, the binary word
1 1 0 1 is encoded using the frequency domain.
[0406] In the time domain, modulation types A, C, W or Y from group
G5 are used for each of time slots t1 and t4, whilst modulation
types B, D, X or Z from group G6 are used for each of the time
slots t2 and t3. Therefore, in this case, the binary word 1 0 0 1
is encoded using the time domain.
[0407] Similarly, only modulation types A, B, W or X from the group
G7 are used for spatial beams s1 and s2, whilst only modulation
types C, D, Y or Z from the group G8 are used for spatial beams s3
and s4. Therefore, in this case, the binary word 1 1 0 0 is encoded
using the spatial domain.
[0408] In this way, three or more dimensions or domains can be used
in order to increase the amount of information from the first block
of information that can be encoded. It will be appreciated that
other domains or techniques may be used instead of or in addition
to the frequency, time and spatial domains described in relation to
FIG. 23.
[0409] Another scheme for encoding the first information portion
using multiple domains is shown in FIG. 24. In this example, each
discrete domain unit in each domain is assigned a value. In the
example of FIG. 10, each sub-carrier in the frequency domain, each
time slot in the time domain and each beam in the spatial domain is
assigned a binary value, such as 0 0, 0 1, 1 0 or 1 1. Therefore,
providing a signal in a bin formed by a particular frequency
sub-carrier, on a particular beam and in a particular time slot
would be indicative of a value formed by combining the values
associated with the particular frequency sub-carrier, time slot and
beam. In the example shown in FIG. 24, a signal is provided using
the frequency sub-carrier associated with the binary value 1 0, in
the time slot associated with the binary value 1 0 and using the
beam associated with the binary value 0 1. Therefore, this signal
is representative of the value 1 0 1 0 0 1. In this way, the total
number of bits that can be encoded is
log.sub.2(N_f)+log.sub.2(N_s)+log.sub.2(N_t), where N_f is the
total number of frequency sub-carriers, N_s is the total number of
beams and N_t is the total number of time slots.
[0410] The signal provided in each bin formed by a frequency
sub-carrier, a time slot and a beam, may be subjected to a second
modulation scheme, such as M-QAM, for encoding the second
information portion. In this case, the total number of bits
transmitted in each transmission is:
log.sub.2(N.sub.--s)+log.sub.2(N.sub.--t)+log.sub.2(N.sub.--f)+log.sub.2-
(M)
[0411] If the number of frequency sub-carriers is sixty four and
the sampling frequency is 25 MHz, then the duration of one time
slot is:
1/f.sub.--s*N.sub.--f=2.56 .mu.s
[0412] If the example is extended such that four time slots are
provided per transmission, thirty two beam transmitters are
provided and 256-QAM is used, then for one transmission, the
maximum data rate for this example is:
log.sub.2(32)+log.sub.2(4)+log.sub.2(64)+log.sub.2(256)/(4*2.56
.mu.s)=2.05 Mbps
[0413] By using this method, significant energy savings can be
achieved. The number of bins in this example is 4*32*64=8192, which
means that 1/8192 of the energy is used for the transmission.
[0414] As in the previous examples, more than one modulation scheme
may be used to encode the data within the bins. For example, if all
the sub-carriers in one of the domains, such as the frequency
domain, were used for the transmission, and a first modulation type
such as OOK was used for a first subset of the frequency
sub-carriers and a second modulation type such as 256-QAM was used
in the remaining frequency subcarriers, the achievable data rate
would be (for 4 time slots, 32 beam transmitters, 32 frequency
sub-carriers encoded using OOK and 32 frequency sub-carriers
encoded using 256-QAM):
log.sub.2(4)+log.sub.2(32)+32sub-carriers(x1 bit per bin of
OOK)+32sub-carriers(x8 bits per bin for 256-QAM)/(4*2.56
.mu.s)=28.81 Mbps
[0415] This example would use 1/(N_t*N_s*2)=1/130 of the energy
that would be used if all time slots, all transmitters and all
frequency sub-carriers were used.
[0416] Suitable apparatus for applying these multi domain
techniques is illustrated in FIG. 25. It will be seen from this
that a transmitter 3905 is provided with a splitter 3910 for
splitting the data into at least a first section and a second
section, a module 3915 for applying inter-carrier modulation to the
first portion by assigning sub-carriers of at least two and
preferably at least three, domains for transmission of the signal.
Each sub-carrier in each domain is associated with a value. A
grammar is used to store the values associated with each
sub-carrier in each domain. The first data is divided into portions
and each portion is assigned to a sub-carrier in a domain with an
associated value equal to the value of the portion of the first
data. A module 3920 for encoding the second data using
intra-carrier modulation, for example, by applying a modulation
scheme such as OOK or 256-QAM to the signal, is also provided.
[0417] By determining the sub-carriers in each domain used to
transmit the signal and applying the grammar that indicates which
sub-carriers in each domain are indicative of which values, the
first data may be reconstructed. On receipt of the signal, the
receiver 3925 de-modulates the received signal using a suitable
demodulator 3935 to recover the second information portion and a
module 3930 is provided for identifying the sub-carriers used for
transmission and comparing the identified sub-carriers with the
grammar in order to determine the first information portion. The
original information signal can then be reconstructed by combining
the portions of the first information and the second information
using a combiner 3940.
[0418] Reference has been made to various examples in the preceding
description. It should be understood that reference to an example
implies that alternative, but not necessarily explicitly disclosed
implementations can be used.
[0419] As used herein the term orthogonal may mean strictly
orthogonal or substantially orthogonal (i.e. not significantly
correlated).
[0420] The blocks illustrated in FIG. 15 may represent steps in a
method and/or sections of code in the computer program. The
illustration of a particular order to the blocks does not
necessarily imply that there is a required or preferred order for
the blocks and the order and arrangement of the block may be
varied. Furthermore, it may be possible for some steps to be
omitted.
[0421] Features described in the preceding description may be used
in combinations other than the combinations explicitly
described.
[0422] Although functions have been described with reference to
certain features, those functions may be performable by other
features whether described or not.
[0423] Although features have been described with reference to
certain embodiments, those features may also be present in other
embodiments whether described or not.
[0424] Whilst endeavoring in the foregoing specification to draw
attention to those features of the invention believed to be of
particular importance it should be understood that the Applicant
claims protection in respect of any patentable feature or
combination of features hereinbefore referred to and/or shown in
the drawings whether or not particular emphasis has been placed
thereon.
[0425] At least one of the following features may apply to one or
more embodiments of the invention described above.
[0426] A method of communicating information optionally comprises
dividing the information into at least a first information portion
and a second information portion; modulating a plurality of domain
resources to encode the first information using an index or
grammar; and encoding the second information by modulation of
domain resources.
[0427] Modulating the plurality of domain resources to encode the
first information optionally comprises allocating at least two
different modulation types to a plurality of sub-carriers. Encoding
the first information portion optionally comprises selecting which
modulation type is allocated to which domain resource according to
the index or grammar, which applies meaning to which modulation
type is allocated to which domain resource.
[0428] Modulating the plurality of domain resources to encode the
first information optionally comprises encoding a part of the first
information portion using one or more domain resources of a first
domain and another part of the first information portion is encoded
using one or more domain resources of a second domain. Modulating
the plurality of domain resources to encode the first information
optionally comprises encoding at least a further part of the first
information portion using one or more domain resources of a third
domain.
[0429] The first, second and/or third domains optionally comprise
the frequency, time and/or spatial domains and/or the domain
resources comprise sub-channels of a frequency domain, beams
arranged in a spatial domain and/or time slots in a time domain.
Values are optionally associated with domain resources of at least
one domain and wherein at least part of the first information
portion is encoded by providing a signal using a domain resource
indicative of the value of the portion of the data. Values are
optionally associated with domain resources of at least two
domains, and wherein the first information portion is at least
partially encoded by providing a signal using domain resources from
the at least two domains indicative of the value of parts of the
first information portion.
[0430] A method of communicating information optionally comprises
dividing the information into at least a first information portion
and a second information portion; encoding the first information
portion by allocating at least two different modulation types to a
plurality of subcarriers; and encoding the second information by
modulation of subcarriers belonging to a subset of the plurality of
subcarriers.
[0431] Encoding the first information portion optionally comprises
selecting which modulation type is allocated to which subcarrier
according to a grammar that applies meaning to which modulation
type is allocated to which subcarrier. Each subcarrier is
optionally associated with an index in a ordered series of indexes,
wherein the modulation type of a subcarrier assigns a value to the
associated index and wherein assigned values, ordered according to
the ordered series of indexes, provide a data word representing the
first information portion.
[0432] A method optionally comprises: encoding the first
information portion by allocating only two different modulation
types to the subcarriers. The first information portion optionally
comprises a first subset of information bits and a second subset of
information bits, further comprising selecting a first information
encoding scheme from a plurality of different first information
encoding schemes, each of which has a different allocation of
modulation types to the first and second subsets. The plurality of
different first information encoding schemes are optionally
predetermined and shared with a receiver.
[0433] At least one of the above methods optionally comprises
transmitting control information that identifies the selected first
information encoding scheme. At least one of the above methods
optionally further comprises encoding the control information by
modulation of subcarriers.
[0434] At least one of the above methods optionally comprises
identifying within the first information portion a first subset of
information bits and a second subset of information bits; selecting
the majority subset of the first and second subsets; and using the
majority subset to allocate a first modulation type to a first
plurality of subcarriers.
[0435] At least one of the above methods optionally comprises
selecting a first information encoding scheme, for determining the
allocation of the at least two different modulation types amongst
the plurality of subcarriers that maximises the available bandwidth
for the remaining information portions. At least one of the above
methods optionally comprises utilising some of the bandwidth for
encoding control information and/or for degenerate encoding a
portion of the second information portion.
[0436] At least two different modulation types optionally comprise
a first modulation type that modulates a subcarrier with a first
fixed amplitude and a second modulation type that modulates a
subcarrier with a second fixed amplitude.
[0437] Optionally, at least two different modulation types enable
on-off keying (OOK), the first amplitude representing suppression
of subcarriers and the second amplitude representing non
suppression of the subcarriers.
[0438] At least one of the above methods optionally comprises power
reallocation from the suppressed subcarriers to the non suppressed
subcarriers.
[0439] At least two different modulation types optionally comprise
a first modulation type that modulates a subcarrier with a higher
order modulation and a second modulation type that modulates a
subcarrier with a lower order modulation.
[0440] A transmitter apparatus optionally comprising means for
dividing the information into at least a first information portion
and a second information portion; means for encoding the first
information portion by allocating at least two different modulation
types to a plurality of subcarriers; and means for encoding the
second information by modulation of subcarriers belonging to a
subset of the plurality of subcarriers
[0441] The transmitter apparatus optionally further comprises means
for performing at least one of the methods described above.
[0442] A transmitter apparatus optionally comprising splitter
circuitry configured to divide the information into at least a
first information portion and a second information portion;
inter-carrier modulation circuitry configured to encode the first
information portion by allocating at least two different modulation
types to a plurality of subcarriers; and intra-carrier modulation
circuitry configured to encode the second information by modulation
of subcarriers belonging to a subset of the plurality of
subcarriers.
[0443] A method of communicating information optionally comprising
decoding first information portion by determining the allocation of
at least two different modulation types to a plurality of
subcarriers; and decoding second information by demodulation of
subcarriers belonging to a subset of the plurality of subcarriers;
and combining the first information portion and the second
information portion.
[0444] Decoding the first information portion optionally comprises
determining which modulation type is allocated to which subcarrier
and applying meaning according to a grammar that applies meaning to
which modulation type is allocated to which subcarrier.
[0445] Each subcarrier is optionally associated with an index in a
ordered series of indexes, wherein the modulation type of a
subcarrier assigns a value to the associated index and wherein
assigned values, ordered according to the ordered series of
indexes, provide a data word representing the first information
portion.
[0446] At least one method described above optionally comprises
determining which modulation type is allocated to which subcarrier;
decoding the first information portion according to different
encoding schemes by applying meaning according to a grammar which
applies a meaning to which modulation types are allocated to which
subcarriers, wherein each encoding scheme has its own grammar; and
selecting one of the first information portions created according
to different encoding schemes.
[0447] At least one method described above optionally comprises
identifying the encoding scheme is use; and decoding the first
information portion by determining which modulation type is
allocated to which subcarrier and applying meaning according to a
grammar of the identified encoding scheme that applies meaning to
which modulation type is allocated to which subcarrier.
[0448] At least two different modulation types optionally comprise
a first modulation type that modulates a subcarrier with a higher
order modulation and a second modulation type that modulates a
subcarrier with a lower order modulation.
[0449] A receiver apparatus optionally comprises means for decoding
first information portion by determining the allocation of at least
two different modulation types to a plurality of subcarriers; means
for decoding second information by demodulation of subcarriers
belonging to a subset of the plurality of subcarriers; and means
for combining the first information portion and the second
information portion.
[0450] The receiver optionally further comprises means for
performing at least one of the methods described above.
[0451] A receiver apparatus optionally comprising inter-carrier
modulation detection circuitry configured to decode first
information portion by determining the allocation of at least two
different modulation types to a plurality of subcarriers;
intra-carrier demodulation circuitry configured to decode second
information by demodulation of subcarriers belonging to a subset of
the plurality of subcarriers; and combiner circuitry configured to
combine the first information portion and the second information
portion.
[0452] A system optionally comprises at least one of the receiver
apparatus described above and the at least one of the transmitter
apparatus described above.
[0453] At least one method described above optionally comprises
dividing the information into at least a first information portion
and a second information portion; communicating the first
information portion by allocating an operational pattern to a
plurality of wireless channels; and communicating the second
information by wireless communication via one or more of the
wireless channels.
[0454] The wireless channels optionally have a plurality of
potential different states and wherein the first information is
communicated by allocating a state for each wireless channel,
wherein the collection of states form the operational pattern. The
wireless channels optionally include orthogonal subcarriers that
have different modulation states. The wireless channels optionally
include time slots that have different modulation states.
[0455] A method of communicating information optionally comprises
dividing the information into at least a first information portion
and a second information portion; encoding the first information
portion by allocating at least two different modulation types to a
plurality of orthogonal multiplexing domain subunits; and encoding
the second information by modulation of orthogonal multiplexing
domain subunits belonging to a subset of the plurality of
orthogonal multiplexing domain subunits.
[0456] A method of communicating information optionally comprises
dividing the information into at least a first information portion
and a second information portion; encoding the first information
using domain resource-index modulation; and encoding the second
information by modulation of domain resources.
[0457] A method of communicating information optionally comprises
dividing the information into at least a first information portion
and a second information portion; encoding the first information
using inter-domain resource differentiation; and encoding the
second information by modulation of domain resources.
[0458] A method of communicating information optionally comprises
dividing the information into at least a first information portion,
a second information portion and a third information portion;
encoding the first information portion by allocating at least two
different modulation types to a plurality of first orthogonal
multiplexing domain subunits; encoding the second information
portion by allocating at least two different modulation types to a
plurality of second orthogonal multiplexing domain subunits; and
encoding the third information by modulation of orthogonal
multiplexing domain subunits belonging to a subset of the plurality
of first orthogonal multiplexing domain subunits and second
orthogonal multiplexing domain subunits.
[0459] A method of communicating information optionally comprises
dividing the information into at least N portions; encoding the
each one of M of the N portions by allocating at least two
different modulation types to a plurality of orthogonal
multiplexing domain subunits; and encoding the N-M portion(s) by
modulation of orthogonal multiplexing domain subunits.
[0460] At least one method described above optionally comprises
encoding at least one of the M portions by allocating at least
2.sup.M different modulation types to a plurality of orthogonal
multiplexing domain subunits. The plurality of orthogonal
multiplexing domain subunits optionally comprise domain subunits
from two or more domains. The plurality of orthogonal multiplexing
domain subunits optionally comprise domain subunits from two or
more of the frequency domain, the time domain and the spatial
domain.
[0461] The plurality of subcarriers optionally comprise subcarriers
of two or more domains. The plurality of subcarriers optionally
comprise subcarriers of two or more of the frequency domain, the
time domain and the spatial domain.
[0462] A first information portion is optionally encoded using one
or more sub-units of a first domain and at least a second
information portion is encoded using one or more sub-units of a
second domain. At least one method described above optionally
comprises encoding at least a third information portion using one
or more sub-units of a third domain. The domains optionally
comprise the frequency, time and/or spatial domains.
[0463] Values are optionally associated with subcarriers of at
least one domain and wherein a respective portion of the data is
optionally encoded by providing a signal using a sub-unit
indicative of the value of the portion of the data.
[0464] A portion of the data is optionally provided by allocating
at least two different modulation types to a plurality of sub-units
of at least one domain.
[0465] At least one embodiment of the present relates to invention
wireless communication systems. At least one embodiment of the
present invention relates to a novel and improved system and/or a
method to enhance the performance of wireless communication systems
employing multiple transmitter elements and at least one receiver
element, sometimes referred to as multiple input multiple output
(MIMO) systems for multiple receiver elements and multiple input
single output (MISO) systems for a single receiver element.
[0466] In multiple input multiple output and multiple input single
output systems (MIMO and MISO), Spatial Modulation is used. The
fundamental component of Spatial Modulation is the exploitation of
the spatial domain, i.e., the spatial position of the antenna at
the transmitter-side, as a means for sending information through a
wireless fading channel. In particular, the underlying principle of
Spatial Modulation is twofold: i) at the transmitter-side, a
one-to-one mapping of information data to transmit antennas, thus
allowing them to convey information, and ii) at the receiver-side,
the exploitation, thanks to the stochastic properties of wireless
fading channels, of distinct multipath profiles received from
different transmit antennas.
[0467] Contributions which are available for Spatial Modulation
have been based on the same assumption: a uniform power allocation
mechanism among the active transmit antennas is assumed a priori.
The key problem of optimizing the effective spatial constellation
pattern of Spatial Modulation has been addressed. However, the
known optimization is based on the activation and de-activation of
a set of antennas rather than on power allocation mechanisms. A
uniform power allocation strategy is implicitly retained in the
analysis.
[0468] Known solutions do also not fully exploit all degrees of
freedom and potentialities of Spatial Modulation (SM) concept. In
particular, the common limitation of all above techniques for SM is
not taking maximum advantage of multiple antennas at the
transmitter-side to obtain transmit-diversity gains. As a matter of
fact, signal designs and optimal detectors available so far offer a
diversity order that depends on the number of receive antennas
only. As a consequence, SM methods proposed to date might find
limited applicability to low-complexity and low-cost downlink
settings and operations, where it is more economical to add
equipment to base stations rather than to remote mobile units.
[0469] Spatial modulation is considered in: Y. Chau and S.-H. Yu,
"Space shift keying modulation", U.S. Pat. No. 9,985,988, Filed
Nov. 7, 2001, Pub. Date Jul. 18, 2002; and Y. A. Chau and S.-H. Yu,
"Space modulation on wireless fading channels", IEEE Vehicular
Technology Conference--Fall, vol. 3, pp. 1668-1671, October
2001.
[0470] The disadvantages of such SM methods are as follows:
[0471] Even though two transmit-antennas are employed in the
communication link, the SM proposal of Chau and Yu offers a
diversity order only equal to one.
[0472] The error probability in depends only on the channel power
gain of the wireless link related to the antenna that can be either
switched on or off during data transmission. As a consequence, in
an adaptive system and for optimizing the system performance, the
antenna with the best (average) channel conditions may be chosen as
the one to be switched on and off.
[0473] The SM concept introduced by Chau and Yu, which is called
Space Shift Keying (SSK) in which only one transmit-antenna is
activated when message 1 has to be sent, while both
transmit-antennas are activated when a message 2 needs to be sent.
Thus when message 2 has to be sent, each antenna at the
transmitter-side is required to transmit a signal with a
corresponding energy. This leads to a power consumptions cost which
is twice with respect to that required when message 1 is sent.
[0474] Other SM methods are proposed in: C.-W. Ahn, S.-B. Yun,
E.-S. Kim, H. Haas, R. Mesleh, T.-I. Hyon, and S. McLaughlin,
"Spatial modulation method and transmitting and receiving
apparatuses using the same in a multiple input multiple output
system", Filed Jul. 10, 2007, Pub. Date Feb. 14, 2008; R. Y.
Mesleh, H. Haas, S. Sinanovic, C. W. Ahn, and S. Yun, "Spatial
modulation", IEEE Transactions on Vehicular Technology, vol. 57,
no. 4, pp. 2228-2241, July 2008; and J. Jeganathan, A. Ghrayeb, and
L. Szczecinski, "Spatial modulation: Optimal detection and
performance analysis", IEEE Communications Letters, vol. 12, no. 8,
pp. 545-547, August 2008.
[0475] The disadvantages of such SM methods are as follows:
[0476] Even though two transmit-antennas are employed in the
communication link, the SM proposal in Mesleh et al and Jeganathan
et al offers, similar to Chau and Yu, a diversity order only equal
to 1.
[0477] The error probability depends on both complex channel gains
and, in particular, is a function of the difference of them. As a
consequence, depending on the instantaneous channel conditions,
constructive and destructive combinations can take place, thus
preventing the full exploitation of the two transmit-antennas for
diversity purposes.
[0478] The error probability is a function of the spatial
correlation coefficient and, in particular, the more the wireless
links are correlated, the worse the error probability is.
[0479] At least one embodiment described herein has an object of
improving an error probability for wireless fading channels, i.e.
increase the robustness and reliability of data transmission.
[0480] At least one embodiment described herein has an object of
improving spectral efficiency for varying channel conditions.
[0481] At least one embodiment described herein has an object of
providing a wireless communication system, based on Spatial
Modulation, with transmit-diversity capabilities.
[0482] At least one method described herein optionally comprises a
method of spatial modulation to identify a transmitter element
within a transmission array of at least two transmitter elements,
each transmitter element being located at a transmitter, wherein a
signal is transmitted by one active transmitter element at a time
over a channel to a receiver, the method comprising: allocating
power to the transmitter elements wherein the power is allocated
differently between at least two transmitter elements, receiving
transmitted data at the receiver and detecting location of the
active transmitter element using knowledge of the power allocated
to the transmitter elements.
[0483] At least one method described herein optionally comprises
determining an error probability of the signal and determining a
power allocation model for the transmitter elements by optimizing
the average bit error probability of the signal, wherein the step
of allocating power to the transmitter elements is performed
according to the determined power allocation model.
[0484] The error probability optionally comprises an average bit
error probability.
[0485] Optionally, the channel comprises a wireless link. More
preferably, the channel comprises an optical wireless link, for
example using a coherent or incoherent light source.
[0486] Preferably, the transmitter elements could include radio
frequency antennae or other signal emitters, such as loudspeakers,
ultrasound transmitters, multiple LEDs (light emitting diodes),
etc.
[0487] The receiver optionally comprises at least one receiver
element, which can be for example a detector for radio frequency,
acoustic or ultrasound signals or a photo detector.
[0488] Preferably, each channel has a defined impulse response
which can include a fading condition of the related channel as
gain. The fading condition is optionally determined by a
statistical method, such as Rayleigh fading or Nakagami-m fading.
If little or no fading is present, an additive white Gaussian noise
distribution can optionally be used.
[0489] The impulse response of each channel optionally includes a
delay of the related channel which can be independent and uniformly
distributed at least in a predetermined interval. The delay can be
known at the receiver. A time-synchronization is optionally
performed at the receiver.
[0490] The impulse response of each channel optionally includes a
phase of the related channel.
[0491] Preferably, the optimizing of the average bit error
probability, also referred to as ABEP, includes a minimization of
the average bit error probability.
[0492] Preferably, the average bit error probability of the signal
is a function of the impulse responses of the respective
channels.
[0493] Preferably, the receiver provides full information of the
channel state, which is also referred to as channel state
information (CSI). The full channel state information optionally
comprises the knowledge of the gain, phase and delay of each
channel. Preferably, phase information is not necessarily required,
for example when incoherent light sources are used as transmitter
elements. In alternative embodiments, other receivers, for example
with partial channel state information, which have been developed
in relation with spatial modulation performance analyses, can be
used.
[0494] The respective channels are optionally correlated or
uncorrelated. Correlation coefficients between the channels can
optionally be determined at the receiver.
[0495] Preferably, the total power, i.e. the sum of the energies
transmitted when data is transmitted from each transmitter element
of the transmission array to the receiver, is fixed to a given
value. In an alternative embodiment, a power control is optionally
applied in which the total power depends on the channel environment
and/or propagation of the signal.
[0496] The optimization or minimization of the average error
probability is optionally obtained by using one or more either
analytical or numerical methods. Preferably, a model of power
allocation is generated at the receiver. The model of power
allocation may consider the correlation coefficients between the
channels. The model of power allocation may also consider the
impulse responses, and/or the fading condition in particular, of
the channels.
[0497] More preferably, the location of a transmitter element is
detected by using data of the model of power allocation.
Preferably, the active transmitter element can transmit modulated
data, thereby conveying data by both an index of the transmitter
element and by the modulated signal.
[0498] Preferably, each channel has a unique impulse response, and
the method further comprises: predetermining a power allocation
sequence of a transmitter element, in the step of allocating power
to the transmitter elements, allocating the power allocation
sequence to the transmitter elements, and in the step of detecting
location of the active transmitter element, using knowledge of the
power allocation sequence of the active transmitter element.
[0499] Preferably, the channel comprises a wireless link. More
preferably, the channel comprises an optical wireless link, for
example using a coherent or incoherent light source.
[0500] Preferably, the transmitter elements could include radio
frequency antennae or other signal emitters, such as loudspeakers,
ultrasound transmitters, multiple LEDs (light emitting diodes),
etc.
[0501] The receiver preferably comprises at least one receiver
element, which can be for example a detector for radio frequency,
acoustic or ultrasound signals or a photo detector.
[0502] Preferably, each channel has a defined impulse response
which can include a fading condition of the related channel as
gain. The fading condition is optionally determined by a
statistical method, such as Rayleigh fading or Nakagami-m fading.
In an alternative embodiment, in which little or no fading is
present, an additive white Gaussian noise distribution can be
used.
[0503] The impulse response of each channel may also include a
delay of the related channel which can be independent and uniformly
distributed at least in a predetermined interval. The delay can be
known at the receiver. A time-synchronization may be performed at
the receiver.
[0504] The impulse response of each channel optionally also
includes a phase of the related channel. The respective channels
are optionally correlated or uncorrelated.
[0505] The power allocation sequences allocated to the transmitter
element optionally comprises a random sequence. Preferably, the
power allocation sequences allocated to different transmitter
elements are provided such that they are easily distinguishable
from each other by using a corresponding detection method. The
power allocation sequence allocated to a transmitter element is
optionally stored in a storage, such as a look-up table, at the
receiver to be accessed when the location of a transmitter element
is to be detected.
[0506] Preferably, the active transmitter element can transmit
modulated data, thereby conveying data by both an index of the
transmitter element and by the modulated signal.
[0507] Preferably, the method further comprises receiving
transmitted training data at the receiver during a training phase,
predetermining a power allocation sequence according to received
training data, in the step of allocating power to the transmitter
elements, allocating the power allocation sequence to the
transmitter elements, terminating the training phase, receiving
transmitted data at the receiver after the training phase, and in
the step of detecting location of the active transmitter element,
using knowledge of the power allocation sequence of the active
transmitter element.
[0508] Preferably, the channel comprises a wireless link. More
preferably, the channel comprises an optical wireless link, for
example using a coherent or incoherent light source.
[0509] Preferably, the transmitter elements could include radio
frequency antennae or other signal emitters, such as loudspeakers,
ultrasound transmitters, multiple LEDs (light emitting diodes),
etc.
[0510] The receiver preferably comprises at least one receiver
element, which can be for example a detector for radio frequency,
acoustic or ultrasound signals or a photo detector.
[0511] Preferably, each channel has a defined impulse response
which can include a fading condition of the related channel as
gain. The fading condition is optionally determined by a
statistical method, such as Raleigh fading or Nakagami-m
fading.
[0512] The impulse response of each channel optionally also
includes a delay of the related channel which can be independent
and uniformly distributed at least in a predetermined interval. The
delay may be known at the receiver. A time-synchronization may be
performed at the receiver.
[0513] The impulse response of each channel optionally also include
a phase of the related channel. Preferably, in one embodiment, the
receiver provides full information of the channel state, which is
also referred to as channel state information (CSI). The full
channel state information optionally comprises the knowledge of the
gain, phase and delay of each channel. Preferably, phase
information is not necessarily required, for example when
incoherent light sources are used as transmitter elements. In
alternative embodiments, other receivers, for example with partial
channel state information, which have been developed in relation
with spatial modulation performance analyses, can be used.
Preferably, the full channel state information (full CSI) is
obtained during the training phase prior to transmitting data.
[0514] The respective channels are optionally correlated or
uncorrelated. The channel state information may comprise
correlation coefficients between the channels. The received data
prior to allocating the power allocation sequence to the
transmitter element optionally comprises and may consider channel
state information, such as correlation coefficients between the
channels or the impulse response of the channels to determine the
power allocation sequence of the related transmitter elements. The
power allocation sequence allocated to a transmitter element is
optionally stored in a storage, such as a look-up table, at the
receiver to be accessed when the location of a transmitter element
is to be detected.
[0515] Preferably, the active transmitter element can transmit
modulated data, thereby conveying data by both an index of the
transmitter element and by the modulated signal.
[0516] The invention as stated above may improve the performance of
Spatial Modulation by, for example, allocating in an opportunistic
fashion the transmission power, while keeping the simplicity of
Spatial Modulation at the transmitter and receiver side. The
invention allows to artificially create unbalanced fading
conditions to make the transmit-receive wireless links more
distinguishable to each other. Furthermore, it may help to reduce
the effect of spatial channel correlation.
[0517] Thus, the determination of the location of an active
transmitter element within an array of at least two transmitter
elements is facilitated.
[0518] The present invention may be applied to an arbitrary number
of transmitter elements at the transmitter and receiver elements at
the receiver. Moreover, the invention can be applied to distributed
and virtual MIMO systems using Spatial Modulation.
[0519] At least one embodiment comprises a transmission apparatus
for identifying a transmitter-element within a transmission array
of at least two transmitter-elements, each transmitter-element
being located at a transmitter, wherein a signal is transmitted by
one active transmitter-element at a time over a channel to a
receiver, the transmission apparatus comprising a power allocator
to allocate power to the transmitter-elements wherein the power is
allocated differently between at least two
transmitter-elements.
[0520] Preferably, the transmission apparatus further comprises a
calculator to determine a power allocation model for the
transmitter-elements by optimizing an average bit error probability
of the signal, wherein the power allocator allocates power to the
transmitter-elements according to the determined power allocation
model.
[0521] Preferably, each channel has a unique impulse response, and
the transmission apparatus further comprises a sequence
predetermination module to predetermine a power allocation sequence
of a transmitter element, wherein the power allocator allocates the
power allocation sequence to the transmitter elements.
[0522] At least one embodiment of the present invention comprises a
receiver apparatus for identifying a transmitter-element within a
transmission array of at least two transmitter-elements, each
transmitter-element being located at a transmitter, wherein a
signal is transmitted by one active transmitter-element at a time
over a channel to the receiver apparatus, the receiver apparatus
being configured to receive transmitted data and comprising a
location detector to detect location of the active
transmitter-element using knowledge of the power allocated to the
transmitter-elements.
[0523] Preferably, the receiver apparatus further comprises a
channel estimator to determine an error probability of the signal
and a model generator to determine a power allocation model for the
transmitter-elements by optimizing the average bit error
probability of the signal. Preferably, each channel has a unique
impulse response, and the location detector, in detecting location
of the active transmitter element, is operable to use knowledge of
a power allocation sequence of the active transmitter element.
[0524] Preferably, the receiver apparatus is operable to receive
transmitted training data at the receiver apparatus during a
training phase, to feed back training data to a transmitter
apparatus for predetermining a power allocation sequence according
to the training data and for allocating a power allocation sequence
to the transmitter elements, and the location detector, in
detecting the location of the active transmitter element, is
operable to use knowledge of the power allocation sequence of the
active transmitter element.
[0525] At least one embodiment optionally comprises a spatial
modulation system for identifying a transmitter-element within a
transmission array of at least two transmitter-elements, each
transmitter-element being located at a transmitter, wherein a
signal is transmitted by one active transmitter-element at a time
over a channel to a receiver, the system comprising the transmitter
apparatus and the receiver apparatus.
[0526] The spatial modulation system optionally incorporates any of
the features described herein.
[0527] One or more embodiments optionally comprise a computer
program product containing one or more sequences of
machine-readable instructions for spatial modulation to identify a
transmitter-element within a transmission array of at least two
transmitter-elements, each transmitter-element being located at a
transmitter, wherein a signal is transmitted by one active
transmitter-element at a time over a channel to a receiver, the
instructions being adapted to cause one or more processors to:
allocate power to the transmitter-elements wherein the power is
allocated differently between at least two transmitter-elements,
receive transmitted data at the receiver and detect location of the
active transmitter-element using knowledge of the power allocated
to the transmitter-elements.
[0528] The computer program product optionally incorporates any
feature described herein
[0529] One or more embodiments optionally comprise or implement a
method of spatial modulation to identify a transmitter within a
transmission array of at least two transmitter elements, wherein a
signal is transmitted by one active transmitter element at a time
over a channel to a receiver, the method comprising predetermining
a waveform of the signal prior to transmitting it to the receiver,
wherein channel signatures of two different channels form a pair of
channel signatures respectively, and wherein the waveform is
predetermined to exploit the pair of channel signatures to cause a
difference between signals transmitted over the different channels,
receiving transmitted data at the receiver, and detecting location
of the active transmitter element using the difference between the
transmitted signals.
[0530] Preferably, the channels signatures comprise propagation
delays. Preferably, the pair of channel signatures is subject to an
orthogonality condition. Preferably, the channel comprises a
wireless link. More preferably, the channel comprises an optical
wireless link, for example using a coherent or incoherent light
source. Preferably, the transmitter elements could include radio
frequency antennae or other signal emitters, such as loudspeakers,
ultrasound transmitters, multiple LEDs (light emitting diodes),
etc.
[0531] The receiver preferably comprises at least one receiver
element, which can be for example a detector for radio frequency,
acoustic or ultrasound signals or a photo detector.
[0532] The waveform of the signal is optionally subject to a delta
function, such as a Dirac's delta function.
[0533] Preferably, the signals transmitted to the receiver are sent
with a similar, more preferably with an equal, energy from each of
the transmitter elements. Preferably, each channel has a defined
impulse response which can include a fading condition of the
related channel as gain. The fading condition may be determined by
a statistical method, such as Rayleigh fading or Nakagami-m
fading.
[0534] The propagation delay of the related channel may be
independent and uniformly distributed at least in a predetermined
time interval, in particular in the interval [0, T.sub.m), wherein
T.sub.m is the signalling internal for all signals being sent from
the transmitter elements. The impulse response of each channel
optionally also includes a phase of the related channel.
[0535] Preferably, in one embodiment, the receiver provides full
information of the channel state, which is also referred to as
channel state information (CSI). The full channel state information
optionally comprises the knowledge of the gain, phase and delay of
each channel. Preferably, phase information is not necessarily
required, for example when incoherent light sources are used as
transmitter elements. In alternative embodiments, other receivers,
for example with partial channel state information, which have been
developed in relation with spatial modulation performance analyses,
can be used. Preferably, the full channel state information (full
CSI) is obtained during the training phase prior to transmitting
data.
[0536] The respective channels can be correlated or uncorrelated.
The channel state information optionally comprises correlation
coefficients between the channels.
[0537] The Spatial Modulation concept is further improved by the
introduction of an optimal signal design at the transmitter and an
optimal detection algorithm at the receiver.
[0538] Furthermore, transmit-diversity gains for Spatial Modulation
are allowed according to one aspect of the invention. The Spatial
Modulation according to this invention is inherently more robust
than other Spatial Modulation proposals to spatial correlation of
fading.
[0539] When the receiver is equipped with multiple antennas, the
invention offers transmit- and receive-diversity at the same
time.
[0540] With regard to the Spatial Modulation assisted by
time-orthogonal signal design, a feedback channel from the receiver
to the transmitter may be required to have the relative times of
arrival (i.e., timing information) of all transmitter element
indexes after propagation through the wireless channel. The
synchronization unit at the receiver optionally estimates these
delays during a training phase before data transmission, and send
back them to the transmitter via a control channel. Upon reception
of this information, the transmitter optionally selects the best
signal design to guarantee signal orthogonality at the receiver via
a Maximum Likelihood (ML) optimal detector described in further
detail below.
[0541] Optionally, at least one embodiment of the present invention
comprises a transmission apparatus for identifying a transmitter
within a transmission array of at least two transmitter elements,
wherein a signal is transmitted by one active transmitter element
at a time over a channel to a receiver, the transmission apparatus
comprising a waveform calculator to predetermine a waveform of the
signal prior to transmitting it to the receiver, wherein channel
signatures of two different channels form a pair of channel
signatures respectively, and wherein the waveform is predetermined
to exploit the pair of channel signatures to cause a difference
between signals transmitted over the different channels.
[0542] Optionally, at least one embodiment of the present invention
comprises a spatial modulation system for identifying a
transmitter-element within a transmission array of at least two
transmitter-elements, each transmitter-element being located at a
transmitter, wherein a signal is transmitted by one active
transmitter-element at a time over a channel to a receiver, the
system comprising the transmitter apparatus and a receiver
apparatus operable to receive transmitted data and detect location
of the active transmitter element using the difference between the
transmitted signals.
[0543] The spatial modulation system may incorporate any of the
features described herein.
[0544] Optionally, at least one embodiment of the present invention
comprises a computer program product containing one or more
sequences of machine-readable instructions for spatial modulation
to identify a transmitter within a transmission array of at least
two transmitter elements, wherein a signal is transmitted by one
active transmitter element at a time over a channel to a receiver,
the instructions being adapted to cause one or more processors to
predetermine a waveform of the signal prior to transmitting it to
the receiver, wherein channel signatures of two different channels
form a pair of channel signatures respectively, and wherein the
waveform is predetermined to exploit the pair of channel signatures
to cause a difference between signals transmitted over the
different channels, receive transmitted data at a receiver, and
detect location of the active transmitter element using the
difference between the transmitted signals.
[0545] The computer program product may incorporate any of the
features described herein.
[0546] To understand the motivation of the proposed power
allocation method and the substantial performance improvement that
can be achieved with it over wireless channels, some numerical
examples are now described. The examples are shown in FIGS. 26 to
29. These figures represent the ABEP of a N t.times.1 SM-MIMO
system, with N t denoting the number of transmitter elements at a
transmitter. Throughout this application, when the term
"transmitters" or "transmit-antenna" is used, this refers to
transmitter elements at the transmitter. Accordingly, the term
"receivers" or "receive-antenna" refers to receiver elements.
[0547] FIGS. 26 to 29 are obtained by considering a Nakagami-m
fading channel with parameters {m.sub.i}.sub.i=1.sup.N.sup.t and
{.OMEGA..sub.i}.sub.i=1.sup.N.sup.t on a wireless link between an
i-th transmit-antenna and a single receive-antenna. FIGS. 26 to 29
are obtained by assuming a uniform power allocation mechanism among
the transmit-antennas (E.sub.m/N.sub.0 denotes the average
Signal-to-Noise Ratio, SNR, for each wireless link), but for
balanced (i.e., {.OMEGA..sub.i}.sub.i=1.sup.N.sup.t are the same in
all wireless links) and unbalanced (i.e.,
{.OMEGA..sub.i}.sub.i=1.sup.N.sup.t can be different among the
various wireless links) fading channels. It will be described in
further detail below that the latter system setup is equivalent to
have balanced fading channels with a non-uniform power allocation
among the transmit-antennas. FIGS. 26 to 29 show both Monte Carlo
simulations and an accurate analytical framework. The analytical
model will be described in more detail below/with regard to FIG.
30.
[0548] In FIG. 26, the ABEP is shown when a fading correlation
model is considered. The following scenarios are applied:
Scenario a: f.sub.1=f.sub.2=1, .OMEGA..sub.1.OMEGA..sub.22/3,
.rho..sub.1=0.50, .rho..sub.2=.rho..sub.3=.rho..sub.4=0.45.
Scenario b: f.sub.1=1, f.sub.2=2.5, .OMEGA..sub.1=1,
.OMEGA..sub.2=10,
.rho..sub.1=.rho..sub.2=.rho..sub.3.rho..sub.4=0.45. Scenario c:
f.sub.1=f.sub.2=1, .OMEGA..sub.1=2/3, .OMEGA..sub.2=20/3,
.rho..sub.1=0.50, .rho..sub.2=.rho..sub.3=.rho..sub.4=0.45.
N.sub.t=2.
[0549] Both Monte Carlo simulation (markers) and analytical model
(solid lines) are shown. The ABEP changes significantly with the
correlation coefficient (.rho..sub.1, .rho..sub.2, .rho..sub.3,
.rho..sub.4) and the fading severity (f.sub.1, f.sub.2). This
latter phenomenon can be observed by comparing Scenario b and
Scenario c in FIG. 26, where a non-negligible performance gap is
observable (for the same power imbalance ratio
.OMEGA..sub.2/.OMEGA..sub.1). This result emphasizes that the
performance of SM is strongly affected by the characteristics of
the wireless channel and the fading distribution as well, since
Scenario c is representative of a Rayleigh fading channel.
[0550] In FIG. 27, the ABEP is shown for uncorrelated fading
channels and for a different power imbalance among the wireless
links. For the balanced fading channel,
.OMEGA..sub.1=.OMEGA..sub.1=.OMEGA..sub.1=.OMEGA..sub.1=1 is
applied. For the unbalanced fading channel, .OMEGA..sub.1=1,
.OMEGA..sub.1=4, .OMEGA..sub.1=8, .OMEGA..sub.1=12 is applied. The
fading channels are uncorrelated,
f.sub.1=f.sub.2=f.sub.3=f.sub.4=2.5 and N.sub.t=4. Both Monte Carlo
simulation and analytical model are shown. A significant
performance improvement can be observed when the power links are
unbalanced. The reason for this behaviour lies in the fact that
power imbalance makes the wireless links more distinguishable to
each other, thus making the decision process at the receiver more
reliable and, thus, improving the ABEP.
[0551] In FIG. 28, the ABEP is shown for correlated fading channels
and for a different power imbalance among the wireless links. The
following scenarios are applied:
Scenario a: f.sub.1=1, f.sub.2=5, .OMEGA..sub.1=.OMEGA..sub.1=1.
Scenario b: f.sub.1=2, f.sub.2=5, .OMEGA..sub.1=10,
.OMEGA..sub.1=1. Scenario c: f.sub.1=5, f.sub.2=2,
.OMEGA..sub.1=10, .OMEGA..sub.1=1.
[0552] Further, .rho..sub.1=.rho..sub.2=.rho..sub.3=0.45,
.rho..sub.4=-0.45 is applied and N.sub.t=2. Both Monte Carlo
simulation and analytical model are shown. Similar to FIG. 27, a
better performance can be achieved for unbalanced fading channels.
Furthermore, an interesting comment can be made by carefully
observing, in particular, the curves related to Scenario b and
Scenario c. In both system setups the first wireless link has a
greater power gain, but in Scenario b the fading severity (f.sub.1)
of the first link is smaller than the fading severity (f.sub.2) of
the second link. A significant performance difference, which
highlights that besides the average power also the fading severity
can remarkably alter the system performance.
[0553] In FIG. 29, the ABEP for correlated fading is shown. For the
balanced fading channel,
.OMEGA..sub.1=.OMEGA..sub.2=.OMEGA..sub.3=.OMEGA..sub.4=1 is
applied. For the unbalanced fading channel, .OMEGA..sub.1=1,
.OMEGA..sub.2=4, .OMEGA..sub.3=8, .OMEGA..sub.4=12 is applied. The
fading channels are correlated with the correlation coefficient
.rho..sub.i,j=exp(-d.sub.0|i-j|) with d.sub.0=0.22 and (i, j) are
antenna's indexes (i, j=1, 2, 3, 4). N.sub.t=4. Both Monte Carlo
simulation and analytical model are shown. The results in FIG. 29
confirm the conclusions already drawn in FIGS. 26 to 28, but also
highlight that the fading severity (t) can have a different impact
on the ABEP, as far as balanced and unbalanced system setups are
considered: A different relation among the curves is noticed.
[0554] In summary, the numerical results shown in FIGS. 26 to 29
lead to the following conclusions:
1. The ABEP of SM gets better for unbalanced wireless fading
channels. 2. The ABEP of SM depends on the fading severity and
channel correlation of the wireless links.
[0555] Optionally, in at least one embodiment, ABEP-driven
opportunistic power allocation mechanisms are described that aim at
distributing the available power at the transmitter in an optimal
fashion by taking into account the actual characteristics of all
transmit-receive wireless links. In particular, the method
according to this aspect of the present invention will help to make
the wireless links more distinguishable among each other in order
to emulate unbalanced fading conditions for those scenarios where
the channel fading is actually identically distributed.
[0556] In FIG. 30, a system 4010 for SM-MIMO is shown by way of
example, which is composed by two transmit-antennas 4012, 4014
(TX.sub.1, TX.sub.2) at the transmitter 4016 (TX) and one
receive-antenna 4018 at the receiver 4020 (RX). The system 4010 is
used to describe the basic and fundamental idea behind the proposed
power allocation method herein below. For analytical simplicity, a
Rayleigh fading channel model with correlated channels 4022, 4024
is considered. However, it is emphasized that the invention
outlined herein is applicable to generic MIMO and MISO systems that
can be deployed in any wireless fading channel.
[0557] The main aim is to show that power imbalance and non-uniform
power allocation are equivalent to each other.
[0558] The symbols and terms used herein are defined as follows:
[0559] A complex-envelope representation of signals is used
throughout the description. [0560] j= {square root over (-1)} the
unitary unit.
[0560] .delta. ( t ) = { 1 if t = 0 0 if t .noteq. 0
##EQU00010##
is the Dirac's delta function. [0561] (x{circle around
(x)}y)(t)=.intg..sub.-.infin..sup.+.infin.x(.xi.)y(t-.xi.)d.xi. is
the convolution operator of signals x(.cndot.) and y(.cndot.).
[0562] (.cndot.)* denotes complex-conjugate. [0563] | |.sup.2
denotes the square absolute value. [0564] E{.cndot.} denotes
expectation operator. [0565] Re{.cndot.} denotes real part
operator. [0566] Pr{.cndot.} means probability. [0567]
G.about.N(.mu..sub.G, .mu..sub.G.sup.2) is a Gaussian distributed
Random Variable (RV) with mean .mu..sub.G and standard deviation
.sigma..sub.G. [0568] A.about.R (.cndot.; .sigma..sub.G.sup.2) is a
Raleigh distributed RV with E{A.sup.2}=2.sigma..sub.A.sup.2 [0569]
P.sub.AB denotes the correlation coefficient of RVs A and B. [0570]
Q(x)=(1/ {square root over (2.pi.)}).intg..sub.0.sup.-.infin.
exp(-t.sup.2/2)dt is the Q-function. [0571]
.GAMMA.(x)=.intg..sub.0.sup.+.infin.t.sup.x-3 exp (-t)dt is the
Gamma function. [0572] m.sub.1 and m.sub.2 denote the two
information messages that the transmitter (TX) in FIG. 30 can emit.
[0573] {circumflex over (m)} Denotes the message estimated at the
receiver. [0574] E.sub.m.sub.1=.zeta..sub.1E.sub.m and
E.sub.m.sub.2=.zeta..sub.2E.sub.m are the energies transmitted for
the information messages m.sub.1 and m.sub.2, respectively. [0575]
.zeta..sub.1 and .zeta..sub.2 are power scaling variables. E.sub.m
is a constant factor. [0576] T.sub.m=T.sub.m.sub.1=T.sub.m.sub.2
denote the signalling internal for both information messages
m.sub.1 and m.sub.2. [0577] h.sub.1(t)=.beta..sub.1
exp(j.phi..sub.1).delta.(t-.tau..sub.1) is the channel impulse
response from antenna TX.sub.1 to the receive-antenna,
.beta..sub.1, .phi..sub.1, and .tau..sub.1 denote the gain, phase,
and delay of the related wireless link. Moreover,
.alpha..sub.1=.beta..sub.1 exp(j.phi..sub.1) denotes the channel's
complex gain of the first wireless link. [0578] Analogously,
h.sub.2(t)=.beta..sub.2 exp(j.phi..sub.2).delta.(t-.tau..sub.2) is
the channel impulse response from antenna TX.sub.2 to the receive
antenna, and .alpha..sub.2, .phi..sub.2, and .tau..sub.2 denote the
gain, phase, and delay of the related wireless link. [0579]
Moreover, .alpha..sub.2=.beta..sub.2 exp (j.phi..sub.2) denotes the
channel's complex-gain of the second wireless link. [0580] For the
sake of simplicity, but without loss of generality, a Rayleigh
fading is assumed for both wireless links TX.sub.1-RX and
TX.sub.2-X. In particular,
.alpha..sub.1=.alpha..sub.1.sup.R+j.alpha..sub.1.sup.I and
.alpha..sub.2=.alpha..sub.2.sup.R+j.alpha..sub.2.sup.I with
.alpha..sub.1.sup.R.about.N(0,.sigma..sub.1.sup.2),
.alpha..sub.1.sup.I.about.N(0,.sigma..sub.1.sup.2),
.alpha..sub.2.sup.R.about.N(0,.sigma..sub.2.sup.2),
.alpha..sub.2.sup.I.about.N(0,.sigma..sub.2.sup.2) and
.rho..sub..alpha..sub.1.sup.R.alpha..sub.1.sup.I=.rho..sub..alpha..sub.2.-
sup.R.alpha..sub.2.sup.I=.rho..sub..alpha..sub.1.sup.R.alpha..sub.2.sup.I=-
.rho..sub..alpha..sub.2.sup.R.alpha..sub.1.sup.I=0,
.rho..sub..alpha..sub.1.sup.R.alpha..sub.2.sup.R=.rho..sub..alpha..sub.1.-
sup.I.alpha..sub.1.sup.I=.rho.. [0581] .tau..sub.1 and .tau..sub.2
are assumed to be independent and uniformly distributed in [0,
T.sub.m], but known at the receiver, i.e. perfect
time-synchronization is considered. [0582] The receiver is assumed
to have full channel state information (CSI), i.e. two triples
(.beta..sub.1,.phi..sub.1,.tau..sub.1) and
(.beta..sub.2,.phi..sub.2,.tau..sub.2) are perfectly known at the
receiver. CSI can be obtained during a training phase before data
transmission. [0583] The signals transmitted by antennas TX.sub.1
and TX.sub.2 are denoted by s.sub.1(.cndot.) and s.sub.2(.cndot.),
respectively, which after passing through the wireless channel
become s.sub.1(t)=s.sub.n{circle around (x)}
h.sub.1)(t)=.beta..sub.1 exp(j.phi..sub.1)s.sub.1(t-.tau..sub.1)
[0584] and {tilde over (s)}.sub.2(t)=(s.sub.2{circle around (x)}
h.sub.2)(t)=.beta..sub.2 exp(j.phi.)s.sub.2(t-.tau..sub.2),
respectively. [0585] The noise at the receiver input is denoted by
n(.cndot.), and is assumed to be Additive White Gaussian (AWG)
distributed, with both real and imaginary parts having a
double-sided power spectral density equal to N.sub.0. [0586] The
receiver signal is denoted by r(.cndot.), and is equal to
r(t)={tilde over (s)}.sub.1(t)+{tilde over (s)}.sub.2(t)+n(t).
[0587] For ease of notation, .gamma..infin.E.sub.m/(4N.sub.0) is
set.
[0588] The Spatial Modulation (SM) concept is based on the rule as
follows: i) when message m.sub.1 has to be transmitted, a properly
designed signal s.sub.1(t).noteq.0 is sent by only the
transmit-antenna TX.sub.1 (i.e., s.sub.2(t)=0), and ii) when
message m.sub.2 has to be transmitted, a properly designed signal
s.sub.2(t).noteq.0 is sent by only the transmit-antenna TX.sub.2
(i.e., s.sub.1(t)=0). In other words, only one transmit-antenna is
activated when either m.sub.1 or m.sub.2 have to be sent: there is
only one active transmit-antenna for each signalling time-interval
T.sub.m. Furthermore, the active transmit-antenna is also allowed
to transmit modulated data and, as a consequence, information is
conveyed by both a transmit-antenna index and the modulated signal
transmitted by that transmit-antenna. This is incorporated in the
power allocation method described herein.
[0589] It is assumed that the transmitted signals, when different
from zero, are pure sinusoidal tones, i.e., s.sub.1(t)= {square
root over (E.sub.m.sub.2)}exp(j.omega..sub.ct) and s.sub.2(t)=
{square root over (m.sub.2)}exp (j.omega..sub.ct). In such a case,
we have:
{ s ~ 1 ( t ) = .beta. 1 E m 1 exp ( j.PHI. 1 ) exp ( j.omega. c t
) s ~ 2 ( t ) = .beta. 2 E m 2 exp ( j.PHI. 2 ) exp ( j.omega. c t
) ( 1 ) ##EQU00011##
where, with a slight abuse of notation, both delays .tau..sub.1 and
.tau..sub.2 have been embedded into the channel phases .phi..sub.1
and .phi..sub.2, respectively.
[0590] As a consequence, the received signal is:
{ r ( t ) m 1 = .beta. 1 E m 1 exp ( j.PHI. 1 ) exp ( j.omega. c t
) s ~ 1 ( ) + n ( t ) r ( t ) m 2 = .beta. 2 E m 2 exp ( j.PHI. 2 )
exp ( j.omega. c t ) s ~ 2 ( ) + n ( t ) ( 2 ) ##EQU00012##
[0591] The Maximum Likelihood (ML) optimal detector with perfect
channel knowledge and time-synchronization at the receiver is as
follows:
m ^ = { m 1 if D 1 .gtoreq. D 2 m 2 if D 2 < D 1 where : ( 3 ) {
D 1 = Re { .intg. T m r ( t ) s ~ 1 * ( t ) t } - 1 2 .intg. T m s
~ 1 ( t ) s ~ 1 * ( t ) t D 2 = Re { .intg. T m r ( t ) s ~ 2 * ( t
) t } - 1 2 .intg. T m s ~ 2 ( t ) s ~ 2 * ( t ) t ( 4 )
##EQU00013##
[0592] So, the probability of error P.sub.E (.cndot.,.cndot.)
conditioned upon the channel impulse responses h.sub.1(.cndot.)and
h.sub.2(.cndot.) is as follows:
P E ( h 1 , h 2 ) = 1 2 P E ( h 1 , h 2 ) | m 1 + 1 2 P E ( h 1 , h
2 ) | m 2 = 1 2 Pr { D 1 | m 1 < D 2 | m 1 } + 1 2 Pr { D 2 | m
2 < D 1 | m 2 } ( 5 ) ##EQU00014##
[0593] After some analytical calculations, the following result can
be obtained:
PR { D 1 | m 1 < D 2 | m 1 } = Pr { D 2 | m 2 < D 1 | m 2 } =
Q ( E m 4 N 0 .zeta. 2 .alpha. 2 - .zeta. 1 .alpha. 1 2 ) ( 6 )
##EQU00015##
which yields the following overall probability of error:
P E ( h 1 , h 2 ) = Q ( E m 4 N 0 .zeta. 2 .alpha. 2 - .zeta. 1
.alpha. 1 2 ) ( 7 ) ##EQU00016##
[0594] Then, the ABEP over correlated Rayleigh fading channels can
be obtained as follows (ABEP=E.sub.h.sub.1,
h.sub.h.sub.2{P.sub.E(h.sub.1,h.sub.2)}):
ABEP = 1 2 - 1 2 .sigma. _ 2 .gamma. _ 1 + .sigma. _ 2 .gamma. _ (
8 ) ##EQU00017##
where we have defined .sigma..sup.2={tilde over
(.sigma.)}.sub.1.sup.2+{tilde over
(.sigma.)}.sub.2.sup.2-2.rho.{tilde over (.sigma.)}.sub.1{tilde
over (.sigma.)}.sub.2 with {tilde over
(.sigma.)}.sub.1.sup.2=.zeta..sub.1.sigma..sub.1.sup.2 and {tilde
over (.sigma.)}.sub.2.sup.2=.zeta..sub.2.sigma..sub.2.sup.2.
[0595] The result shown in (8) takes into account an arbitrary
power allocation between the two transmit-antennas, i.e., in
general, .zeta..sub.1.noteq..zeta..sub.2. A uniform power
allocation strategy is obtained by setting
.zeta..sub.1=.zeta..sub.2=1.
[0596] The power imbalance between the transmit-receive wireless
links is equivalent to non-uniform power allocation. This can be
readily proven by considering the following facts:
1. If .alpha..sub.1 and .alpha..sub.2 are Rayleigh distributed RVs
with Probability Density Function (PDF))
f.sub..alpha..sub.2(.xi..sub.2).about.R(.xi..sub.2;.sigma..sub.2)
and
f.sub..alpha..sub.1(.xi..sub.1).about.R(.xi..sub.1;.sigma..sub.1)
as follows, respectively:
f .alpha. 1 ( .xi. 1 ) = .xi. 3 .sigma. 1 2 exp ( - .xi. 1 2 2
.sigma. 1 2 ) ( 9 ) f .alpha. 2 ( .xi. 2 ) = .xi. 2 .sigma. 2 2 exp
( - .xi. 2 2 2 .sigma. 2 2 ) ( 10 ) ##EQU00018##
2. Then, .alpha..sub.1= {square root over
(.zeta..sub.1)}.alpha..sub.1 and .alpha..sub.2= {square root over
(.zeta..sub.2)}.alpha..sub.2 are still Rayleigh distributed RVs
with PDFs equal to
f.sub..alpha..sub.1(.xi..sub.1).about.R(.xi..sub.1; {square root
over (.zeta..sub.1)}.sigma..sup.1) and
f.sub..alpha..sub.2(.xi..sub.2).about.R(.xi..sub.2; {square root
over (.zeta..sub.2)}.sigma..sub.2), respectively.
[0597] In the light of 1. and 2., it follows that even though
.sigma..sub.1=.sigma..sub.2, which corresponds to a balanced fading
scenario, the net effect of a non-uniform power allocation,
.zeta..sub.1.noteq.f.sub.2, in (8) is equivalent to have an
unbalanced fading scenario with power imbalance ratio equal to
E{{tilde over (.alpha.)}.sub.1.sup.2}/E{{tilde over
(.alpha.)}.sub.2.sup.2}=.zeta..sub.1/.zeta..sub.2. As a
consequence, the ABEP depicted in FIGS. 26 to 29 for the balanced
fading scenario can be moved towards the ABEP of the unbalanced
fading scenario via an adequate and opportunistic (i.e., which
depends also on the fading parameters--see, e.g., FIG. 28) power
allocation scheme.
[0598] To formalize the power allocation mechanism, a general
system setup with N.sub.t transmit-antennas and one receive-antenna
is considered, as shown for example in FIG. 30. Further
generalizations to an arbitrary number of receive-antennas are
possible provided that closed-form and accurate expressions of the
ABEP are computed. A general fading scenario with Nakagami-m fading
(the same channel model considered in FIGS. 26 to 29) is also
considered. For illustrative purposes, the power allocation
optimization is formulated by considering the ABEP for uncorrelated
fading channels and for a receiver having full channel state
information (CSI). However, the same method can be used for
correlated fading channels and for other sub-optimal receiver
architectures.
[0599] In FIGS. 30, 31 and 32, a.sub.n, and a.sub.1 and a.sub.2
respectively, denote the power scaling variables. Furthermore,
c.sub.1,2 denotes the correlation coefficient between the
channels.
[0600] The transmitter 4016 comprises a spatial modulator 4026 and
a signal modulator 4028. At the receiver 4020, a signal demodulator
4030 is located. The receiver also comprises a channel estimator
4032 which determines correlation coefficients between the channels
4022, 4024. These correlation coefficients are transmitted to the
transmitter 4016. The transmitter 4016 comprises a calculator 4034
to determine a power allocation to be allocated to the
transmit-antennas 4012, 4014 on the basis of data 4035 comprising
correlation coefficients measured at the receiver 4020. The
transmitter 4016 further comprises a power allocator 4036 to
allocate the determined power to the transmit-antennas 4012, 4014.
A modulated signal is then transmitted from one transmit-antenna
4012, 4014 at a time with the corresponding allocated power. At the
receiver 4020, a model of power allocation is generated in a model
generator 4038, in which the data 4035 of the channel estimator
4032 is applied. The location of the transmit-antennas 4012, 4014
is then determined in a detector 4040 located at the receiver 4020
by using the power allocation model.
[0601] In the depicted scenario, the ABEP can be written in
closed-form as follows:
ABEP ( .zeta. 1 , .zeta. 2 , , .zeta. N t ) .apprxeq. 1 2 ( N t - 1
) i = 1 N i i .noteq. j = 1 N t PEP ( i -> j ; .zeta. 1 , .zeta.
2 , , .zeta. N t ) ( 11 ) ##EQU00019##
where it is denoted E.sub.m.sub.1=.zeta..sub.1E.sub.m,
E.sub.m.sub.2=.zeta..sub.2E.sub.m, . . . , and
E.sub.m.sub.i=.zeta..sub.N.sub.1E.sub.m emphasized that the above
ABEP is conditioned upon a fixed power allocation among the
transmit-antennas, i.e., the set (.zeta..sub.1, .zeta..sub.2, . . .
, .zeta..sub.N.sub.1).
[0602] Moreover, we have defined:
PEP ( i -> j ; .zeta. 1 , .zeta. 2 , , .zeta. N t ) = 1 .pi.
.intg. 0 .pi. / 2 M i -> j ( .gamma. _ 2 sin 2 ( .theta. ) )
.theta. and : ( 12 ) M i -> j ( s ) = A i A j 4 ( s + B i ) - (
1 2 + C i 2 ) ( s + B j ) - ( 1 2 + C j 2 ) G 2 , 2 1 , 2 ( - s 2 (
s + B i ) ( s + B j ) | 0.5 - 0.5 C j 0.5 - 0.5 C i 0 0 ) with G p
, q m , n ( | ( a p ) ( a q ) ) ( 13 ) ##EQU00020##
being the Meijer-G function, and
A.sub.i=2m.sub.i.sup.m.sup.t/({tilde over
(.OMEGA.)}.sub.i.sup.m.sup.t.GAMMA.(m.sub.i)),
B.sub.i=m.sub.i/{tilde over (.OMEGA.)}.sub.i, C.sub.i=2m.sub.i-1,
{tilde over (.OMEGA.)}.sub.i=.zeta..sub.i.OMEGA..sub.i, for i=1, 2,
. . . , N.sub.t. Moreover, (m.sub.i, .OMEGA..sub.i) are the
parameters of the Nakagami-m distribution for the i-th wireless
link.
[0603] The optimal and opportunistic power allocation mechanism
based on the optimization of the ABEP can be analytically
formalized as follows:
( .zeta. 1 ( opt ) , .zeta. 2 ( opt ) , , .zeta. N t ( opt ) ) =
argmin ( .zeta. 1 , .zeta. 2 , , .zeta. N t ) .zeta. 1 + .zeta. 2 +
+ .zeta. N t = 1 { ABEP ( .zeta. 1 , .zeta. 2 , , .zeta. N t ) } (
14 ) ##EQU00021##
where the constraint that the total power is fixed to a given
value, .SIGMA..sub.i=1.sup.N.sup.tE.sub.m.sub.i=E.sub.m i.e., has
been added.
[0604] The minimization in (14) turns out to be a constrained
optimization problem, which can be solved via either analytical or
numerical methods.
[0605] In FIG. 31, an embodiment of a system 4100 according to the
present invention is shown. A transmitter 4102 is provided with two
transmit-antennas 4104, 4106 and further comprises a spatial
modulator 4108 and a signal modulator 4110. At a receiver 4112, a
signal demodulator 114 is located. The receiver 112 has a
receive-antenna 4116 and also comprises a storage 4118 for storing
power allocation sequences, for example in a look-up table. The
transmitter 4102 comprises a sequence predetermination module 4120
in which a power allocation sequence 4122, 4124 to be allocated to
the transmit-antennas 4104, 4106 is predetermined. This power
allocation sequence 4122, 4124 is randomly chosen prior to any data
transmission from the transmit-antennas to the receiver 4112 and
does not consider channel conditions. The transmitter 4102 further
comprises a power allocator 4126 to allocate the predetermined
power allocation sequences to the transmit-antennas 4104, 4106. A
modulated signal is then transmitted from the transmit-antennas
4104, 4106, i.e. from the transmit-antenna 4104, 4106 which is
active at a certain time interval, with the corresponding power
allocation sequence. The power allocation sequences 4122, 4124 of
two transmit-antennas, which are shown in more detail, are only an
example of possible power allocation sequences. The power
allocation sequences 4122, 4124 are shown as a function of time
(t). They have a staircase-shaped characteristic which differs for
each of the transmit-antennas. For example, in a first time
interval 4128, the power (a.sub.1) of the first transmit-antenna
4104 is lower than the power (a.sub.2) of the second
transmit-antenna 4106. Then, in a second time interval 4130, the
power (a.sub.1) of the first transmit-antenna 4104 is higher than
the power (a.sub.2) of the second transmit-antenna 4106. The power
allocation sequences 4122, 4124 of each transmit-antenna 4104, 4106
are transmitted to the receiver 4110 and stored in the storage
4118. The receiver 4112 further comprises a detector 4132 to detect
location of the active transmit-antenna by using the power
allocation sequence data stored in the storage 4118.
[0606] FIG. 32 shows a further embodiment of a system 4101
according to the present invention by way of example. In this
embodiment is similar to the embodiment shown in FIG. 31 and
therefore, the same reference signs are used for similar items.
However, channel conditions are considered prior to the allocation
of a power allocation sequence 4122, 4124 to the transmit-antennas
4104, 4106. Therefore, the receiver 4112 further comprises a
channel estimator 4134. Training data 4136 is transmitted from the
receiver 4112 to the transmitter 4102, containing channel
information that has been analyzed in the channel estimator 4134.
The channel information mainly comprises gain, phase and/or delay
of the signals received at the receiver 4112 from different
transmit-antennas. The training data provides a feedback to the
transmitter 4102 such that the power allocation sequence 4124, 4126
is randomly chosen based the channel information. The predetermined
power allocation sequence 4124, 4126 is transmitted to the receiver
4112 again to be stored in the storage 4118 which is accessible for
the detector 4132 to determine the location of the active
transmit-antenna.
[0607] With regard to the embodiments shown in FIGS. 31 and 32, it
is also possible that a constant power (a.sub.1, a.sub.2) is
allocated to each of the transmit-antennas 4104, 4106, such that
a.sub.1.noteq.a.sub.2. The difference between these constant powers
can depend on the channel correlation, for example, the difference
can be direct proportional to the channel correlation.
[0608] Another aspect of the invention focuses on proposing a novel
Spatial Modulation method, which is called Time-Orthogonal Signal
Design assisted Spatial Modulation (TOSD-SM) and, differently from
all other SM techniques, can exploit the antennas at the
transmitter to get transmit-diversity. The basic idea behind
TOSD-SM is not restricting the transmitted signal to be a pure
sinusoidal tone, but to properly design it in order to exploit, in
an efficient way, the different propagation delays (.tau..sub.1 and
.tau..sub.2) of the wireless links TX.sub.1-RX and TX.sub.2-RX.
[0609] Similar to the first aspect of the invention, TOSD-SM
retains the main assumption that only one transmit-antenna is
activated for every signalling interval T.sub.m. In particular,
also in this aspect, the following rule is adopted: i) when message
m.sub.1 has to be transmitted, a properly designed signal s.sub.1
(t).noteq.0 is sent by only the antenna TX.sub.1 (i.e., s.sub.2
(t)=0), and ii) when message m.sub.2 has to be transmitted, a
properly designed signal s.sub.2 (t).noteq.0 is sent by only the
antenna TX.sub.2 (i.e., s.sub.1 (t)=0). The assumption that only
one transmit-antenna is activated for every signalling interval
T.sub.m can also be used for more than two antennas.
[0610] TOSD-SM does not restrict the transmitted signals to be pure
sinusoidal tones, but s.sub.1(.cndot.) and s.sub.2 (.cndot.) are
properly optimized for performance improvement. In particular, the
novel TOSD-SM concept relies on the following signal design (when
s.sub.1(.cndot.) and s.sub.2 (.cndot.) are different from
zero):
s.sub.1(t)=s.sub.2(t)=w(t) (18)
where w (.cndot.) is a generic signal waveform, which is chosen to
satisfy the following condition:
R w ( .tau. ) = .intg. - .infin. + .infin. w ( .xi. ) w * ( .xi. -
.tau. ) d .xi. = .delta. ( .tau. ) ( 19 ) ##EQU00022##
[0611] The design condition in (19) simply states that w (.cndot.)
is required to have a very peaky time auto-correlation function
R.sub.W (.cndot.), which under ideal signal design conditions can
be assumed to be a Dirac's delta function.
[0612] According to (18), the signals after propagation through the
wireless channels can be written as follows:
{ s ~ 1 ( t ) = .beta. 1 E m exp ( j .PHI. 1 ) w ( t - .tau. 1 ) s
~ 2 ( t ) = .beta. 2 E m exp ( j.PHI. 2 ) w ( t - .tau. 2 ) ( 20 )
##EQU00023##
[0613] As a consequence, the received signal is:
{ r ( t ) | m 1 = .beta. 1 E m exp ( j .PHI. 1 ) w ( t - .tau. 1 )
s ~ 1 ( ) + n ( t ) r ( t ) | m 2 = .beta. 2 E m exp ( j .PHI. 2 )
w ( t - .tau. 2 ) s ~ 2 ( ) + n ( t ) ( 21 ) ##EQU00024##
[0614] The ML optimal detector with perfect channel knowledge and
synchronization at the receiver is as follows:
m ^ = { m 1 if D 1 .gtoreq. D 2 m 2 if D 2 < D 1 where : ( 22 )
{ D 1 = Re { .intg. T m r ( t ) s ~ 1 * ( t ) t } - 1 2 .intg. T m
s ~ 1 ( t ) s ~ 1 * ( t ) t D 2 = Re { .intg. T m r ( t ) s ~ 2 * (
t ) t } - 1 2 .intg. T m s ~ 2 ( t ) s ~ 2 * ( t ) t ( 23 )
##EQU00025##
[0615] Similar to (5), the probability of error P.sub.E
(.cndot.,.cndot.) conditioned upon the channel impulse responses
h.sub.1 (.cndot.) and h.sub.2 (.cndot.) is as follows:
P E ( h 1 , h 2 ) = 1 2 P E ( h 1 , h 2 ) | m 1 + 1 2 P E ( h 1 , h
2 ) | m 2 = 1 2 Pr { D 1 | m 1 < D 2 | m 1 } + 1 2 Pr { D 2 | m
2 < D 1 | m 2 } ( 24 ) ##EQU00026##
[0616] After some analytical calculations and the exploitation of
the orthogonality condition in (19) for every pair of delays
(.tau..sub.1,.tau..sub.2) with .tau..sub.1.noteq..tau..sub.2,
(i.e., propagation through the wireless links TX.sub.T-RX and
TX.sub.2-RX is subject to different delays), i.e.,:
.intg. - .infin. + .infin. w ( .xi. - .tau. 1 ) w * ( .xi. - .tau.
2 ) .xi. = .intg. - .infin. + .infin. w ( .xi. ) w * [ .xi. - (
.tau. 2 - .tau. 1 ) ] .xi. = .delta. ( .tau. 2 - .tau. 1 ) = { 1 if
.tau. 1 = .tau. 2 0 if .tau. 1 .noteq. .tau. 2 ( 25 )
##EQU00027##
the result in what follows can be obtained:
Pr { D 1 | m 1 < D 2 | m 1 } = Pr { D 2 | m 2 < D 1 | m 2 } =
Q ( E b 4 N 0 ( .beta. 1 2 + .beta. 2 2 ) ) ( 26 ) ##EQU00028##
which yields the following overall probability of error:
P E ( h 1 , h 2 ) = Q ( E b 4 N 0 ( .beta. 1 2 + .beta. 2 2 ) ) (
27 ) ##EQU00029##
[0617] Then, P.sub..epsilon. over Rayleigh fading channels can be
obtained as follows:
P _ E = 1 .pi. .intg. 0 .pi. / 2 M ( .gamma. _ 2 sin 2 ( .theta. )
) .theta. ( 28 ) ##EQU00030##
where we have defined
M(s)=[1+2(.sigma..sub.1.sup.2+.sigma..sub.2.sup.2)s+4(1-.rho..sup.2).sigm-
a..sub.1.sup.2.sigma..sub.2.sup.2s.sup.2].sup.-1, which is the
Moment Generating Function (MGF) of RV
.beta.=.beta..sub.1+.beta..sub.2, i.e., M (s)=E{exp (-s
.beta.)}.
[0618] The main advantage of this aspect of the invention is to
provide transmit-diversity. In particular, for a 2.times.1 MISO
system a transmit-diversity order equal to 2 is obtained. In
particular, the diversity order can be computed by analyzing the
behaviour of M (.cndot.) for large values of |s|. It can be readily
proven that:
lim s -> + .infin. { M ( s ) } .apprxeq. 1 4 ( 1 - .rho. 2 )
.sigma. 1 2 .sigma. 2 2 s - 2 ( 29 ) ##EQU00031##
and it is known that the system's diversity order is equal to the
negative exponent of |s|, i.e., 2 in (29).
[0619] As a result of the higher diversity order, the error
probability is expected to have a steeper slope for increasing
SNRs, which results in substantial improvements in system's
performance. This aspect of the invention turns out to be also more
robust to channel correlation. As a matter of fact, the error
probability in (28) depends on only the square value of the
correlation coefficient, i.e., .rho..sup.2. So, since
0.ltoreq..rho..ltoreq.1 the performance drop for increasing .rho.
is expected to be smaller than in known solutions. As opposed to
known solutions in which the performance of the SM scheme is
independent of .rho., a transmit-diversity is achieved.
[0620] With respect to other SM schemes, it might be required that
the propagation delays (.tau..sub.1,.tau..sub.2) are known at the
transmitter. When needed, this can be easily obtained via a
feedback channel from the receiver to the transmitter, such that
the orthogonality condition in (25) can always be verified. On the
other hand, when the signal design condition in (19) can be
guaranteed, a priori, for every (.tau..sub.1,.tau..sub.2) pair, no
feedback channel is required since the condition in (25) is
implicitly verified for every pair (.tau..sub.1,.tau..sub.2) at the
receiver-side. In addition to .tau..sub.1,.tau..sub.2) of
propagation delays, other channel signatures (or spectral
characteristics) such as phase rotation, amplitude or frequency may
be used.
[0621] Below, numerical results which are obtained from the
analytical frameworks described above, are described. The following
system setup is used to obtain the simulation results:
i) .sigma..sub.1=.sigma..sub.2=1, ii) .rho.={0.00, 0.25, 0.50,
0.75, 0.99}, iii) N.sub.0=-204 dBW/Hz, and iv) the error
probability from Monte Carlo simulations is obtained by requiring a
number of wrong detections equal to 10.sup.4.
[0622] Numerical results are shown in FIG. 33, FIG. 34, and FIG. 35
for known SM schemes and the proposed TOSD-SM, respectively.
Markers show a Monte Carlo simulation and solid lines show the
analytical model.
[0623] In particular, FIG. 33 shows the error probability of a SM
scheme which is called Space Shift Keying (SSK) and based on the
rule that only one transmit-antenna is activated when m.sub.1 has
to be sent, while both transmit-antennas are activated when m.sub.2
needs to be sent. In this scheme, the transmitted signals, when
different from zero, are always pure sinusoidal tones. This allows
to embed both delays (.tau..sub.1 and .tau..sub.2) into the channel
phases (.phi..sub.1 and .phi..sub.2, respectively. Numerical
results confirm that no performance degradation can be observed for
increasing values of the correlation coefficient.
[0624] In FIG. 34, the error probability of another SM scheme,
based on the rule that only one transmit-antenna is activated when
either m.sub.1 or m.sub.2 have to be sent: there is only one active
transmit-antenna for every signalling interval T.sub.m. Similar to
the SSK scheme the transmitted signals, when different from zero,
are always pure sinusoidal tones, i.e., s.sub.1(t)=s.sub.2(t)=
{square root over (E.sub.m)}exp (j.omega..sub.ct). Thus, also in
this case both propagation delays and .tau..sub.2 can be embedded
into the channel phases (.phi..sub.1 and .phi..sub.2, respectively.
It is observed that the spatial correlation between the wireless
links can remarkably increase the error probability. When the
wireless links are subject to high correlation, the error
probability can be very high. More in detail, the SNR penalty with
respect to spatial correlation is 1.24 dB, 3 dB, and 6 dB for
.rho.=0.25, .rho.=0.5, and .rho.=0.75, respectively.
[0625] In FIG. 35, the error probability for the novel TOSD-SM
scheme is depicted. Numerical results confirm that the proposed
invention shows a higher diversity order than other SM schemes: the
error probability shows a steeper slope than the other two
proposals. This yields a substantial performance gain with respect
to other solutions. Moreover, we can observe that spatial
correlation of wireless links has a significant less impact than
the SM proposal as described in relation with FIG. 34.
[0626] In FIG. 36 a comparison among the various SM proposals is
shown in order to understand the different behaviour of them as a
function of channel spatial correlation. The following facts can be
observed:
i) The proposed TOSD-SM yields a significant performance gain with
respect to all other SM proposals and, even in the presence of
channel correlation, it offers better error probabilities than
other SM schemes over independent wireless links: this is a clear
indication of the robustness of the proposed invention to spatial
correlation of fading. ii) The SM proposal referring to Space Shift
Keying offers worse performance than the SM scheme of which results
are shown in FIG. 9 when the wireless links are uncorrelated.
However, in the presence of channel correlation the situation is
reversed: SM for SSK offers a better error probability than for the
SM scheme which results are shown in relation to FIG. 34.
[0627] FIG. 37 shows a system 4200 for SM-MIMO, in accordance with
an embodiment of the TOSD-SM scheme. The system 4200 comprises a
transmitter 4202 with two transmit-antennas 4204, 4206 and a
receiver 4208 with one receive-antenna 4210. The transmitter 4202
comprises a spatial modulator 4212 and a signal modulator 4214. The
receiver 4208 comprises a signal demodulator 4216 for demodulating
a signal which is sent over a channel 4218, 4220 from one
transmit-antenna 4204, 4206 to the receive-antenna 4210. Each
signal transmitted over the channels 4218, 4220 has a specific
waveform. According to the TOSD-SM scheme, the waveform of the
signal is predetermined prior to any data transmission between the
transmitter 4202 and the receiver 4208 such that a propagation
delay of the signals is modified. Corresponding data 4222, which
comprise orthogonal pairs (.tau..sub.1, .tau..sub.2) of the
propagation delay between the two channels 4218, 4220 are supplied
to a waveform calculator 4224 located at the transmitter 4202. The
predetermined waveform is then allocated to the corresponding
signal sent by the transmit-antennas 4204, 4206 via a waveform
allocator 4226 located at the transmitter 4202. The receiver 4208
comprises a database 4228 in which a model of waveforms is
provided. The receiver 4208 further comprises a detector 4230 which
uses knowledge of the conditions of the channels 4218, 4220 and the
waveform model to detect location of the active transmit-antenna
4204, 4206. Furthermore, a time-synchronisation at the receiver
4208 is considered. The resulting time-orthogonal design of the
signals reduces the error probability as described above.
[0628] It should be noted that other numbers of transmit-antennas,
i.e. more than two transmit-antennas at the transmitter, and
receivers, i.e. more than one receive-antenna at the receiver,
could equally be used.
[0629] It will be appreciated that the transmitters, receivers
and/or communications system and/or methods described above may be
used in conjunction with other embodiments described above.
[0630] A skilled person will appreciate that variations of the
disclosed arrangements are possible without departing from the
invention.
[0631] For example, although the above embodiments have been
described in relation to a system that uses a light source that
comprises LEDs 25a, 25b, 25c, other light sources may be used,
particularly light sources having a fast switching time that allows
for modulation of the output.
[0632] In addition, although embodiments are described above that
use intensity modulation and specifically on-off keying, it will be
appreciated that other modulation schemes may be alternatively or
additionally used, such as spatial modulation, colour modulation,
multi-level intensity modulation and the like.
[0633] Although the above example uses a portable electronics
device 40, it will be appreciated that the electronics device need
not be portable but that any suitably programmable or configurable
device that comprises a camera 35 and is capable of implementing a
rolling shutter as described above may be used.
[0634] Alternative embodiments of the invention can be implemented
as a computer program product for use with a computer system, the
computer program product being, for example, a series of computer
instructions stored on a tangible data recording medium, such as a
diskette, CD-ROM, ROM, or fixed disk, or embodied in a computer
data signal, the signal being transmitted over a tangible medium or
a wireless medium, for example, microwave or infrared. The series
of computer instructions can constitute all or part of the
functionality described above, and can also be stored in any memory
device, volatile or non-volatile, such as semiconductor, magnetic,
optical or other memory device.
[0635] It will also be well understood by persons of ordinary skill
in the art that whilst the preferred embodiment implements certain
functionality by means of software, that functionality could
equally be implemented solely in hardware (for example by means of
one or more ASICs (application specific integrated circuit)) or
indeed by a mix of hardware and software. As such, the scope of the
present invention should not be interpreted as being limited only
to being implemented in software.
[0636] Lastly, it should also be noted that whilst the accompanying
claims set out particular combinations of features described
herein, the scope of the present invention is not limited to the
particular combinations hereafter claimed, but instead extends to
encompass any combination of features or embodiments herein
disclosed irrespective of whether or not that particular
combination has been specifically enumerated in the accompanying
claims at this time.
* * * * *