U.S. patent application number 13/916972 was filed with the patent office on 2013-10-17 for calibration mode.
The applicant listed for this patent is Neul Ltd.. Invention is credited to William Webb.
Application Number | 20130272156 13/916972 |
Document ID | / |
Family ID | 44764551 |
Filed Date | 2013-10-17 |
United States Patent
Application |
20130272156 |
Kind Code |
A1 |
Webb; William |
October 17, 2013 |
CALIBRATION MODE
Abstract
A communication device for communicating with a plurality of
terminals via a series of frames, the communication device being
configured to indicate to the terminals, at the commencement of a
frame, that communication is to be suspended for the remainder of
the frame in order for a calibration to be performed, and to
perform said calibration during the remainder of the time allotted
to that frame.
Inventors: |
Webb; William; (Cambridge,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Neul Ltd. |
Cambridge |
|
GB |
|
|
Family ID: |
44764551 |
Appl. No.: |
13/916972 |
Filed: |
June 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2012/058203 |
May 4, 2012 |
|
|
|
13916972 |
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Current U.S.
Class: |
370/252 |
Current CPC
Class: |
H04L 67/28 20130101;
H04L 69/18 20130101; H04B 7/2656 20130101; H04W 56/001 20130101;
H04L 25/061 20130101; Y04S 40/20 20130101; H04W 16/14 20130101;
H04L 47/10 20130101; H04W 72/1242 20130101; H04W 24/02 20130101;
H04W 36/0066 20130101; H04L 5/0032 20130101; H04W 36/22 20130101;
H04W 56/0015 20130101; H04B 1/713 20130101; H04W 8/18 20130101;
H04W 4/70 20180201; H04W 64/00 20130101; H04W 72/048 20130101; H04W
28/065 20130101; H04W 72/1215 20130101; H04W 12/06 20130101; H04L
7/041 20130101; H04W 88/10 20130101 |
Class at
Publication: |
370/252 |
International
Class: |
H04W 24/02 20060101
H04W024/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2011 |
GB |
1109829.0 |
Jun 13, 2011 |
GB |
1109830.8 |
Jun 13, 2011 |
GB |
1109836.5 |
Jun 13, 2011 |
GB |
1109837.3 |
Jun 13, 2011 |
GB |
1109840.7 |
Jun 13, 2011 |
GB |
1109844.9 |
Jun 13, 2011 |
GB |
1109848.0 |
Jun 13, 2011 |
GB |
1109850.6 |
Jun 13, 2011 |
GB |
1109853.0 |
Jun 13, 2011 |
GB |
1109854.8 |
Jun 13, 2011 |
GB |
1109863.9 |
Jun 13, 2011 |
GB |
1109867.0 |
Jun 13, 2011 |
GB |
1109874.6 |
Sep 6, 2011 |
GB |
1115382.2 |
Sep 30, 2011 |
GB |
1116910.9 |
Claims
1. A communication device for communicating with a plurality of
terminals via a series of frames, the communication device being
configured to indicate to the terminals, at the commencement of a
frame, that communication is to be suspended for the remainder of
the frame in order for a calibration to be performed, and to
perform said calibration during the remainder of the time allotted
to that frame by transmitting a constant power signal at a
particular frequency or by measuring a constant power signal
transmitted at a particular frequency by another communication
device.
2. A communication device as claimed in claim 1, configured to
indicate that communication is to be suspended by instructing the
terminals not to transmit for the remainder of the frame.
3. A communication device as claimed in claim 1, configured to
indicate that communication is to be suspended by indicating to the
terminals that it will not be transmitting for the remainder of the
frame.
4. A communication device as claimed in claim 1, configured to
indicate in a header of the frame that communication is to be
suspended for the remainder of the frame.
5. A communication device as claimed in claim 1, configured to
indicate that communication is to be suspended for the remainder of
the frame by not including in the frame any allocations of time
slots for one or more of the plurality of terminals.
6. A communication device as claimed in claim 1, configured to
indicate that communication is to be suspended for the remainder of
a frame by including in the frame an indication that all timeslots
in the remainder of the frame are reserved.
7. A communication device as claimed in claim 1, configured to
perform said calibration by determining a level of interference
present on one or more frequencies.
8. A communication device as claimed in claim 1, configured to
perform said calibration by measuring a noise level across one or
more frequencies.
9. A communication device as claimed in claim 1, configured to
instruct one or more of the terminals that are experiencing
interference on a designated carrier frequency to monitor that
frequency and transmit to the communication device an indication
when it is no longer experiencing interference on that
frequency.
10. A communication device as claimed in claim 9, configured to
instruct said one or more terminals to transmit the indication via
contended access.
11. A communication device as claimed in claim 1, configured to:
report a result of said calibration to a network controller;
receive a new frequency hopping sequence from the network
controller; and communicate that new frequency hopping sequence to
the plurality of terminals.
12. A communication network device as claimed in claim 1,
configured to operate in whitespace.
13. A communication device as claimed in claim 1, configured for
machine-to-machine communication.
14. A method for communicating with a plurality of terminals via a
series of frames, the method comprising: indicating to the
terminals, at the commencement of a frame, that communication is to
be suspended for the remainder of the frame in order for a
calibration to be performed; and performing said calibration during
the remainder of the time allotted to that frame by transmitting a
constant power signal at a particular frequency or by measuring a
constant power signal transmitted at a particular frequency by
another communication device.
15-16. (canceled)
Description
BACKGROUND
[0001] The invention relates to an apparatus and method for
calibrating a network that is subject to interference on one or
more frequencies.
[0002] A wireless network may be configured to operate without
having been specifically allocated any part of the electromagnetic
spectrum. Such a network may be permitted to operate in so-called
whitespace: a part of the spectrum that is made available for
unlicensed or opportunistic access. Typically whitespace is found
in the UHF TV band and spans 450 MHz to 800 MHz, depending on the
country. A large amount of spectrum has been made available for
unlicensed wireless systems in this frequency range.
[0003] A problem with operating in whitespace is that the available
bandwidth is variable and cannot be guaranteed. These limitations
are well-matched to the capabilities of machine-to-machine networks
in which there is no human interaction. Machine-to-machine networks
are typically tolerant of delays, dropped connections and high
latency communications.
[0004] Any network operating in the UHF TV band has to be able to
coexist with analogue and digital television broadcast
transmitters. The density of the active television channels in any
given location is relatively low (resulting in the availability of
whitespace that can be used by unlicensed systems). The FCC has
mandated that systems operating in the whitespace must reference a
database that determines which channels may be used in any given
location. This is intended to avoid interference with the TV
transmissions and certain other incumbent systems such as wireless
microphones.
[0005] For TV receivers (including those for digital TV (DTV),
there will inevitably be adjacent channels on which a strong
transmission close to the TV receiver will interfere with TV
reception. For example, the TV receivers may have image frequencies
and poor adjacent channel rejection (ACR) on certain frequencies
due to spurs on their local oscillators and limitations in their
receive filters. These frequencies are often dependent on the
specific receiver implementation.
[0006] Digital TV typically uses a channel bandwidth of 6 to 8 MHz.
It also uses OFDM modulation in which the overall channel bandwidth
is split into a large number of narrower channels (so-called
sub-carriers), each of which is individually modulated. The system
is designed so that, if a certain number of sub-carriers are
subject to multipath fading, with the result that their
signal-to-noise ratio is poor, the overall data can still be
recovered. This is typically achieved by using interleaving and
error correction codes, which mean that bit errors localized to a
limited number of sub-carriers can be corrected. OFDM modulation
can therefore achieve considerable robustness to multipath
fading.
[0007] OFDM is only able to recover the transmitted data when the
interferer is relatively narrowband compared with the bandwidth of
the overall TV signal, such that a limited number of sub-carriers
are affected. OFDM does not provide a similar performance benefit
when the interferer occupies a relatively large proportion of the
DTV channel bandwidth because in this case the error control coding
may be incapable of correcting the bit errors due to the higher
proportion of bits that may be corrupt. If the bandwidth of the
transmitted signal from the terminal can be reduced to a small
fraction of the DTV channel bandwidth, there is a lower chance of
the DTV receiver being unable to decode the signal correctly.
Another perspective on this is that the narrowband whitespace
transmitter can be located much closer to the DTV receiver before
causing noticeable degradation of the decoded DTV signal. This can
be of particular benefit for mobile or portable whitespace devices
whose exact location and antenna orientation cannot be easily
constrained.
[0008] There is a potential issue with reducing the bandwidth
occupied by the whitespace device's transmitter: transmitting on a
narrow bandwidth channel makes the whitespace device sensitive to
poor reception due to multipath fading. This is because the entire
bandwidth could be in a long-term fade (lasting multiple frames),
resulting in poor signal-to-noise ratio.
[0009] Both of these problems may be addressed using frequency
hopping. Frequency hopping minimizes the interference to TV
reception, since no communication will be permanently causing
interference to any given TV receiver. Frequency hopping also
reduces the probability of the terminal being in a long-term fade.
It provides a form of interleaving that enables more efficient
error correction to be used.
[0010] The channels used for frequency hopping may be selected by
the base station based upon information from the whitespace
database on the available channels and associated power levels
(which in turn are based upon the licensed spectrum use in the
area). However, the whitespace database does not include
information about every possible source of interference.
[0011] For example, a television transmitter may be intended to
broadcast to only a particular coverage area, but may in fact leak
into other nearby areas where the use of the frequencies in use by
that transmitter are not prohibited in the whitespace database;
major TV stations can be well above the thermal noise at distances
of 100 km. Although the signal from this transmitter may not be
strong enough to be reliably received by television antennas in
those nearby areas, it is often strong enough to cause severe
interference to whitespace base stations in those areas,
particularly if they have elevated antennas (which they may have in
order to increase their own coverage area). On nominally free
channels, reception is far more likely to be dominated by distant
TV broadcasts rather than thermal noise, especially in rural
regions. This interference can render many of the whitespace
channels unusable or severely compromised.
[0012] Interference from other unlicensed whitespace networks can
also be a problem as all whitespace networks compete for use of
those frequencies the whitespace database marks as available.
[0013] Interference may also be caused by the unintended emissions
of devices that are not part of a wireless network, e.g. spurious
emissions from faulty electric drills.
[0014] Apart from all these interferers external to the network,
there can also be problems for devices located close to the edge of
cells. Neighboring base stations are likely to have similar
whitespace channel assignments. (As the distance between base
stations increases, the assignments tend to change as the base
stations are located in different TV service areas.) Therefore, if
base stations pick their own frequency hopping sequences based on
only the frequencies available in the whitespace database, the base
stations of neighboring cells are likely to make similar choices.
If neighboring cells use the same frequency hopping sequences then
terminals at cell edges may receive multiple weak signals from both
the base station for their own cell, and any neighboring base
stations in range, and have no way of distinguishing between them.
Two neighboring base stations may use the same frequencies on
approximately one in ten frames. Each base station is surrounded by
a number of others, typically around six, meaning interference is
likely to occur somewhere within each cell around fifty percent of
the time. This can result in a significant loss in capacity.
[0015] What is needed is a method and apparatus for monitoring the
various types of interference suffered by devices in communication
networks such as whitespace networks.
SUMMARY
[0016] According to a first embodiment of the invention, there is
provided a communication device for communicating with a plurality
of terminals via a series of frames, the communication device being
configured to indicate to the terminals, at the commencement of a
frame, that communication is to be suspended for the remainder of
the frame in order for a calibration to be performed, and to
perform said calibration during the remainder of the time allotted
to that frame.
[0017] The communication device may be configured to indicate that
communication is to be suspended by instructing the terminals not
to transmit for the remainder of the frame.
[0018] The communication device may be configured to indicate that
communication is to be suspended by indicating to the terminals
that it will not be transmitting for the remainder of the
frame.
[0019] The communication device may be configured to indicate in a
header of the frame that communication is to be suspended for the
remainder of the frame.
[0020] The communication device may be configured to indicate that
communication is to be suspended for the remainder of the frame by
not including in the frame any allocations of time slots for one or
more of the plurality of terminals.
[0021] The communication device may be configured to indicate that
communication is to be suspended for the remainder of a frame by
including in the frame an indication that all timeslots in the
remainder of the frame are reserved.
[0022] The communication device may be configured to perform said
calibration by determining a level of interference present on one
or more frequencies.
[0023] The communication device may be configured to perform said
calibration by measuring a noise level across one or more
frequencies.
[0024] The communication device may be configured to perform said
calibration by transmitting a constant power signal at a particular
frequency.
[0025] The communication device may be configured to perform said
calibration by measuring a constant power signal transmitted at a
particular frequency by another base station.
[0026] The communication device may be configured to instruct one
or more of the terminals that are experiencing interference on a
designated carrier frequency to monitor that frequency and transmit
to the communication device an indication when it is no longer
experiencing interference on that frequency.
[0027] The communication device may be configured to instruct said
one or more terminals to transmit the indication via contended
access.
[0028] The communication device may be configured to report a
result of said calibration to a network controller, receive a new
frequency hopping sequence from the network controller and
communicate that new frequency hopping sequence to the plurality of
terminals.
[0029] The communication device may be configured to operate in
whitespace.
[0030] The communication device may be configured for
machine-to-machine communication.
[0031] According to a second embodiment of the invention, there is
provided a method for communicating with a plurality of terminals
via a series of frames, the method comprising indicating to the
terminals, at the commencement of a frame, that communication is to
be suspended for the remainder of the frame in order for a
calibration to be performed, and performing said calibration during
the remainder of the time allotted to that frame.
[0032] According to a third embodiment of the invention, there is
provided a controller for a communication network, the
communication network comprising: a plurality of cells, each cell
comprising a communication device and at least one terminal and
each communication device being configured to communicate with the
at least one terminal in its respective cell according to a
frequency hopping sequence associated with that cell; and the
controller being configured to: instruct a communication device to
perform a calibration; receive a result of that calibration; and
generate a new frequency hopping sequence for the cell comprising
the communication device in dependence on that result.
[0033] The controller may be configured to communicate the new
frequency hopping sequence to the communication device.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Aspects of the present invention will now be described by
way of example with reference to the accompanying drawings. In the
drawings:
[0035] FIG. 1 shows an example of a machine-to-machine network;
[0036] FIG. 2 shows an example of a process that may be implemented
by a controller;
[0037] FIG. 3 shows an example of a process for monitoring
interference;
[0038] FIG. 4 shows an example of a frame structure;
[0039] FIG. 5 shows an example of a central controller; and
[0040] FIG. 6 shows an example of a communication device.
DETAILED DESCRIPTION
[0041] A communication device may be arranged to communicate
periodically with a plurality of terminals. Those periodic
communications may take the form of frames. The communication
device may be configured to indicate to the terminals, at the
commencement of a particular one of those periodic communications,
that communication is to be suspended for the remainder of that
periodic communication in order for a calibration to be performed.
The remainder of the time that would normally have been dedicated
to terminal transmissions or routine base station traffic can then
be used for calibration instead.
[0042] Suitably the calibration operation involves communication
devices throughout the network taking measurements on one or more
frequencies that indicate what, if any, interference is being
suffered on those frequencies at different locations within the
network. Suspending communication for a limited period of time is
beneficial because it enables those measurements to be made in the
absence of routine terminal and base station traffic, enabling a
more accurate picture of the underlying interference conditions to
be obtained.
[0043] A communication device may be configured to communicate with
a plurality of terminals by means of a series of periodic
communications having a predetermined structure. A single instance
of that periodic communication structure may be termed a "frame". A
typical frame may start with a preamble and end with an uplink
section.
[0044] An example of a wireless network is shown in FIG. 1. The
network, shown generally at 104, comprises one or more base
stations 105 that are each capable of communicating wirelessly with
a number of terminals 106. Each base station may be arranged to
communicate with terminals that are located within a particular
geographical area or cell. The base stations transmit to and
receive radio signals from the terminals. The terminals are
suitably entities embedded or machines or similar that communicate
with the base stations. Suitably the wireless network is arranged
to operate in a master-slave mode where the base station is the
master and the terminals are the slaves.
[0045] The base station controller 107 is a device that provides a
single point of communication to the base stations and then
distributes the information received to other network elements as
required. That is, the network is based around a many-to-one
communication model. The network may be arranged to communicate
with a client-facing portion 101 via the internet 102. In this way
a client may provide services to the terminals via the wireless
network.
[0046] Other logical network elements shown in this example are:
[0047] Core network. This routes traffic information between base
stations and client networks. [0048] Billing system. This records
utilization levels and generates appropriate billing data. [0049]
Authentication system. This holds terminal and base station
authentication information. [0050] Location register. This retains
the last known location of the terminals. [0051] Broadcast
register. This retains information on group membership and can be
used to store and process acknowledgements to broadcast messages.
[0052] Operations and maintenance center (OMC). This monitors the
function of the network and raises alarms when errors are detected.
It also manages frequency and code planning, load balancing and
other operational aspects of the network. [0053] Whitespace
database. This provides information on the available whitespace
spectrum. [0054] Client information portal. This allows clients to
determine data such as the status of associated terminals, levels
of traffic etc.
[0055] In practice, many of the logical network elements may be
implemented as databases running software and can be provided on a
wide range of platforms. A number of network elements may be
physically located within the same platform.
[0056] A network such as that shown in FIG. 1 may be used for
machine-to-machine communications, i.e. communications that do not
involve human interaction. Machine-to-machine communications are
well-matched to the limitations of operating in whitespace, in
which the bandwidth available to the network may vary from one
location to another and also from one time instant to the next. As
the network does not have any specific part of the spectrum
allocated to it, even unallocated parts of the spectrum may become
unavailable, e.g. due to a device in the vicinity that is operating
outside of the network but using the same part of the spectrum.
Machines are able to tolerate the delays and breaks in
communication that can result from these varying communication
conditions. Services can be provided in not real time; low latency
is not important as long as data is reliably delivered.
[0057] In order to increase the number of messages that are
reliably delivered, a central controller may be provided to make
intelligent frequency hopping allocations to cells in the network
by analyzing frequency availability. A suitable process that may be
performed by the controller is shown in FIG. 2. The process
commences in step 201. In step 202, the controller determines, for
each and every cell, which frequencies are permitted for whitespace
use. The controller may perform this step by accessing the
whitespace database to rule out those frequencies reserved for
licensed users. The controller may then determine what frequencies
are otherwise excluded as being unsuitable (step 203). It may, for
example, rule out as being unsuitable frequencies on which an
unacceptably high level of interference has been found. In step 204
the controller produces a finalized list of frequencies that are
available to each cell. The controller uses this list to generate a
frequency hopping sequence for each cell (step 205), which is then
communicated to the appropriate base station (step 206). The
process finishes in step 207.
[0058] Once the controller has established which frequencies are
available for use in each cell it can start to allocate frequency
hopping sequences. It is preferable for the sequences to contain as
many frequencies as possible to reduce the impact of fading etc.,
as discussed above. However, the sequences should also be generated
so as to minimize the occasions on which neighboring cells will be
transmitting on the same frequency, as this can cause interference
to the terminals in each cell (particularly those located near to a
cell boundary). The controller may employ an algorithm to determine
every possible frequency sequence across the cells of the network
to analyses which arrangement will generate the least amount of
overlap between neighboring cells.
[0059] A preferred option is for the available frequencies to be
arranged in a predetermined order, with each cell starting its
respective hopping sequence at a different frequency in the order
from its neighboring cells. The predetermined order might be random
or worked out according to some rule. For example, the available
frequencies might simply be organized into ascending or descending
order. Preferably, each cell commences its respective sequence at a
different offset from its neighbor, so that at any given time each
cell is using a different frequency in the sequence from its
neighbor. Simulations have shown that cyclic frequency hopping
sequences function very well with such an offset, without the
unfeasible computational burden associated with looking at all
possible frequency hopping sequences across all cells.
[0060] Generating the sequences to simply comprise a list of
available frequencies arranged in a predetermined order and then
applying a respective offset for each cell works particularly well
in networks arranged to operate in whitespace. This is because the
frequencies available for use in whitespace are largely dictated by
the frequencies that are already allocated to TV channels.
Different TV transmitters may use different frequencies (which is
why the spectrum available to whitespace networks is dependent on
the location of that network); however, each TV transmitter is
associated with a large geographical region. Typically, a
transmitter may cover an area having a radius of around 50 miles.
This means that neighboring cells will largely have the same set of
frequencies available to them. When this is the case, neighboring
cells can be prevented from overlapping in their frequency hopping
sequences simply by applying an offset in each cell.
[0061] The central controller is preferably arranged to review the
frequency hopping sequences from time to time. This is recommended
due to the variability of the frequency spectrum available to the
network, which means that different frequencies will be subject to
interference at different times and in different locations. A
frequency hopping sequence will tend not to be optimal for long
periods of time. The network is preferably arranged to periodically
monitor what frequencies are subject to interference, and
where.
[0062] A preferred option is for the network to have a calibration
mode, when different devices across the network are arranged to
make interference measurements that can be used to develop a
picture of interference across the network. This information may be
used to adapt the frequency hopping pattern assigned to each base
station to the interference conditions.
[0063] It may be helpful for the network to specifically measure:
[0064] 1. The noise on each channel due to licensed users (e.g. the
residual signal from distant TV transmitters); and [0065] 2. The
propagation from one base station to another to understand better
which base stations can share frequencies.
[0066] Preferably the calibration mode involves a period of `quiet`
being established across all or part of the network so that
interference measurements can be made without the presence of
routine traffic between the base stations and the terminals. The
base stations may therefore be configured to indicate to each of
the terminals in their cells that communication will be suspended
for some predetermined period of time in order for calibration to
be performed. This may be indicated to the terminals directly, for
example by broadcasting a `calibration mode` message to all of the
terminals in the cell. Alternatively, it may be indicated
indirectly by transmitting to the terminals a normal frame header,
but without any resources allocated to the terminals. This may be
achieved by marking all resources as being `reserved`. This ensures
that there will be no terminal transmissions or routine base
station traffic for the remainder of the frame. Additional
information on a suitable frame structure for the network is given
below.
[0067] Calibration can then be performed according to one or more
of the following: [0068] 1. When all base stations are not
transmitting, each base station can be instructed to measure its
noise floor across a number of channels. Depending on how quickly a
measurement can be made on each channel this could be performed
across the network in just a few seconds. [0069] 2. A selected base
station can be asked to transmit a constant power carrier and all
the others in the vicinity asked to make measurements on the
frequency used. This can then be repeated for all base stations of
interest. This could take some time but will typically only need to
be performed once on initial network deployment. Assuming one
measurement per frame and that clusters of around 100 base stations
are analyzed, it is anticipated that the process is likely to take
around 200 seconds.
[0070] An example of a process that may be performed by a network
in calibration mode is shown in FIG. 3. The process starts in step
301. In step 302 the central controller determines that the network
should perform a calibration and instructs the base stations
accordingly. The base stations then indicate to the terminals in
their respective cells that a calibration is to be performed, so
that communication will be suspended (step 303). Each base station
then measures the noise floor across a range of channels (step
304), transmits a constant power signal on a designated carrier
frequency (step 305) or measures the constant power signal being
transmitted by another base station on a designated carrier
frequency (step 306) in dependence on what the central controller
instructed it to do. The relevant measurements are then reported
back to the central controller by the base stations (step 307). The
central controller may then determine a new frequency hopping
sequence and communicate it to the base stations (step 308). The
process terminates in step 309.
[0071] Interference on a particular frequency in a particular cell
may be detected in a number of ways. For example, a base station
may measure a constant power signal being transmitted by another
base station by measuring an indication of the strength of that
signal at the receiving base station (e.g. by measuring an RSSI).
If the RSSI increases above some predetermined threshold then an
unacceptable level of interference may be determined to be present.
The constant power carrier could be transmitted to include some
predetermined bit sequence so that the bit error rate (BER) could
be monitored and unacceptable interference could be determined to
be present if it reaches a predetermined threshold. Similarly the
constant power carrier might contain cyclic redundancy checks
(CRCs). If less than a predetermined number of these fail within a
predetermined time period or number of frames then, again, an
unacceptable level of interference could be determined to be
present since this would indicate the interfering signal is being
strongly received. If there are periods when a frequency is not
being used, interference could be determined more directly by
measuring the noise floor on that frequency during a quiet
period.
[0072] Due to the dynamic nature of the whitespace environment, it
may be necessary for the network to enter the calibration mode
periodically so that the frequency hopping sequences can be updated
where needed. The controller could alternatively, or in addition,
cause the network to enter into calibration mode in response to
information received from one or more of the base stations, or any
other network device. For example, if a predetermined threshold
associated with a number of missed ACKs or failed CRCs, or an RSSI
or BER level, is crossed in one part of the network, this could
trigger a redetermination of frequency hopping sequences across a
greater part of the network.
[0073] The base stations may also be configured to make their own
adjustments to their frequency hopping sequence. Any adjustments
may suitably involve the base station scheduling communications
with particular terminals so as to avoid frequencies on which those
terminals are suffering interference. If a large number of
terminals are suffering interference on a particular channel, the
base station may exclude that channel from the frequency hopping
sequence for a time. This determination may be made, for example,
in dependence on an RSSI or BER level measured by one or more
terminals exceeding a predetermined threshold, or on a number of
failed CRCs exceeding a predetermined threshold. Communications
between a base station and its terminals could alternatively/also
require acknowledgements (ACKs). If more than a predetermined
proportion of ACKs are missed then this too could indicate an
unacceptable level of interference. This raises the question of
when to return to the channel. It channel should not be reinstated
until the interference has ceased, which may typically take around
15 minutes or so. While the base station could try to measure the
interference, it may be local to a particular part of the cell and
so not visible at the base station. Therefore, the interference
measurements are preferably made by the terminals.
[0074] This may be achieved by instructing powered terminals in
areas that had previously seen interference to periodically measure
the noise level on the channel and report back to the base station
when it had fallen to acceptable values. The base station may be
configured to transmit a message to a particular terminal
instructing it to measure a particular channel (e.g. during the
time period between one frame header and the next) and to send a
contended access message when the interference appears to have
stopped.
[0075] Preferably the frequency hopping sequences are communicated
to each base station, and the base stations pass the information on
to any terminals in their cells. This communication is suitably
achieved by the base station including information defining the
sequence in each frame it transmits, so that a terminal can obtain
the frequency hopping sequence by listening to only one frame.
[0076] The base station could inform terminals of the channels and
hopping sequence to be used, and any changes, in a number of ways.
A preferred embodiment is for the frequency hopping sequence to be
communicated in every frame so that a terminal need only listen to
one frame to obtain all the information about the frequency hopping
sequence that it needs. One advantage of having each base station
simply use an ascending or descending sequence of frequencies is
that it can be particularly easily communicated to the terminals.
For example, such a hopping sequence may be indicated simply by
having a channel bitmap in every frame. For more complex sequences,
it may be necessary for the base station to transmit the actual
list of channels in the order in which hopping will occur. This can
be transmitted in each frame, but with the risk that the resource
requirement is high if there are a large number of channels.
[0077] An alternative for more complex hopping sequences is to
transmit the full hopping sequence as part of a broadcast control
channel frame transmitted by the base station to all the terminals
in the cell at regular intervals. This frame could inform terminals
of a forthcoming change to the channel assignment/hopping sequence
in the cell, and page terminals if they are required to respond
outside of their normal allocated slot, amongst other things. In
every other frame, the base station may transmit the periodicity of
the hopping sequence, the frequency of the next frame and
optionally the frequency that the next broadcast control channel
frame will be on. This approach allows for greater flexibility in
the hopping sequences that can be adopted, but does mean that the
terminals cannot gain complete knowledge of the hopping sequence
from simply listening to one frame.
[0078] A further option is to transmit the hopping sequence as a
combination of a 48-bit channel map (with a bit being set if that
channel is in use in the base station), and a log2(n)-bit seed. In
order to generate the sequence, a terminal may input the seed to a
pseudo random noise generator. The chosen channel would then be the
(MacFrame)'th value in the PRN sequence, modulo the number of bits
set in the channel map. The base station might transmit the 48-bit
channel map in a broadcast frame and the seed in every frame. This
approach results in a relatively small amount of data being needed
to characterize the hopping sequence, allowing it to be transmitted
in each frame and hence ensuring devices can determine the future
frequency usage from monitoring a single frame.
[0079] The network may use medium access control (MAC) to share the
same radio resource between multiple terminals. An example of a
suitable frame structure is shown in FIG. 4. The frame (shown
generally at 401) comprises time to ramp-up to full output power
402 (T_IFS), a synchronization burst 403 (DL_SYNC), an information
field providing the subsequent channel structure 404 (DL_FCH), a
map of which information is intended for which terminal 405
(DL_MAP), a field to allow acknowledgement of previous uplink
transmissions 406 (DL_ACK) and then the actual information to be
sent to terminals 407 (DL_ALLOC). There is then a guard period for
ramp-down of the downlink and ramp-up on the uplink 408 (T_SW),
followed by the allocated uplink data transmissions 410 (UL_ALLOC)
in parallel with channels set aside for uplink contended access 409
(UL_CA).
[0080] A suitable hopping rate for the downlink channels may be the
frame rate, so that each frame is transmitted on a different
frequency from the preceding frame. The frames for a network
designed to operate in whitespace for machine-to-machine
communication may be particularly long. In one example the frames
may each be 2 seconds long, giving a frequency hop on the downlink
every 2 seconds (which is 30 hops per minute).
[0081] The DL_FCH may include information to enable the terminals
to determine the hopping sequence. The DL_FCH may include a list of
the frequencies that are included in the sequence. One efficient
way of communicating this information is by means of a channel map,
with a bit being set if the channel is in use in the base station.
The DL_FCH may also include a MAC Frame count (16-bit) enabling
terminals to determine where the base station is in its hopping
pattern.
[0082] The DL_MAP informs terminals as to whether there is any
information for them in the frame and whether they have an uplink
slot reserved for them to transmit information. It comprises a
table of terminal identities, the number of slots that their
information is spread over and the transmission mode and spreading
factors used. All terminals monitoring the frame decode this field
to determine whether they need to decode subsequent information.
The length of the DL_MAP may be included as part of the DL.sub.--
FCH. A terminal can determine the position of its assigned slots
from the DL_MAP by adding up the number of slots allocated in prior
rows in the table.
[0083] On the uplink the slots may be numbered from 0 to n on the
first FDMA channel, then on the subsequent FDMA channel and so on.
The terminal can determine how many slots there are each channel
from the length of the frame available for the uplink (that
remaining after completion of the downlink) divided by the length
of each slot. If a terminal has data requiring multiple slots it
would normally be given these consecutively on the same carrier as
this both simplifies the terminal transmission and minimizes the
control information required to describe the slot location.
However, it is possible to give the terminal multiple allocations
on different carriers (so long as they are not simultaneous) to
achieve frequency hopping on the uplink.
[0084] To indicate to the terminals that the network is entering a
calibration mode, the DL_MAP preferably does not include any
terminal allocations. Instead, the DL_MAP preferably indicates that
all slots are reserved, thus providing for a period in which
communications are effectively suspended. This results in the
required `quiet period` during which the calibration measurements
can be made.
[0085] The communication links between the various components of
the networks may be wired or wireless. The terminals may be located
in fixed positions or mobile, roaming throughout and/or between
cells.
[0086] Interference between network devices in neighboring cells
could also be reduced by the controller assigning different
spreading codes for use in different cells. This can help to avoid
complete packet loss in the event of direct frequency clashes
between neighboring cells. Preferably neighboring cells are
assigned orthogonal spreading codes.
[0087] An example of the functional blocks that may be comprised in
a controller according to one embodiment of the invention are shown
in FIG. 5. The controller, shown generally at 501, comprises a
communication unit 503 connected to an antenna 502 for transmitting
and receiving messages. The controller might equally communicate
the frequency hopping sequences to the communication devices via a
wired connection. The controller further comprises an availability
unit 504 for determining what frequencies are available in each
cell, an analysis unit 505 for analyzing the data returned by the
base stations and determining from this which available frequencies
should be avoided in the frequency hopping sequences, and a
generation unit 506 for generating the frequency hopping sequences.
The communication unit may effectively act as a central controller
and may pass information between the other functional blocks.
[0088] An example of the functional blocks that may be comprised in
a communication device according to one embodiment of the invention
are shown in FIG. 6. The communication device, shown generally at
601, comprises a communication unit 603 connected to an antenna 602
for transmitting and receiving messages. The communication device
further comprises a calibration unit 604 for ensuring that the
relevant calibration indication is sent to the terminals and for
making the appropriate measurements to return to the central
controller. The base station also comprises an analysis unit 605
for analyzing the interference conditions in the cell in dependence
on the information received from its own terminals and for adapting
the frequency hopping sequence accordingly. The communication unit
may effectively act as a central controller and may pass
information between the other functional blocks.
[0089] The apparatus in FIGS. 5 and 6 are shown illustratively as
comprising a number of interconnected functional blocks. This is
for illustrative purposes and is not intended to define a strict
division between different parts of hardware on a chip. In
practice, the communication controller and communication device
preferably use a microprocessor acting under software control for
implementing the methods described herein. In some embodiments, the
algorithms may be performed wholly or partly in hardware.
[0090] The applicant hereby discloses in isolation each individual
feature described herein and any combination of two or more such
features, to the extent that such features or combinations are
capable of being carried out based on the present specification as
a whole in the light of the common general knowledge of a person
skilled in the art, irrespective of whether such features or
combinations of features solve any problems disclosed herein, and
without limitation to the scope of the claims. The applicant
indicates that aspects of the present invention may consist of any
such individual feature or combination of features. In view of the
foregoing description it will be evident to a person skilled in the
art that various modifications may be made within the scope of the
invention.
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