U.S. patent number RE45,299 [Application Number 13/867,015] was granted by the patent office on 2014-12-23 for receiving a pilot design and channel estimation.
This patent grant is currently assigned to Texas Instruments Incorporated. The grantee listed for this patent is Texas Instruments Incorporated. Invention is credited to Anand Dabak, Eko N Onggosanusi, Aris Papasakellariou, Timothy M. Schmidl.
United States Patent |
RE45,299 |
Papasakellariou , et
al. |
December 23, 2014 |
Receiving a pilot design and channel estimation
Abstract
A receiver in an OFDM based communication system is adapted to
perform channel estimation using a received reference signal
transmitted from at least one antenna The reference signal is
substantially located into at least two OFDM symbols of a
transmission time interval comprising of more than two OFDM
symbols. A power level of said reference signal is divided into
said non-consecutive OFDM symbols in said transmission time
interval and adapted to use the reference signal located in a first
OFDM symbol in succeeding transmission time intervals in addition
to the reference symbols in a current transmission time interval
and a preceding transmission time interval.
Inventors: |
Papasakellariou; Aris (Houston,
TX), Schmidl; Timothy M. (Dallas, TX), Onggosanusi; Eko
N (Allen, TX), Dabak; Anand (Plano, TX) |
Applicant: |
Name |
City |
State |
Country |
Type |
Texas Instruments Incorporated |
Dallas |
TX |
US |
|
|
Assignee: |
Texas Instruments Incorporated
(Dallas, TX)
|
Family
ID: |
37573247 |
Appl.
No.: |
13/867,015 |
Filed: |
April 19, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11424939 |
Jun 19, 2006 |
7660229 |
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60692184 |
Jun 20, 2005 |
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60709085 |
Aug 16, 2005 |
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60723891 |
Oct 5, 2005 |
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Reissue of: |
12639422 |
Dec 16, 2009 |
7929416 |
Apr 19, 2011 |
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Current U.S.
Class: |
370/208; 370/345;
370/343; 370/334; 375/316; 375/260 |
Current CPC
Class: |
H04L
1/06 (20130101); H04L 5/0048 (20130101); H04B
7/0452 (20130101); H04L 25/0226 (20130101); H04B
7/12 (20130101); H04L 5/0007 (20130101) |
Current International
Class: |
H04J
11/00 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Thierry Lestable et al., Adaptive Pilot Pattern for Multi-Carrier
Spread-Spectrum (MC-SS) Transmission Systems; 2004 IEEE; pp.
385-388. cited by applicant .
Downlink Multiple Access Parameterisation; R1-050384-3GPP TSG RAN
WG1#41, Athens, Greece; May 9-13, 2005. cited by applicant .
EUTRA Downlink Numerology; R1-050520-3GPP TSG RAN1#41 Meeting,
Athens, Greece, May Sep. 13, 2005. cited by applicant .
Performance and Implementation Aspects for Scattered and TDM Pilot
Formats in EUTRA OFDMA Downlink; R1-051060-3GPP TSG RAN WG1, San
Diego, California, USA; Oct. 10-14, 2005. cited by applicant .
TP on Pilot Structure for OFDM based E-UTRA Downlink Unicast;
R1-051489-3GPP TSG-RAN WG1 #43, Seoul, Korea; Nov. 7-11, 2005.
cited by applicant .
On Pilot Structure for IFDM Based E-UTRA Downlink Multicast;
R1-051490-3GPP TSG-RAN WG1 #43; Seoul, Korea; Nov. 7-11, 2005.
cited by applicant .
Boosting the Uplink Pilot Transmission Power for Higher Mobility
UEs; R1-060924-3GPP TSG-RAN WG1 Meeting #44bis; Athens, Greece;
Mar. 27-31, 2006. cited by applicant .
PCT/US 06/23901 International Search Report, Dec. 14, 2006. cited
by applicant.
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Primary Examiner: Pham; Chi
Assistant Examiner: Hom; Shick
Attorney, Agent or Firm: Denker; David Telecky, Jr.;
Frederick J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Divisional of U.S. application Ser. No.
11/424,939 filed on Jun. 19, 2006, .Iadd.now issued as U.S. Pat.
No. 7,660,229, .Iaddend.which claims priority to U.S. Provisional
Application No. 60/692,184 entitled "Pilot design and channel
estimation for OFDM" filed Jun. 20, 2005, U.S. Provisional
Application No. 60/709,085 entitled "Pilot design and channel
estimation for OFDM" filed Aug. 16, 2005, and U.S. Provisional
Application No. 60/723,891 entitled "Pilot design and channel
estimation for OFDM" filed Oct. 5, 2005. All applications assigned
to the assignee hereof and hereby incorporated by reference.
Claims
What is claimed is:
1. A receiver in an orthogonal frequency division multiplexing OFDM
based communication system including: a processor adapted to
perform channel estimation using a received reference signal
transmitted from at least one antenna, said reference signal being
substantially located into at least two non-consecutive OFDM
symbols of a transmission time interval comprising of more than two
OFDM symbols and a power level of said reference signal is divided
into said non-consecutive OFDM symbols in said transmission time
interval and adapted to use the reference signal located in a first
OFDM symbol in succeeding transmission time intervals in addition
to the reference symbols in a current transmission time interval
and a preceding transmission time intervals.
2. An apparatus for an orthogonal frequency division multiplexing
(OFDM) based communication system, said apparatus: adapted to
perform channel estimation using a received reference signal
transmitted from a plurality of transmitting antennas, said
reference signal being substantially located into two
non-consecutive OFDM symbols of a current transmission time
interval for each transmit antenna, said transmission time interval
comprising of more than two OFDM symbols and a power level of said
reference signal is divided into said non-consecutive OFDM symbols
in said transmission time interval, wherein a first portion of a
plurality of pilot symbols of at least one OFDM symbol is near a
beginning of the transmission time interval and a second portion of
said plurality of pilot symbols of at least one other OFDM symbol
is near a middle of said transmission time interval, wherein a
power level of said plurality of pilot symbols has been divided
into said two OFDM symbols in said transmission time interval, said
apparatus comprising: a receiver adapted to using the first of the
two OFDM symbols carrying reference signals in a succeeding
transmission time interval in addition to the reference signals in
a current transmission time interval and a preceding transmission
time interval.
3. The apparatus of claim 2, wherein said input signal and
plurality of data symbols are comprised of sub-sets, each sub-set
intended for a unique receiver.
4. An apparatus .[.using.]. .Iadd.comprising: a processor using
.Iaddend.information for channel estimation in an orthogonal
frequency division multiplexing OFDM system, said information in
the form of a pilot structure with a transmission time interval of
seven OFDM symbols, said pilot structure comprising a plurality of
pilot signals from at least .Iadd.two transmitting antennas, the
pilot structure from .Iaddend.one transmitting antenna located in a
first OFDM symbol and fifth OFDM symbol of said transmission time
interval, .[.said apparatus comprising: at least two transmit
antennas.]. and wherein said plurality of pilot signals from any
antenna are located in said first OFDM symbol and said fifth OFDM
symbol or a second OFDM symbol and a sixth OFDM symbol of said
transmission time interval.
5. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having a plurality of antennas, said method
comprising: receiving a frame composed with a time domain and a
frequency domain, wherein said frame has a transmission time
interval in the time domain and occupies a bandwidth in said
frequency domain, said transmission time interval comprising of
more than two orthogonal frequency division multiplexing symbols,
wherein said transmission time interval comprises of seven
orthogonal frequency division multiplexing symbols; and receiving a
pilot signal, having pilot power level, from .[.a.]. .Iadd.said
.Iaddend.plurality of transmitting .[.antenna.]. .Iadd.antennas
.Iaddend.into two orthogonal frequency division multiplexing
symbols of said frame, wherein a plurality of pilot signals from
all transmitting antennas have a plurality of same locations in
said time domain and wherein at least one pilot signal from at
least .[.one transmitting antenna.]. .Iadd.a first antenna of said
plurality of transmitting antennas .Iaddend.has been located in a
first OFDM symbol and a fifth OFDM symbol of said transmission time
interval and said power level of said pilot signal has been divided
into said a first OFDM symbol and a fifth OFDM symbol in said
transmission time interval, wherein said transmitter has at least
two antennas and a pilot signal from a second antenna .Iadd.of said
plurality of transmitting antennas .Iaddend.has been located in
said first and fifth orthogonal frequency division multiplexing
symbols of said frame such that said pilot power of said pilot
signal from said second antenna is in said first and said fifth
orthogonal frequency division multiplexing symbols of said
transmission time interval.
6. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having at least two antennas, said method comprising:
receiving a frame composed with a time domain and a frequency
domain, wherein said frame has a transmission time interval in the
time domain and occupies a bandwidth in said frequency domain, said
transmission time interval comprising of more than two orthogonal
frequency division multiplexing symbols, wherein said transmission
time interval comprises of seven orthogonal frequency division
multiplexing symbols; and receiving a pilot signal, having a pilot
power level, from a plurality of transmitting antenna into two
orthogonal frequency division multiplexing symbols of said frame,
wherein a plurality of pilot signals from all transmitting antennas
have a plurality of same locations in said time domain and wherein
at least one pilot signal from at least one transmitting antenna
has been located in a first OFDM symbol and a fifth OFDM symbol of
said transmission time interval and said power level of said pilot
signal has been divided into said a first OFDM symbol and a fifth
OFDM symbol in said transmission time interval, wherein a pilot
signal from a second antenna has been located in a second and sixth
orthogonal frequency division multiplexing symbols of said frame
such that said pilot power of said pilot signal from said second
antenna is in said second and said sixth orthogonal frequency
division multiplexing symbols of the transmission time
interval.
7. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having at least four antennas, said method
comprising: receiving a frame composed with a time domain and a
frequency domain, wherein said frame has a transmission time
interval in the time domain and occupies a bandwidth in said
frequency domain, said transmission time interval comprising of
more than two orthogonal frequency division multiplexing symbols,
wherein said transmission time interval comprises of seven
orthogonal frequency division multiplexing symbols; and receiving a
pilot signal, having pilot power level, from a plurality of
transmitting antenna into two orthogonal frequency division
multiplexing symbols of said frame, wherein a plurality of pilot
signals from all transmitting antennas have a plurality of same
locations in said time domain, wherein a pilot signal from a third
antenna has been located in a second and sixth orthogonal frequency
division multiplexing symbols of said frame such that the pilot
power of the pilot signal from said third antenna is in a second
orthogonal frequency division multiplexing symbol and a sixth
orthogonal frequency division multiplexing symbols of the
transmission time interval; and further wherein said pilot signal
from a fourth antenna has been located in a first orthogonal
frequency division multiplexing symbol and a fifth orthogonal
frequency division multiplexing symbols of said frame such that the
pilot power of the pilot signal from the fourth antenna is in the
first and fifth orthogonal frequency division multiplexing symbols
of the transmission time interval.
8. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having at least four antennas, said method
comprising: receiving a frame composed with a time domain and a
frequency domain, wherein said frame has a transmission time
interval in the time domain and occupies a bandwidth in said
frequency domain, said transmission time interval comprising of
more than two orthogonal frequency division multiplexing symbols,
wherein said transmission time interval comprises of seven
orthogonal frequency division multiplexing symbols; and receiving a
pilot signal, having pilot power level, from a plurality of
transmitting antenna into two orthogonal frequency division
multiplexing symbols of said frame, wherein a plurality of pilot
signals from all transmitting antennas have a plurality of same
locations in said time domain, wherein a pilot signal from a third
antenna has been located in a first orthogonal frequency division
multiplexing symbol and a fifth orthogonal frequency division
multiplexing symbol of said frame such that the pilot power of the
pilot signal from said third antenna is in said first and fifth
orthogonal frequency division multiplexing symbols of the
transmission time interval; and a pilot signal from a fourth
antenna is located in a second orthogonal frequency division
multiplexing symbol and a sixth orthogonal frequency division
multiplexing symbols of said frame such that said pilot power of
said pilot signal from said fourth antenna is in said second and
sixth orthogonal frequency division multiplexing symbols of said
transmission time interval.
9. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having at least four antennas, said method
comprising: receiving a frame composed with a time domain and a
frequency domain, wherein said frame has a transmission time
interval in the time domain and occupies a bandwidth in said
frequency domain, said transmission time interval comprising of
more than two orthogonal frequency division multiplexing symbols,
wherein said transmission time interval comprises of seven
orthogonal frequency division multiplexing symbols; and receiving a
pilot signal, having pilot power level, from a plurality of
transmitting antenna into two orthogonal frequency division
multiplexing symbols of said frame, wherein a plurality of pilot
signals from all transmitting antennas have a plurality of same
locations in said time domain and wherein at least one pilot signal
from at least one antenna has been located in a first OFDM symbol
and a fifth OFDM symbol of said transmission time interval and said
power level of said pilot signal has been divided into a first OFDM
symbol and a fifth OFDM symbol in said transmission time interval,
wherein a frequency location of a pilot signal from a third antenna
is the same as the frequency location of a pilot signal from a
first antenna and a frequency location of a pilot signal from a
fourth antenna is the same as a frequency location of the pilot
from a second antenna.
.Iadd.10. A method for receiving a pilot structure in an orthogonal
frequency division multiplexing (OFDM) communication system having
a transmitter having a plurality of antennas, the method
comprising: receiving a frame composed with a time domain and a
frequency domain, wherein the frame has a transmission time
interval in the time domain and occupies a bandwidth in the
frequency domain, wherein the transmission time interval comprises
seven OFDM symbols; and receiving a reference signal, from said
plurality of transmitting antennas wherein the reference signal
includes first antenna reference symbols from at least a first
antenna of said plurality of transmitting antennas, the first
antenna reference symbol located in a first OFDM symbol being at a
first frequency, and the first antenna reference symbol located in
a fifth OFDM symbol of the transmission time interval being at a
second frequency; wherein the reference signal includes second
antenna reference symbols from a second antenna of said plurality
of transmitting antennas, the second antenna reference symbol
located in the first OFDM symbol being at the second frequency, and
the second antenna reference symbol located in the fifth OFDM
symbol of the transmission time interval being at the first
frequency; and using at least one of the received symbols to
perform a channel estimate..Iaddend.
.Iadd.11. The method of claim 10, wherein the transmission time
interval has a 0.5 millisecond duration..Iaddend.
.Iadd.12. The method of claim 10, further comprising providing
Channel Quality Indication (CQI) feedback..Iaddend.
.Iadd.13. The method of claim 10, wherein each symbol of the
reference signal at the first frequency also occurs at a third
frequency and each symbol of the reference signal at the second
frequency also occurs at a fourth frequency..Iaddend.
.Iadd.14. The method of claim 13, further comprising providing
Channel Quality Indication (CQI) feedback..Iaddend.
.Iadd.15. The method of claim 10, wherein the reference signal
further includes a third antenna reference symbol from a third
antenna, the third antenna reference symbol located in a second
OFDM symbol of the transmission time interval and being at the
first frequency; and the reference signal further includes a fourth
antenna reference symbol from a fourth antenna, the fourth antenna
reference symbol located in a second OFDM symbol of the
transmission time interval and being at the second
frequency..Iaddend.
.Iadd.16. The method of claim 15, further comprising providing
Channel Quality Indication (CQI) feedback..Iaddend.
.Iadd.17. The method of claim 15, wherein each symbol of the
reference signal at the first frequency also occurs at a third
frequency and each symbol of the reference signal at the second
frequency also occurs at a fourth frequency..Iaddend.
.Iadd.18. The method of claim 17, further comprising providing
Channel Quality Indication (CQI) feedback..Iaddend.
Description
BACKGROUND
Embodiments of the invention are directed, in general, to
communication systems and, more specifically, to pilot design used
in communications systems.
The global market for both voice and data communication services
continues to grow as does users of the systems which deliver those
services. As communication systems evolve, system design has become
increasingly demanding in relation to equipment and performance
requirements. Future generations of communication systems, will be
required to provide high quality high transmission rate data
services in addition to high quality voice services. Orthogonal
Frequency Division Multiplexing (OFDM) is a technique that will
allow for high speed voice and data communication services.
Orthogonal Frequency Division Multiplexing (OFDM) is based on the
well-known technique of Frequency Division Multiplexing (FDM). OFDM
technique relies on the orthogonality properties of the fast
Fourier transform (FFT) and the inverse fast Fourier transform
(IFFT) to eliminate interference between carriers. At the
transmitter, the precise setting of the carrier frequencies is
performed by the IFFT. The data is encoded into constellation
points by multiple (one for each carrier) constellation encoders.
The complex values of the constellation encoder outputs are the
inputs to the IFFT. For wireless transmission, the outputs of the
IFFT are converted to an analog waveform, up-converted to a radio
frequency, amplified, and transmitted. At the receiver, the reverse
process is performed. The received signal (input signal) is
amplified, down converted to a band suitable for analog to digital
conversion, digitized, and processed by a FFT to recover the
carriers. The multiple carriers are then demodulated in multiple
constellation decoders (one for each carrier), recovering the
original data. Since an IFFT is used to combine the carriers at the
transmitter and a corresponding FFT is used to separate the
carriers at the receiver, the process has potentially zero
inter-carrier interference.
FIG. 1 is a diagram illustrative of the Frequency 103--Time 101
Representation 100 of an OFDM Signal. In FDM different streams of
information are mapped onto separate parallel frequency channels
140. Each FDM channel is separated from the others by a frequency
guard band to reduce interference between adjacent channels.
The OFDM technique differs from traditional FDM in the following
interrelated ways: 1. multiple carriers (called sub-carriers 150)
carry the information stream; 2. the sub-carriers 150 are
orthogonal to each other; and 3. a Cyclic Prefix (CP) 110 (also
known as guard interval) is added to each symbol 120 to combat the
channel delay spread and avoid OFDM inter-symbol interference
(ISI).
The data/information carried by each sub-carrier 150 may be user
data of many forms, including text, voice, video, and the like. In
addition, the data includes control data, a particular type of
which is discussed below. As a result of the orthogonality, ideally
each receiving element tuned to a given sub-carrier does not
perceive any of the signals communicated at any other of the
sub-carriers. Given this aspect, various benefits arise. For
example, OFDM is able to use orthogonal sub-carriers and, as a
result, thorough use is made of the overall OFDM spectrum. As
another example, in many wireless systems, the same transmitted
signal arrives at the receiver at different times having traveled
different lengths due to reflections in the channel between the
transmitter and receiver. Each different arrival of the same
originally-transmitted signal is typically referred to as a
multi-path. Typically, multi-paths interfere with one another,
which is sometimes referred to as InterSymbol Interference (ISI)
because each path includes transmitted data referred to as symbols.
Nonetheless, the orthogonality implemented by OFDM considerably
reduces ISI and, as a result, often a less complex receiver
structure, such as one without an equalizer, may be implemented in
an OFDM system.
A Cyclic Prefix (CP) (also known as guard interval) is added to
each symbol to combat the channel delay spread and avoid OFDM
inter-symbol interference (ISI). FIG. 2 is a diagram illustrative
of using Cyclic Prefix (CP) to eliminate ISI and perform frequency
domain equalization. Blocks 200 each comprising cyclic prefix 210
coupled to data symbols 220 to perform frequency domain
equalization. OFDM typically allows the application of simple,
1-tap, frequency domain equalization (FDE) through the use of a
Cyclic Prefix (CP) 210 at every FFT processing block 200 to
suppress multi-path interference. Two blocks are shown for drawing
convenience. CP 210 eliminates inter-data-block interference and
multi-access interference using Frequency Division Multiple Access
(FDMA).
Since orthogonality is guaranteed between overlapping sub-carriers
and between consecutive OFDM symbols in the presence of
time/frequency dispersive channels, the data symbol density in the
time-frequency plane can be maximized and high data rates can be
very efficiently achieved for high Signal-to-Interference and Noise
Ratios (SINR).
FIG. 3 is a diagram illustrative of Cyclic Prefix (CP) Insertion. A
number of samples is typically inserted between useful OFDM symbols
320 (guard interval) to combat OFDM ISI induced by channel
dispersion, assist receiver synchronization, and aid spectral
shaping. The guard interval 310 is typically a prefix that is
inserted 350 at the beginning of the useful OFDM symbol (OFDM
symbol without the CP) 320. The CP duration 315 should be
sufficient to cover most of the delay-spread energy of a radio
channel impulse response. It should also be as small as possible
since it represents overhead and reduces OFDM efficiency. Prefix
310 is generated using a last block of samples 340 from the useful
OFDM symbol 330 and is therefore a cyclic extension to the OFDM
symbol (cyclic prefix).
When the channel delay spread exceeds the CP duration 315, the
energy contained in the ISI should be much smaller than the useful
OFDM symbol energy and therefore, the OFDM symbol duration 325
should be much larger than the channel delay spread. However, the
OFDM symbol duration 325 should be smaller than the minimum channel
coherence time in order to maintain the OFDM ability to combat fast
temporal fading. Otherwise, the channel may not always be constant
over the OFDM symbol and this may result in inter-sub-carrier
orthogonality loss in fast fading channels. Since the channel
coherence time is inversely proportional to the maximum Doppler
shift (time-frequency duality), this implies that the symbol
duration should be much smaller than the inverse of the maximum
Doppler shift.
The large number of OFDM sub-carriers makes the bandwidth of
individual sub-carriers small relative to the total signal
bandwidth. With an adequate number of sub-carriers, the
inter-carrier spacing is much narrower than the channel coherence
bandwidth. Since the channel coherence bandwidth is inversely
proportional to the channel delay spread, the sub-carrier
separation is generally designed to be much smaller that the
inverse of the channel coherence time. Then, the fading on each
sub-carrier appears flat in frequency and this enables 1-tap
frequency equalization, use of high order modulation, and effective
utilization of multiple transmitter and receiver antenna techniques
such as Multiple Input/Multiple Output (MIMO). Therefore, OFDM
effectively converts a frequency-selective channel into a parallel
collection of frequency flat sub-channels and enables a very simple
receiver. Moreover, in order to combat Doppler effects, the
inter-carrier spacing should be much larger than the maximum
Doppler shift.
The baseband representation 400 of the OFDM signal generation using
an N-point IFFT 460 is shown in FIG. 4, where n refers to the
n.sup.th sub-channel modulated data symbol 420, during the time 401
period 0<t.ltoreq.T.sub.u where Tu is OFDM useful symbol
duration. The vector S is defined as the useful OFDM symbol and is
practically the time superposition of the N narrowband modulated
sub-carriers. Therefore, from a parallel stream of N data sources,
a waveform composed of N orthogonal sub-carriers 407 is obtained.
At the receiver, a computationally efficient Fast Fourier Transform
(FFT) may be used to demodulate the multi-carrier information and
to recover the transmitted data.
FIG. 5 shows the concepts of frequency diversity 500 and multi-user
diversity 505. Using link adaptation techniques based on the
estimated dynamic channel properties, the OFDM transmitter can
adapt the transmitted signal to each User Equipment (UE) to match
channel conditions and approach the ideal capacity of
frequency-selective channel. Thanks to such properties as flattened
channel per sub-carrier, high-order modulation, orthogonal
sub-carriers, and MIMO; it is possible to improve spectrum
utilization and increase achievable peak data rate in OFDM system.
Also, OFDM can provide scalability for various channel bandwidths
(i.e. 1.25, 2.5, 5, 10, 20 MHz) without significantly increasing
complexity.
OFDM may be combined with Frequency Division Multiple Access (FDMA)
in an Orthogonal Frequency Division Multiple Access (OFDMA) system
to allow multiplexing of multiple UEs over the available bandwidth.
Because OFDMA assigns UEs to isolated frequency sub-carriers,
intra-cell interference may be avoided and high data rate may be
achieved. The base station (or Node B) scheduler assigns physical
channels based on Channel Quality Indication (CQI) feedback
information from the UEs, thus effectively controlling the
multiple-access mechanism in the cell. For example, in FIG. 5,
transmission to each of the three UEs 501, 502, 503 is scheduled at
frequency sub-bands where the channel frequency response allows for
higher SINR relative to other sub-bands. This is represented by the
Received signal levels R501, R502, and R503 for users 501, 502 and
503 at Frequencies F501, F502, and F503 respectively.
OFDM can use frequency-dependent scheduling with optimal per
sub-band Modulation & Coding Scheme (MCS) selection. For each
UE and each Transmission Time Interval (TTI), the Node B scheduler
selects for transmission with the appropriate MCS a group of the
active UEs in the cell, according to some criteria that typically
incorporate the achievable SINR based on the CQI feedback. In
addition, sub-carriers or group of sub-carriers may be reserved to
transmit pilot, signaling or other channels. Multiplexing may also
be performed in the time dimension, as long as it occurs at the
OFDM symbol rate or at a multiple of the symbol rate (i.e. from one
IFFT computation to the next). The MCS used for each sub-carrier or
group of sub-carriers can also be changed at the corresponding
rate, keeping the computational simplicity of the FFT-based
implementation. This allows 2-dimensional time-frequency
multiplexing, as shown in FIG. 6 and FIG. 7.
Transmission Time Interval (TTI) may also be referred to as a
frame.
Turning now to FIG. 6, which is a diagram illustrative of a
configuration for multi-user diversity. The minimum frequency
sub-band used for frequency-dependent scheduling of a UE typically
comprises of several sub-carriers and may be referred to as a
Resource Block (RB) 620. Reference number 620 is only pointing to
one of the 8 RBs per OFDM symbol shown as example and for drawing
clarity. RB 620 is shown with RB bandwidth 625 in frequency
dimension and TTI duration 610 in time dimension. Each RB may be
comprised of continuous sub-carriers and thus be localized in
nature to afford frequency-dependent scheduling. A high data rate
UE may use several RBs within same TTI 630. UE #1 is shown as an
example of a high rate UE. Low data rate UEs may be multiplexed
within the same RB 640.
Alternatively referring to FIG. 7, which is a diagram illustrative
of a configuration for frequency diversity, an RB 720 may
correspond to a number of sub-carriers substantially occupying the
entire bandwidth thereby offering frequency diversity. This may be
useful in situations where CQI feedback is not available or it is
unreliable (as is the case for high speed UEs).
To facilitate data-aided methods, OFDM systems periodically insert
reference (or pilot) symbols that are known a priori, into the
transmission signal. The receiver can thus estimate the channel
response based on the received pilot symbols and the known
transmitted pilot symbols. In an OFDM based communication system,
pilot symbols are transmitted in addition to data symbols in order
to serve, inter aila, in providing a reference for the receiver to
estimate the channel medium and accordingly demodulate the received
signal. A pilot signal also referred to as reference signal is
composed of the pilot symbols.
The DownLink (DL) pilot signal should provide effective performance
for the following functions: Channel estimation at all possible
operating carrier frequencies for all physical channels for all
channel multipath delay spreads (frequency selectivity) encountered
in practice and for all UE speeds of interest. CQI measurement for
link adaptation and channel-dependent scheduling. Sector
identification of sector within the same cell. Measurements for
cell search and handover.
UE dedicated pilot signals may also be used for UE-dependent
adaptive beam-forming. Moreover, as the pilot signal is actually
overhead consuming resources that could have been otherwise
dedicated for data transmission, it should have minimum
time/frequency and power overhead.
Two types of pilot structure have been previously examined; i) a
Time-Division Multiplexed (TDM) pilot structure where the pilot
signal is placed on a single OFDM symbol per TTI (FIG. 8). ii) a
scattered pilot structure where the pilot signal is placed in every
OFDM symbol per TTI (FIG. 9)
In the example shown in FIG. 8, the TTI is assumed to comprise of
seven OFDM symbols 810. Moreover, the TTI duration is assumed to be
0.5 milliseconds. Shaded dots indicate pilot locations for 1
transmit antenna. In the example of FIG. 9, the TTI is assumed to
comprise of seven OFDM symbols 910. Shaded dots indicate pilot
locations for 1 transmit antenna. Channel estimation is based on
time and frequency interpolation among pilot sub-carriers in order
to obtain the channel estimates at the position corresponding to
data sub-carriers. In order to be able to perform frequency
interpolation, the pilot sub-carrier spacing in the frequency
domain should be smaller than the 50% correlation coherence
bandwidth of the channel for all channels of interest. Similarly,
in order to be able to perform time interpolation, the pilot
sub-carrier spacing in the time domain should be smaller than the
50% coherence time of the channel at the operating carrier
frequency for all UE speeds of interest. The example shown in FIG.
8 has a spacing in frequency domain .DELTA.F.sub.pil 830 of 4
sub-carriers and spacing in time domain .DELTA.T.sub.pil 820 of 7
OFDM symbols. The example shown in FIG. 9 has a spacing in
frequency domain .DELTA.F.sub.pil 930 of 4 sub-carriers and spacing
in time domain .DELTA.T.sub.pil 920 of 1 OFDM symbol.
Additional requirements for the pilot signal design may relate to
the ability to demodulate only an initial sub-set of the TTI
without having to receive the entire TTI. This is applicable, for
example, when the control channel associated with scheduling of UEs
in the current TTI at various RBs is transmitted in the first few
OFDM symbols in every TTI. Then, it may be beneficial to demodulate
and decode the control channel prior to the reception of the
remaining OFDM symbols in the referenced TTI in order to reduce
latency. Moreover, in order to improve channel estimation
performance, it is desirable to capture as much of the transmitted
pilot signal power as possible without additional latency. Clearly,
the pilot signal power from preceding TTIs may be assumed available
to the UE but the UE will have to incur additional decoding latency
if it were to obtain the pilot signal power from succeeding TTIs.
However, this would be particularly desirable for channel
estimation performance as it would result to pilot signal
availability that is more symmetric relative to the TTI of
interest.
Based on the above discussion, the following disadvantages can be
directly identified for the pilot structures of prior art: 1) The
TDM pilot structure cannot provide reliable channel estimation and
communication support at high UE speeds. For example, for a UE
speed of 250 Kmph and carrier frequency of 2.6 GHz, the channel at
the fourth OFDM symbol in the TTI structure of FIG. 8 has very
little correlation with the channel at the first OFDM symbol of the
same TTI or the channel at the first OFDM symbol of the next TTI
where the pilot sub-carriers are located. The same problem would
persist if the pilot sub-carriers were placed at another OFDM
symbol in the TTI (not the first symbol). 2) The nature of the
scattered pilot structure results to very little pilot signal power
concentration per OFDM symbol. As a result, if the control channel
is transmitted in the first few OFDM symbols in the TTI, there may
not be enough pilot signal power to demodulate is prior to the
reception of the entire TTI. Moreover, if any substantial pilot
signal power from the succeeding TTI is to be obtained in order to
provide some symmetry in the channel estimated for the TTI of
interest, substantial latency will be incurred as the pilot
sub-carriers in several OFDM symbols of the succeeding TTI will
need to be captured.
There is a need for an improved pilot structure design in order to
achieve accurate channel estimates for high user equipment (UE)
speeds in mobile operations while also achieve the ability to use
substantial pilot energy from succeeding TTI with minimum
latency.
SUMMARY
In light of the foregoing background, embodiments of the invention
provide a method for generating a structure in an orthogonal
frequency division multiplexing communication system having a
transmitter with a least one transmitting antenna, said method
comprising; composing a frame with a time domain and a frequency
domain, wherein the frame has a transmission time interval in the
time domain with a beginning and an ending; and locating a pilot,
having pilot power level, from a first at least one antenna into
two orthogonal frequency division multiplexing symbols of said
frame.
Therefore, the system and method of embodiments of the present
invention solve the problems identified by prior techniques and
provide additional advantages.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus described the invention in general terms, reference
will now be made to the accompanying drawings, which are not
necessarily drawn to scale, and wherein:
FIG. 1 is a diagram illustrative of the Frequency-Time
Representation of an OFDM Signal;
FIG. 2 is a diagram illustrative of using cyclic prefix (CP) to
eliminate ISI and perform frequency domain equalization;
FIG. 3 is a diagram illustrative of Cyclic Prefix (CP)
Insertion
FIG. 4 shows OFDM Useful Symbol Generation Using an inverse fast
Fourier transform (IFFT);
FIG. 5 shows the concepts of frequency and multi-user
diversity;
FIG. 6 is a diagram illustrative of a configuration for Multi-User
Diversity;
FIG. 7, which is a diagram illustrative of a configuration for
frequency diversity;
FIG. 8 shows an exemplary Time-Division Multiplexed (TDM) pilot
structure;
FIG. 9 shows an exemplary scattered pilot structure;
FIG. 10 shows a staggered pilot structure in accordance with
embodiments of the invention using one transmit antenna;
FIG. 11 shows a staggered pilot structure in accordance with
embodiments of the invention using two transmit antennas; and
FIG. 12 shows a staggered pilot structure in accordance with
embodiments of the invention using four transmit antennas.
DETAILED DESCRIPTION
The invention now will be described more fully hereinafter with
reference to the accompanying drawings. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. In this disclosure, the term
pilot parameters can mean pilot power, number of pilot fields,
pilot position, power of each pilot field, etc. The term speed and
velocity may be used interchangeably. One skilled in the art may be
able to use the various embodiments of the invention to use both
the speed and direction of a mobile to adjust other parameters to
vary the power and direction of signal transmission.
A novel pilot structure circumventing the aforementioned
shortcomings is presented in embodiments of this invention. FIG. 10
shows a staggered pilot structure for the exemplary case of one
transmit antenna, but several obvious extensions are discussed for
more transmit antennas. Staggered pilot structure 1000 comprises a
TTI of seven OFDM symbols 1010. Shaded dots 1060 indicate pilot
locations for 1 transmit antenna. Non-shaded dots 1050 indicate
data sub-carrier. The exemplary embodiment shown in FIG. 1000 has a
spacing in frequency domain .DELTA.F.sub.pil 1030 of 4 sub-carriers
and spacing in time domain .DELTA.T.sub.pil 1020 of 4 OFDM
symbols.
The attributes of the staggered pilot signal structures disclosed
in the embodiments can be summarized as follows: i) The pilot
signal power is divided in the beginning and middle of the TTI. In
the exemplary embodiment, 50% of the pilot signal power from the
transmit antenna is placed at the first OFDM symbol 1001 and the
remaining 50% is placed at the fifth OFDM symbol 1005 of the seven
OFDM symbol exemplary TTI 1010. Placing the pilot signal power at
the second and the sixth OFDM symbols instead of the first and
fifth OFDM symbols is an alternative of dividing the pilot signal
power between the beginning and middle of the TTI. Asymmetric power
allocation may also be possible. ii) For the exemplary TTI
structure 1010 having 0.5 milliseconds duration, the staggered
pilot signal structure can maintain good channel estimation quality
even for very high speeds of interest as channel estimates can be
always obtained well before the 50% coherence time period of the
channel. iii) The pilot sub-carrier spacing in the frequency domain
can be easily designed to be smaller that the 50% correlation
coherence bandwidth for the longest channel among the channels of
interest. iv) Sufficient pilot signal energy exists in the first
OFDM symbol 1001 of the
TTI of interest 1010A and the preceding TTI 10108 to decode a
control channel that may be located in the first few OFDM symbols
with minimal performance degradation and without additional latency
from the absence of the pilot sub-carriers at the fifth OFDM symbol
of the TTI. v) Sufficient pilot signal energy exists in the first
OFDM symbol 1001 of the succeeding TTI 10108 to materially improve
channel estimation performance while resulting to minimal
additional decoding latency of one OFDM symbol and being applicable
to most of the current TTI 1010A even at high UE speeds.
The above and other properties of the staggered pilot signal design
can assist in the development of OFDM systems offering reliable and
robust communication from a Node B to the receiving UEs. Node B may
be a base station, access point or the like network entity.
FIG. 11 and FIG. 12 further expand the concept of materially
dividing the pilot signal power transmitted by an antenna to two
OFDM symbols per TTI for the cases of two transmit FIG. 11 and four
transmit antennas FIG. 12.
In FIG. 11, similarly to the one transmit antenna case of FIG. 10,
the pilot sub-carriers from the two antennas are placed on the
first OFDM symbol 1101 and fifth OFDM symbol 1105. Alternatively,
the pilot sub-carriers from the second antenna could be placed on
the second OFDM symbol 1102 and sixth OFDM symbol 1106.
In FIG. 12, similarly to that two transmit antennas case of FIG.
11, the pilot sub-carriers from the first two antennas are placed
on the first OFDM symbol 1201 and fifth OFDM symbol 1205 while the
pilot sub-carriers from the third and fourth antennas are placed on
the second OFDM symbol 1202 and sixth OFDM symbol 1206.
Embodiments of the invention can be implemented in either the
transmitter or the receiver, or in both, of a multi-carrier system,
such as an OFDM system, using software, hardware, or a combination
of software and hardware. The software is assumed to be embodied as
a lookup table, an algorithm, or other program code that defines
the pilot structure in a time transmission interval or frame.
An apparatus for an OFDM based communication system operating in
accordance with an OFDM transmission technique would be coupled to
a plurality of transmitting antennas and comprise a mapper for
converting an input signal to a plurality of data symbols,
transmitter circuitry adapted to insert pilot symbols with the data
symbols for each transmitting antenna, a modulator for modulating
said pilot symbols and data symbols in a transmission time interval
in accordance with an OFDM transmission technique. The transmission
time interval has multiple OFDM symbols. The power level of the
pilot symbols is divided into two OFDM symbols in the transmission
time interval. The input signal and plurality of data symbols are
comprised of sub-sets, each sub-set intended to a unique receiver
in the OFDM based communication system.
Embodiments of the invention may be utilized in a receiver in an
OFDM based communication system adapted to perform channel
estimation using a received reference signal transmitted from at
least one antenna, said reference signal being substantially
located into two OFDM symbols. The receiver may also be adapted to
use the reference signal located in the first OFDM symbol in
succeeding transmission time intervals in addition to the reference
symbols in the current and preceding transmission time
intervals.
Many modifications and other embodiments of the invention will come
to mind to one skilled in the art to which this invention pertains
having the benefit of the teachings presented in the foregoing
descriptions, the associated drawings, and claims. Therefore, it is
to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
* * * * *