U.S. patent application number 13/419024 was filed with the patent office on 2012-07-05 for multicarrier transceiver and method for precoding spatially multiplexed ofdm signals in a wireless access network.
Invention is credited to Lakshmipathi Sondur.
Application Number | 20120170672 13/419024 |
Document ID | / |
Family ID | 34217999 |
Filed Date | 2012-07-05 |
United States Patent
Application |
20120170672 |
Kind Code |
A1 |
Sondur; Lakshmipathi |
July 5, 2012 |
MULTICARRIER TRANSCEIVER AND METHOD FOR PRECODING SPATIALLY
MULTIPLEXED OFDM SIGNALS IN A WIRELESS ACCESS NETWORK
Abstract
A multicarrier transceiver for communicating spatially
multiplexed orthogonal frequency division multiplexed (OFDM)
signals over two or more spatial channels in a wireless access
network in accordance with an orthogonal frequency division
multiple access technique with a plurality of antennas is generally
disclosed herein. In some embodiments, the multicarrier transceiver
comprises a matrix generator to generate a matrix for precoding
transmission symbols based an indicator received from a receiving
station, the indicator being based at least in part on channel
characteristics. The multicarrier transceiver also includes a
precoder to multiply the transmission symbols by the matrix for
transmission on groups of subcarriers that comprise the OFDM
signals using the antennas for transmission through two or more
spatial channels, each of the two or more spatial channels to
concurrently convey separate spatially multiplexed data streams
over a same set of subcarriers.
Inventors: |
Sondur; Lakshmipathi;
(US) |
Family ID: |
34217999 |
Appl. No.: |
13/419024 |
Filed: |
March 13, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12245050 |
Oct 3, 2008 |
8165233 |
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13419024 |
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10654037 |
Sep 3, 2003 |
7453946 |
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12245050 |
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Current U.S.
Class: |
375/260 |
Current CPC
Class: |
H04B 7/0697 20130101;
H04B 7/086 20130101; H04B 7/0617 20130101; H04L 25/0242 20130101;
H04L 27/265 20130101; H04L 25/0226 20130101 |
Class at
Publication: |
375/260 |
International
Class: |
H04L 27/28 20060101
H04L027/28 |
Claims
1. A multicarrier transceiver for communicating spatially
multiplexed orthogonal frequency division multiplexed (OFDM)
signals over two or more spatial channels in a wireless access
network in accordance with an orthogonal frequency division
multiple access technique with a plurality of antennas, the
transceiver comprising: a matrix generator to generate a matrix for
precoding transmission symbols based an indicator received from a
receiving station, the indicator being based at least in part on
channel characteristics; and a precoder to multiply the
transmission symbols by the matrix for transmission on groups of
subcarriers that comprise the OFDM signals using the antennas for
transmission through two or more spatial channels, each of the two
or more spatial channels to concurrently convey separate spatially
multiplexed data streams over a same set of subcarriers.
2. The multicarrier transceiver of claim 1 further comprising RF
circuitry coupled to the plurality of antennas to generate RF
signals for transmission from baseband signals provided by the
precoder, the RF signals being precoded for receipt by the
receiving station.
3. The multicarrier transceiver of claim 2 wherein the indicator is
a precoding matrix indicator that is generated from the channel
characteristics by the receiving station.
4. The multicarrier transceiver of claim 3 wherein the indicator
corresponds to a codebook index of a precoding matrix for use in
beamforming
5. The multicarrier transceiver of claim 4 further comprising
circuitry to select a modulation level based on channel quality
information provided by the receiving station.
6. The multicarrier transceiver of claim 4 wherein the multicarrier
transceiver is configured to receive an indicator from the
receiving station to indicate a number of transmission layers for
transmission using the antennas.
7. The multicarrier transceiver of claim 6 wherein the precoding
matrix is applied to the OFDM signals at baseband prior to
upconversion and amplification by the RF circuitry and transmission
by to the plurality of antennas.
8. A method for communicating spatially multiplexed orthogonal
frequency division multiplexed (OFDM) signals over two or more
spatial channels in a wireless access network in accordance with an
orthogonal frequency division multiple access technique with a
plurality of antennas, the method comprising: generating a matrix
for precoding transmission symbols based an indicator received from
a receiving station, the indicator being based at least in part on
channel characteristics; and multiplying the transmission symbols
by the matrix for transmission on groups of subcarriers that
comprise the OFDM signals using the antennas for transmission
through two or more spatial channels, each of the two or more
spatial channels to concurrently convey separate spatially
multiplexed data streams over a same set of subcarriers.
9. The method of claim 8 further comprising RF circuitry coupled to
the plurality of antennas to generate RF signals for transmission
from baseband signals provided by the precoder, the RF signals
being precoded for receipt by the receiving station.
10. The method of claim 9 wherein the indicator is a precoding
matrix indicator that is generated from the channel characteristics
by the receiving station.
11. The method of claim 10 wherein the indicator corresponds to a
codebook index of a precoding matrix for use in beamforming
12. The method of claim 11 further comprising selecting a
modulation level based on channel quality information provided by
the receiving station.
13. The method of claim 11 further comprising receiving an
indicator that indicates a number of transmission layers for
transmission to the receiving station using the antennas.
14. A method for precoding beamforming in a wireless access network
that communicates using orthogonal frequency division multiplexed
(OFDM) communication signals over a plurality of spatial channels,
the method comprising: transmitting OFDM training symbols over the
spatial channels through a plurality of associated antennas;
receiving an indicator from a receiving station, the indicator
being based at least in part on channel characteristics of the
spatial channels measured by the receiving station based on the
training symbols; generating a matrix based on the indicator, the
matrix being associated with a codebook; applying the matrix at
baseband to the transmission symbols to precode the transmission
symbols for subsequent transmission to the receiving station
through two or more spatial channels.
15. A mobile device for receiving precoded orthogonal frequency
division multiplexed (OFDM) transmissions over a plurality of
spatial channels, the mobile device comprising: a receiver to
receive OFDM training symbols over the spatial channels through a
plurality of associated antennas; processing circuitry to generate
an indicator that is based at least in part on channel
characteristics of the spatial channels measured from the training
symbols, the indicator to indicate a matrix associated with a
codebook for precoding of transmission symbols; and a transmitter
to transmit the indicator to a transmitting station, wherein the
receiver is configured to receive transmission symbols from the
transmitting station that are precoded by application of the
matrix.
16. The mobile device of claim 15 wherein the indicator is a
precoding matrix indicator.
17. The mobile device of claim 16 wherein the transmission symbols
received from the transmitting station are beamformed signals.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/245,050, filed Oct. 3, 2008, which is a
continuation of U.S. patent application Ser. No. 10/654,037, filed
Sep. 3, 2003, issued as U.S. Pat. No. 7,453,946, all of which are
incorporated herein by reference in their entireties.
TECHNICAL FIELD
[0002] Embodiments of the present invention pertain to wireless
communications, and some embodiments pertain to systems using
symbol-modulated orthogonal subcarrier communications.
BACKGROUND
[0003] Orthogonal frequency division multiplexing is an example of
a multi-carrier transmission technique that uses symbol-modulated
orthogonal subcarriers to transmit information within an available
spectrum. When the subcarriers are orthogonal to one another, they
may be spaced much more closely together within the available
spectrum than, for example, the individual channels in a
conventional frequency division multiplexing (FDM) system. To
achieve orthogonality, a subcarrier may have a null at the center
frequency of the other subcarriers. Orthogonality of the
subcarriers may help reduce inter-subcarrier interference within
the system. Before transmission, the subcarriers may be modulated
with a low-rate data stream. The transmitted symbol rate of the
symbols may be low, and thus the transmitted signal may be highly
tolerant to multipath delay spread within the channel. For this
reason, many modern digital communication systems are using
symbol-modulated orthogonal subcarriers as a modulation scheme to
help signals survive in environments having multipath reflections
and/or strong interference.
[0004] Communication systems that use symbol-modulated orthogonal
subcarrier communications may have reduced channel capacity due to
multipath fading and other channel conditions. Thus, there are
general needs for apparatus and methods that increase channel
capacity, improve channel equalization and/or reduce the effects of
multipath fading, especially in systems using symbol-modulated
orthogonal subcarrier communications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The appended claims are directed to some of the various
embodiments of the present invention. However, the detailed
description presents a more complete understanding of embodiments
of the present invention when considered in connection with the
figures, wherein like reference numbers refer to similar items
throughout the figures and:
[0006] FIG. 1 illustrates a wireless communication environment in
which some embodiments of the present invention may be
practiced;
[0007] FIG. 2 is a block diagram of a communication node in
accordance with some embodiments of the present invention;
[0008] FIG. 3 illustrates a block diagram of a transceiver in
accordance with some embodiments of the present invention;
[0009] FIG. 4 illustrates a time-frequency structure of an
orthogonal frequency division multiplexed packet suitable for use
with some embodiments of the present invention; and
[0010] FIG. 5 is a flow chart of a communication procedure in
accordance with some embodiments of the present invention.
DETAILED DESCRIPTION
[0011] The following description and the drawings illustrate some
specific embodiments of the invention sufficiently to enable those
skilled in the art to practice them. Other embodiments may
incorporate structural, logical, electrical, process, and other
changes. Examples merely typify possible variations. Individual
components and functions are optional unless explicitly required,
and the sequence of operations may vary. Portions and features of
some embodiments may be included in or substituted for those of
others. The scope of embodiments of the invention encompasses the
full ambit of the claims and all available equivalents of those
claims.
[0012] FIG. 1 illustrates a wireless communication environment in
which some embodiments of the present invention may be practiced.
Communication environment 100 includes one or more wireless
communication devices (WCD) 102 which may communicate with access
point (AP) 104 over communication links 108, which may be
bi-directional links. WCDs 102 may include, for example, personal
digital assistants (PDAs), laptop and portable commuters with
wireless communication capability, web tablets, wireless
telephones, wireless headsets, pagers, instant messaging devices,
MP3 players, digital cameras, and other devices that may receive
and/or transmit information wirelessly. WCDs 102 may communicate
with AP 104 using a multi-carrier transmission technique, such as
an orthogonal frequency division multiplexing (OFDM) technique that
uses orthogonal subcarriers to transmit information within an
assigned spectrum. WCDs 102 and AP 104 may also implement one or
more communication standards, such as one of the IEEE 802.11a, b or
g standards, the Digital Video Broadcasting Terrestrial (DVB-T)
broadcasting standard, or the High performance radio Local Area
Network (HiperLAN) standard. Other local area network (LAN) and
wireless area network (WAN) communication techniques may also be
suitable for communication over links 108.
[0013] In addition to facilitating communications between WCDs 102,
in some embodiments, AP 104 may be coupled with one or more
networks 114, such as an intranet or the Internet, allowing WCDs
102 to access such networks. For convenience, the term "downstream"
is used herein to designate communications in the direction from AP
104 to WCDs 102 while the term "upstream" is used herein to
designate communications in the direction from WCDs 102 to AP 104,
however, the terms downstream and upstream may be interchanged.
WCDs 102 may support duplex communications utilizing different
spectrum for upstream and downstream communications, although this
is not a requirement. In some embodiments, upstream and downstream
communications may share the same spectrum for communicating in
both the upstream and downstream directions. Although FIG. 1
illustrates point-to-multipoint communications, embodiments of the
present invention are suitable to both point-to-multipoint and
point-to-point communications.
[0014] In some embodiments, a communication node (e.g., access
point 104) of a wireless local area network (WLAN) may utilize
multi-element array antenna 106 to estimate angle-of-arrival 110
(e.g., theta (.theta.)) for communication signals received over
links 108 from one or more signal sources (e.g., WCDs 102). Angle
110 may be measured relative to end-fire direction 116 of the
antenna 106, although the scope of the invention is not limited in
this respect. The signal sources may be wireless communication
devices which communicate on symbol-modulated orthogonal
subcarriers. Channel coefficients may be estimated from the
angle-of-arrival for the one or more signal sources to increase
channel capacity, improve channel equalization and/or reduce the
effects of multipath fading. In some embodiments, the channel
coefficients may be generated from one symbol modulated on a
plurality of subcarriers received by different elements of antenna
106. In some embodiments, AP 104 may provide communications within
a range of up to 500 feet, and even greater, for wireless
communication devices, although the scope of the invention is not
limited in this respect.
[0015] In some embodiments, beamforming coefficients may also be
generated from the angle-of-arrival for improved reception and/or
transmission of communication signals with the one or more signal
sources using multi-element array antenna 106. The beamforming
coefficients may be used to direct the reception and/or
transmission of signals in a direction of the particular signal
source. The angle-of-arrival may be estimated by sampling the
response from the antenna elements of the array for at least one
symbol at the subcarrier frequencies, although the scope of the
invention is not limited in this respect. The sampled symbol may be
a training symbol having a known value. The sampling may be
performed on the same symbol at all subcarrier frequencies after
demodulation by a fast Fourier transform (FFT) although the scope
of the invention is not limited in this respect. With beamforming,
frequency reuse may be realized using space-division multiple
access techniques.
[0016] Multi-element array antenna 106 may be a phased-array
antenna comprising at least two directional or omnidirectional
antenna elements 112. Elements 112 may comprise dipole antennas,
monopole antennas, loop antennas, microstrip antennas or other type
of antenna suitable for reception and/or transmission of RF signals
which may be processed by AP 104. In some embodiments, a beamformer
may be used to control phasing between elements 112 to provide
directional communications with WCDs 102. In some embodiments, the
phasing may be controlled at baseband, although the scope of the
invention is not limited in this respect.
[0017] FIG. 2 is a block diagram of a communication node in
accordance with some embodiments of the present invention.
Communication node 200 may be suitable for use as AP 104 (FIG. 1),
although other communication nodes may also be suitable. In some
embodiments, communication node 200 may also be suitable for use as
one or more of WCDs 102 (FIG. 1), although the scope of the
invention is not limited in this respect.
[0018] Communication node 200 receives and/or transmits radio
frequency (RF) communications with multi-element array antenna 202.
RF signals received from antenna 202 may be converted to baseband
signals and eventually to data signals comprising a bit stream by
transceiver 204. Transceiver 204 may also convert data signals
comprising a bit stream to baseband signals and RF signals for
transmission by antenna 202. Communication node 200 may also
include signal separator 206 to separate received and transmitted
communication signals. Communication node 200 may also include data
processing portion 208 to process data signals received through
transceiver 204 and generate data signals for transmission by
transceiver 204. Antenna 202 may comprise a plurality of antenna
elements 212, which may correspond to antenna elements 112 (FIG.
1). Although signal separator 206 is illustrated as a separate
element of node 200, the present invention is not limited in this
respect. In some embodiments, signal separator 206 may be part of
antenna 202, while in other embodiments, antenna 202 may comprise
one set of antenna elements for transmission of signals, and
another set of antenna elements for reception of signals
eliminating the need for signal separator 206.
[0019] In some embodiments, communication node 200 may include
interfaces 210 to wireline devices and wireline networks, such as
to a personal computer, a server, or the Internet, for example. In
these embodiments, communication node 200 may facilitate
communications between WCDs 102 (FIG. 1) and these wireline devices
and/or networks.
[0020] FIG. 3 illustrates a block diagram of a transceiver in
accordance with some embodiments of the present invention.
Transceiver 300 may be suitable for use as transceiver 204 (FIG. 2)
although other transceiver configurations may also be suitable.
Transceiver 300 may include RF circuitry 302 to receive a signal
from a signal source through a multi-element antenna having a
plurality of antenna elements. The signal may comprise a plurality
of subcarriers modulated with at least one symbol. Transceiver 300
may also include angle-of-arrival (AOA) estimator 304 to estimate
an angle-of-arrival for a signal source from a subcarrier level of
the symbol received by at least two of the antenna elements.
Transceiver 300 may also include channel coefficient generator 306
to generate channel coefficients for communications received from
the signal source based on the angle-of-arrival. The channel
coefficients may compensate for at least some of the channel
effects between a signal source and the access point. Transceiver
300 may also include channel equalizer 308 which may be responsive
to the channel coefficients to provide equalized frequency-domain
symbol-modulated subcarriers 310 resulting in improved
reception.
[0021] In some embodiments, transceiver 300 may further include
beamformer coefficient generator 312 to generate beamforming
coefficients for elements of the multi-element antenna based on the
angle-of-arrival. The beamforming coefficients may be used to help
direct the reception and/or transmission of signals in a direction
of a particular signal source. In these embodiments, transceiver
300 may further include beamformer 314. Beamformer 314 may change
the directionality of the antenna based on the beamforming
coefficients, and in some embodiments, beamformer 314 may change
phasing of received and/or transmitted signals. In some
embodiments, beamforming may be done prior to conversion to
corresponding RF signals by RF circuitry 302 and transmission of
the signals by the elements of the multi-element antenna. In some
embodiments, beamformer 314 may change the directionality of the
antenna by changing the phasing of baseband-level signals that
comprise a plurality of symbol-modulated subcarriers for use in
generating and/or receiving an orthogonal-frequency division
multiplexed signal by RF circuitry 302 for transmission and/or
reception by a multi-element antenna. With beamforming, frequency
reuse may be realized using space-division multiple access
techniques.
[0022] In some embodiments, angle-of-arrival estimator 304 may
include one or more processors and memory to generate an initial
matrix (e.g., X) comprising demodulated pilot subcarriers for a
symbol provided by FFT 328 corresponding to each of the antenna
elements. The processor and memory may also generate a response
matrix (e.g., A) substantially from the equation X=AD+N. In the
equation, `D` may represent a diagonal matrix having elements
corresponding to the pilot subcarriers of the symbol, and `N` may
represent an uncorrelated noise matrix. The processor and memory
may use a search function to identify a peak corresponding to the
angle-of-arrival. The search function may be based on a
decomposition of the response matrix. This is described in more
detail below.
[0023] FIG. 4 illustrates a time-frequency structure of an
orthogonal frequency division multiplexed packet suitable for use
with some embodiments of the present invention. Time-frequency
structure 400 is an example of a packet in accordance with the IEEE
802.11(a) standard; however, other time-frequency structures for
packets may be equally suitable for use with some embodiments of
the present invention. As illustrated in structure 400, symbols
having known training values are crosshatched/shaded. Structure 400
illustrates a packet starting with ten short training symbols 402
modulated on twelve subcarriers 404. These symbols may contain
known pilot subcarriers. Short training symbols 402 are followed by
two long training symbols 406 which are followed by data symbols
408. Data symbols 408 may include four pilot subcarriers 410.
[0024] In some embodiments, angle-of-arrival estimator 304 (FIG. 3)
may estimate the angle-of-arrival based the antenna response for
subcarriers 404 for one of training symbols 402 or based on one of
training symbols 406, although the scope of the invention is not
limited in this respect. A training symbol may have known training
values. In some embodiments, channel equalizer 308 (FIG. 3) may
provide equalized frequency-domain symbol-modulated subcarriers for
subsequent data symbols (e.g., symbols 408) of a data packet
received from the signal source.
[0025] Referring back to FIG. 3, in some embodiments, RF receive
circuitry 302 receives signals through a multi-element antenna, and
generates serial symbol stream 320 representing symbols. In some
embodiments, a packet may include short training symbols 402 (FIG.
4) and long training symbols 406 (FIG. 4) followed by data symbols
408 (FIG. 4). In some embodiments, the received signal may have a
carrier frequency ranging between 5 and 6 GHz, although embodiments
of the present invention are equally suitable to carrier
frequencies, for example, ranging between 2 and 20 GHz, and even
greater. In some embodiments, a symbol-modulated signal may include
up to a hundred or more subcarriers. The short training symbols may
be transmitted on a portion of the subcarriers, and data symbols
may contain one or more known pilot subcarriers although this is
not a requirement. In some embodiments, the long training symbols
may have a duration of approximately 4 microseconds and the short
training symbols may have a duration of approximately one
microsecond. In some embodiments, the signals may be infrared (IR)
signals.
[0026] The receiver portion of transceiver 300 may include serial
to parallel (S/P) converter 322 to convert a symbol of serial
symbol stream 320 into parallel groups of time-domain samples 324.
Cyclic-redundancy prefix (C/P) element 326 removes a
cyclic-redundancy prefix from each symbol. Fast Fourier Transform
(FFT) element 328 performs an FFT on parallel groups of time-domain
samples 330 to generate frequency-domain symbol-modulated
subcarriers 332 for use by equalizer 308 and angle-of-arrival
estimator 304.
[0027] Angle-of-arrival estimator 304 may generate an
angle-of-arrival estimate for a signal source which may be used by
channel coefficient generator 306 for generating channel
coefficients for use by equalizer 308 for improved demodulation of
the subcarriers. In some embodiments, a channel estimator (not
illustrated) may be used, in addition to generator 306, to generate
channel estimates for use by equalizer 308.
[0028] Equalizer 308 may perform a channel equalization on
frequency-domain symbol-modulated subcarriers 332 provided by FFT
element 328. Equalizer 308 may generate equalized frequency-domain
symbol-modulated subcarriers 310 using the channel coefficients
provided by channel coefficient generator 306. For example,
equalization in the frequency domain may be performed by division
of the frequency domain subcarriers 332 with complex values that
represent the channel estimation. Accordingly, the magnitudes of
equalized frequency-domain symbol-modulated subcarriers 332 may be
normalized and the phases of equalized frequency-domain
symbol-modulated subcarriers 310 may be aligned to a zero origin to
allow for further processing by demapper 334.
[0029] Equalized frequency-domain symbol-modulated subcarriers 310
may be demapped by demapper 334 to produce a plurality of parallel
symbols. Demapper 334 may demap the parallel symbols in accordance
with a particular modulation order in which the transmitter
modulated the subcarriers. Modulation orders, for example, may
include binary phase shift keying (BPSK), which communicates one
bit per symbol, quadrature phase shift keying (QPSK), which
communicates two bits per symbol, 8-PSK, which communicates three
bits per symbol, 16-quadrature amplitude modulation (16-QAM), which
communicates four bits per symbol, 32-QAM, which communicates five
bits per symbol, and 64-QAM, which communicates six bits per
symbol. Modulation orders may also include differentially-coded
star QAM (DSQAM). Modulation orders with lower and even higher
communication rates may also be used. The parallel symbols from
demapper 334 may be converted from a parallel form to a serial
stream by parser 336, which may perform a de-interleaving operation
on the serial stream. Parser 336 generates decoded serial bit
stream 338 for use by data processing elements (not
illustrated).
[0030] The transmitter portion of transceiver 300 may include
parser 342 to encode serial bit-stream 340 to generate parallel
symbols. Mapper 344 maps the parallel symbols to frequency-domain
symbol-modulated subcarriers 346. IFFT element 348 performs an
inverse fast Fourier transform (IFFT) on frequency-domain
symbol-modulated subcarriers 346 to generate parallel groups of
time-domain samples 350. CP circuit 352 adds a cyclic-redundancy
prefix to each symbol, and parallel-to-serial (P/S) circuit 354
converts the parallel groups of time-domain samples 356 to serial
symbol stream 358 for RF circuitry 302. In accordance with
embodiments, the length of the cyclic-redundancy prefix is greater
than the length of intersymbol interference.
[0031] Although communication node 200 (FIG. 2) and transceiver 300
are illustrated as having several separate functional circuit
elements, one or more of the functional elements may be combined
and may be implemented by combinations of software-configured
elements, such as processing elements including digital signal
processors (DSPs), and/or other hardware elements and software. For
example, circuit elements may comprise one or more processing
elements such as microprocessors, DSPs, application specific
integrated circuits (ASICs), and combinations of various hardware
and logic circuitry for performing at least the functions described
herein.
[0032] FIG. 5 is a flow chart of a communication procedure in
accordance with some embodiments of the present invention.
Communication procedure 500 may be performed by a communication
node, such as AP 104 (FIG. 1) although other communication nodes
may also be suitable for performing procedure 500. In some
embodiments, communication procedure 500 may be performed by
communication devices, such as WCDs 102 (FIG. 1). Although the
individual operations of procedure 500 are illustrated and
described as separate operations, one or more of the individual
operations may be performed concurrently and nothing requires that
the operations be performed in the order illustrated.
[0033] In operation 502, a signal comprising at least one symbol of
a data packet comprising symbol-modulated subcarriers is received
through a multi-element antenna from a signal source. In operation
504, an FFT may be performed on parallel groups of time-domain
samples that represent the symbol as received by the elements of
the multi-element antenna. The FFT may generate frequency-domain
symbol-modulated subcarriers for each antenna element. The symbol
may be a training symbol having known training values. In operation
506, an angle-of-arrival estimate is generated for the signal
source. The angle-of-arrival may be relative to an end-fire
direction of the multi-element antenna. The angle-of-arrival may be
estimated based on the antenna response for the antenna elements
for more than one subcarrier frequency of the symbol, although the
scope of the invention is not limited in this respect. In operation
508, channel coefficients may be generated from the
angle-of-arrival estimate, and in operation 510, the channel
coefficients may be used for equalization of symbols, including
data symbols, subsequently received from the signal source. In
operation 512, beamforming coefficients may be generated based on
the angle-of-arrival, and in operation 514, a communication signal
comprising symbol-modulated subcarriers may be directionally
transmitted to the signal source (e.g., in a direction of the
signal source) using the beamforming coefficients. In some
embodiments, a communication signal comprising symbol-modulated
subcarriers may be directionally received from the signal source
using the beamforming coefficients.
[0034] In some embodiments, the operations of procedure 500 may be
repeated or performed concurrently for one or more of a plurality
of signal sources. In these embodiments, angles-of-arrival may be
individually estimated for the different signal sources, and
channel and beamforming coefficients may be generated for the
different signal sources and used for communicating with the signal
sources. Accordingly, increased channel capacity, improved channel
equalization and/or reduced the effects of multipath fading may be
achieved, although the scope of the invention is not limited in
this respect.
[0035] In some embodiments of the present invention, an
angle-of-arrival may be estimated by angle-of-arrival estimator 304
(FIG. 2) and channel coefficients may be generated by channel
coefficient generator 306 (FIG. 3) as illustrated in the following
example. Consider an N-element adaptive antenna receiving J--user
signals having J distinct directions .theta..sub.1, . . .
.theta..sub.J, where the angles .theta..sub.j are measured with
respect to end-fire direction 116 (FIG. 1). In this example, let Q
be number of subcarriers used to carry known pilot subsymbols
transmission. The remaining (K-Q) subcarriers may be used for
information bearing subsymbols. In this example, consider N>Q.
For the sake of generality, a single sample case is illustrated
which may be further extended for multiple samples in which an
average estimate may be obtained. In the single-sample case, the
signals may be collected after demodulation by an FFT in the form
of matrix, which may be described by the following equations.
X=AD+N (1)
[0036] In equation (1), X is a matrix in which the i.sup.th column
may correspond to the antenna-array response to the i.sup.th
subcarrier. D is a diagonal matrix whose elements may correspond to
the known pilot symbols scaled by channel coefficients along with
the phase shift. A is an array-response matrix for the subcarrier
frequencies. m corresponds to the m th symbol. N may be a spatially
and temporally uncorrelated noise matrix.
X = [ x 1 ( m , 0 ) x 1 ( m , 1 ) x 1 ( m , Q - 1 ) x 2 ( m , 0 ) x
2 ( m , 1 ) x 2 ( m , Q - 1 ) x N ( m , 0 ) x N ( m , 1 ) x N ( m ,
Q - 1 ) ] ( 2 ) A = [ 1 1 1 k 1 d cos .theta. k 2 d cos .theta. k Q
d cos .theta. k 1 d ( N - 1 ) cos .theta. k 2 d ( N - 1 ) cos
.theta. k Q d ( N - 1 ) cos .theta. ] ( 3 ) ##EQU00001##
D=diag(p(m, 0), p(m, 1), . . . , p(m, Q-1)) (4)
p(m, 0)=s(m, 0)h.sub.0e.sup.-i.delta..sup.1, . . . , p(m, Q-1)=s(m,
Q-1)h.sub.Q-1e.sup.-.delta..sup.Q-1 (5)
N = [ n 11 ( m , 0 ) n 12 ( m , 0 ) n 1 Q ( m , 0 ) n 21 ( m , 0 )
n 22 ( m , 0 ) n 2 Q ( m , 0 ) n N 1 ( m , 0 ) n N 2 ( m , 0 ) n NQ
( m , 0 ) ] ( 6 ) ##EQU00002##
[0037] In some embodiments, equation (1) may be multiplied by unit
vectors, e.g., e=[1 . . . 1].sup.T and shown as Xe=ADe+Ne which
reduces to:
x=Ap (7)
[0038] where x=x.sub.1+x.sub.2+ . . . +x.sub.Q-1 and
p=p.sub.1+p.sub.2+ . . . +p.sub.Q-1
x .di-elect cons. span{A} (8)
[0039] A matrix B may be formed.
B=[A(.theta.)x] where .theta. .di-elect cons.(0, 2.pi.). (9)
[0040] The size of matrix B may be N(Q+1), where
N.gtoreq.(Q+1).
[0041] Matrix B may become rank deficient (e.g., undetermined) when
.theta.=.theta..sub.true. The .theta..sub.true estimate can be
found from the following QR-decomposition and search function.
B(.theta.)=Q(.theta.)R(.theta.), (10)
[0042] with search function as
G ( .theta. ) - max [ 1 r ( Q ) ( Q ) ( .theta. ) ] . ( 11 )
##EQU00003##
[0043] r.sub.Q,Q(.theta.) is the Q-th diagonal element of the upper
triangular matrix R(.theta.). The search function G(.theta.) may
have J-highest peaks that may correspond to the angle-of-arrival
estimates, {circumflex over (.theta.)}.sub.1, . . . {circumflex
over (.theta.)}.sub.J of J-sources. Note that the estimates
({circumflex over (.theta.)}.sub.1, . . . {circumflex over
(.theta.)}.sub.J) may be obtained from processing of four pilot
subcarriers that are spaced quite apart in the frequency spectrum.
Such a property may provide a good approximation of the
angle-of-arrival corresponding to the complete set of subcarriers
in the OFDM. In other words the estimates may correspond to
angle-of-arrival of broadband signal sources.
[0044] Having obtained {circumflex over (.theta.)} .di-elect
cons.({circumflex over (.theta.)}.sub.1, . . . {circumflex over
(.theta.)}.sub.J), {circumflex over (.theta.)} may be substituted
in the matrix A in equation (7) as
{circumflex over (x)}=A({circumflex over (.theta.)})p. (12)
[0045] Thus, in equation (12) p remains unknown, and may be
obtained as follows:
p=[A({circumflex over (.theta.)})A({circumflex over
(.theta.)})].sup.-1A.sup.T({circumflex over (.theta.)})x (13)
[0046] The k th element of p is p(m, k)=s(m,
k)h.sub.ke.sup.-1.delta..sup.k. Since s(m, k) is known, the channel
estimation may be obtained as
h k - .delta. k = p ( m , k ) s ( m , k ) ( 14 ) ##EQU00004##
[0047] where s(m, k), k=1, . . . , Q , are known pilot symbols.
[0048] In the second stage, the next set of Q-subcarriers may be
chosen to obtain the channel coefficients as,
p=[A({circumflex over (.theta.)})A({circumflex over
(.theta.)})].sup.-1A.sup.T({circumflex over (.theta.)})x, (13)
[0049] where A({circumflex over (.theta.)}) corresponds to the
columns as a function of next subset of subcarriers. This channel
estimation process may be repeated for this next subset of
subcarriers.
[0050] In some embodiments, the estimation of angular information
{circumflex over (.theta.)} of a signal source may be performed for
only one sample. The angular estimate {circumflex over (.theta.)}
may be repeatedly used for each subset of subcarrier matrices and
accordingly the channel estimates for entire subcarrier channels
may be found. For increased reliability, in some embodiments, the
angular estimation may be performed for each sample and the average
estimate can be obtained as follows:
.theta..sub.estimate=E[{circumflex over (.theta.)}]. (15)
[0051] In some embodiments, beamforming in the direction
.theta..sub.estimate is performed to increase the channel capacity.
With beamforming, frequency reuse may be realized using
space-division multiple access techniques. The beamforming may be
used for both reception and transmission.
[0052] It is emphasized that the Abstract is provided to comply
with 37 C.F.R. Section 1.72(b) requiring an abstract that will
allow the reader to ascertain the nature and gist of the technical
disclosure. It is submitted with the understanding that it will not
be used to limit or interpret the scope or meaning of the
claims.
[0053] In the foregoing detailed description, various features are
occasionally grouped together in a single embodiment for the
purpose of streamlining the disclosure. This method of disclosure
is not to be interpreted as reflecting an intention that the
claimed embodiments of the subject matter require more features
that are expressly recited in each claim. Rather, as the following
claims reflect, inventive subject matter lies in less than all
features of a single disclosed embodiment. Thus the following
claims are hereby incorporated into the detailed description, with
each claim standing on its own as a separate preferred
embodiment.
* * * * *