Note: Descriptions are shown in the official language in which they were submitted.
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METHODS AND SYSTEMS FOR CHOOSING CYCLIC DELAYS IN
MULTIPLE ANTENNA OFDM SYSTEMS
CLAIM OF PRIORITY
[0001] This application claims benefit of priority from U.S.
Provisional Patent
Application Serial No. 61/036,895, entitled "Method and apparatus for
transmitting
pilots from multiple antennas" and filed March 14, 2008õ
TECHNICAL FIELD
[0002] Certain embodiments of the present disclosure generally
relate to a wireless
communication and, more particularly, to a method to choose appropriate values
of
cyclic delays for a multi-antenna transmission in order to accurately estimate
channel
gains.
SUMMARY
= [0003] Certain embodiments provide a method of
transmitting pilots in a wireless
communication system. The method generally includes generating a first pilot
for a first
transmit antenna based on a first cyclic delay, and generating a second pilot
for a second
transmit antenna based on a second cyclic delay larger than the first cyclic
delay by at
least a cyclic prefix length.
[0004] Certain embodiments provide a method of performing channel
estimation in
a wireless communication system. The method generally includes obtaining first
input
samples comprising first and second pilots, the first pilot being generated
based on a
first cyclic delay and sent from a first transmit antenna, the second pilot
being generated
based on a second cyclic delay and sent from a second transmit antenna, the
second
cyclic delay being larger than the first cyclic delay by at least a cyclic
prefix length, and
the first input samples being from a first receive antenna, and processing the
first input
samples to obtain a first channel estimate for the first transmit antenna and
a second
channel estimate for the second transmit antenna.
[0005] Certain embodiments provide an apparatus for transmitting
pilots in a
wireless communication system. The apparatus generally includes logic for
generating a
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first pilot for a first transmit antenna based on a first cyclic delay, and
logic for
generating a second pilot for a second transmit antenna based on a second
cyclic delay
larger than the first cyclic delay by at least a cyclic prefix length.
[0006] Certain embodiments provide an apparatus for performing channel
estimation in a wireless communication system. The apparatus generally
includes logic
for obtaining first input samples comprising first and second pilots, the
first pilot being
generated based on a first cyclic delay and sent from a first transmit
antenna, the second
pilot being generated based on a second cyclic delay and sent from a second
transmit
antenna, the second cyclic delay being larger than the first cyclic delay by
at least a
cyclic prefix length, and the first input samples being from a first receive
antenna, and
logic for processing the first input samples to obtain a first channel
estimate for the first
transmit antenna and a second channel estimate for the second transmit
antenna.
[0007] Certain embodiments provide an apparatus for transmitting pilots in
a
wireless communication system. The apparatus generally includes means for
generating
a first pilot for a first transmit antenna based on a first cyclic delay, and
means for
generating a second pilot for a second transmit antenna based on a second
cyclic delay
larger than the first cyclic delay by at least a cyclic prefix length.
[0008] Certain embodiments provide an apparatus for performing channel
estimation in a wireless communication system. The apparatus generally
includes means
for obtaining first input samples comprising first and second pilots, the
first pilot being
generated based on a first cyclic delay and sent from a first transmit
antenna, the second
pilot being generated based on a second cyclic delay and sent from a second
transmit
antenna, the second cyclic delay being larger than the first cyclic delay by
at least a
cyclic prefix length, and the first input samples being from a first receive
antenna, and
means for processing the first input samples to obtain a first channel
estimate for the
first transmit antenna and a second channel estimate for the second transmit
antenna.
[0009] Certain embodiments provide a computer-program product for
transmitting
pilots in a wireless communication system, comprising a computer readable
medium
having instructions stored thereon, the instructions being executable by one
or more
processors. The instructions generally include instructions for generating a
first pilot for
a first transmit antenna based on a first cyclic delay, and instructions for
generating a
second pilot for a second transmit antenna based on a second cyclic delay
larger than the
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first cyclic delay by at least a cyclic prefix length.
[0010] Certain embodiments provide a computer-program product for
performing
channel estimation in a wireless communication system, comprising a computer
readable
medium having instructions stored thereon, the instructions being executable
by one or more
processors. The instructions generally include instructions for obtaining
first input samples
comprising first and second pilots, the first pilot being generated based on a
first cyclic delay
and sent from a first transmit antenna, the second pilot being generated based
on a second
cyclic delay and sent from a second transmit antenna, the second cyclic delay
being larger
than the first cyclic delay by at least a cyclic prefix length, and the first
input samples being
from a first receive antenna, and instructions for processing the first input
samples to obtain a
first channel estimate for the first transmit antenna and a second channel
estimate for the
second transmit antenna.
[0010a] According to one aspect of the present invention, there is
provided a method of
transmitting pilots in a wireless communication system, comprising: generating
a first pilot
for a first transmit antenna based on a first cyclic delay; and generating a
second pilot for a
second transmit antenna based on a second cyclic delay larger than the first
cyclic delay by at
least a cyclic prefix length; wherein the generating the first pilot comprises
generating a first
OFDM symbol comprising the first pilot and having the first cyclic delay, and
wherein the
generating the second pilot comprises generating a second OFDM symbol
comprising the
second pilot and having the second cyclic delay; wherein the generating the
first OFDM
symbol comprises mapping pilot symbols to subcarriers spaced apart by p, where
p is a prime
number that does not divide Nõ,, and N FFT is an FFT size for the first OFDM
symbol;
wherein the generating the second OFDM symbol comprises mapping pilot symbols
to
subcarriers spaced apart by p.
[0010b] According to another aspect of the present invention, there is
provided a
method of performing channel estimation in a wireless communication system,
comprising:
obtaining first input samples comprising a first pilot sent from a first
transmit antenna and a
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second pilot sent from a second transmit antenna, the first input samples
being from a first
receive antenna; and processing the first input samples to obtain a first
channel estimate for
the first transmit antenna and a second channel estimate for the second
transmit antenna;
wherein the processing the first input samples comprises: processing the first
input samples to
obtain observations for pilot subcarriers; and processing the observations to
obtain the first
and second channel estimates; and wherein the pilot subcarriers are spaced
apart by p, where p
is a prime number that does not divide NF, and N FFT is an FFT size for an
OFDM symbol.
[0010c] According to still another aspect of the present invention,
there is provided an
apparatus for transmitting pilots in a wireless communication system,
comprising: logic for
generating a first pilot for a first transmit antenna based on a first cyclic
delay; and logic for
generating a second pilot for a second transmit antenna based on a second
cyclic delay larger
than the first cyclic delay by at least a cyclic prefix length; wherein the
logic for generating
the first pilot comprises logic for generating a first OFDM symbol comprising
the first pilot
and having the first cyclic delay, and wherein the logic for generating the
second pilot
comprises logic for generating a second OFDM symbol comprising the second
pilot and
having the second cyclic delay; wherein the logic for generating the first
OFDM symbol
comprises logic for mapping pilot symbols to subcarriers spaced apart by p,
where p is a
prime number that does not divide N FFT , and N FFT is an FFT size for the
first OFDM
symbol; and wherein the logic for generating the second OFDM symbol comprises
logic for
mapping pilot symbols to subcarriers spaced apart by p.
iootoqi According to yet another aspect of the present invention,
there is provided an
apparatus for performing channel estimation in a wireless communication
system, comprising:
logic for obtaining first input samples comprising a first pilot sent from a
first transmit
antenna and a second pilot sent from a second transmit antenna, the first
input samples being
from a first receive antenna; and logic for processing the first input samples
to obtain a first
channel estimate for the first transmit antenna and a second channel estimate
for the second
transmit antenna; wherein the logic for processing the first input samples
comprises: logic for
processing the first input samples to obtain observations for pilot
subcarriers; and logic for
processing the observations to obtain the first and second channel estimates;
wherein the pilot
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subcarriers are spaced apart by p, where p is a prime number that does not
divide N FFT , and
N FFT is an FFT size for an OFDM symbol.
[0010e] According to a further aspect of the present invention, there
is provided an
apparatus for transmitting pilots in a wireless communication system,
comprising: means for
generating a first pilot for a first transmit antenna based on a first cyclic
delay; and means for
generating a second pilot for a second transmit antenna based on a second
cyclic delay larger
than the first cyclic delay by at least a cyclic prefix length; wherein the
means for generating
the first pilot comprises means for generating a first OFDM symbol comprising
the first pilot
and having the first cyclic delay, and wherein the means for generating the
second pilot
comprises logic for generating a second OFDM symbol comprising the second
pilot and
having the second cyclic delay; wherein the means for generating the first
OFDM symbol
comprises means for mapping pilot symbols to subcarriers spaced apart by p,
where p is a
prime number that does not divide N õT , and N FFT is an FFT size for the
first OFDM
symbol; and wherein the means for generating the second OFDM symbol comprises
means
for mapping pilot symbols to subcarriers spaced apart by p.
[00101] According to yet a further aspect of the present invention,
there is provided an
apparatus for performing channel estimation in a wireless communication
system, comprising:
means for obtaining first input samples comprising a first pilot sent from a
first transmit
antenna and a second pilot sent from a second transmit antenna, the first
input samples being
from a first receive antenna; and means for processing the first input samples
to obtain a first
channel estimate for the first transmit antenna and a second channel estimate
for the second
transmit antenna; wherein the means for processing the first input samples
comprises: means
for processing the first input samples to obtain observations for pilot
subcarriers; and means
for processing the observations to obtain the first and second channel
estimates; wherein the
pilot subcarriers are spaced apart by p, where p is a prime number that does
not divide N hpy,
and Nn7 is an FFT size for an OFDM symbol.
[0010g] According to still a further aspect of the present invention,
there is provided a
computer-program product for transmitting pilots in a wireless communication
system,
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comprising a computer readable medium having instructions stored thereon, the
instructions
being executable by one or more processors and the instructions comprising:
instructions for
generating a first pilot for a first transmit antenna based on a first cyclic
delay; and
instructions for generating a second pilot for a second transmit antenna based
on a second
15 [001011] According to another aspect of the present invention,
there is provided a
computer-program product for performing channel estimation in a wireless
communication
system, comprising a computer readable medium having instructions stored
thereon, the
instructions being executable by one or more processors and the instructions
comprising:
instructions for obtaining first input samples comprising a first pilot sent
from a first transmit
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BRIEF DESCRIPTION OF THE DRAWINGS
[0011] So that the manner in which the above-recited features of the
present disclosure
can be understood in detail, a more particular description, briefly summarized
above, may be
had by reference to embodiments, some of which are illustrated in the appended
drawings. It
is to be noted, however, that the appended drawings illustrate only certain
typical
embodiments of this disclosure and are therefore not to be considered limiting
of its scope, for
the description may admit to other equally effective embodiments.
[0012] FIG. I illustrates an example wireless communication system,
in accordance
with certain embodiments of the present disclosure.
[0013] FIG. 2 illustrates an example Orthogonal Frequency Division
Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) frame
for
Time Division Duplex (TDD) in accordance with certain embodiments of the
present
disclosure.
[0014] FIG. 3 illustrates an example transmitter and an example
receiver that may be
used within a wireless communication system in accordance with certain
embodiments of the
present disclosure.
[0015] FIG. 4 illustrates a block diagram of a design of an OFDM
modulator in
accordance with certain embodiments of the present disclosure.
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[0016] FIG. 5 illustrates an example of cyclic delay diversity in
accordance with
certain embodiments of the present disclosure.
[0017] FIG. 6 illustrates an example pilot subcarrier structure for one
OFDM
symbol in accordance with certain embodiments of the present disclosure.
[0018] FIG. 7 illustrates a block diagram of a design of modulators at a
base station
in FIG. 3 in accordance with certain embodiments of the present disclosure.
[0019] FIG. 8 illustrates a process for generating pilots for multiple-
input single-
output (MISO) or multiple-input multiple-output (MIMO) systems in accordance
with
certain embodiments of the present disclosure.
[0020] FIG. 8A illustrates example components capable of performing the
operations illustrated in FIG. 8.
[0021] FIG. 9 illustrates a block diagram of a design of a channel
estimator in
accordance with certain embodiments of the present disclosure.
[0022] FIG. 10 illustrates a process for performing channel estimation in
MISO or
MIMO systems in accordance with certain embodiments of the present disclosure.
[0023] FIG. 10A illustrates example components capable of performing the
operations illustrated in FIG. 10.
DETAILED DESCRIPTION
[0024] The word "exemplary" is used herein to mean "serving as an example,
instance, or illustration." Any embodiment described herein as "exemplary" is
not
necessarily to be construed as preferred or advantageous over other
embodiments.
[0025] A Cyclic Delay Diversity (CDD) scheme can be applied for a multi-
antenna
Orthogonal Frequency Division Multiplexing (OFDM) transmission in order to
provide
higher frequency diversity and improve error rate performance. Multiple
artificial
channel paths can be generated by transmitting cyclically delayed data from a
plurality
of antennas. Estimation of channel gains associated with the plurality of
transmit
antennas can be performed at a receiver side using known pilot or training
sequences.
However, in certain cases, time domain channel paths cannot be fully separated
at the
receiver if cyclically delayed pilot sequences match path delays of a channel
profile.
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Exemplary Wireless Communication System
[0026] The techniques described herein may be used for various broadband
wireless
communication systems, including communication systems that are based on an
orthogonal multiplexing scheme. Examples of such communication systems include
Orthogonal Frequency Division Multiple Access (OFDMA) systems, Single-Carrier
Frequency Division Multiple Access (SC-FDMA) systems, and so forth. An OFDMA
system utilizes orthogonal frequency division multiplexing (OFDM), which is a
modulation technique that partitions the overall system bandwidth into
multiple
orthogonal sub-carriers. These sub-carriers may also be called tones, bins,
etc. With
OFDM, each sub-carrier may be independently modulated with data. A SC-FDMA
system may utilize interleaved FDMA (IFDMA) to transmit on sub-carriers that
are
distributed across the system bandwidth, localized FDMA (LFDMA) to transmit on
a
block of adjacent sub-carriers, or enhanced FDMA (EFDMA) to transmit on
multiple
blocks of adjacent sub-carriers. In general, modulation symbols are sent in
the
frequency domain with OFDM and in the time domain with SC-FDMA.
[0027] One specific example of a communication system based on an
orthogonal
multiplexing scheme is a WiMAX system. WiMAX, which stands for the Worldwide
Interoperability for Microwave Access, is a standards-based broadband wireless
technology that provides high-throughput broadband connections over long
distances.
There are two main applications of WiMAX today: fixed WiMAX and mobile WiMAX.
Fixed WiMAX applications are point-to-multipoint, enabling broadband access to
homes and businesses, for example. Mobile WiMAX offers the full mobility of
cellular
networks at broadband speeds.
[0028] IEEE 802.16 is an emerging standard organization to define an air
interface
for fixed and mobile broadband wireless access (BWA) systems. These standards
define at least four different physical layers (PHYs) and one medium access
control
(MAC) layer. The OFDM and OFDMA physical layer of the four physical layers are
the most popular in the fixed and mobile BWA areas respectively.
[0029] FIG. 1 illustrates an example of a wireless communication system 100
in
which embodiments of the present disclosure may be employed. The wireless
communication system 100 may be a broadband wireless communication system. The
wireless communication system 100 may provide communication for a number of
cells
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102, each of which is serviced by a base station 104. A base station 104 may
be a fixed
station that communicates with user terminals 106. The base station 104 may
alternatively be referred to as an access point, a Node B or some other
terminology.
[0030] FIG. 1 depicts various user terminals 106 dispersed throughout the
system
100. The user terminals 106 may be fixed (i.e., stationary) or mobile. The
user
terminals 106 may alternatively be referred to as remote stations, access
terminals,
terminals, subscriber units, mobile stations, stations, user equipment,
subscriber
stations, etc. The user terminals 106 may be wireless devices, such as
cellular phones,
personal digital assistants (PDAs), handheld devices, wireless modems, laptop
computers, personal computers, etc.
[0031] A variety of algorithms and methods may be used for transmissions in
the
wireless communication system 100 between the base stations 104 and the user
terminals 106. For example, signals may be sent and received between the base
stations
104 and the user terminals 106 in accordance with OFDM/OFDMA techniques. If
this
is the case, the wireless communication system 100 may be referred to as an
OFDM/OFDMA system.
[0032] A communication link that facilitates transmission from a base
station 104 to
a user terminal 106 may be referred to as a downlink (DL) 108, and a
communication
link that facilitates transmission from a user terminal 106 to a base station
104 may be
referred to as an uplink (UL) 110. Alternatively, a downlink 108 may be
referred to as a
forward link or a forward channel, and an uplink 110 may be referred to as a
reverse
liffl( or a reverse channel.
[0033] A cell 102 may be divided into multiple sectors 112. A sector 112 is
a
physical coverage area within a cell 102. Base stations 104 within a wireless
communication system 100 may utilize antennas that concentrate the flow of
power
within a particular sector 112 of the cell 102. Such antennas may be referred
to as
directional antennas.
[0034] FIG. 2 shows an example frame structure 200 for a time division
duplex
(TDD) mode in IEEE 802.16. The transmission timeline may be partitioned into
units
of frames. Each frame may span predetermined time duration, e.g., 5
milliseconds (ms),
and may be partitioned into a downlink subframe and an uplink subframe. In
general,
the downlink and uplink subframes may cover any fraction of a frame. The
downlink
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and uplink subframes may be separated by a transmit transmission gap (TTG) and
a
receive transmission gap (RTG).
[0035] A number of physical subchannels may be defined. Each physical
subchannel may include a set of subcarriers that may be contiguous or
distributed across
the system bandwidth. A number of logical subchannels may also be defined and
may
be mapped to the physical subchannels based on a known mapping. The logical
subchannels may simplify the allocation of resources.
[0036] As shown in FIG. 2, a downlink subframe may include a preamble, a
frame
control header (FCH), a downlink map (DL-MAP), an uplink map (UL-MAP), and
downlink (DL) bursts. The preamble may carry a known transmission that may be
used
by subscriber stations for frame detection and synchronization. The FCH may
carry
parameters used to receive the DL-MAP, the UL-MAP, and the downlink bursts.
The
DL-MAP may carry a DL-MAP message, which may include information elements
(IEs) for various types of control information (e.g., resource allocation or
assignment)
for downlink access. The UL-MAP may carry a UL-MAP message, which may include
IEs for various types of control information for uplink access. The downlink
bursts may
carry data for the subscriber stations being served. An uplink subframe may
include
uplink bursts, which may carry data transmitted by the subscriber stations
scheduled for
uplink transmission.
[0037] The pilot transmission techniques described herein may be used for
multiple-
input multiple-output (MIMO) transmission as well as multiple-input single-
output
transmission (MISO) transmission. The techniques may also be used for pilot
transmission on the downlink as well as the uplink. For clarity, certain
aspects of the
techniques are described below for pilot transmission on the downlink with
MIMO.
[0038] FIG. 3 shows a block diagram of a design of a base station 104 and a
subscriber station 106, which are one of the base stations and one of the
subscriber
stations in FIG. 1. Base station 104 is equipped with multiple (M) antennas
334a
through 334m. Subscriber station 106 is equipped with multiple (R) antennas
352a
through 352r.
[0039] At base station 104, a transmit (TX) data processor 320 may receive
data
from a data source 312, process (e.g., encode and symbol map) the data based
on one or
more modulation and coding schemes, and provide data symbols. As used herein,
a
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data symbol is a symbol for data, a pilot symbol is a symbol for pilot, and a
symbol may
be a real or complex value. The data and pilot symbols may be modulation
symbols
from a modulation scheme such as PSK or QAM. Pilots may comprise data that is
known a priori by both the base station and the subscriber station. A TX MIMO
processor 330 may process the data and pilot symbols and provide M output
symbol
streams to M modulators (MOD) 332a through 332m. Each modulator 332 may
process its output symbol stream (e.g., for OFDM) to obtain an output sample
stream.
Each modulator 332 may further condition (e.g., convert to analog, filter,
amplify, and
upconvert) its output sample stream and generate a downlink signal. M downlink
signals from modulators 332a through 332m may be transmitted via antennas 334a
through 334m, respectively.
[0040] At subscriber station 106, R antennas 352a through 352r may receive
the M
downlink signals from base station 104, and each antenna 352 may provide a
received
signal to an associated demodulator (DEMOD) 354. Each demodulator 354 may
condition (e.g., filter, amplify, downconvert, and digitize) its received
signal to obtain
input samples and may further process the input samples (e.g., for OFDM) to
obtain
received symbols. Each demodulator 354 may provide received data symbols to a
MIMO detector 360 and provide the received pilot symbols to a channel
processor 394.
Channel processor 394 may estimate the response of a MIMO channel from base
station
104 to subscriber station 120 based on the received pilot symbols and provide
a MIMO
channel estimate to MIMO detector 360. MIMO detector 360 may perform MIMO
detection on the received symbols based on the MIMO channel estimate and
provide
detected symbols, which are estimates of the transmitted data symbols. A
receive (RX)
data processor 370 may process (e.g., symbol de-mapping and decode) the
detected
symbols and provide decoded data to a data siffl( 372.
[0041] Subscriber station 106 may evaluate the channel conditions and
generate
feedback information, which may comprise various types of information. The
feedback
information and data from a data source 378 may be processed (e.g., encoded
and
symbol mapped) by a TX data processor 380, spatially processed by a TX MIMO
processor 382, and further processed by modulators 354a through 354r to
generate R
uplink signals, which may be transmitted via antennas 352a through 352r. At
base
station 104, the R uplink signals from subscriber station 106 may be received
by
antennas 334a through 334m, processed by demodulators 332a through 332m,
spatially
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processed by a MIMO detector 336, and further processed (e.g., symbol demapped
and
decoded) by an RX data processor 338 to recover the feedback information and
data
sent by subscriber station 106. Controller/processor 340 may control data
transmission
to subscriber station 106 based on the feedback information.
[0042] Controllers/processors 340 and 390 may direct the operation at base
station
104 and subscriber station 106, respectively. Memories 342 and 392 may store
data and
program codes for base station 104 and subscriber station 106, respectively. A
scheduler 344 may schedule subscriber station 106 and/or other subscriber
stations for
data transmission on the downlink and/or uplink based on the feedback
information
received from all subscriber stations.
[0043] IEEE 802.16 utilizes orthogonal frequency division multiplexing
(OFDM)
for the downlink and uplink. OFDM partitions the system bandwidth into
multiple
(Nõ,) orthogonal subcarriers, which may also be referred to as tones, bins,
etc. Each
subcarrier may be modulated with data or pilot. The number of subcarriers may
be
dependent on the system bandwidth as well as the frequency spacing between
adjacent
subcarriers. For example, NFFT may be equal to 128, 256, 512, 1024 or 2048.
Only a
subset of the Nõ, total subcarriers may be usable for transmission of data and
pilot, and
the remaining subcarriers may serve as guard subcarriers to allow the system
to meet
spectral mask requirements. In the following description, a data subcarrier is
a
subcarrier used for data, and a pilot subcarrier is a subcarrier used for
pilot. An OFDM
symbol may be transmitted in each OFDM symbol period (or simply, a symbol
period).
Each OFDM symbol may include data subcarriers used to send data, pilot
subcarriers
used to send pilot, and/or guard subcarriers not used for data or pilot.
[0044] FIG. 4 shows a block diagram of a design of an OFDM modulator 400,
which may be included in each of modulators 332a through 332m and modulators
354a
through 354r in FIG. 3. Within OFDM modulator 400, a symbol-to-subcarrier
mapper
410 receives and maps output symbols to the N FFT total subcarriers. In each
OFDM
symbol period, a unit 412 transforms N FFT output symbols for the N FFT total
subcarriers to the time domain with an N FFT -point inverse discrete Fourier
transform
(IDFT) and provides a useful portion containing N FFT time-domain samples.
Each
sample is a complex value to be transmitted in one chip period. A parallel-to-
serial
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(P/S) converter 414 serializes the N FFT samples in the useful portion. A
cyclic prefix
generator 416 copies the last Arcp samples of the useful portion and appends
these Arcp
samples to the front of the useful portion to form an OFDM symbol containing
N FFT N CP samples. Each OFDM symbol thus contains a useful portion of NFFT
samples and a cyclic prefix ofNcp samples. The cyclic prefix is used to combat
intersymbol interference (ISI) and inter-carrier interference (ICI) caused by
delay spread
in a wireless channel.
[0045] Referring back to FIG. 3, on the downlink, a MIMO channel is formed
by
the /11 transmit antennas at base station 104 and the R receive antennas at
subscriber
station 106. This MIMO channel is composed ofill =R single-input single-output
(SISO) channels or one SISO channel for each possible pair of transmit and
receive
antennas. The channel response for each SISO channel may be characterized by
either a
time-domain channel impulse response or a corresponding frequency-domain
channel
frequency response. The channel frequency response is the discrete Fourier
transform
(DFT) of the channel impulse response.
[0046] The channel impulse response for each SISO channel may be
characterized
by L time-domain channel taps, where L is typically much less than N FFT .
That is, if
an impulse is applied at a transmit antenna, then L time-domain samples at the
sample
rate taken at a receive antenna for this impulse stimulus would be sufficient
to
characterize the response of the SISO channel. The required number of channel
taps
(L) for the channel impulse response is dependent on the delay spread of the
system,
which is the time difference between the earliest and latest arriving signal
instances of
sufficient energy at the receive antenna.
[0047] Each SISO channel may include one or more propagation paths between
the
transmit antenna and the receive antenna for that SISO channel, with the
propagation
paths being determined by the wireless environment. Each path may be
associated with
a particular complex gain and a particular delay. For each SISO channel, the
complex
gains of the L channel taps are determined by complex gains of paths for that
SISO
channel. Each SISO channel thus has a channel profile with paths do through
di, 1,
where the complex gain of each path di may be a zero or non-zero value.
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[0048]
Cyclic delay diversity (CDD) may be used to create frequency diversity in a
MIMO transmission, which may improve error rate performance. With cyclic delay
diversity, the OFDM symbols for each transmit antenna may be cyclically
delayed by a
different amount, as described below. M different cyclically delayed signals
may be
transmitted from the M transmit antennas. However, cyclic delay diversity may
adversely impact MIMO channel estimation in some instances. In particular, it
may not
be possible to separate paths if a cyclically delayed signal matches a path
delay in the
channel profile. For example, for a given receive antenna, it may not be
possible to
determine whether a complex gain for a delay of two samples is from (i) a
downlink
signal from transmit antenna 0 with no cyclic delay and received via a path
with a delay
of two samples, or (ii) a downlink signal from transmit antenna 1 with a
cyclic delay of
one sample and received via a path with a delay of one sample, or (iii) a
downlink signal
from transmit antenna 2 with a cyclic delay of two samples and received via a
path with
no delay.
[0049] If
the channel profile has paths do through d, and if the M downlink
signals from the M transmit antennas have cyclic delays of to through tm 1,
then the L
channel taps for each SISO channel can be determined without ambiguity if
(di+c) mod Ts is distinct for all values of indices / and m, where / = 0,...,L
¨1,
m=0,...,M ¨1, Ts is the duration of the useful portion and is equal to N FFT
samples,
and "mod" denotes a modulo operation. This condition is applicable for full
frequency
reuse.
[0050] For
certain embodiments, the cyclic delay tm for each transmit antenna
(except for one transmit antenna with cyclic delay of zero) may be selected to
be equal
to or greater than the maximum expected delay spread in the system. The cyclic
prefix
length Arcp may be selected such that it is equal to or greater than the
maximum
expected delay spread in the system, so that L N cp . Thus, for certain
embodiments,
the cyclic delay for each transmit antenna may be selected to be as follows:
t. = EArc for m= 0,1.....M-1 (1)
i=0
where Arcs, 0 , and Arc,,Arcp Vi .
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[0051] FIG.
5 shows cyclic delay diversity for one exemplary case of equation (1)
when Arcs, = 0 and = Ncp
for i =1,...,M ¨1, with M = 4 transmit antennas.
Transmit antenna 0 has a cyclic delay of 0, and the useful portion is
cyclically
shifted/delayed by zero samples for this transmit antenna. Transmit antenna 1
has a
cyclic delay of kp , and the useful portion is cyclically shifted by N cp
samples for this
transmit antenna. Transmit antenna 2 has a cyclic delay of 2 .kp, and the
useful
portion is cyclically shifted by 2 N cp samples for this transmit antenna.
Transmit
antenna 3 has a cyclic delay of 3 N cp , and the useful portion is cyclically
shifted by
3 kir, samples for this transmit antenna.
[0052]
Following equation (1), the cyclic delays for the M transmit antennas may be
selected as:
tm+1 ¨ tm N cp for m = 0,...,M ¨ 2 , (2)
while tm N FFT N CP =
[0053] The
design in equation (2) ensures that di +t. is distinct for all values of /
and m. Channel estimation for all L paths from all M transmit antennas (which
is
referred to as complete channel estimation) may then be possible without
ambiguity. If
the cyclic delays for the M transmit antennas are standardized or known a
priori, then
there is no need to explicitly send signaling for the cyclic delays.
[0054] Base
station 104 may transmit pilot symbols from the M transmit antennas
in a manner to facilitate complete channel estimation by subscriber station
106. The
pilot symbols may be sent on S subcarriers ko through ks 1, where in general
S N FFT = The S pilot subcarriers may be determined as described below.
Ai -1
[0055] A set of Q = E Arc, coefficients may be defined as follows:
m=0
,,¨J2.7c.(di-Ftõ,)/ Ts (3)
ug
where / = 0, , ¨1, for m = 0,...,M ¨1, and N
C ,M CP ,
q = 1 = M +m = 0.....Q-1, and b, is the qth coefficient in the set. Since L N
cp , there
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may be fewer than Nc, channel taps. A thresholding may be used to zero out
channel
taps that are not present.
[0056] An S x Q matrix B may be defined for the S pilot subcarriers as
follows:
bo bl b2 = = = kiok
U0 U1b1k1 b2 k1 k
B /30 k2 k2 b2 k2 k ik2
5 (4)
=
=
=
=
k ss1 k 1 ks1 ks1
"0 "1 u2 uQ-1
where bi,, = bqk is an element in the ith row and qth column of matrix B, with
i = 0,...,S ¨1 and q = 0,...,Q ¨1.
[0057] A
sufficient condition for complete channel estimation is that the rank of
matrix B is equal to L = M. This leads to a necessary condition that b, be
distinct,
which means that di +t. should be distinct up to modulo Ts .
[0058] The
system may operate with full frequency reuse, and each cell may
transmit on all Nõ, total subcarriers (except for guard subcarriers). For full
frequency
reuse, pilot symbols may be sent on each subcarrier usable for transmission,
or
S = N FFT 5 and matrix B may be an S x S Vandermonde matrix V having the
following
form:
1 1 1 = = = 1
a0 al a2 = = = a S-1
2 22
V 2 = a
0 a 1 a 2
= = = a s-1 =
(5)
=
s-i
S-1 S-1
a0 al a 2 = = = a s-i _
[0059] For
full frequency reuse, the necessary condition of distinct b, is sufficient
to allow for complete channel estimation. Even if some subcarriers are
reserved for
guard but all other subcarriers are used and there are more than Q such
subcarriers, then
the matrix V will be full rank.
[0060] The
system may operate with partial frequency reuse, and each cell may
transmit on a subset of the N FFT total subcarriers. For example, with partial
frequency
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reuse factor of 3, each cell may transmit on approximately one third of the N
FFT total
subcarriers. For partial frequency reuse, pilot symbols may be sent on a
subset of the
N FFT total subcarriers, matrix B may be a submatrix of the Vandermonde
matrix, and
the necessary condition of distinct bq may not be sufficient. However, the S
pilot
subcarriers If() through ks 1 may be selected such that the necessary
condition becomes
sufficient for complete channel estimation.
[0061] For
certain embodiments, the Spilot subcarriers may be spaced apart by p
subcarriers, where p is a prime number that does not divide N FFT . The pilot
subcarriers
may be selected as follows:
k, =i = p, for i = 0,...,S ¨1, (6)
where k, is an index of the ith pilot subcarrier, S =LNFFT / 19] and "U
"denotes a floor
operator.
[0062] FIG. 6
shows an example pilot subcarrier structure for one OFDM symbol n
for the design shown in equation (6). In this example, p = 3 and the pilot
subcarriers
are spaced apart by three subcarriers. Pilot symbols may be sent on
subcarriers 0, 3, 6,
etc. The same set of pilot subcarriers may be used for each of the M transmit
antennas,
as shown in FIG. 6. The OFDM symbol with the pilot subcarriers may be for the
preamble shown in FIG. 2 or some other OFDM symbol.
[0063] For
the design shown in equation (6), matrix B is the same as the first Q
columns of an SxS Vandermonde matrix formed with elements a q = b qP
for q = 0,...,Q ¨1, and with elements for the Qth to Sth columns formed with
any
elements that are all different from each of the bqP elements. Complete
channel
estimation may then be possible with the following conditions:
p = (d,+t.) mod N FFT
1. should be distinct for all values of 1 and m , and
2. The number of rows 5 in matrix B should be equal to or greater than the
number of columns Q in matrix B, or
[0064] The
two conditions above may be satisfied if p is a prime number that does
not divide N, and N, /
regardless of the cyclic prefix length L. However,
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the maximum value of Ncp (Ncp,) may be limited by the total number of
subcarriers
(NFFT), the number of transmit antennas (M), and the pilot subcarrier spacing
(p), as
follows:
NFFT (7)
=
NCP,max
_P*111 _
[0065] For
example, N cp max = 170 for a case withM = 2 ,NFFT =1024 and p = 3.
A cyclic prefix length of 128 may be selected for this example. As another
example,
Ncp,maõ = 85 for a case with M = 2, NFFT = 1024 and p = 3. A cyclic prefix
length of
64 may be selected for this example. As yet another example, N cp = 102 for a
case
with M = 2, NFFT = 1024 and p = 5 for a lower reuse factor. A cyclic prefix
length of
64 may be selected for this example.
[0066] The
pilot subcarrier spacing may be selected based on cyclic delay lengths
applied on M transmit antennas and the total number of subcarriers N,, as
follows:
< N FFT
P ¨ m-1 = (8)
ENc,
m=0
[0067] FIG.
7 shows a block diagram of a design of modulators 332a through 332m
at base station 104 in FIG. 3. For simplicity, FIG. 7 shows only the
processing to
generate pilots for the M transmit antennas. Within modulator 332a for
transmit
antenna 0, a symbol-to-subcarrier mapper 710a maps pilot symbols to pilot
subcarriers
(e.g., determined as shown in equation (6)) and maps zero symbols to remaining
subcarriers. An IDFT unit 712a performs an NFFT -point IDFT on the NFFT pilot
and
zero symbols and provides NFFT time-domain samples. A P/S converter 714a
serializes
the NFFT samples. For certain embodiments, a cyclic delay unit 716a cyclically
shifts
the NFFT samples by Nc,0 samples for transmit antenna 0. A cyclic prefix
generator
718a appends a cyclic prefix and provides an OFDM symbol comprising a first
pilot for
transmit antenna 0.
[0068]
Modulator 332b may similarly generate an OFDM symbol comprising a
second pilot for transmit antenna 1. However, a cyclic delay unit 716b
cyclically shifts
the NFFT samples by Nc,0 + Nc,1 Ncp samples for transmit antenna 1. Each
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remaining modulator 332 may similarly generate an OFDM symbol comprising a
pilot
for its transmit antenna but may cyclically shift the NFFT samples by EN
samples
=0
for transmit antenna m , where m= 0,1,...,M ¨1.
[0069] FIG. 8 shows a design of a process 800 for generating pilots for
MISO or
MIMO systems. Process 800 may be performed by base station 104 for pilot
transmission on the downlink, by subscriber station 106 for pilot transmission
on the
uplink, or by some other entity.
[0070] At 810, a first pilot for a first transmit antenna may be generated
based on a
first cyclic delay, e.g., of zero samples. At 820, an mth pilot sequence may
be generated
for an mth transmit antenna based on an mth cyclic delay of a length that is
larger than
an (m-1)th cyclic delay length by at least the cyclic prefix length Ncp, where
m >1.
For certain embodiments, the cyclic delay for each transmit antenna is given
as shown
by equation (1), where Nc,0 = 0 and Nc,õ,, = m= Ncp5 VM=1,...,M ¨1. Additional
pilots for additional transmit antennas may be generated based on suitable
cyclic delays.
[0071] At 810, a first sample sequence comprising the first pilot may be
generated
and cyclically delayed by the first cyclic delay. A first OFDM symbol
comprising the
first pilot and having the first cyclic delay may be generated based on the
cyclically
delayed first sample sequence. At 820, the mth sample sequence comprising the
mth
pilot may be generated and cyclically delayed by the mth cyclic delay, where
m>1.
The mth OFDM symbol comprising the mth pilot and having the mth cyclic delay
may
be generated based on the cyclically delayed mth sample sequence, where m>1.
For
the first OFDM symbol, pilot symbols may be mapped to subcarriers spaced apart
by p,
where p may be a prime number that does not divide NFFT. For the mth OFDM
symbol, pilot symbols may be mapped to subcarriers spaced apart by p, where m
>1.
The same set of pilot subcarriers may be used for all OFDM symbols. The number
of
pilot subcarriers (S) may be equal to or greater than M = Ncp. The pilot
subcarrier
spacing (p) may be selected as shown in equation (8).
[0072] Subscriber station 106 may derive a channel estimate for each of the
M = R
SISO channels in the MIMO channel between base station 104 and subscriber
station
106. For each receive antenna, subscriber station 106 may obtain S received
pilot
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symbols from the S pilot subcarriers and may remove the pilot modulation to
obtain S
observations for the S pilot subcarriers. The S observations for each receive
antenna j
may be expressed as:
y j =Bh j +n, (9)
where y1 is an S x 1 vector of observations for the S pilot subcarriers on
receive
antenna j, B is an S x Q matrix defined in equation (4), h1 is a Q x 1 vector
of channel
gains for the M transmit antennas, and n is a S x 1 noise vector.
Af-1
[0073] Vector h1 includes Q=EN
elements h1,0 through h 1. The first
m=0
Nc,0 N CP elements h1,0 through hj,Nc,o_i are channel gains for transmit
antenna 0, the
next Nc,1 Ncp elements hi,Nc,0 through hj,Nc,o+Nc., 1 are channel gains for
transmit
antenna 1, and so on, and the last Ncx N p
elements h Ncx_t through hj,Q I are
channel gains for transmit antenna M ¨1. An estimate of h1 may be obtained
from y1
based on various techniques. In one design, an estimate of h1 may be obtained
from
y1 based such as a minimum mean square error (MMSE) technique, as follows:
h = D [ BH B +a2 j]1 BH y1, (10)
where D = diag { [ BH B + crn2 I II BH B}1, and h1 is an estimate of h j .
[0074] The
same processing may be performed for each receive antenna to obtain
M channel estimates for M SISO channels between the M transmit antennas and
that
receive antenna.
[0075] FIG.
9 shows a block diagram of a design of a channel estimator 900.
Within channel estimator 900, R units 910a through 910r obtain S received
pilot
symbols for the S pilot subcarriers from R receive antennas 0 through R-1,
respectively. Each unit 910 removes the pilot modulation on the S received
pilot
symbols from its receive antenna and provides S observations. The pilot
modulation
removal may be achieved by multiplying each received pilot symbol with a
complex
conjugate of the transmitted pilot symbol. R channel estimators 912a through
912r
receive the S observations from units 910a through 910r, respectively. Each
channel
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estimator 912 derives an estimate of h1 for its receive antenna j, e.g., as
shown in
equation (10), and provides h1. R demultiplexers (Demux) 914a through 914r
receive
from channel estimators 912a through 912r, respectively. Each demultiplexer
914
demultiplexes the channel gains in and
provides M channel estimates for the M
transmit antennas.
[0076] FIG.
10 shows a design of a process 1000 for performing channel estimation
for MISO or MIMO systems. Process 1000 may be performed by subscriber station
106
for downlink channel estimation, by base station 104 for uplink channel
estimation, or
by some other entity. At 1010, M cyclically delayed pilot sequences may be
transmitted from M transmit antennas, where the mth pilot sequence is
cyclically
delayed based on the mth cyclic delay (m =1,...,M ) of a length that is larger
than the
(m-1)th cyclic delay length by at least a cyclic prefix length N cp .
[0077] At
1020, received samples may be processed for all R receive antennas to
obtain estimates channel gains for M utilized transmit antennas. In general,
received
samples may be obtained from any number of receive antennas and processed to
obtain
channel estimates for any number of transmit antennas for each receive
antenna. At
1020, the received samples may be processed to obtain observations for pilot
subcarriers, e.g., by (i) performing OFDM demodulation on the received samples
to
obtain received pilot symbols for the pilot subcarriers and (ii) removing
pilot
modulation from the received pilot symbols to obtain the observations for the
pilot
subcarriers. The observations may be processed (e.g., based on the MMSE
technique as
shown in equation (10)) to obtain channel estimates for all utilized transmit
antennas.
[0078] The
various operations of methods described above may be performed by
various hardware and/or software component(s) and/or module(s) corresponding
to
means-plus-function blocks illustrated in the Figures. For example, blocks 810-
820
illustrated in FIG. 8 correspond to means-plus-function blocks 810A-820A
illustrated in
FIG. 8A. Similarly, blocks 1010-1020 illustrated in FIG. 10 correspond to
means-plus-
function blocks 1010A-1020A illustrated in FIG. 10A. More generally, where
there are
methods illustrated in Figures having corresponding counterpart means-plus-
function
Figures, the operation blocks correspond to means-plus-function blocks with
similar
numbering.
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[0079] The various illustrative logical blocks, modules and circuits
described in
connection with the present disclosure may be implemented or performed with a
general
purpose processor, a digital signal processor (DSP), an application specific
integrated
circuit (ASIC), a field programmable gate array signal (FPGA) or other
programmable
logic device (PLD), discrete gate or transistor logic, discrete hardware
components or
any combination thereof designed to perform the functions described herein. A
general
purpose processor may be a microprocessor, but in the alternative, the
processor may be
any commercially available processor, controller, microcontroller or state
machine. A
processor may also be implemented as a combination of computing devices, e.g.,
a
combination of a DSP and a microprocessor, a plurality of microprocessors, one
or
more microprocessors in conjunction with a DSP core, or any other such
configuration.
[0080] The steps of a method or algorithm described in connection with the
present
disclosure may be embodied directly in hardware, in a software module executed
by a
processor, or in a combination of the two. A software module may reside in any
form
of storage medium that is known in the art. Some examples of storage media
that may
be used include random access memory (RAM), read only memory (ROM), flash
memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk,
a CD-ROM and so forth. A software module may comprise a single instruction, or
many instructions, and may be distributed over several different code
segments, among
different programs, and across multiple storage media. A storage medium may be
coupled to a processor such that the processor can read information from, and
write
information to, the storage medium. In the alternative, the storage medium may
be
integral to the processor.
[0081] The methods disclosed herein comprise one or more steps or actions
for
achieving the described method. The method steps and/or actions may be
interchanged
with one another without departing from the scope of the claims. In other
words, unless
a specific order of steps or actions is specified, the order and/or use of
specific steps
and/or actions may be modified without departing from the scope of the claims.
[0082] The functions described may be implemented in hardware, software,
firmware or any combination thereof If implemented in software, the functions
may be
stored as one or more instructions on a computer-readable medium. A storage
media
may be any available media that can be accessed by a computer. By way of
example,
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and not limitation, such computer-readable media can comprise RAM, ROM,
EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other
magnetic storage devices, or any other medium that can be used to carry or
store desired
program code in the form of instructions or data structures and that can be
accessed by a
computer. Disk and disc, as used herein, include compact disc (CD), laser
disc, optical
disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks
usually
reproduce data magnetically, while discs reproduce data optically with lasers.
[0083] Software or instructions may also be transmitted over a transmission
medium. For example, if the software is transmitted from a website, server, or
other
remote source using a coaxial cable, fiber optic cable, twisted pair, digital
subscriber
line (DSL), or wireless technologies such as infrared, radio, and microwave,
then the
coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies
such as
infrared, radio, and microwave are included in the definition of transmission
medium.
[0084] Further, it should be appreciated that modules and/or other
appropriate
means for performing the methods and techniques described herein can be
downloaded
and/or otherwise obtained by a user terminal and/or base station as
applicable. For
example, such a device can be coupled to a server to facilitate the transfer
of means for
performing the methods described herein. Alternatively, various methods
described
herein can be provided via storage means (e.g., RAM, ROM, a physical storage
medium
such as a compact disc (CD) or floppy disk, etc.), such that a user terminal
and/or base
station can obtain the various methods upon coupling or providing the storage
means to
the device. Moreover, any other suitable technique for providing the methods
and
techniques described herein to a device can be utilized.
[0085] It is to be understood that the claims are not limited to the
precise
configuration and components illustrated above. Various modifications, changes
and
variations may be made in the arrangement, operation and details of the
methods and
apparatus described above without departing from the scope of the claims.
What is claimed is:
