U.S. patent application number 12/916400 was filed with the patent office on 2011-02-24 for methods and apparatuses for reducing inter-carrier interference in an ofdm system.
This patent application is currently assigned to Qualcomm Incorporated. Invention is credited to Raghuraman Krishnamoorthi, Fuyun Ling, Krishna Kiran Mukkavilli, Tao TIAN.
Application Number | 20110044411 12/916400 |
Document ID | / |
Family ID | 39583993 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044411 |
Kind Code |
A1 |
TIAN; Tao ; et al. |
February 24, 2011 |
METHODS AND APPARATUSES FOR REDUCING INTER-CARRIER INTERFERENCE IN
AN OFDM SYSTEM
Abstract
An OFDM telecommunications system includes a transmitter and a
receiver. The receiver includes a canceller configured to reduce
inter-carrier interference (ICI) in an OFDM symbol in the frequency
domain.
Inventors: |
TIAN; Tao; (San Diego,
CA) ; Krishnamoorthi; Raghuraman; (San Diego, CA)
; Mukkavilli; Krishna Kiran; (San Diego, CA) ;
Ling; Fuyun; (San Diego, CA) |
Correspondence
Address: |
McDermott Will & Emery LLP
600 13th Street, NW
Washington
DC
20005-3096
US
|
Assignee: |
Qualcomm Incorporated
San Diego
CA
|
Family ID: |
39583993 |
Appl. No.: |
12/916400 |
Filed: |
October 29, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11674632 |
Feb 13, 2007 |
7844018 |
|
|
12916400 |
|
|
|
|
60883137 |
Jan 2, 2007 |
|
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|
Current U.S.
Class: |
375/346 |
Current CPC
Class: |
H04L 27/2657
20130101 |
Class at
Publication: |
375/346 |
International
Class: |
H04B 1/10 20060101
H04B001/10 |
Claims
1. A receiver, comprising: a canceller configured to reduce
inter-carrier interference (ICI) in an OFDM symbol in the frequency
domain.
2. The receiver of claim 1 wherein the OFDM symbol comprises a
plurality of sub-carriers, and wherein the canceller is further
configured to reduce ICI by performing an iterative process on one
or more of the sub-carriers.
3. The receiver of claim 2 wherein the sub-carriers are distributed
over a spectrum having a mid-band and two edge-bands, and wherein
the one or more of the sub-carriers are limited to the sub-carriers
in the edge-bands.
4. The receiver of claim 3 wherein the one or more of the sub-bands
include all of the sub-carriers carrying non-zero symbols in the
edge-bands.
5. The receiver of claim 4 wherein the canceller is further
configured to determine the number of the sub-carriers in each of
the edge-bands.
6. The receiver of claim 2 wherein the canceller is further
configured to determine the number of iterations to be performed on
each of the one or more of the sub-carriers.
7. The receiver of claim 2 wherein the canceller is further
configured to perform the iterative process by initializing R' to
the symbols carried by each of the one or more of the sub-carriers,
computing a value as a function of R', and setting R' to the
value.
8. The receiver of claim 7 wherein the canceller is further
configured to compute the value using the following formula:
Y-.eta..PSI..OMEGA.R'.
9. The receiver of claim 1 further comprising a FFT configured to
convert the OFDM symbol from the time domain to the frequency
domain.
10. The receiver of claim 9 wherein the OFDM symbol is received
over a radio channel, and wherein the receiver further comprises a
processor configured to recover data from the OFDM symbol output
from the canceller and an estimate of the radio channel's impulse
response.
11. The receiver of claim 1 wherein the ICI is caused by sampling
clock error for the OFDM symbol.
12. An inter-carrier interference (ICI) canceller, comprising:
means for receiving an OFDM symbol in the frequency domain; and
means for reducing ICI in the OFDM symbol in the frequency
domain.
13. The ICI canceller of claim 12 wherein the OFDM symbol comprises
a plurality of sub-carriers, and wherein the means for reducing ICI
is configured to perform an iterative process on one or more of the
sub-carriers.
14. The ICI canceller of claim 13 wherein the sub-carriers are
distributed over a spectrum having a mid-band and two edge-bands,
and wherein the one or more of the sub-carriers are limited to the
sub-carriers in the edge-bands.
15. The ICI canceller of claim 14 wherein the one or more of the
sub-bands include all of the sub-carriers carrying non-zero symbols
in the edge-bands.
16. The ICI canceller of claim 15 further comprising means for
determining the number of the sub-carriers in each of the
edge-bands.
17. The ICI canceller of claim 13 further comprising means for
determine the number of iterations to be performed on each of the
one or more of the sub-carriers.
18. The ICI canceller of claim 13 wherein the means for reducing
ICI is further configured to perform the iterative process by
initializing R' to the symbols carried by each of the one or more
of the sub-carriers, computing a value as a function of R', and
setting R' to the value.
19. The ICI canceller of claim 18 wherein the means for reducing
ICI is further configured to compute the value using the following
formula: Y-.eta..PSI..OMEGA.R'.
20. The ICI canceller of claim 12 wherein the ICI is caused by
sampling clock error for the OFDM symbol.
21. A method of receiving communications, comprising: receiving an
OFDM symbol in the frequency domain; and reducing inter-carrier
interference (ICI) in the OFDM symbol in the frequency domain.
22. The method of claim 21 wherein the OFDM symbol comprises a
plurality of sub-carriers, and wherein the reduction of the ICI
comprises performing an iterative process on one or more of the
sub-carriers.
23. The method of claim 22 wherein the sub-carriers are distributed
over a spectrum having a mid-band and two edge-bands, and wherein
the one or more of the sub-carriers are limited to the sub-carriers
in the edge-bands.
24. The method of claim 23 wherein the one or more of the sub-bands
include all of the sub-carriers carrying non-zero symbols in the
edge-bands.
25. The method of claim 24 further comprising determining the
number of the sub-carriers in each of the edge-bands.
26. The method of claim 22 further comprising determining the
number of iterations to be performed on each of the one or more of
the sub-carriers.
27. The method of claim 22 wherein the iterative process for
reducing ICI comprises initializing R' to the symbols carried by
each of the one or more of the sub-carriers, computing a value as a
function of R', and setting R' to the value.
28. The method of claim 27 wherein the iterative process for
reducing ICI further comprising computing the value using the
following formula: Y-.eta..PSI..OMEGA.R'.
29. The method receiver of claim 21 wherein the ICI is caused by
sampling clock error for OFDM symbol.
30. Computer readable media containing a set of instructions for a
processor to cancel inter-carrier interference (ICI), the
instructions comprising: code to receive an OFDM symbol in the
frequency domain; and code to reduce ICI in the OFDM symbol in the
frequency domain.
31. The computer readable media of claim 30 wherein the OFDM symbol
comprises a plurality of sub-carriers, and wherein the code to
reduce ICI performs an iterative process on one or more of the
sub-carriers.
32. The computer readable media of claim 31 wherein the code to
reduce ICI performs the iterative process by initializing R' to the
symbols carried by each of the one or more of the sub-carriers,
computing a value using the following formula:
Y-.eta..PSI..OMEGA.R', and setting R' to the value.
33. A computer program product, comprising: computer readable
medium comprising, code to receive an OFDM symbol in the frequency
domain; and code to reduce ICI in the OFDM symbol in the frequency
domain.
34. The computer program product of claim 33 wherein the OFDM
symbol comprises a plurality of sub-carriers, and wherein the code
to reduce ICI performs an iterative process on one or more of the
sub-carriers.
35. The computer program product of claim 34 wherein the code to
reduce ICI performs the iterative process by initializing R' to the
symbols carried by each of the one or more of the sub-carriers,
computing a value using the following formula:
Y-.eta..PSI..OMEGA.R', and setting R' to the value.
Description
RELATED APPLICATIONS
[0001] This application is a continuation application from pending
U.S. patent application Ser. No. 11/674,632, filed Feb. 13, 2007,
which claims benefit of priority under 35 U.S.C. .sctn.119(e) to
U.S. Provisional Patent Application No. 60/883,137, filed Jan. 2,
2007, the disclosure of which is hereby incorporated herein by
reference in its entirety.
BACKGROUND
[0002] 1. Field
[0003] The present disclosure relates generally to
telecommunications, and more particularly, to various concepts for
reducing inter-carrier interference (ICI) an OFDM system.
[0004] 2. Background
[0005] In recent years, there has been an increased interest in
multi-carrier telecommunication systems. One example is a
telecommunication system using Orthogonal Frequency Division
Multiplexing (OFDM) technology. OFDM is a multi-carrier modulation
technique that effectively partitions the overall system bandwidth
into multiple sub-carriers. Data is modulated onto each sub-carrier
by adjusting the sub-carrier's phase, amplitude or both. Typically,
quadrature phase shift keying (QPSK) or quadrature amplitude
modulation (QAM) is used, but other modulation schemes may also be
used. These sub-carriers are spaced apart at precise frequencies to
provide orthogonality, thereby eliminating crosstalk between the
sub-carriers. This reduces the complexity of both the transmitter
and receiver by eliminating the need for separate filters for each
sub-channel typically required in Frequency Division Multiplexing
(FDM) systems. Instead, efficient modulation and demodulation
techniques may be employed using a Fast Fourier Transform (FFT)
algorithm, making it ideal for low cost wideband communications.
Today, OFDM is used in many telecommunication systems including
Qualcomm's MediaFLO, Wireless Local Area Networks (WLAN), such as
IEEE 802.11a, 802.11g, 802.16 (Wi-Max), IEEE 802.20 (Mobile
Broadband Wireless Access), etc., Ultra wideband (UWB) systems, and
others.
[0006] OFDM relies heavily on the transmitted sub-carriers being
aligned with the demodulating sub-carriers at the receiver. An
error in the receiver master clock frequency may cause the spacing
of the demodulating sub-carriers to differ from those transmitted,
resulting in a loss of orthogonality and introducing inter-carrier
interference (ICI). Various solutions have been proposed in the
past for reducing ICI in multi-carrier telecommunication systems.
These solutions generally involve complex techniques to derive an
error signal from the received signal to drive an Automatic
Frequency Control (AFC) loop or perform some type of digital
resampling process. Accordingly, there is a need in the art for
more efficient means for reducing ICI in multi-carrier
telecommunication systems.
SUMMARY
[0007] An aspect of a receiver is disclosed. The receiver includes
a canceller configured to reduce inter-carrier interference (ICI)
in an OFDM symbol in the frequency domain.
[0008] An aspect of an inter-carrier interference (ICI) canceller
is disclosed. The ICI canceller includes means for receiving an
OFDM symbol in the frequency domain, and means for reducing ICI in
the OFDM symbol in the frequency domain.
[0009] An aspect of a method of receiving communications is
disclosed. The method includes receiving an OFDM symbol in the
frequency domain, and reducing inter-carrier interference (ICI) in
the OFDM symbol in the frequency domain.
[0010] An aspect of computer readable media is disclosed. The
computer readable media includes a set of instructions for a
processor to cancel inter-carrier interference (ICI). The
instructions include code to receive an OFDM symbol in the
frequency domain, and code to reduce ICI in the OFDM symbol in the
frequency domain.
[0011] An aspect of a computer program product is disclosed. The
computer program product includes computer-readable medium. The
computer readable medium including code to receive an OFDM symbol
in the frequency domain, and code to reduce ICI in the OFDM symbol
in the frequency domain.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Various aspects of a wireless communications system are
illustrated by way of example, and not by way of limitation, in the
accompanying drawings, wherein:
[0013] FIG. 1 is a high-level block diagram of a transmitter in a
multi-carrier telecommunications system;
[0014] FIG. 2 is a high-level block diagram of a receiver in a
multi-carrier telecommunications system;
[0015] FIG. 3 is a flow diagram illustrating an example of an
algorithm using an iterative process to cancel inter-carrier
interference (ICI); and
[0016] FIG. 4 is a functional block diagram of an ICI
canceller.
DETAILED DESCRIPTION
[0017] The detailed description set forth below in connection with
the appended drawings is intended as a description of various
configurations of the invention and is not intended to represent
the only configurations in which the invention may be practiced.
The detailed description includes specific details for the purpose
of providing a thorough understanding of the invention. However, it
will be apparent to those skilled in the art that the invention may
be practiced without these specific details. In some instances,
well known structures and components are shown in block diagram
form in order to avoid obscuring the concepts of the invention.
[0018] FIGS. 1 and 2 are high-level block diagrams of a transmitter
102 and receiver 104 in a multi-carrier telecommunications system.
The transmitter 102 may be part of a base station and the receiver
104 may be part of a access terminal. Conversely, the transmitter
102 may be part of an access terminal and the receiver 104 may be
part of a base station. An access terminal may be any fixed or
mobile radio device, such as a mobile telephone, a personal digital
assistant (PDA), a personal or laptop computer, a television
receiver, a game console, a camera, a MP3 player, or any other
video, audio, or data device capable of radio communications. A
base station may be a fixed or mobile transceiver that serves one
or more access terminals in its geographic region. The base station
may be used to provide multimedia broadcasts, enable access
terminals to communicate with one another, or serve as a gateway to
wired packet-based and/or circuit-switched networks.
[0019] Referring to FIG. 1, a transmit (TX) data processor 106
processes (e.g., encodes, interleaves, and symbol maps) data into a
stream of data symbols. As used herein, a "data symbol" is a
modulation symbol for data, and a "modulation symbol" is a complex
value for a point in a signal constellation (e.g., for PSK or
QAM).
[0020] The transmit (TX) data provides the data symbol stream to a
serial-to-parallel converter 108. The serial-to-parallel converter
108 maps a data symbol to each of the N sub-carriers. Because the
sub-carriers at the edges of the spectrum may impose severe
constraints on transmit filtering required to meet regulatory
spectral masks, the skilled artisan may chose not to send any data
on those sub-carriers. Instead, the edges of the spectrum, referred
to as "guard bands," may carry zero symbols and hence no
energy.
[0021] The serial-to-parallel converter 108 provides a set of N
symbols to an inverse FFT 110. The inverse FFT 110 is used to
generate an OFDM symbol comprising N independently modulated
sub-carriers in the time domain. A parallel-to-serial converter 111
is used to convert the time domain sub-carriers into a serial
format for framing. A TX pilot processor 112 is used to generate a
pilot signal comprising a number of pilot symbols. A multiplexer
114 may be used to frame the pilot signal from the TX pilot
processor 110 with one or more OFDM symbols from the inverse FFT
110. Alternatively, the TX pilot processor 112 may be used to
generate a pilot signal in the frequency domain by providing pilot
symbols to the serial-to-parallel converter 108 for mapping to a
portion of the N sub-carriers.
[0022] A digital-to-analog (D/A) converter 116 is used to convert
the OFDM symbol stream to the analog domain. As used herein, an
"OFDM symbol stream" means an OFDM symbol stream with or without a
time domain pilot signal. The sum of the N orthogonal sub-carriers
for each OFDM symbol is used to modulate a carrier frequency in an
analog front end (AFE) 118 to generate a transmission signal. The
transmission signal is transmitted by the AFE 118 over a radio
channel through an antenna 120.
[0023] Referring to FIG. 2, an antenna 202 receives the transmitted
signal and provides it to an AFE 204. The AFE 204 recovers the OFDM
symbol stream from the carrier frequency and provides it to an
analog-to-digital (A/D) converter 206. The A/D converter 206 is
used to convert the OFDM symbol steam to the digital domain.
[0024] Each OFDM symbol is converted to a parallel format using a
serial-to-parallel converter 208 before being provided to a FFT
210. The FFT 210 is used to convert the OFDM symbol stream back
into the frequency domain. Specifically, the FFT 210 generates N
independently modulated sub-carriers for each OFDM symbol. The N
independently modulated sub-carriers are provided to a
parallel-to-serial converter 212 to produce a stream of symbol
estimates.
[0025] The symbol estimates output from the parallel-to-serial
converter 212 may not correspond to the exact location of a point
in the original signal constellation due to noise and other
disturbances in the radio channel. A receive (RX) data processor
216 may be used to determine the data symbol in the signal
constellation most likely transmitted for each non-zero symbol
estimate. This determination is based on the data symbol estimate
and the radio channel's impulse response provided by a channel
estimator 218. The channel estimator 218 estimates the radio
channel's impulse response from the time domain pilot signal in the
OFDM symbol stream. The RX data processor 216 also provides other
signal processing functions such as deinterleaving and decoding, to
recover the original data.
[0026] The receiver 104 may also include an ICI canceller 214. In
the example shown in FIG. 2, the ICI canceller 214 is placed
between the parallel-to-serial converter 212 and the RX data
processor 214, but may be used elsewhere in the receiver 104. The
ICI canceller 214 provides a means for reducing ICI resulting from
sampling clock error at the receiver. The manner in which the ICI
canceller 214 performs this functions is best understood with
reference to the following mathematical analysis.
[0027] The baseband model of the transmitted signal s(t) can be
expressed as:
s ( t ) = k = 0 N - 1 j 2 .pi. T u ( k - N 2 ) t b k , 0 .ltoreq. t
.ltoreq. T u + T g , ( 1 ) ##EQU00001##
where: [0028] N is the size of the transmitter IFFT (assumed to be
the same as receiver FFT), [0029] T.sub.u is the useful OFDM symbol
time; [0030] T.sub.g is the guard band time; [0031] b.sub.k is the
transmitted data/pilot/zero symbol on sub-carrier k; [0032] N/2 is
used to represent centering the carriers around DC, which
corresponds to the carrier index and the physical location of the
downconverted signal at baseband.
[0033] Assuming an ideal receiver with no local oscillator (LO)
noise, and assuming the guard band of the OFDM symbol is
sufficiently long, the received signal y(t) is affected only by the
radio channel's impulse response H.sub.k. The received signal can
be expressed as:
y ( t ) = k = 0 N - 1 j 2 .pi. T U ( k - N 2 ) t b k H k , ( 2 )
##EQU00002##
where H.sub.k is the complex impulse response of the radio channel
at the frequency of the k.sup.th carrier. Replacing b.sub.kH.sub.k
with R.sub.k, the received signal becomes:
y ( t ) = k = 0 N - 1 j 2 .pi. T U ( k - N 2 ) t R k . ( 3 )
##EQU00003##
[0034] At the receiver, the received signal y (t) is correlated
with each of the possible sub-carrier waveforms to recover R.sub.k
(i.e., the received modulation symbol on sub-carrier k). However,
the sub-carrier waveforms (demodulating sub-carriers) may be
subject to a sampling frequency error which causes the spacing of
the demodulating sub-carriers to differ from those transmitted. The
sampling frequency error
f s f s ' ##EQU00004##
may be expressed as (1+.epsilon.), where f.sub.s. (the transmitter
master clock frequency) is equal to
N T u . ##EQU00005##
Also, note that f'.sub.s here refers to the receiver master clock
frequency. Taking into consideration the frequency sampling error,
the received signal can be expressed as follows:
y n = k = 0 N - 1 j 2 .pi. T u ( k - N 2 ) n ( 1 + ) T u N R k = k
= 0 N - 1 j 2 .pi. N ( k - N 2 ) n ( 1 + ) R k . ( 4 ) .
##EQU00006##
[0035] Using an FFT, the samples y.sub.n are correlated with the
demodulating sub-carriers to obtain the following result for the
l.sup.th sub-carrier:
Y l = n = 0 N - 1 j.pi. n - j 2 .pi. N nl k = 0 N - 1 j 2 .pi. N (
k - N 2 ) n ( 1 + ) R k , ( 5 ) . ##EQU00007##
[0036] Taking the FFT shift performed through (-1).sup.n
multiplication into account, equation (5) can be manipulated to
separate the desired (signal) component from the ICI component
as:
Y l = n = 0 N - 1 R l j 2 .pi. N ( l - N 2 ) n + n = 0 N - 1 k = 0
k .noteq. l N - 1 j 2 .pi. N n ( k ( 1 + ) - l - N 2 ) R k , ( 6 )
. ##EQU00008##
[0037] Equation (6) can be simplified using a finite geometric
series as:
Y l = R l j.pi. ( l - N 2 ) j .pi. N ( l - N 2 ) sin ( .pi. ( l - N
2 ) ) N sin ( .pi. N ( l - N 2 ) ) + k = 0 k .noteq. l N - 1 R k
j.pi. ( k - N 2 ) j .pi. N ( ( k - N 2 ) + k - l ) sin ( .pi. ( k -
N 2 ) ) N sin ( .pi. N ( ( k - N 2 ) + k - l ) ) = R l j.pi. ( l -
N 2 ) j .pi. N ( l - N 2 ) sin ( .pi. ( l - N 2 ) ) N sin ( .pi. N
( l - N 2 ) ) + k = 0 k .noteq. l N - 1 R k j.pi. ( ( k - N 2 ) + k
- l ) j .pi. N ( ( k - N 2 ) + k - l ) sin ( .pi. ( ( k - N 2 ) + k
- l ) ) N sin ( .pi. N ( ( k - N 2 ) + k - l ) ) . ( 7 )
##EQU00009##
[0038] A Dirichlet function can be defined as
diric N ( x ) = sin ( N .pi. x ) N sin ( .pi. x ) .
##EQU00010##
If x<<1, the Dirichlet can be expressed as:
diric N ( x ) .apprxeq. sin ( N .pi. x ) N .pi. x = sin c ( Nx ) .
( 8 ) ##EQU00011##
[0039] Assuming .epsilon.N<<1 and neglecting the phase
ramping term, the following equation can be derived using the
Dirichlet function:
Y l .apprxeq. R l diric N ( ( l N - 1 2 ) ) + k = 0 k .noteq. l N -
1 R k - j .pi. N ( k - 1 ) ( - 1 ) k - l diric N ( ( k N - 1 2 ) +
k - l N ) . ( 9 ) ##EQU00012##
[0040] The term in the summation can be defined as:
ICI k , l = R k - j .pi. N ( k - l ) ( - 1 ) k - l diric N ( ( k N
- 1 2 ) + k - l N ) . ( 10 ) ##EQU00013##
[0041] As can be seen from equation (10), most of the ICI energy is
concentrated at neighboring sub-carriers. Thus, ICI.sub.k,l can be
approximated under the condition |k-1|<<N as follows:
ICI k , l .apprxeq. R k ( - 1 ) k - l sin c ( ( k - N 2 ) + k - l )
= R k ( - 1 ) k - l sin .pi. ( ( k - N 2 ) + k - l ) .pi. ( ( k - N
2 ) + k - l ) .apprxeq. R k .pi. ( ( k - N 2 ) ) .pi. ( k - l ) = R
k 2 2 k - N k - l . ( 11 ) ##EQU00014##
[0042] Next, the periodicity of ICI.sub.k,l is be examined. The
following relationship can be defined due to its FFT nature:
R.sub.k=R.sub.k+N (12).
[0043] Applying this definition, the following expression can be
derived:
ICI k + N , l = R k + N - j .pi. N ( k + N - l ) ( - 1 ) k + N - l
diric N ( ( k + N N - 1 2 ) + k + N - l N ) = R k - j .pi. N ( k -
l ) ( - 1 ) ( - 1 ) k - l ( - 1 ) N diric N ( ( k N - 1 2 ) + k - l
N + + 1 ) = R k - j .pi. N ( k - l ) ( - 1 ) k - l diric N ( ( k N
+ 1 2 ) + k - l N ) , ( 13 ) ##EQU00015##
where the following property of the Dirichlet function is used:
diric.sub.N(x+1)=-diric.sub.N(x) (14).
As one can readily see, the ICI.sub.k,l term is not exactly
periodic. However, if k and l are neighbor sub-carriers in the
modulo-N sense (e.g., the two sub-carriers are located around the
lowest frequency and the highest frequency respectively), the
following approximation applies:
ICI k + N , l .apprxeq. R k - j .pi. N ( k - l ) ( - 1 ) k - l sin
c ( ( k + N 2 ) + k - l ) = R k ( - 1 ) k - l sin .pi. ( ( k + N 2
) + k - l ) .pi. ( ( k + N 2 ) + k - l ) .apprxeq. R k .pi. ( ( k +
N 2 ) ) .pi. ( k - l ) = R k 2 2 k + N k - l = R k 2 2 ( k + N ) -
N k - l , ( 15 ) ##EQU00016##
Therefore, periodicity holds approximately in the wrap-around
sense.
[0044] If the contribution from distant neighbor sub-carriers is
neglected and only the .DELTA. close neighbor sub-carriers are
considered, where .DELTA.<<N, the ICI-contaminated signal
reduces to:
Y l .apprxeq. R l + 2 k = l - .DELTA. k .noteq. l l + .DELTA. mod (
2 k - N , 2 N ) k - l R k . ( 16 ) ##EQU00017##
[0045] Equation (16) can be equivalently represented as matrix
operation.
[ Y 0 Y 1 Y 2 Y N - 1 ] Y .apprxeq. [ R 0 R 1 R 2 R N - 1 ] R + 2
.eta. [ 0 1 1 2 1 3 - 1 3 - 1 2 - 1 - 1 0 1 1 2 - 1 3 - 1 2 - 1 2 -
1 0 1 - 1 3 - 1 3 - 1 2 - 1 1 3 1 3 1 1 2 1 2 1 3 - 1 0 1 1 1 2 1 3
- 1 3 - 1 2 - 1 0 ] .PSI. ##EQU00018## [ - N 2 - N 4 - N N - 4 N -
2 ] .OMEGA. [ R 0 R 1 R 2 R N - 1 ] R ##EQU00018.2##
where .PSI. is Toeplitz by definition.
[0046] One can readily see from the above matrices that the
sampling frequency error impacts the sub-carriers in the edges of
the band most because the energy of the scaling matrix .OMEGA.
concentrates on the outer diagonal taps. The edge-bands can be
selectively treated by clasping the above matrices such that only
ICI for the E lowest-frequency sub-carriers and the E
highest-frequency sub-carriers are reduced, where E<N. The
scaling matrix .OMEGA. can be reduced to a scalar N while flipping
the sign of the top part of R in the second term, i.e.:
[ Y 0 Y 1 Y 2 Y N - 1 ] Y .apprxeq. [ R 0 R 1 R 2 R N - 1 ] R + N 2
.eta. [ 0 1 1 2 1 3 - 1 3 - 1 2 - 1 - 1 0 1 1 2 - 1 3 - 1 2 - 1 2 -
1 0 1 - 1 3 - 1 3 - 1 2 - 1 1 3 1 3 1 1 2 1 2 1 3 - 1 0 1 1 1 2 1 3
- 1 3 - 1 2 - 1 0 ] .PSI. ##EQU00019## [ - 1 - 1 - 1 1 1 1 ]
.OMEGA. [ R 0 R 1 R 2 R N - 1 ] R ##EQU00019.2##
[0047] In this simplified model, the term
N 2 ##EQU00020##
determines the ICI around the lowest and the highest frequency
sub-carriers. Because the impact of ICI is most significant in this
region, .eta. can be set to
N 2 . ##EQU00021##
Alternatively, .eta. can be set to
[0048] ( N - E ) 2 ##EQU00022##
to improve the accuracy of the ICI reduction algorithm in the
middle of the edge-bands.
[0049] The phase ramping term that has been neglected will now be
addressed. With this term, the ICI-contaminated signal is expressed
as:
Y l .apprxeq. R l j.pi. ( l - N 2 ) + 2 k = l - .DELTA. k .noteq. l
l + .DELTA. mod ( 2 k - N , 2 N ) k - l j.pi. ( k - N 2 ) R k . (
17 ) ##EQU00023##
[0050] Defining
R l ' = R l j.pi. ( l - N 2 ) , ##EQU00024##
the following relationship still holds:
Y=R'+.eta..PSI..OMEGA.R' (18).
[0051] An iterative process using equation (18) may be used to
reduce ICI in the edge-bands of an OFDM symbol Y in the frequency
domain. An example of this process will be presented with reference
to FIG. 3. In step 302, the iterative process is initialized by
setting R' to Y. Next, one iteration of R' is performed using
equation (18). Specifically, a value is computed for
Y-.eta..PSI..OMEGA.R' in step 304 and then R' is reset to that
value in step 306. The algorithm then determines, in step 308,
whether a sufficient number of iterations has been performed.
Typically, one iteration should be sufficient, but a further
reduction in ICI may be obtained with additional iterations. Those
skilled in the art will readily be able to assess whether multiple
iterations are required to meet the performance requirements for
any particular application. If another iteration of R' is required,
then the algorithm loops back to step 304. The process continues
until a sufficient number of iterations has been performed on R',
at which point the algorithm exits the loop and outputs R' in step
310. Once this occurs, R.sub.l may be computed from the following
formula:
R l = R l ' - j.pi. ( l - N 2 ) . ( 19 ) ##EQU00025##
[0052] The algorithm may be performed on each sub-carrier of the
OFDM symbol, or alternatively, on the sub-carriers in the
edge-bands (i.e., the E lowest frequency sub-carriers and the E
highest frequency sub-carriers). The number of sub-carriers in the
edge-bands may be readily determined by those skilled in the art
based on the specific application and the overall design
constraints imposed on the system. By way of example, the number of
sub-carriers in the edge-bands can be determined by calculating the
signal-to-noise ratio (SNR) of the sub-carriers and adjusting the
edge-bands to include enough sub-carriers to meet the SNR
requirements of the system.
[0053] Returning to FIG. 2, the output from the ICI canceller 214
R.sub.l is provided to the RX data processor 216. Referring to
equations (2) and (3), the RX data processor 216 can recover the
transmitted data/pilot/zero symbol b.sub.k from R.sub.k using the
channel impulse response H.sub.k provided by the channel estimator
218.
[0054] FIG. 4 is a functional block diagram of an (ICI) canceller.
The ICI canceller 132 includes a module 402 for receiving an OFDM
symbol in the frequency domain, and a module 304 for reducing ICI
in the OFDM symbol in the frequency domain.
[0055] The various illustrative logical blocks, modules, circuits,
elements, and/or components described in connection with the
embodiments disclosed herein 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 (FPGA) or other programmable logic
component, 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
conventional processor, controller, microcontroller, or state
machine. A processor may also be implemented as a combination of
computing components, 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.
[0056] The methods or algorithms described in connection with the
embodiments disclosed herein may be embodied directly in hardware,
in software executed by a processor, or in a combination of the
two. The software may reside in computer-readable.
Computer-readable media includes both computer storage media and
communication media including any medium that facilitates transfer
of a computer program from one place to another. A storage media
may be any available media that can be accessed by a computer. By
way of example, 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 software in
the form of instructions or data structures and that can be
accessed by a processor. Also, any connection is properly termed a
computer-readable 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 medium. Disk and disc,
as used herein, includes 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. Combinations of the above
should also be included within the scope of computer-readable
media.
[0057] The previous description is provided to enable any person
skilled in the art to practice the various embodiments described
herein. Various modifications to these embodiments will be readily
apparent to those skilled in the art, and the generic principles
defined herein may be applied to other embodiments. Thus, the
claims are not intended to be limited to the embodiments shown
herein, but is to be accorded the full scope consistent with the
language claims, wherein reference to an element in the singular is
not intended to mean "one and only one" unless specifically so
stated, but rather "one or more." All structural and functional
equivalents to the elements of the various embodiments described
throughout this disclosure that are known or later come to be known
to those of ordinary skill in the art are expressly incorporated
herein by reference and are intended to be encompassed by the
claims. Moreover, nothing disclosed herein is intended to be
dedicated to the public regardless of whether such disclosure is
explicitly recited in the claims. No claim element is to be
construed under the provisions of 35 U.S.C. .sctn.112, sixth
paragraph, unless the element is expressly recited using the phrase
"means for" or, in the case of a method claim, the element is
recited using the phrase "step for."
* * * * *