Title: Method and apparatus for reducing interference within a communication system
Abstract: A method for reducing interference within a communication system is provided herein. A receiver (100), and method for operating a receiver are provided. The receiver operates by utilizing a filter bank (103-104) to partition a wide-band signal into smaller sub-bands. Interference suppression takes place individually on the sub-bands (frequency bands) instead of on the wideband signal as a whole. By using interference suppression on smaller sub-bands, interference suppression techniques can be utilized with less computational complexity than when performing interference suppression on the broadband signal as a whole.
Patent Number: 6,996,197 Issued on 02/07/2006 to Thomas, et al.
| Inventors:
|
Thomas; Timothy A. (Palatine, IL);
Vook; Frederick W. (Schaumburg, IL)
|
| Assignee:
|
Motorola, Inc. (Schaumburg, IL)
|
| Appl. No.:
|
919198 |
| Filed:
|
August 16, 2004 |
| Current U.S. Class: |
375/346 |
| Current Intern'l Class: |
H03D 1/04 (20060101); H03D 1/06 (20060101); H03K 5/01 (20060101); H04B 1/10 (20060101); H04L 25/08 (20060101) |
| Field of Search: |
370/203,209,320
455/561
375/346,316
|
References Cited [Referenced By]
U.S. Patent Documents
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| |
| 5063560 | Nov., 1991 | Yerbury et al.
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| 5260968 | Nov., 1993 | Gardner et al.
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| 5524023 | Jun., 1996 | Tsujimoto.
| |
| 5528581 | Jun., 1996 | De Bot.
| |
| 5598428 | Jan., 1997 | Sato.
| |
| 5623485 | Apr., 1997 | Bi.
| |
| 5757789 | May., 1998 | Dent.
| |
| 6137785 | Oct., 2000 | Bar-Ness.
| |
| 6141393 | Oct., 2000 | Thomas et al.
| |
| 6363262 | Mar., 2002 | McNicol.
| |
| 6445735 | Sep., 2002 | Whikehart.
| |
| 6788752 | Sep., 2004 | Andre.
| |
| 2002/0150070 | Oct., 2002 | Shattil.
| |
| 2002/0154614 | Oct., 2002 | Jagger et al.
| |
| 2003/0048800 | Mar., 2003 | Kilfoyle et al.
| |
| 2003/0072258 | Apr., 2003 | Tarokh et al.
| |
| 2003/0123384 | Jul., 2003 | Agee.
| |
| 2003/0179840 | Sep., 2003 | Oh et al.
| |
| 2003/0216122 | Nov., 2003 | Cordone et al.
| |
| 2004/0062216 | Apr., 2004 | Nicholis et al.
| |
| 2004/0184570 | Sep., 2004 | Thomas et al.
| |
Other References
Li, Y.; Sollenberger, N. "Adaptive Antenna Arrays for OFDM Systems With Cochannel
Interference" IEEE Transactions on Communications vol. 47, No. 2, Feb. 1999.
Thomas, T.; Vook, F.; "Asynchronous Interference Suppression in Broad-band Cyclic-Prefix
Communications" IEEE WCNC 2003, New Orleans, LA, Mar. 18-20, 2003.
|
Primary Examiner: Chin; Stephen
Assistant Examiner: Lu; Jia
Claims
What is claimed is:
1. An apparatus comprising:
a first antenna outputting a first wideband signal;
a second antenna outputting a second wideband signal;
a first filter bank coupled to the first antenna, receiving the first wideband
signal and outputting a first plurality of narrowband signals;
a second filter bank coupled to the second antenna, receiving the second wideband
signal and outputting a second plurality of narrowband signals;
an interference suppressor receiving a first narrowband signal from the first
filter bank and a second narrowband signal from the second filter bank and outputting
an interference-suppressed signal based on the first and the second narrowband signals;
synthesis circuitry receiving the interference-suppressed signal and a plurality
of other interference-suppressed signals, and outputting a composite interference-suppressed
signal based on the interference-suppressed signal and the plurality of other interference-suppressed signals;
an FFT processor receiving the composite interference-suppressed signal and performing
FFT processing on the signal; and
wherein the first and the second filter banks operate continuously in a linear
convolution fashion prior to FFT processing.
2. The apparatus of claim 1 wherein the first and the second narrowband signals
exist within a same sub-band.
3. The apparatus of claim 2 further comprising:
a second interference suppressor receiving a third narrowband signal from the
first filter bank and a fourth narrowband signal from the second filter bank and
outputting a second interference-suppressed signal based on the first and the second
narrowband signals, wherein the third and the fourth narrowband signals exist within
a same sub-band.
4. The apparatus of claim 1 wherein the interference suppressor suppresses interference
via a blind-only technique that does not require pilot symbols to compute interference
suppression filters.
5. The apparatus of claim 1 wherein the interference suppressor utilizes a cyclic
prefix to suppress interference.
6. The apparatus of claim 5 wherein the interference suppressor additionally
utilizes transmitted pilot symbols in suppressing interference.
7. The apparatus of claim 1 wherein the interference suppressor utilizes transmitted
pilot symbols in suppressing interference.
8. An apparatus for interference suppression, the apparatus comprising:
a first antenna outputting a first wideband time-domain signal r
1(n);
a second antenna outputting a second wideband time-domain signal r
2(n);
a first filter bank coupled to the first antenna, receiving r
1(n)
and outputting X narrowband time-domain signals, wherein each narrowband time-domain
signal has a center frequency of {overscore (Ω)}
l (l=1, . . .
, X);
a second filter bank coupled to the second antenna, recieving r
2(n)
and outputting X narrowband time-domain signals, wherein each narrowband time-domain
signal has a center frequency of {overscore (Ω)}
l (l=1, . . .
, X);
a first interference suppressor receiving a first narrowband time-domain signal
from the first filter bank and a second narrowband time-domain signal from the
second filter bank and outputting a first interference-suppressed time-domain signal
based on the first and the second narrowband time-domain signals, wherein the first
and the second narrowband time-domain signals have a same center frequency; and
a second interference suppressor receiving a third narrowband time-domain signal
from the first filter bank and a fourth narrowband time-domain signal from the
second filter bank and outputting a second interference-suppressed time-domain
signal based on the third and the fourth narrowband time-domain signals, and wherein
the third and the fourth narrowband time-domain signals have a same center frequency
wherein the first and the second filter banks operate continuously in a linear
convolution fashion.
9. The apparatus of claim 8 further comprising:
synthesis circuitry receiving the first and the second interference-suppressed
time-domain signals, and outputting a composite interference-suppressed time-domain
signal based on the first and the second interference-suppressed time domain signals.
10. The apparatus of claim 8 wherein the first and the second interference suppressors
suppress interference via a blind-only technique that does not require pilot symbols
to compute interference suppression filters.
11. The apparatus of claim 8 wherein the first and the second interference suppressors
utilize a cyclic prefix to suppress interference.
12. The apparatus of claim 11 wherein the first and the second interference suppressors
additionally utilizes transmitted pilot symbols in suppressing interference.
13. The apparatus of claim 8 wherein the first and the second interference suppressors
utilize transmitted pilot symbols in suppressing interference.
14. A method comprising the steps of:
receiving a wideband time-domain signal at a first filter bank;
receiving the wideband time-domain signal at a second filter bank;
outputting a first narrowband time-domain signal having a first center frequency
from the first filter bank based on the wideband time-domain signal;
outputting a second narrowband time-domain signal having the first center frequency
from the second filter bank based on the wideband time-domain signal;
receiving the first and the second narrowband time-domain signals and performing
interference suppression on the first and the second narrowband time-domain signals; and
wherein the step of performing interference suppression comprises the step of
utilizing transmitted pilot symbols to perform interference suppression.
15. The method of claim 14 wherein the step of performing interference suppression
comprises the step of utilizing transmitted pilot symbols to perform interference suppression.
16. The method of claim 14 wherein the step of performing interference suppression
comprises the step of utilizing a cyclic prefix in performing interference suppression.
17. An apparatus comprising:
a first antenna outputting a first wideband signal;
a second antenna outputting a second wideband signal;
a first filter bank coupled to the first antenna, receiving the first wideband
signal and outputting a first plurality of narrowband time-domain signals;
a second filter bank coupled to the second antenna, receiving the second wideband
signal and outputting a second plurality of narrowband time-domain signals;
an interference suppressor receiving a first narrowband time-domain signal from
the first filter bank and a second narrowband time-domain signal from the second
filter bank and outputting an interference-suppressed time-domain signal based
on the first and the second narrowband time-domain signals; and
wherein the first and the second filter banks operate continuously in a linear
convolution fashion.
Description
FIELD OF THE INVENTION
The present invention relates generally to interference suppression and in particular,
to a method and apparatus for reducing interference within a communication system.
BACKGROUND OF THE INVENTION
Interference often hinders performance of communication systems. One
type of interference often encountered by a user within a communication system
is interference generated by the transmissions of other users. This is typically
caused by many users transmitting within the same frequency band, and is referred
to as co-channel interference. In order to reduce co-channel interference many
communication systems employ a frequency reuse pattern, where transmitters in adjacent
cells transmit on different frequencies. However, given the price of spectrum,
future communications systems will be characterized by aggressive frequency reuse
patterns that will result in significantly increased levels of co-channel interference.
Notwithstanding the above, more and more system operators are taking
advantage of unlicensed frequency bands for transmitting information. Because the
number of transmitters within an unlicensed frequency band is not restricted, there
exists the potential of greatly increased co-channel interference. Additionally,
because operators within the unlicensed band do not have to synchronize to a common
source, typical co-channel interference is asynchronous in that the interfering
signal does not align in time with the desired signal.
Because interference can greatly reduce the efficiency of a communication
system, and because interference can be both synchronous and asynchronous, a need
exists for a method and apparatus for reducing interference within a communication system.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a receiver for reducing interference.
FIG. 2 illustrates a frequency response for the filter of FIG. 1.
FIG. 3 is a flow chart showing the operation of the receiver of FIG. 1.
DETAILED DESCRIPTION OF THE DRAWINGS
In order to address the above-mentioned need, a method and apparatus for interference
suppression is provided herein. In particular, a receiver, and method for operating
a receiver are provided. The receiver operates by utilizing a filter bank to partition
a wide-band signal into smaller sub-bands. Interference suppression takes place
individually on the sub-bands (frequency bands) instead of on the wideband signal
as a whole. By using interference suppression on smaller sub-bands, interference
suppression techniques can be utilized with less computational complexity than
when performing interference suppression on the broadband signal as a whole.
The present invention encompasses an apparatus comprising a first antenna outputting
a first wideband signal, a second antenna outputting a second wideband signal,
a first filter bank coupled to the first antenna, receiving the first wideband
signal and outputting a first plurality of narrowband signals, a second filter
bank coupled to the second antenna, receiving the second wideband signal and outputting
a second plurality of narrowband signals, and an interference suppressor receiving
a first narrowband signal from the first filter bank and a second narrowband signal
from the second filter bank and outputting an interference-suppressed signal based
on the first and the second narrowband signals.
The present invention additionally encompasses an apparatus for interference
suppression. The apparatus comprises a first antenna outputting a first wideband
signal r
1(n), a second antenna outputting r
2(n), a first
filter bank coupled to the first antenna, receiving r
1(n) and outputting
X narrowband signals, wherein each narrowband signal has a center frequency of
{overscore (ω)}
l(l=1 , . . . , X), a second filter bank coupled
to the second antenna, receiving r
2(n) and outputting X narrowband signals,
wherein each narrowband signal has a center frequency of {overscore (ω)}
l
(l=1, . . . , X), a first interference suppressor receiving a first narrowband
signal from the first filter bank and a second narrowband signal from the second
filter bank and outputting a first interference-suppressed signal based on the
first and the second narrowband signals, wherein the first and the second narrowband
signals have a same center frequency, and a second interference suppressor receiving
a third narrowband signal from the first filter bank and a fourth narrowband signal
from the second filter bank and outputting a second interference-suppressed signal
based on the third and the fourth narrowband signals, wherein the third and the
fourth narrowband signals have a same center frequency.
The present invention additionally encompasses a method comprising the steps
of receiving a wideband signal at a first filter bank, receiving the wideband signal
at a second filter bank, outputting a first narrowband signal having a first center
frequency from the first filter bank based on the wideband signal, and outputting
a second narrowband signal having the first center frequency from the second filter
bank based on the wideband signal. The first and the second narrowband signals
are received and interference suppression is performed on the first and the second
narrowband signals.
Turning now to the drawings, wherein like numerals designate like components,
FIG. 1 is a block diagram of receiver
100. Receiver
100 utilizes
an orthogonal frequency division multiplexing (OFDM) communication system protocol,
however in alternate embodiments of the present invention, other system protocols
utilizing wideband transmission schemes may be utilized as well. Such other system
protocols include, but are not limited to frequency-domain equalized single-carrier
systems with cyclic prefixes (called cyclic prefix single carrier) or without cyclic
prefixes, code division multiple access (CDMA) systems with cyclic prefixes (called
cyclic prefix CDMA) or without cyclic prefixes, multi-carrier CDMA systems, and
spread-OFDM systems. As a result, receiver
100 is applicable and effective
in OFDM systems, single carrier systems, CDMA systems, and any other similar or
hybrid systems.
As shown, receiver
100 comprises at least one receive antenna
101
outputting a wideband signal, at least one filter bank
103-
104 receiving
the wideband signal, at least one interference suppressor
105-
106,
a synthesis filter bank
107, a cyclic prefix remover
108, and a fast
Fourier transformer (FFT)
109. The exact number of the above elements will
vary depending upon the number of signals that are expected to be received simultaneously
by the receiver. For simplicity, only two received signals are shown, with s(n)
being the signal received from the desired transmitter (i.e., the transmitter whose
data the receiver is trying to estimate) and i(n) is the signal received from an
interfering transmitter, and n is an integer indicating a discrete time index.
(For the illustration purposes, s(n) and i(n) are both column vectors having a
length equal to the number of receive antennas.) The desired signal transmitted
by the desired transmitter before being corrupted by the channel is referred to
as d(n). Although only a single desired transmitted signal is shown, the present
invention can also recover multiple data signals sent from a single desired transmitter
(or single or multiple data signals sent from multiple desired transmitters). The
desired transmitter is also referred to as the desired signal or the desired user.
Both s(n) and i(n) are the vectors of the signals received on each receive antenna
from the respective transmitters after having been corrupted by their respective
channels. It should be noted that s(n) and i(n) may, or may not be synchronized
to a common time source. It should also be noted that although a single interfering
transmitted signal is shown, the present invention can also suppress multiple interfering
signals sent from multiple interfering transmitters.
Regardless of the number of signals simultaneously being received, receiver
100 comprises a single filter bank
103-
104 for each channel
(antenna
101-
102). Each filter bank
103-
104 divides
the received wideband signal vector r(n)=s(n)+i(n) into a plurality (X) of sub-bands
(where the effects of receiver noise have been neglected for clarity). Particularly,
where OFDM is utilized, the OFDM bandwidth is divided into small frequency bands
such that the received bank breaks up the received M×1 OFDM wideband signal
vector, r(n), (where M is the number of receive antennas) into X sub-bands, or
narrowband signals, (e.g., 16 sub-bands/narrowband signals). Each sub-band comprises
a narrowband signal having a center frequency {overscore (ω)}
l
(l=1, . . . , X). The narrowband signal at each center frequency may be generated
by shifting r
m(n) (where r
m(n) is the received wideband signal
from antenna m, i.e.,
##EQU1##
in frequency by -{overscore (ω)}
l, next filtering by a low-pass
filter, f(n), and then decimating by X. The filter, f(n), is chosen so that in
the absence of the interference suppressors
105-
106 and for a single
channel (antenna
101-
102), when an arbitrary signal, q(n), that spans
the same bandwidth as the OFDM system is input into the filter bank
103-
104,
the output of the synthesis filter bank
107 equals (or approximately equals)
the arbitrary signal q(n). It should be noted that the filter bank operates continuously
in a linear convolution fashion as opposed to block processing with circular convolution.
In other words, the filter banks operate very differently than traditional OFDM
receiver processing where the portion of the received signal corresponding to the
cyclic prefix is removed before a block of N time-domain signals is transformed
into the frequency domain by an FFT operation. In the present invention, the filter
banks are utilized such that the received signal is continuously processed, meaning
that the portion of the received signal corresponding to the cyclic prefix is processed
(i.e., not discarded) along with the data portion of the received signal. The use
of continuous processing (i.e., linear convolution as opposed to circular convolution)
greatly improves the ability to suppress asynchronous interference at the receiver
while the use of filter banks keeps the computational complexity low.
As an example with X=16, FIG. 2 shows the frequency response of a particular
filter,
f(n), convolved with itself and frequency shifted by center frequencies {overscore
(ω)}
1 through {overscore (ω)}
16. Thus, FIG. 2
shows the 16 particular sub-bands where the interference suppressors
105-
106
will operate for this example. The low-pass filter, f(n), was chosen as a pulse
with a square-root raised-cosine spectrum with a roll-off factor of 0.2 and a symbol
time of 0.8333 μsec. For this example, the center frequencies were given
as:
##EQU2##
where N is an FFT size for the FFT
109 and K is the number of OFDM subcarriers
with data (in FIG. 2, N=1024 and K=760). Note that the total bandwidth is broken
up into 16 overlapping sections. Also note that the first and last sub-bands have
the roll-off portion of the pulse outside of the OFDM signal bandwidth.
Returning to FIG. 1, each filter bank
103-
104 outputs X sub-bands/narrowband
signals, with like sub-bands (i.e., like center frequencies ω
1
through {overscore (ω)}
X) being directed to similar interference
suppressors
105-
106. In particular, each interference suppressor
105-
106 has inputs from the various filter banks
103-
104,
with each input comprising the same particular sub-band. Thus, a first interference
suppressor will receive all sub-bands centered at ω
1, while a
second interference suppressor will receive all sub-bands centered at ω
2
. . . , etc. Interference suppression then takes place on the particular
sub-band, with the output of the interference suppressors
105-
106
being a scalar time sequence for the particular sub-band. Since the bandwidth of
a sub-band is significantly less than the original signal, the effective length
of the channel in each sub-band is much smaller than the original channel length.
Therefore, various interference suppression filters (described below) can be designed
using a frequency-shifted and filtered version of the pilot sequences where a pilot
sequence is a group of symbols transmitted by the desired transmitter that is known
by the receiver.
The output of the interference suppressors
105-
106 is input into
synthesis filter bank
107 where the signals are added to produce a composite
interference-suppressed signal. Particularly, synthesis filter bank
107
serves to create the interference-free, equalized, wideband signal d(n) from the
various interference-free sub-bands. In other words, the output of the synthesis
filter bank
107, d(n), is the equalized time-domain OFDM signal and thus
no further equalization is necessary (only the FFT of the appropriate symbols needs
to be performed). This is accomplished by up-sampling the outputs of the interference
suppressors
105-
106, filtering by f(n), then shifting the frequency
band of the resulting signal by {overscore (ω)}
1 through {overscore
(ω)}
X and finally summing all frequency-shifted signals.
With an estimate of the desired signal d(n) being output from synthesis filter
bank
107, normal OFDM processing can be performed on a "clean" signal. Particularly,
the cyclic prefix portion of the desired signal d(n) is removed by cyclic prefix
remover
108, and standard FFT processing takes place via the FFT
109.
The resulting signal is then processed via normal OFDM decoding.
FIG. 3 is a flow chart showing the operation of the receiver of FIG. 1. The
logic flow begins at step
301 where a first and a second antenna outputs
a first and a second wideband signal, respectively. At step
303, the first
and the second wideband signals are received at a first and a second filter bank
where they are partitioned into a plurality of smaller, narrowband, signals. As
discussed above, each filter bank outputs X narrowband signals, each having the
same bandwidth in the preferred embodiment and a center frequency of {overscore
(ω)}
l (l=1, . . . ,X). At step
305, the various narrowband
signals are output to interference suppression circuitry
105-
106
where interference suppression takes place. Particularly, a single interference
suppressor will receive the same sub-band signal output from all filter banks
103-
104.
For example, interference suppressor
105 will receive all sub-bands centered
at ω
1, while interference suppressor
106 may receive all
sub-bands centered at ω
X. At step
307 interference suppression
takes place on the various sub-bands and the interference-free sub-bands are supplied
to synthesis filter bank
107 where the interference-free wideband signal
is reconstructed (step
309).
As discussed above, the various interference suppressors can be designed using
a frequency-shifted and filtered version of the pilot sequence. Cyclic-prefix communication
systems incorporate redundancy by repeating the last L
cp symbols in
a data block of N symbols at the beginning of the data block. (The interference
suppression techniques being described herein are applicable to systems using other
forms of cyclic redundancy. For example, the first L
cp symbols in a
data block of N symbols can be repeated at the end of the data block. In another
example, the redundancy can be present at both the beginning and end of the data
block. Additionally, these interference suppression techniques are applicable to
systems employing no cyclic redundancy) Even after frequency shifting by -{overscore
(ω)}
l and filtering with f(n) and then decimating, the resulting
signal, y
l(n,b), will still have the cyclic prefix property as long
f(n) has a finite impulse response (FIR) and has reasonable attenuation in its
tails (in the preferred embodiment, f(n) is a pulse with a square-root raised-cosine
spectrum). Note that a time block number, b, was added to the resulting signal,
y
l(n,b), in order to track the placement of the cyclic prefixes (if
present). If the cyclic prefixes are not present, only the pilot-only computation
of the interference suppressor that is described below is possible. In a manner
similar to that described by H. Cheon and D. Hong, "A Blind Spatio-Temporal Equalizer
Using Cyclic Prefix in OFDM Systems,"
IEEE ICASSP-2000, Istanbul, Turkey,
the cyclic redundancy in y
l(n,b) can be exploited in finding space-time
combining weights that equalize the desired signal and suppress signals whose cyclic
prefixes are not time aligned with the desired user (i.e., asynchronous interferers).
Basically the blind space-time combining weights operate by making the cyclic prefix
portion of the equalized data block equal to the last L′
cp symbols
of the same equalized data block (L′
cp is the effective cyclic
prefix length of the OFDM signal after filtering with f(n) and decimating). Let
the M×1 received signal for data block b be given as:
##EQU3##
where s
l(n,b) is the desired user's symbols at time n on block b
(s
l(n,b) is found by multiplying the original OFDM signal, d(n) (at
the appropriate times), by the phase shift e
-j{overscore (ω)}ln,
filtering by f(n), and then decimating), h
ml (0≦m≦l-1)
is the effective channel (of length L) for filter bank l, and z
l(n,b)
incorporates noise plus synchronous and asynchronous interference (the receiver
noise and interference will also be multiplied by the phase shift e
-j{overscore
(ω)}ln and decimated). It is assumed
that the received symbols on block b for 0≦n≦N
f-1 (where
N
f is the effective data block size for the decimated signal) are synchronized
with the data symbols on block b, s
l(n,b) (recall that the cyclic prefix
property implies that s
l(n,b)=s
l(n+N
f,b-1) for
L
cp≦n≦-1). Let the M×1 space-time equalizer taps
be g
l(0) through g
l(L
g-1) (i.e., g
l(0)
through g
1(L
g-1) specify the impulse response of the interference
suppressor
105-
106 for sub-band l), then the equalized symbols are
given by (L
g is the length of the space-time equalizer):
##EQU4##
where D is the equalizer delay (typically equal to (L
g+L-1)/2).
The blind equalizer is found by making ŝ
l(n,b)=ŝ
l(n+N
f,b)
for L
cp′≦n≦-1 and 1≦b≦B.
Using matrix notation the equalizer can be found as the solution to the following
equation:
##EQU5##
where ML
g×1 g
l=[g
lT(0), .
. . , g
lT(L
g-1)]
T, and L
cp×ML
g
Y
l,1(b) and Y
l,2(b) are:
##EQU6##
Simplifying the above equations, g
l is found as the non-trivial
(i.e., non-zero) solution to:
gl=arg min{
glHYlgl} (7)
where:
##EQU7##
One non-trivial solution is to choose g
l as the eigenvector associated
with the smallest eigenvalue of Y
l. Note that this solution is completely
blind and thus will tend to significantly under-perform pilot-based techniques
and will have a scalar and phase ambiguity. In addition, this blind equalizer will
also have difficulties when an unknown interferer is synchronous, meaning that
the interferer's cyclic prefixes line up in time with the desired signal.
However, when a limited number of pilots symbols are available, the above
blind equalizer can be used with the pilots to find an excellent semi-blind equalizer
in the presence of both asynchronous and synchronous interference. The semi-blind
estimator modifies (7) by adding a term to account for the known pilot sequence
(symbols). Assuming that there is one block (including cyclic prefix) of all known
time-domain pilots, (7) becomes:
gl=arg min{γ
glHYlgl+|Yl,3gl-sl|
2} (9)
where γ is a weighting that can emphasize/de-emphasize the blind minimization
versus the pilot minimization (in the preferred embodiment γ=1), and (N
f+L
cp
′)×ML
g Y
l,3 and (N
f+L
cp′)×1
s
l are given by (data block b=1 is assumed to contain the pilot symbols):
##EQU8##
Note that Y
l,3g
l is simply the equalized symbols and the
equalizer is designed to minimize the mean square error between the equalized symbols
and the pilots (along with the blind criteria). The solution to (9) is:
gl=(γ
Yl+Yl,3HYl,3)
-1Yl,3Hsl (12)
Thus, in the various embodiments of the present invention three interference
suppression techniques may be utilized by interference suppressors
105-
106.
The first technique is a blind-only technique in that it does not need pilot symbols
to compute the interference suppression filters but instead uses the cyclic redundancy
(i.e., cyclic prefix) in the transmitted desired signal to design the interference
suppression filter. The second technique is a pilot-only method that designs the
interference suppression filter to operate using pilot symbols while not taking
into account the cyclic redundancy in the transmitted desired signal. (Thus this
pilot-only technique can be used for systems with no cyclic redundancy). Finally,
the third technique is a semi-blind method that designs the interference suppression
filter to operate using pilot symbols while also taking into account the cyclic
redundancy in the transmitted desired signal.
While the invention has been particularly shown and described with reference
to a particular embodiment, it will be understood by those skilled in the art that
various changes in form and details may be made therein without departing from
the spirit and scope of the invention. For example, the specific implementation
of the filter bank in this invention is just one possible implementation. Other
implementations of a filter bank are possible and will result in an equivalent
receiver. For example, as specified in J. G. Proakis and D. G. Manolakis,
Digital
Signal Processing, Second Edition, Macmillan Publishing, New York, 1992, instead
of having a single low-pass filter f(n), an equivalent implementation is to use
a different band-pass filter, f
1(n) through f
X(n), for each
of the X center frequencies. In the equivalent implementation after band-pass filtering,
each of the X signals is then decimated by X and then shifted in frequency (as
opposed to frequency shifting, filtering by low-pass f(n) and then decimating by
X). It is intended that such changes come within the scope of the following claims.
*
