U.S. patent application number 10/337962 was filed with the patent office on 2003-08-28 for spread-spectrum, relative-signal-level data detection using matched-filter obtained side information.
Invention is credited to Schilling, Donald L..
Application Number | 20030161388 10/337962 |
Document ID | / |
Family ID | 23011560 |
Filed Date | 2003-08-28 |
United States Patent
Application |
20030161388 |
Kind Code |
A1 |
Schilling, Donald L. |
August 28, 2003 |
SPREAD-SPECTRUM, RELATIVE-SIGNAL-LEVEL DATA DETECTION USING
MATCHED-FILTER OBTAINED SIDE INFORMATION
Abstract
An improvement to a spread-spectrum base station receiver having
a matched filter. A symbol sampler samples at the symbol time
T.sub.S, a plurality of symbol samples from the matched filter. A
relative-signal-level decoder decodes the plurality of
received-symbol samples, thereby generating a plurality of
decoded-symbol samples. A noise sampler samples at a plurality of
chip times kT.sub.C, but not at the symbol time T.sub.S, a
plurality of noise samples from the matched filter, before, after,
or a combination of before and after, a symbol sample. An estimator
processes the plurality of noise samples. The erasure detector
detects for each decoded-symbol sample from the plurality of
decoded-symbol samples and from the plurality of noise samples, an
erasure condition for the corresponding decoded-symbol sample, and
thereby generates an erasure signal. An erasure decoder erasure
decodes the input data using the erasure signals from the erasure
detector.
Inventors: |
Schilling, Donald L.; (Palm
Beach Gardens, FL) |
Correspondence
Address: |
DAVID NEWMAN CHARTERED
Centennial Square
P. O. Box 2728
La Plata
MD
20646-2728
US
|
Family ID: |
23011560 |
Appl. No.: |
10/337962 |
Filed: |
January 7, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10337962 |
Jan 7, 2003 |
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10038822 |
Jan 8, 2002 |
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6512786 |
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10337962 |
Jan 7, 2003 |
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09265706 |
Mar 9, 1999 |
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6359925 |
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Current U.S.
Class: |
375/147 ;
375/152 |
Current CPC
Class: |
H04L 1/0047 20130101;
H04L 1/0045 20130101 |
Class at
Publication: |
375/147 ;
375/152 |
International
Class: |
H04B 001/707 |
Claims
I claim:
1. An improvement to a spread-spectrum receiver at a base station
in a direct-sequence code-division-multiple-access (DS-CDMA)
system, having a plurality of spread-spectrum signals modulating
differentially-encoded symbols, with each spread-spectrum signal in
the plurality of spread-spectrum signals having a chip-sequence
signal lasting a symbol time T.sub.S, and with each chip-sequence
signal different from other chip-sequence signals used by other
spread-spectrum signals in the plurality of spread-spectrum
signals, with the spread-spectrum receiver including a matched
filter having an impulse response matched to a desired
chip-sequence signal in the plurality of chip-sequence signals, for
detecting a desired spread-spectrum signal in the plurality of
spread-spectrum signals arriving at the spread-spectrum receiver,
the improvement comprising: a symbol sampler, coupled to said
matched filter, for sampling at a plurality of symbol times
nT.sub.S, where n is an index to each symbol time, a plurality of
symbol samples; a relative-signal-level decoder, coupled to said
symbol sampler, for relative-signal-level decoding the plurality of
symbol samples, thereby generating a plurality of decoded-symbol
samples; a noise sampler, coupled to said matched filter, for
sampling at any of before, after, or a combination of before and
after each decoded-symbol sample, at a plurality of chip times
kT.sub.C, but not at the plurality of symbol times nT.sub.S, a
plurality of noise samples; an estimator, coupled to said noise
sampler, for processing the plurality of noise samples to generate
a noise estimate; an erasure detector, coupled to said estimator
and to said symbol sampler, for detecting from a particular symbol
sample corresponding in time to the particular decoded-symbol
sample and the noise estimate, an erasure condition, thereby
generating an erasure signal; and an erasure decoder, having an
erasure input coupled to said erasure detector and a data input
coupled to said relative-signal-level decoder, responsive to the
erasure signal, for erasure decoding the data input.
2. An improvement to a spread-spectrum receiver in a
direct-sequence code-division-multiple-access (DS-CDMA) system
having a plurality of spread-spectrum signals modulating
differentially-encoded data, with each spread-spectrum signal in
the plurality of spread-spectrum signals having a chip-sequence
signal lasting a symbol time T.sub.S, and with each chip-sequence
signal different from other chip-sequence signals used by other
spread-spectrum signals in the plurality of spread-spectrum
signals, with the spread-spectrum receiver including despreading
means for detecting a desired spread-spectrum signal in the
plurality of spread-spectrum signals arriving at the
spread-spectrum receiver, the improvement comprising: sampler
means, coupled to said matched filter, for sampling at a plurality
of symbol times nT.sub.S, a plurality of symbol samples;
relative-signal-level means, coupled to said sampler means, for
relative-signal-level decoding the plurality of symbol samples,
thereby generating a plurality of decoded-symbol samples; said
sampler means for sampling at any of before, after, or a
combination of before and after each symbol sample corresponding to
the particular decoded-symbol sample from a plurality of
decoded-symbol samples, at a plurality of chip times kT.sub.C, but
not at the plurality of symbol times nT.sub.S, a plurality of noise
samples; estimate means, coupled to said sampler means, for
processing the plurality of noise samples to generate a
corresponding noise estimate for each decoded-symbol sample;
erasure-detection means, coupled to said estimate means, for
detecting from the corresponding noise estimate and a symbol sample
corresponding to each decoded-symbol sample from the plurality of
decoded-symbol samples, an erasure condition, thereby generating a
corresponding erasure signal; and an erasure decoder, having an
erasure input coupled to said erasure-detection means and a data
input coupled to said relative-signal-level means, responsive to
the erasure signal, for erasure decoding the plurality of
decoded-symbol samples.
3. The improvement as set forth in claim 2 with said sampler means
including: a noise sampler, coupled to said matched filter, for
sampling at the plurality of chip times kT.sub.C, but not at the
plurality of symbol times nT.sub.C, the plurality of noise samples;
and a symbol sampler, coupled to said matched filter, for sampling
at the plurality of symbol times nT.sub.S, the plurality of symbol
sample.
4. The improvement as set forth in claim 2, with said estimate
means including: a delay device, coupled to said sampler means, for
delaying a symbol sample one symbol time T.sub.S, thereby
generating a delayed-symbol sample; a combiner, coupled to said
delay device and to said symbol sampler, for subtracting the
delayed-symbol sample from the symbol sample, thereby generating a
relative-signal-level sample; and a comparator, coupled to said
combiner and having a threshold input with a threshold, for
comparing the relative-signal-level sample to the threshold,
thereby generating a decoded-symbol sample of the plurality of
decoded-symbol samples.
5. The improvement as set forth in claim 3, with said estimate
means including: a delay device, coupled to said symbol sampler,
for delaying a symbol sample one symbol time T.sub.S, thereby
generating a delayed-symbol sample; a combiner, coupled to said
delay device and to said symbol sampler, for subtracting the
delayed-symbol sample from the symbol sample, thereby generating a
relative-signal-level sample; and a comparator, coupled to said
combiner and having a threshold input with a threshold, for
comparing the relative-signal-level sample to the threshold,
thereby generating a decoded-symbol sample of the plurality of
decoded-symbol samples.
6. The improvement as set forth in claim 2, 3 or 4, with said
estimate means including a register for storing the plurality of
noise samples.
7. A method for improving a spread-spectrum receiver in a
direct-sequence code-division-multiple-access (DS-CDMA) system
having a plurality of spread-spectrum signals modulating
differentially-encoded data, with each spread-spectrum signal in
the plurality of spread-spectrum signals having
relative-signal-level encoded symbol samples and a chip-sequence
signal lasting a symbol time T.sub.S, and with each chip-sequence
signal different from other chip-sequence signals used by other
spread-spectrum signals in the plurality of spread-spectrum
signals, with the spread-spectrum receiver including despreading
means for detecting a desired spread-spectrum signal in the
plurality of spread-spectrum signals arriving at the
spread-spectrum receiver, the improvement comprising the steps of:
sampling, at a plurality of symbol times nT.sub.S, where n is an
index to each symbol time, a plurality of symbol samples;
relative-signal-level decoding the plurality of symbol samples,
thereby generating a plurality of decoded-symbol samples; sampling
at any of before, after, or a combination of before and after each
symbol sample, at a plurality of chip times kT.sub.C, but not at
the plurality of symbol times nT.sub.S, a plurality of noise
samples; processing the plurality of noise samples to generate a
corresponding noise estimate for each decoded-symbol sample;
detecting from the corresponding noise estimate and symbol sample
corresponding to each decoded-symbol sample from the plurality of
decoded-symbol samples, an erasure condition, thereby generating a
corresponding erasure signal; and erasure decoding, in response to
the erasure signals, the input-data symbols.
Description
RELATED PATENTS
[0001] This patent stems from a continuation application of U.S.
patent application Ser. No. 10/038,822, and filing date of Jan. 8,
2002, entitled MATCHED-FILTER OBTAINED SIDE INFORMATION FOR
RELATIVE-SIGNAL-LEVEL DATA DETECTION FROM A SPREAD-SPECTRUM SIGNAL
by inventor, DONALD L. SCHILLING, and from a continuation
application of U.S. patent application Ser. No. 09/265,706, and
filing date of Mar. 9, 1999, entitled RELATIVE-SIGNAL-LEVEL DATA
DETECTION FROM A SPREAD-SPECTRUM SIGNAL USING MATCHED-FILTER
OBTAINED SIDE INFORMATION by inventor, DONALD L. SCHILLING, now
U.S. Pat. No. 6,359,925. The benefit of the earlier filing date of
the parent patent application is claimed for common subject matter
pursuant to 35 U.S.C. .sctn. 120.
BACKGROUND OF THE INVENTION
[0002] In a direct-sequence (DS) code-division-multiple-access
(CDMA) system having a base station and a plurality of remote
stations transmitting to the base station, the spread-spectrum
signals from many of the remote stations arrive at the base station
simultaneously. The spread-spectrum signal from each remote station
may arrive at the base station with a different power level with
different symbol and chip arrival times. Further, the desired
spread-spectrum signal at a particular spread-spectrum receiver
receiving a particular spread-spectrum channel from a particular
remote station, may be fading, and is, on occasion, not detectable,
or has a high error rate.
[0003] Diversity coding, forward-error-correction (FEC) decoding,
and interference cancellation are approaches to reducing the error
rates. RAKE may be used to combine the strongest signal paths in a
fading or multipath environment. These approaches do not, in
general, take advantage of the unique noise environment of a
DS-CDMA system, in which noise, on the average, is due to the
multiple spread-spectrum signals from the plurality of remote
stations.
SUMMARY OF THE INVENTION
[0004] A general object of the invention is to reduce error rate in
a direct-sequence code-division-multiple-access (DS-CDMA)
spread-spectrum system.
[0005] Another object of the invention is to use the noise
interference from the multiple users in the DS-CDMA system as side
information in reducing error rate for decoding
differentially-encoded data.
[0006] According to the present invention, as embodied and broadly
described herein, an improvement to a spread-spectrum receiver at
the base station in a direct-sequence code-division-multiple-access
(DS-CDMA) system is provided. The spread-spectrum receiver, in a
DS-CDMA system has, at an input, a plurality of spread-spectrum
signals, arriving from a plurality of remote users, respectively.
Each spread-spectrum signal in the plurality of spread-spectrum
signals has a differentially encoded-data symbol. Each
differentially encoded-data symbol is spread-spectrum processed by
a chip-sequence signal lasting a symbol time T.sub.S. Each remote
user may be operating at a different symbol time T.sub.si, where i
is an index for the different symbol time. Each chip-sequence
signal in the plurality of chip-sequence signals is different, due
to a different chip sequence, from other chip-sequence signals used
by other spread-spectrum signals in the plurality of
spread-spectrum signals.
[0007] Each spread-spectrum receiver in the base station includes a
matched filter having an impulse response matched to a desired
chip-sequence signal in the plurality of chip-sequence signals. The
matched filter detects a desired spread-spectrum signal in the
plurality of spread-spectrum signals arriving at the
spread-spectrum receiver at the base station. The desired
spread-spectrum signal is spread-spectrum processed with a desired
chip-sequence signal.
[0008] The improvement comprises a symbol sampler, a noise sampler,
a relative-signal-level decoder, an estimator, an erasure detector,
and an erasure decoder. The symbol sampler samples at a plurality
of symbol times nT.sub.S, a plurality of symbol samples from the
desired matched filter. The integer n indexes the plurality of
symbol times. Each symbol sample has time duration T.sub.S. The
relative-signal-level decoder decodes, with reference to the
relative-signal-level of the current and previously received symbol
samples, the plurality of symbol samples, thereby generating a
plurality of decoded-symbol samples. As a result of noise and
interference, these samples are non-binary. Hard limiting these
samples prior to processing is not a preferred embodiment, but is
an option included herein.
[0009] The noise sampler samples before, after, or a combination of
before and after each symbol sample at a plurality of chip times
kT.sub.C, but not at the symbol time T.sub.S, a plurality of noise
samples. The estimator processes the plurality of noise samples to
generate a noise estimate. The erasure detector detects, for each
symbol sample and from the noise estimate, an erasure condition,
and thereby generates an erasure signal. In response to the data
and the erasure signals, the erasure FEC decoder, erasure decodes
the symbols, as is well known in the art.
[0010] Additional objects and advantages of the invention are set
forth in part in the description which follows, and in part are
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention also may be
realized and attained by means of the instrumentalities and
combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate preferred
embodiments of the invention, and together with the description
serve to explain the principles of the invention.
[0012] FIG. 1 is a block diagram of a relative-signal-level data
symbol detector using matched-filter obtained side information;
[0013] FIG. 2 shows sampling at chip time T.sub.C and symbol time
T.sub.S;
[0014] FIG. 3 shows average noise power during a symbol time
T.sub.S;
[0015] FIG. 4 shows a threshold between a 1 and 0 bit; and
[0016] FIG. 5 shows an erasure region.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] Reference now is made in detail to the present preferred
embodiments of the invention, examples of which are illustrated in
the accompanying drawings.
[0018] The present invention provides an improvement to a
spread-spectrum receiver in a direct-sequence
code-division-multiple-access (DS-CDMA) system. The DS-CDMA system
is assumed to have a base station and a plurality of remote
stations. At the base station a plurality of spread-spectrum
signals arrive from the plurality of remote stations, respectively.
Each spread-spectrum signal modulates differentially encoded
symbols. The differentially encoded symbols typically are data to
be transmitted over a particular spread-spectrum channel. More
particularly, the input data are differentially encoded, using
techniques well-known in the art.
[0019] The present invention is anticipated to be implemented with
a digital signal processor (DSP) or application specific integrated
circuit (ASIC). The means plus functions, and their embodiment as a
"device", "detector", "decoder" and/or "estimator", include the
digital signal processor or application specific integrated circuit
with software. Thus, a device, detector, decoder, and/or estimator,
may be a part or a portion of a digital signal processor or ASIC,
and software.
[0020] Each spread-spectrum signal in the plurality of
spread-spectrum signals has a chip-sequence signal lasting a symbol
time T.sub.S. Each remote user may be operating at a different
symbol time T.sub.si, where i is an index for the different symbol
time. Each chip-sequence signal in the plurality of chip-sequence
signals is different from other chip-sequence signals used by other
spread-spectrum signals in the plurality of spread-spectrum
signals. Each chip-sequence signal is different since a different
chip sequence is used for each chip-sequence signal in the
plurality of chip-sequence signals.
[0021] The invention anticipates the use of interfering
spread-spectrum signals from the DS-CDMA system as side information
to reduce error rate. Consider a received plurality of
spread-spectrum signals r(t), which includes a desired
spread-spectrum signal s.sub.o(t) and a multiplicity of interfering
spread-spectrum signals s.sub.i(t), where i is an index referring
to each of the multiplicity of interfering signals. Then the
received plurality of spread-spectrum signals r(t) may be expressed
as a sum of the desired spread-spectrum signal s.sub.o(t) plus the
sum of the interfering spread-spectrum signals s.sub.i(t):
r(t)=s.sub.o(t)+.SIGMA.s.sub.i(t)
[0022] On the average, only half of the interfering spread-spectrum
signals are changing a data bit, or data symbol, from a +1 to a -1
symbol or bit, or from a -1 to a +1 symbol or bit, at any point in
time. Thus, during a symbol time, 0<t.ltoreq.T.sub.S, the
interfering spread-spectrum signals s.sub.1(t), s.sub.2(t),
s.sub.3(t), . . . s.sub.N(t), on average are a "one" ("1") bit half
of the time. Thus, half of the interfering spread-spectrum signals
change from -1 to +1 and +1 to -1, and the other half of the
interfering spread-spectrum signals do not change state, and go
from +1 to +1 and -1 to -1.
[0023] When the DS-CDMA system is operating at or near capacity,
then the signal-to-interference ratio (SIR) at the output of the
matched filter of the spread-spectrum receiver may be 3 dB. With
the observation, for the DS-CDMA system, that if half of the
interfering signals changed state from +1 to -1 or vice versa, then
the interference level does not change for the other half of the
interfering spread-spectrum signals. This is because half of the
interfering spread-spectrum signals transition from a -1 to a -1 or
from a +1 to a +1, which result in no change in signal level. Thus,
on the average, half of the interfering spread-spectrum signals did
not change state, then on the average, half of the interfering
spread-spectrum signals are constant. Thus, half of the noise,
caused by the multiuser interference, is correlated, and half of
the noise is not correlated. This observation from the DS-CDMA
system is used to reduce error rate with the present invention.
[0024] In the exemplary arrangement shown in FIG. 1, the
spread-spectrum receiver includes despreading means, which may be
embodied as a matched filter 21. The matched filter 21 has an
impulse response matched to a desired chip-sequence signal in the
plurality of chip-sequence signals. The matched filter 21 detects a
desired spread-spectrum signal in the plurality of spread-spectrum
signals arriving at the spread-spectrum receiver. In general, the
matched filter 21 is for a complex signal, that is, signals having
an in-phase component and a quadrature-phase component. Designing a
particular embodiment for a complex signal is anticipated by the
present invention. References to the various signals, symbols and
estimate, in this disclosure includes embodiments as a complex
signal, to embodiments as a real signal, i.e., a real component of
a complex signal, and to embodiments as a magnitude of a complex
signal.
[0025] While the matched filter is the preferred embodiment, the
despreading means may be embodied as a correlator or a bank of
correlators. The correlator(s) would include a chip-sequence
generator, for generating a chip-sequence signal matched to the
desired chip-sequence signal in the plurality of chip-sequence
signals, as is well-known in the art.
[0026] Each spread-spectrum signal in the plurality of
spread-spectrum signals has a chip-sequence signal lasting a symbol
time T.sub.S. Each remote user may be operating at a different
symbol time T.sub.si, where i is an index for the different symbol
time. Each chip-sequence signal in the plurality of chip-sequence
signals is different from other chip-sequence signals used by other
spread-spectrum signals in the plurality of spread-spectrum
signals. Each chip-sequence signal is different since a different
chip sequence is used for each chip-sequence signal in the
plurality of chip-sequence signals.
[0027] In the exemplary arrangement shown in FIG. 1, the
spread-spectrum receiver includes a matched filter 21, which has an
impulse response matched to a desired chip-sequence signal in the
plurality of chip-sequence signals. The desired chip-sequence
signal is for the desired spread-spectrum signal to be received by
the receiver. The matched filter 21 detects the desired
spread-spectrum signal from the plurality of spread-spectrum
signals arriving at the spread-spectrum receiver.
[0028] The improvement comprises sampler means,
relative-signal-level means, estimate means, erasure-detection
means, and an erasure decoder 28. The sampler means is coupled to
the matched filter 21. The relative-signal-level means is coupled
to the sampler means. The estimate means is coupled to the sampler
means. The erasure-detection means is coupled to the estimate means
and to the sampler means. The erasure decoder 28 has an erasure
input coupled to the erasure-detection means and a data input
coupled to the relative-signal-level means.
[0029] The sampler means samples, as shown in FIG. 2, at a
plurality of symbol times nT.sub.S, the plurality of symbol samples
from the matched filter 21. The plurality of symbol times nT.sub.S
is the time occurrence of a plurality of symbol samples, and
repeats every symbol time T.sub.S. The integer n is an index to
each symbol time.
[0030] The sampler means samples at a plurality of chip times
kT.sub.C, but not at a plurality of symbol times nT.sub.S, a
plurality of noise samples, from the matched filter 21. The chip
time T.sub.C is the time duration of a chip, and repeats every chip
time T.sub.C. The sequence of chip times is indexed by factor k.
The sampling of the plurality of noise samples may occur before,
after, or a combination of before and after, the sampling at each
symbol time for each symbol sample. FIG. 3 shows that the symbol
sample for a particular sequence of symbols may be non-synchronous
for symbol samples for other sequences of symbol samples, from
other spread-spectrum channels.
[0031] The relative-signal-level means decodes adjacent symbol
samples of the plurality of symbol samples, thereby generating a
plurality of decoded-symbol samples. The decoding preferably is
from subtracting the signal level of adjacent symbol samples. The
result is preferably a non-binary word, although hard limiting,
which produces a binary word, could be used in a poorer quality
system in which cost is of primary concern.
[0032] The estimate means estimates, or filters, a plurality of
noise samples from the sampler means, to generate a noise estimate.
The noise estimate may be a low-pass filtered version of the
plurality of noise samples. Alternatively, using a digital signal
processor embodiment or application specific integrated circuit
(ASIC) embodiment, the estimate means may use a mathematical
algorithm for estimating the level of noise. The mathematical
algorithm may include, but is not limited to, straight averaging;
root means square (RMS) averaging; and determining a median value
in the plurality of noise samples.
[0033] The erasure-detection means detects from the noise estimate
corresponding to a particular decoded-symbol sample from the
plurality of decoded-symbol samples, an erasure condition, and
thereby generates an erasure signal. The erasure condition might
occur when the ratio of the particular decoded-symbol sample to the
noise estimate, an SIR, is below a threshold, or when the magnitude
of the difference between the decoded-symbol sample corresponding
to the noise estimate is below the threshold.
[0034] The erasure decoder 28 may be embodied as an FEC decoder,
and has an erasure input and a data input. The erasure input is
coupled to the erasure-detection means, and the data input is
coupled to the relative-signal-level means. The erasure decoder 28
erasure decodes each decoded-symbol sample, using a corresponding
erasure signal. Typically, if the erasure signal from the
erasure-detection means indicated a high probability of error, that
is, the signal level falls between levels .DELTA..sub.1 and
.DELTA..sub.2 in FIG. 5, then the erasure decoder 28 employs this
added information when processing the syndrome formed in the FEC
decoder. FEC erasure decoders are well known in the art and can be
purchased commercially.
[0035] As illustratively shown in FIG. 1, the sampler means may
include a noise sampler 22 and symbol sampler 23. The symbol
sampler 23 is coupled to the matched filter 21. The symbol sampler
23 samples at a plurality of symbol times nT.sub.S, a plurality of
symbol samples. In a typical embodiment employing a digital signal
processor or an application specific integrated circuit (ASIC), the
symbol sampler 23 might be a gate, for gating the symbol sample
from the matched filter 21. The timing for sampling with the gate
comes from timing circuit 33.
[0036] The noise sampler 22 is coupled to the matched filter 21.
The noise sampler 22 typically is a gate for gating the output data
signal from the matched filter 21, at particular times. The gating
is the sampling of the digital output of the matched filter 21. The
noise sampler 22 samples, as illustrated in FIGS. 2 and 3, for each
symbol sample at the plurality of chip times kT.sub.C, but not at
the plurality of symbol times nT.sub.S, the plurality of noise
samples. The sampling of the plurality of noise samples may occur
before, after, or a combination of before and after, sampling of
the corresponding symbol sample.
[0037] Timing for the noise sampler 22 and for the symbol sampler
23 may be derived from acquisition and tracking circuits 31 of the
spread-spectrum receiver. The acquisition and tracking circuits may
derive timing from a header portion of a packet signal, or from a
separate synchronization channel. The acquisition and tracking
circuits 31 generate timing which controls a chip clock 32 for the
desired spread-spectrum signal to be received. The timing circuit
33, based on timing from the chip clock 32, generates appropriate
timing signals for triggering sampling of noise sampler 22 and
symbol sampler 23.
[0038] The relative-signal-level means is embodied as
relative-signal-level detector, which includes a delay device 41, a
combiner 42, and a comparator 43. The delay device 41 is coupled to
the symbol sampler 23. The delay device 41 delays an n-bit symbol
sample, one symbol time T.sub.S, thereby generating a
delayed-symbol sample.
[0039] The combiner 42 is coupled to the delay device 41 and to the
symbol sampler 23. The combiner 42 subtracts the delayed-symbol
sample from the symbol sample, thereby generating a
relative-signal-level sample.
[0040] The comparator 43 is coupled to the combiner 42. The
comparator 43 has a threshold input with a threshold, typically a
voltage level. The comparator 43 compares the relative-signal-level
sample to the threshold, thereby generating each decoded-symbol
sample of the plurality of decoded-symbol samples.
[0041] While the invention broadly applies to n-bit symbol samples,
where n is the number of bits per symbol, when the symbol samples
are binary digits or bits, then the relative-signal-level means
might be embodied as a differential decoder. Differential decoders
are well known in the art.
[0042] The estimate means may be embodied as an estimator 44, such
as a register or memory circuit, for storing and averaging the
plurality of noise samples. The estimate means may include a low
pass filter, or an algorithm for computing or determining an
average. The algorithm may be, by way of example, root means square
averaging, means square averaging, straight averaging, weighted
averaging, or determining a median value.
[0043] The erasure-detection means may be embodied as an erasure
detector 45. The erasure detector 45 is coupled to the symbol
sampler 23, the estimator 44 and the erasure decoder 28. The
erasure detector 45, using a particular symbol sample, from the
symbol sampler 23, and a corresponding noise estimate from the
estimator 44, generates an erasure signal. Typically, the erasure
detector 45 compares the symbol sample to the noise estimate, and
if the comparison failed to meet a certain criterion or crosses a
threshold, then the erasure detector 45 generates the erasure
signal to erasure decode the corresponding symbols.
[0044] FIG. 4 illustrates detection between a symbol=1 and a
symbol=0, without erasure decoding, by comparing the output of the
matched filter 21 to a threshold. FIG. 5 illustrates detection
between a symbol=1 and a symbol=0, with erasure decoding. With
erasure decoding, there is an in-between region, where an error has
a likelihood of occurring. The comparison of the symbol sample and
the noise estimate might be from a signal-to-interference ratio
(SIR) or energy ratio, and if the SIR for the particular symbol
sample and noise estimate failed to cross a threshold, then the
erasure signal indicates to erasure decode the particular symbol
sample. The criterion also may be based on the energy of the symbol
sample, and noise estimate, or from subtracting the noise estimate
from the symbol sample. Other algorithms or criteria may be used,
based on the symbol sample and the noise estimate, to determine if
the symbol sample were to be erasure decoded.
[0045] In use, a plurality of spread-spectrum signals arrive at the
input to the receiver. The matched filter 21 detects the desired
spread-spectrum signal from the plurality of spread-spectrum
signals, by having an impulse response matched to the desired
chip-sequence signal. At the output of the matched filter, the
symbol sampler 23 samples at each symbol time, nT.sub.S, to
generate a plurality of symbol samples. The noise sampler 22, for
each symbol sample, samples at a plurality of chip times kT.sub.C,
to generate a plurality of noise samples. The estimator averages or
filters, for each symbol sample, the plurality of noise samples, to
generate a noise estimate.
[0046] The erasure detector 45, for each symbol sample, uses a
noise estimate to generate an erasure signal. The erasure signal is
fed to the erasure input of the FEC decoder 28.
[0047] The relative-signal-level detector decodes the plurality of
symbol samples, to generate a relative-signal-level sample. The
n-bit relative-signal-level sample, or magnitude of the
relative-signal-level sample, is fed to the data input of the FEC
decoder 28. If the erasure signal were present to erase the symbol
sample, then the symbol sample is erased at the FEC decoder 28
input.
[0048] The invention includes a method for improving a
spread-spectrum receiver in a DS-CDMA system having a plurality of
spread-spectrum signals arriving at a base station from a plurality
of remote stations. Each spread-spectrum signal in the plurality of
spread-spectrum signals has relative-signal-level encoded-symbol
samples and a chip-sequence signal lasting a symbol time. The
chip-sequence signal is different from other chip-sequence signals
in the plurality of chip signals used by other spread-spectrum
signals in the plurality of spread-spectrum signals. The
spread-spectrum receiver has a matched filter with an impulse
response matched to a desired chip-sequence signal in the plurality
of chip-sequence signals. The matched filter detects a desired
spread-spectrum signal in the plurality of spread-spectrum signals
arriving at the spread-spectrum receiver.
[0049] The method comprises the steps of sampling at a plurality of
symbol times nT.sub.S, a plurality of symbol samples;
relative-signal-level decoding the plurality of symbol samples,
thereby generating a plurality of decoded-symbol samples; sampling,
for each symbol sample, at a plurality of chip times kT.sub.C, but
not at the plurality of symbol times nT.sub.S, a plurality of noise
samples; averaging the plurality of noise samples; detecting from
the symbol sample and from the plurality of noise samples, an
erasure condition, and thereby generating an erasure signal; and
erasure decoding the plurality of decoded-symbol samples using the
erasure signals. Erasure decoding is well known to those versed in
the art.
[0050] It will be apparent to those skilled in the art that various
modifications can be made to relative-signal-level data detection
from a spread-spectrum signal using matched-filter obtained side
information of the instant invention without departing from the
scope or spirit of the invention, and it is intended that the
present invention cover modifications and variations of
relative-signal-level data detection from a spread-spectrum signal
using matched-filter obtained side information, provided they come
within the scope of the appended claims and their equivalents.
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