Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-03-28
2001-10-02
Ghebretinsae, Temesghen (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C714S794000
Reexamination Certificate
active
06298102
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for performing an improved approximated maximum likelihood decoding in digital communications applications and to a maximum likelihood receiver embodying means for carrying out said method. More particularly, the invention relates to such a method and receiver, when a significant source of channel interference is the co-channel interference.
BACKGROUND OF THE INVENTION
Interference is an unavoidable occurrence in communication systems. Therefore, communications systems designed for today's needs are provided with means for dealing with interference. Interference can be of different types. Prior art methods provide means for dealing with this problem, which are implemented in existing equipment.
One detection method used in the art utilizes a Maximum Likelihood Receiver.
As is known, under the white Gaussian noise assumption, the standard decision metric is given by
∑
n
&LeftBracketingBar;
y
n
-
d
n
&RightBracketingBar;
2
(
1
)
wherein d
n
is the hypothized received signal excluding the additive channel interference, and y
n
denotes the received signal, i.e. y
n
=d
n
+e
n
, where e
n
is the additive channel interference. The maximum likelihood receiver seeks for the input sequence that brings (1) to a maximum.
The metric of equation (1) is optimal in a maximum likelihood sense for additive white Gaussian noise, but is not optimal in the presence of significant co-channel interference (CCI).
CCI occurs when two or more signals overlap in frequency and are present concurrently. CCI is a severe hindrance to frequency and time-division multiple-access communications and is typically minimized by interference rejection/suppression techniques. One case in which this disturbance is severe is that of cellular radio networks where one or more secondary signals from nearby cells can interfere with the desired primary signal.
WO 96/11533 discloses a method for signal detection, in which a primary signal and at least one interfering co-channel signal are received on the same channel. The received signals have different, but known training sequences, and different transmission channels on the radio path. The transmission channel estimates of the primary signal and the interfering co-channel signal are determined by their training sequences. The primary signal is detected by utilizing the transmission channel estimates of both primary signal and the interfering co-channel signal.
EP 667683 discloses a communication system in which known data sequences characterise both wanted signals and interfering co-channel signals. The system comprises a receiver with sequence identifier means which includes a store for known data sequences and a correlator for correlating received signals with the stored known data sequences, so as to identify the relative location in time of received signal sequences; impulse response estimation meansfor estimating the impulse response of the received signal for which the location has been identified; means for producing a set of possible signal configurations in response to the estimated impulse responses; and detector means to filter the wanted signal by comparing the signals of each set with the received signal.
Although the art has dealt with this problem in various ways, no method has been provided which is on the one hand efficient when CCI is the dominant interference source, and on the other hand is robust and does not require a complicated receiver.
It is therefore an object of this invention to provide an approximated maximum likelihood decoding method which improves over the prior art methods and produces a better approximation when CCI is a significant or is the dominant interference source.
It is another purpose of this invention to provide a metric which permits to carry out said method.
It is a further purpose of this invention to provide a metric for carrying out said method.
It is a still further purpose of this invention to provide a Maximum Likelihod Receiver, which is more efficient than the prior art ones when CCI is a significant or the dominant interference source.
It is a still further purpose of this invention to provide such a receiver which is not complicated, and is robust.
Other purposes and advantages of this invention will appear as the description proceeds.
SUMMARY OF THE INVENTION
The invention comprises a method for conducting an approximated maximum likelihood decoding on a received, digitally modulated signal at the receiving side, in the presence of co-channel interference, using an improved decision metric.
Let the additive channel interference be denoted by e
n
=&egr;
n
+c
n
, wherein &egr;
n
is the white noise interference component and c
n
is the CCI component, whose envelope is denoted by C. The method is especially suited for the case where the CCI signal is constant envelope, but may be used in other scenarios as well, in which case, C denotes the average envelope of the signal. The decision method of the invention is based on an improved decision metric which is given by:
∑
n
(
&LeftBracketingBar;
y
n
-
d
n
&RightBracketingBar;
-
β
C
)
2
wherein:
y
n
is the received signal;
d
n
is the hypothesized received signal excluding the additive channel interference;
&bgr; is an empirical coefficient, wherein 0<&bgr;, preferably 0<&bgr;≦1.
C is the envelope of the CCI signal.
Practically, the amplitude of the CCI signal (C) is usually unknown. Therefore, it needs to be estimated. For that purpose, one may use the average of |y
n
−d
n
|
2
, for determining C
2
. For example, in a decision directed detection mode, this average is formed by utilizing the knowledge of the values of previous data symbols, d
1
, d
2
, . . . d
n−1
at time index n, e.g., Ĉ ;
n
2
=&agr;Ĉ ;
n−1
2
+(1−&agr;)|y
n−1
−d
n−1
|
1
, where 20<&agr;<1 is some constant, e.g., &agr;=0.95, and Ĉ ; is the estimated CCI envelope. In Viterbi based detection scheme, an estimation for C
2
may be obtained on a per survivor basis, in the same manner as channel estimates are obtained, e.g. in Raheli R. et. al, “Per Survivor Processing”, Digital Signal Processing 3, pg. 175×187 (1993).
When &bgr;=0, viz. the CCI is not significant, the metric is reduced to the standard one. Therefore, in the method according to the invention &bgr;>0. On the other hand, &bgr;=1 is optimal if the CCI term is dominant compared to the additive noise interference. Typically, the value of &bgr; is about 0.8.
The invention also comprises an approximate Maximum Likelihood Receiver which carries out said method and comprises means for calculating the improved decision metric
∑
n
(
&LeftBracketingBar;
y
n
-
d
n
&RightBracketingBar;
-
β
C
)
2
.
REFERENCES:
patent: 5251233 (1993-10-01), Labedz et al.
patent: 5263026 (1993-11-01), Parr et al.
patent: 5818876 (1998-10-01), Love
patent: 0667683 (1995-04-01), None
patent: 9611533 (1996-04-01), None
patent: 9726718 (1997-07-01), None
“Maximum likelihood sequence estimation of digital sequences in the presence of intersymbol interference” IEEE vol. IT 18 No. 3, May 1972.*
Raheli et al. “Per Survivor Processing” Digital Signal Processing 3, pp. 175-187 (1993).
Burshtein David
Rainish Doron
Baker & Botts L.L.P.
DSPC Israel Ltd.
Ghebretinsae Temesghen
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