Combined equalization and decoding techniques

Details Combined equalization and decoding techniques Combined equalization and decoding techniques

1999-09-10

2001-12-04

Pham, Chi (Department: 2631)

Pulse or digital communications

Receivers

Particular pulse demodulator or detector

C375S262000, C714S795000

Reexamination Certificate

active

06327317

ABSTRACT:

BACKGROUND
The present invention relates to coded modulation techniques, and more particularly to techniques for performing equalization and decoding in a communications system that utilizes coded modulation.
The concept of multi-level coding or block coded modulation (BCM) was introduced in H. Imai and S. Hirakawa, “A new multi-level coding method using error correcting codes,”
IEEE Transactions on Information Theory
, vol. IT-23, pp. 371-377, May 1977; S. Sayegh, “A class of optimum block codes in signal space,”
IEEE Transactions on Communications
, vol. COM-34, pp. 1043-1045, October 1986; and A. R. Calderbank, “Multi-level codes and multi-stage decoding,”
IEEE Transactions on Communications
, vol. COM-37, pp. 222-229, March 1989. Recent publications have demonstrated that BCM is an attractive approach for combining modulation and coding for Rayleigh fading channels. Such publications include N. Seshadri and C.-E. W. Sundberg, “Multi-level coded modulations for fading channels,”
Proceedings of the Fifth Tirennia International Workshop on Digital Communications
(E. Biglieri and M. Luise, eds.), pp. 341-352, Elsevier Science Publishers B.V., 1992; N. Seshadri and C.-E. W. Sundberg, “Coded modulation with time diversity, unequal error protection and low delay for the Rayleigh fading channel,”
Proceedings of First Universal Conference on Portable and Mobile Communications
, pp. 283-287, September 1992; and N. Seshadri and C.-E. W. Sundberg, “Multi-level block coded modulations with unequal error protection for the Rayleigh fading channel,”
European Transactions on Communications
, vol. 4, pp. 325-334, May 1993. One of the main issues in evaluating the applicability of BCM to the North American TDMA cellular standard (IS-136) or to similar wireless radio applications is the issue of equalization, which is necessitated by the delay spread of the channel.
The conventional approach for equalization of coded modulation systems is to perform the equalization and decoding as two independent steps with soft information being passed from the equalizer to the decoder. Such a technique is described in Y. Liu, “Performance of adaptive equalization and soft decision decoding techniques over TDMA digital cellular radio channels,”
Proceedings of IEEE GLOBECOM
'92, pp. 27.6.1-27.6.5, December 1992.
By performing combined equalization (demodulation) and decoding it is possible to eliminate the step of soft information generation. Unless soft information generation is accurate, there is the potential for information loss and subsequent loss in decoder performance. In this sense, combined equalization and decoding is closer to optimal.
A combined equalization and decoding technique was described for trellis coded modulation (TCM) systems in R. Mehlan and H. Meyr, “Combined equalization/decoding of trellis coded modulation on frequency-selective fading channels,”
Proc. of the Fifth Tirrenia International Workshop on Digital Communications
(E. Biglieri and M. Luise, eds.), pp. 341-352, Elsevier Science Publishers B.V., 1992. The algorithm has been developed further in S. A. Fechtel and H. Meyr, “A new mobile radio transceiver concept using low-complexity combined equalization/trellis decoding and a near-optimal receiver sync strategy,”
Proceedings of IEEE PIMRC
'92, pp. 382-386, October 1992 [9, 10]; and in S. A. Fechtel and H. Meyr, “Combined equalization, decoding and antenna diversity combining for mobile/personal digital radio transmission using feedforward synchronization,”
Proceedings of IEEE VTC
'93, pp. 633-636, May 1993, and has been applied to the European GSM system.
The novel techniques disclosed herein improve upon the performance of the above disclosed techniques.
In order to perform equalization in a Rayleigh fast-fading environment, it is essential that the channel impulse response (CIR) variation over the entire slot be estimated accurately. One of the approaches that has been successfully demonstrated for CIR estimation is interpolation of the CIR using the initial estimates obtained over the training periods. This approach has been described in N. W. K. Lo, D. D. Falconer, and A. U. H. Sheikh, “Adaptive equalization and diversity combining for a mobile radio channel,”
Proceedings of IEEE GLOBECOM
'90, pp. 507A.2.1-507A.2.5, Dec. 1990; N. W. K. Lo, D. D. Falconer, and A. U. H. Sheikh, “Channel interpolation for digital mobile radio communications,”
Proceedings of IEEE International Conference on Communications
(ICC 91), pp. 25.3.1-25.3.5, June 1991; and in R. D. Koilpillai, S. Chennakeshu, and R. L. Toy, “Equalizer performance with diversity for U.S. digital cellular,” in
Proceedings of IEEE PIMRC
'92, pp. 255-258, October 1992.
SUMMARY
It is an object of the present invention to provide a combined equalization and decoding algorithm that is tailored for BCM with the CIR estimation being done via CIR interpolation.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatus for performing combined equalization and decoding of codewords encoded in a block coded modulation (BCM) symbol sequence that has been received from a channel, wherein each of the codewords comprises a plurality of symbols. In one embodiment, this is accomplished by determining channel impulse response estimates corresponding to each received symbol of the BCM symbol sequence. A codeword of the received BCM symbol sequence is then decoded using a maximum likelihood decoding trellis with a path metric that depends upon the channel impulse response estimates and that includes at least one term representing intersymbol interference resulting from one or more symbols of one or more previously decoded codewords. The decoding step is then repeated until all of the codewords of the received BCM symbol sequence have been decoded.
In another aspect of the invention, the path metric is determined in accordance with an equation that is a sum of squared differences associated with one BCM codeword, each one of said differences being a difference between a current received sample and a sum term, wherein the sum term is a sum of one or more symbols of a candidate BCM codeword and one or more symbols of at least one previously decoded BCM codeword. For example, the path metric, &Ggr;
k
(j)
, may be determined in accordance with the equation:
Γ
k
(
j
)
=
∑
i
=
1
N
block
⁢
&LeftBracketingBar;
r
⁡
(
n
i
+
k
)
-
c
1
⁡
(
n
i
+
k
)
⁢
s
(
j
)
⁡
(
n
i
+
k
)
-
c
2
⁡
(
n
i
+
k
)
⁢
s
^
⁡
(
n
i
+
k
-
1
)
&RightBracketingBar;
2
,
where r(n
i
+k) are received signal samples corresponding to symbols of a BCM codeword at a time n
i
+k, N
block
is the block length of one BCM codeword, s
j
(n
i
+k) are the symbols corresponding to the j
th
candidate for BCM codeword, and ŝ(n
i
+k−1) are the symbols of the BCM codeword (k−1) decoded in a prior step.
In still another aspect of the invention, the above-mentioned decoding step may be a forward decoding step, and the technique may further comprise the step of performing backward decoding of the received BCM signal. Alternatively, the above-mentioned decoding step may be a backward decoding step.
In yet another aspect of the invention, the technique further includes retaining a list of N
s
best previously decoded BCM codewords. In this case, the step of decoding is performed for each of the N
s
best BCM codewords.
In still another aspect of the invention, the path metric may be determined in accordance with a function that includes a term corresponding to received signal samples at fractional symbol spacing and one or more terms that are proportional to channel impulse response coefficients at fractional symbol sampling locations.
In yet another aspect of the invention, the block coded modulation symbol sequence that has been received from the channel includes deliberately introduced intersymbol interference.
In other embodiments of the invention, combined equalization and decoding of codewords encoded

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