Versatile serial concatenated convolutional codes

Details Versatile serial concatenated convolutional codes Versatile serial concatenated convolutional codes

2001-02-09

2004-08-31

Decady, Albert (Department: 2133)

Error detection/correction and fault detection/recovery

Pulse or data error handling

Digital data error correction

C375S262000

Reexamination Certificate

active

06785861

ABSTRACT:

TECHNICAL FIELD
The present invention relates to modulation and encoding/decoding of signals. More specifically, the invention relates to a novel method and apparatus for obtaining flexible-rate/bandwidth-efficient encoding and decoding plus modulation.
BACKGROUND OF THE INVENTION
Several techniques for combining concatenated convolution codes with interleaver and spectrally efficient modulations have been proposed, due to the attractiveness of achieving high spectral efficiency and high coding gain at the same time.
Concatenated convolutional codes with interleaver (known as “turbo codes” in their parallel concatenated structure) represent one of the most important results in code theory achieved during the last decade; in fact, the performance of optimized schemes can be within 1 dB from the theoretical Shannon limit. The main ingredients of concatenated codes with interleaver consist of two Convolutional Codes (CCs) and one interleaver. The impressive performance of those codes is achieved through an iterative decoding process that refines the decisions taken by each constituent decoding block. Among the turbo codes two different classes can be distinguished depending on the concatenation scheme.
The first class is based on Parallel Concatenation: the information sequence enters the first systematic encoder, which generates a coded sequence; at the same time, the information sequence is transformed by the interleaver into a permutation of itself and is successively encoded by the second systematic encoder. A block diagram of the encoder in question is shown in FIG.
1
. The code sequence of the parallel turbo code is obtained through concatenation, in some suitable order, of the two code sequences. This turbo code was first proposed in 1993 in C. Berrou, A. Glavieux and P. Thitimajshima, “Near Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes,” in
Proceedings of ICC '
93 (Geneva, Switzerland), pp. 1064-1070, May 1993, and further described in U.S. Pat. No. 5,446,747.
The second class is based on Serial Concatenation, as first proposed in S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, “Serial concatenation of interleaved codes: performance analysis, design and iterative decoding,”
IEEE Transactions on Information Theory
, pp. 909-926, May 1998, which is incorporated herein by reference. In this case the output of the outer encoder is encoded by the inner encoder, after a permutation through a suitable interleaver. A block diagram of such an encoder is shown in FIG.
2
.
The constituent codes (C
1
, C
2
) can be block or convolutional codes for both classes (parallel and serial).
Pragmatic Trellis-Coded Modulation (PTCM), proposed in the work by Andrew Viterbi, et al., “A Pragmatic Approach to Trellis-Coded Modulation,” IEEE Communications Magazine, July 1989, pp. 11-19, is a combined error-correction coding and modulation scheme that utilizes a standard convolutional code applied to certain bits of M-ary Phase Shift Keyed (M-PSK) or M-ary Quadrature Amplitude Modulation (M-QAM) symbol mappings.
The earlier studies on concatenated codes with interleavers dealt with parallel concatenated convolutional codes (PCCC) with medium-low rates (½, ⅓ and below) used in conjunction with binary (or, equivalently, independent quaternary) modulations, such as 2-PAM, BPSK, QPSK, with the aim of obtaining very large coding gains. Subsequently, several attempts have been undertaken to merge PCCC with high-level modulation schemes in order to conjugate significant coding gains with high spectral efficiencies. The first paper on this subject is S. Le Goff, A. Glavieux and C. Berrou, “Turbo-codes and high spectral efficiency modulation,” in
Communications
, 1994
. ICC '
94
, SUPERCOMM/ICC
'94
, Conference Record, “Serving Humanity Through Communications.” IEEE International Conference on
, pp. 656-649 vol.2, 1994, which adopts a pragmatic approach to obtain 2,3 and 4 bits/signal by means of 8PSK and 16QAM modulations with turbo codes (PCCC schemes).
Several techniques to merge turbo codes with classical Ungerboeck Trellis-Coded Modulation (TCM) are described in the literature for both parallel and serial concatenated convolutional coding (SCCC) schemes.
Exemplary of the former coding schemes are the works by P. Robertson and T. Worz, “Bandwidth-efficient turbo trellis-coded modulation using punctured component codes,”
IEEE Journal on Selected Areas in Communications
, vol. 16 2, pp. 206-218, February 1998, and S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, “Parallel concatenated trellis coded modulation,” in
Proceedings of ICC '
96, (Dallas, Tex.), June 1996.
Exemplary of the latter coding schemes are the works S. Benedetto, D. Divsalar, G. Montorsi and F. Pollara, “Serial concatenated trellis coded modulation with iterative decoding: design and performance,” in
Proceedings of GLOBECOM '
97
—Communications Theory Miniconference
, (Phoenix, Ariz.)., October 1997, and S. Benedetto, D. Divsalar, R. Garello, G. Montorsi and F. Pollara, “Self-Concatenated Trellis Coded Modulation with Self-Iterative Decoding,” in
GLOBECOM '
98
—Communications Theory Miniconference
, (Sydney, Australia), October 1998.
Also, references of interest are U.S. Pat. No. 6,088,387 to Gelblum et al., and U.S. Pat. No. 6,023,783 to Divsalar et al.
Specifically, this latter patent discloses several improved turbo code apparatuses and methods encompassing several classes such as:
a data source is applied to two or more encoders with an interleaver between the source and each of the second and subsequent encoders. Each encoder outputs a code element which may be transmitted or stored. A parallel decoder provides the ability to decode the code elements to derive the original source information without the use of a received data signal corresponding to such information. The output may be coupled to a multilevel trellis-coded modulator (TCM);
a data source is applied to two or more encoders with an interleaver between the source and each of the second and subsequent encoders. Each of the encoders outputs a code element. In addition, the original data source is output from the decoder. All of the output elements are coupled to a TCM;
at least two data sources are applied to two or more encoders with an interleaver between each source and each of the second and subsequent encoders. The output may be coupled to a TCM;
at least two data sources are applied to two or more encoders with at least two interleavers between each source and each of the second and subsequent encoders; and
at least one data source is applied to one or more serially linked encoders through at least one interleaver; the output may be coupled to a TCM.
Specifically, the solution described in the captioned patent includes a novel way of terminating a turbo coder.
SCCCs with interleavers have been shown to yield some advantages with respect to PCCCs, especially when very low bit error probabilities are being pursued: see in that respect the first work by Benedetto et al. referred to in the foregoing.
Besides, the need of combining large coding gains with high spectral efficiency is becoming increasingly important, owing to applications for which both power and bandwidth are precious and scarce resources. Moreover, applications in which the channel reliability is subject to large variations ask for coding-modulation schemes able to move back and forth the trade-off point between bandwidth and power efficiency in favor of either resource. In other words, rather than precisely tuned designs, those applications require versatile schemes yielding good, though not optimized, performance. For example, this is the case of wireless communications, like third generation cellular communications and digital video broadcasting.
SUMMARY OF THE INVENTION
Thus, the prior art fails to propose a “universal” encoding/decoding scheme that can yield the very good bit error rate performances of the SCCCs for a wide range of spectral efficiencies, and with the same hardware implementation.
An embodiment the

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