Pulse or digital communications – Receivers – Interference or noise reduction
Patent
1997-12-03
2000-08-22
Bocure, Tesfaldet
Pulse or digital communications
Receivers
Interference or noise reduction
375341, H03D 106, H04L 2706
Patent
active
061083887
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF INVENTION
The field of the invention is that of the transmission and broadcasting of digital signals, especially in the presence of transmission noises. More specifically, the invention relates to the reception of digital signals encoded by means of a convolutive type error correction code.
The invention can be applied in all cases where digital signals are transmitted or broadcast on noise-infested channels. For example, the present invention can be implemented in receivers of systems for digital radiocommunications with mobile units such as the GSM system.
Other possible applications are, for example, the reception of signals transmitted by RF channels, satellites, etc. More generally, the invention can be applied advantageously in all cases where a convolutive type code is implemented at transmission.
In the known type of receivers, the symbols retrieved at output of the demodulator take the form of analog samples which, after quantification, are generally processed by an equalizer (a transversal filter) given the task of eliminating the inter-symbol interference introduced by the channel. The equalized samples are then de-interleaved if necessary and decoded before being given to the addressee.
The principle of equalization consists in estimating the response of the transmission channel for the application, to the received signal, of a filtering operation symmetrical to this response so as to: obtain a corrected signal. In particular, the equalization is aimed at eliminating or, at least, at limiting the inter-symbol interference introduced by the channel.
The response of the channel is generally estimated by the analysis of a reference signal known to the receivers. Of course, the transmission of these reference signals leads to a reduction of the useful bit rate.
The equalized items of data are then de-interleaved (in an operation symmetrical to the interleaving done at the time of encoding, if such an interleaving is planned) and then decoded.
In certain situations, it happens that the equalization of this type is not sufficient to ensure high quality source decoding.
The article by P. Jung and P. W. Baier, "VLSI implementation of soft output Viterbi equalizers for mobile radio applications" (Proc. of IEEE 42nd Vehicular Technology Society Conference, Denver, Colo., pp. 577-585, May 1992) proposes the use, instead of a transversal filter, of a symbol detector working according to the principle of maximum likelihood.
This technique is more efficient if the coefficients representing the channel are properly estimated. However, once again, the equalized items of data may prove to be of insufficient quality. Furthermore, it is more complicated to make and requires more space than with the transverse filter technique.
Convolutive codes are codes that associate at least one encoded item of data with each item of source data to be encoded. This encoded item of data is obtained by the modulo 2 summation of this item of source data with at least one of the previous items of source data. Thus, each encoded signal is a linear combination of the item of source data to be encoded and previous items of source data taken into account.
In the decoder, the original items of data are most usually reconstructed by means of a maximum likelihood algorithm, for example the Viterbi algorithm, whose decisions may be weighted if necessary. The Viterbi algorithm, in taking account of a sequence of received encoded symbols, provides an estimation of each item of data encoded at transmission, in defining the source sequence most probably corresponding to the received sequence.
The Viterbi algorithm may also be used to detect sequences affected by inter-symbol interference. The invention can be also applied in this case.
Clearly, the greater the number of symbols taken into account, the more reliable is the decision. By contrast, the greater this number, the more complicated is the decoder or the detector (hereinafter, the term decoder is used to describe decoders themselves as well as detectors. This observation
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Berrou Claude
Douillard Catherine
Glavieux Alain
Jezequel Michel
Bocure Tesfaldet
France Telecom
Telediffusion de France
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