Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
2001-07-20
2003-01-28
Moise, Emmanuel L. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C714S780000, C714S791000, C714S794000, C714S795000, C375S341000
Reexamination Certificate
active
06513140
ABSTRACT:
The invention lies in the telecommunications and signal processing fields. More specifically, the invention relates to a method and device for decoding convolutional codes.
In communication systems, for example such as radio systems, the signal to be transmitted (for example voice signal) is submitted to channel coding after conditioning in a source coder. The channel coding serves the purpose of adapting the signal to be transmitted to the properties of the transmission channel. In the process, effective error protection is achieved by specific introduction of redundancy into the signal to be transmitted.
Binary parallel-concatenated recursive convolutional codes have been investigated for channel coding only for a few years. The designation “turbo codes” has become established for these convolutional codes. In particular, when transmitting large blocks with more than one thousand bits (symbols), a substantially better error protection can be achieved with turbo codes than with the (conventional) convolutional coding. However, it is disadvantageous that the decoding of turbo codes is more complicated than the decoding of (conventional) convolutional codes.
It has been known to utilize an iterative turbo decoder for the purpose of decoding turbo codes. The iterative turbo decoder thereby contains two individual convolutional decoders which are interleaved with one another in a feedback fashion. At least the convolutional decoder provided on the input side must permit soft decoding, that is to say be capable in the case of each received data symbol to determine in addition to, or instead of, a binary output value a value-continuous estimated value for the original, uncoded data symbol on which the received data symbol is based. It is characteristic of iterative turbo decoders that these value-continuous estimated values are fed to the second convolutional decoder as input information in the course of the iteration. Estimated values for original, uncoded data symbols are also denoted below as a first item of reliability information.
The article “Near Shannon Limit Error-Correcting, Coding and Decoding: Turbo-codes (1)” C. Berrou et al., Proc. IEEE Int. Conf. on Communications ICC′93, Genua, 1993, pages 1064 to 1070 describes an iterative turbo decoder whose convolutional decoder on the input side is used to produce the first item of reliability information according to a modified Bahl et al.-algorithm. The second convolutional decoder, which need not produce any reliablity information, can operate, for example, according to the known Viterbi algorithm.
Convolutional decoders which operate according to a symbol-by-symbol MAP (maximum a posteriori) algorithm, are likewise capable of producing a first item of reliability information. Such convolutional decoders are denoted as MAP symbol estimator (or else MAP symbol decoder). They have the advantage that they can be used to achieve the lowest possible bit error ratio.
A detailed description of an iterative turbo decoder with two recursively interleaved MAP symbol estimators can be found in the book “Analyse und Entwurf digitaler Mobilfunksysteme” [“Analysis and Design of Digital Mobile Radio Systems”], by P. Jung, Stuttgart, B. G. Teubner, 1997 on pages 343-68, in particular FIG. E.
2
.
The problem arises in mobile radio applications that the mobile radio channel is highly time variant, that is to say its transmission properties change continuously because of changing environmental influences. The constant changes in the transmission properties of the mobile radio channel must be taken into account as early as during data detection. For this purpose, a communication terminal used in mobile radio has a channel estimator which is coupled to the data detector and continuously determines the transmission properties (pulse responses) of the mobile radio channel and communicates them to the data detector. Such data detection, which takes account of the instantaneous transmission channel properties, is denoted as adaptive data detection or adaptive equalization.
However, the time variance of the mobile radio channel also influences the decoding which takes place downstream of the adaptive equalization. It is disadvantageous in this regard that the high degree of error protection which can in principle be achieved by turbo decoding is nullified again by the time variance of the mobile radio channel, at least in part.
The article “Combined Turbo Equalization and Turbo Decoding” by D. Raphaeli and Y. Zarai, IEEE Communications Letters, Vol. 2, No. 4, 1998, pages 107-09 describes an iterative receiver structure which is constructed from a combination of an adaptive equalizer and an (iterative) turbo decoder connected downstream of the latter. The term “turbo equalization” has been coined as keyword for such a combined receiver structure. The iterative turbo decoder is also structured here, in turn, from two MAP symbol decoders. In addition to the first item of reliability information, the two MAP symbol decoders, which are denoted in this article as MAP blocks, also calculate a second item of reliability information. The second item of reliability information constitutes a value-continuous estimated value for the original, coded data symbol on which the detected data symbol is based.
The coupling between adaptive equalization and data decoding is realized by virtue of the fact that in each iteration step the iterative turbo decoder generates from the second item of reliability information of the two convolutional decoders a combined item of reliability information which it feeds to the adaptive equalizer as information which is extrinsic (that is to say not produced in the equalizer itself), and that the adaptive equalizer for its part feeds extrinsic information into the turbo decoder. The time variance of the mobile radio channel can be taken into account during the turbo decoding by means of this feedback between the equalizer and turbo decoder. However, it is disadvantageous that the computational outlay, which is already very high in any case for turbo decoding, is further substantially increased by the fact that the equalizer is also incorporated into the iteration cycle.
A simplified version of an iterative turbo decoder for decoding turbo codes is proposed in the article “Novel low complexity decoder for turbo-codes” by P. Jung, Electronics Letters, Vol. 31, No. 2, 1995, pages 86-87. That turbo decoder differs from the previously known turbo decoders in that the two convolutional decoders contained in the turbo decoder operate using a novel, so-called SUBMAP algorithm which, in conjunction with an acceptable deterioration of the decoding performance (that is to say increase in the bit error ratio), permits a substantial saving on computational outlay in the calculation of the first item of reliability information.
In the publication “Combined turbo equalization and turbo decoding”, Global Telecommunications Conference (GLOBE-COM), US, New York, IEEE, 1997, pages 639-43, XP00208195 ISBN: 0-7803-4199-6 by Raphaeli et al., a decoder structure is described wherein a sequence of received coded symbols is accepted in accordance with FIG.
3
and the input C
1
, illustrated there, of the code MAP, a first item of reliability information is calculated for each uncoded symbol in accordance with FIG.
3
and the output L, illustrated therein, of the code MAP block, and a second item of reliability information is calculated for each coded symbol in accordance with FIG.
3
and the output F, illustrated therein, of the code MAP block. Reference is made in the publication to the prior art MAP decoder with reference to determining the second item of reliability information.
A specific modification of the Viterbi decoding algorithm for binary trellis diagrams is proposed in the publication “Source-controlled Channel Decoding” in IEEE Transactions on Communications, US, IEEE INC. New York, Vol. 43, No. 9, Sep. 1, 1995 (1995-09-01), pages 2449-57, XP000525669 ISSN: 0090-6778 by J. Hagenauer. The calculation of a second item of reliabi
Dötsch Markus
Jung Peter
Plechinger Jörg
Schmidt Peter
Greenberg Laurence A.
Infineon - Technologies AG
Mayback Gregory L.
Moise Emmanuel L.
Stemer Werner H.
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