18,455 research outputs found
Iterative channel equalization, channel decoding and source decoding
The performance of soft source decoding is evaluated over dispersive AWGN channels. By employing source codes having error-correcting capabilities, such as Reversible Variable-Length Codes (RVLCs) and Variable-Length Error-Correcting (VLEC) codes, the softin/soft-out (SISO) source decoder benefits from exchanging information with the MAP equalizer, and effectively eliminates the inter-symbol interference (ISI) after a few iterations. It was also found that the soft source decoder is capable of significantly improving the attainable performance of the turbo receiver provided that channel equalization, channel decoding and source decoding are carried out jointly and iteratively. At SER = 10-4, the performance of this three-component turbo receiver is about 2 dB better in comparison to the benchmark scheme carrying out channel equalization and channel decoding jointly, but source decoding separately. At this SER value, the performance of the proposed scheme is about 1 dB worse than that of the ½-rate convolutional coded non-dispersive AWGN channel.<br/
Protograph-Based LDPC Code Design for Ternary Message Passing Decoding
A ternary message passing (TMP) decoding algorithm for low-density
parity-check codes is developed. All messages exchanged between variable and
check nodes have a ternary alphabet, and the variable nodes exploit soft
information from the channel. A density evolution analysis is developed for
unstructured and protograph-based ensembles. For unstructured ensembles the
stability condition is derived. Optimized ensembles for TMP decoding show
asymptotic gains of up to 0.6 dB with respect to ensembles optimized for binary
message passing decoding. Finite length simulations of codes from TMP-optimized
ensembles show gains of up to 0.5 dB under TMP compared to protograph-based
codes designed for unquantized belief propagation decoding
Synchronization recovery and state model reduction for soft decoding of variable length codes
Variable length codes exhibit de-synchronization problems when transmitted
over noisy channels. Trellis decoding techniques based on Maximum A Posteriori
(MAP) estimators are often used to minimize the error rate on the estimated
sequence. If the number of symbols and/or bits transmitted are known by the
decoder, termination constraints can be incorporated in the decoding process.
All the paths in the trellis which do not lead to a valid sequence length are
suppressed. This paper presents an analytic method to assess the expected error
resilience of a VLC when trellis decoding with a sequence length constraint is
used. The approach is based on the computation, for a given code, of the amount
of information brought by the constraint. It is then shown that this quantity
as well as the probability that the VLC decoder does not re-synchronize in a
strict sense, are not significantly altered by appropriate trellis states
aggregation. This proves that the performance obtained by running a
length-constrained Viterbi decoder on aggregated state models approaches the
one obtained with the bit/symbol trellis, with a significantly reduced
complexity. It is then shown that the complexity can be further decreased by
projecting the state model on two state models of reduced size
Codes robustes et codes joints source-canal pour transmission multimédia sur canaux mobiles
Some new error-resilient source coding and joint source/channel coding techniquesare proposed for the transmission of multimedia sources over error-prone channels.First, we introduce a class of entropy codes providing unequal error-resilience, i.e.providing some protection to the most sensitive information. These codes are thenextended to exploit the temporal dependencies. A new state model based on the aggregation of some states of the trellis is thenproposed and analyzed for soft source decoding of variable length codes with a lengthconstraint. It allows the weighting of the compromise between the estimation accuracyand the decoding complexity.Next, some paquetization methods are proposed to reduce the error propagationphenomenon of variable length codes.Finally, some re-writing rules are proposed to extend the binary codetree representationof entropy codes. The proposed representation allows in particular the designof codes with improved soft decoding performances.Cette thèse propose des codes robustes et des codes conjoints source/canal pourtransmettre des signaux multimédia sur des canaux bruités. Nous proposons des codesentropiques offrant une résistance intrinsèque aux données prioritaires. Ces codes sontétendus pour exploiter la dépendance temporelle du signal.Un nouveau modèle d’état est ensuite proposé et analysé pour le décodage souplede codes à longueur variable avec une contrainte de longueur. Il permet de réglerfinement le compromis performance de décodage/complexité.Nous proposons également de séparer, au niveau du codage entropique, les étapesde production des mots de codes et de paquétisation. Différentes stratégies de constructionde train binaire sont alors proposées.Enfin, la représentation en arbre binaire des codes entropiques est étendue enconsidérant des règles de ré-écriture. Cela permet en particulier d’obtenir des codesqui offrent des meilleures performances en décodage souple
Reduced-complexity trellises for the joint source-channel decoding of variable-length encoded data
Many trellis-based soft decoding techniques have been proposed for data encoded using variable-length codes (VLC).
However, for actual VLC tables, these trellises are too complex to allow real-time soft decoding. This paper presents the
principle of an algorithm for grouping VLC codewords into classes, which allows significant reductions of the complexity
of the resulting trellises and of the soft decoding techniques. The adapation of decoding algorithms such as SOVA or
BCJR to the reduced-complexity trellises is detailed. Illustrations are provided on the VLC table used for texture encoding
in H.263+. The performance of a decoding technique for the localization of block frontiers in a bitstream generated by
an H.263+ coder is also presented.De nombreux algorithmes ont été proposés pour le décodage souple de données codées à l’aide de codes
à longueur variable (CLV), la plupart travaillant avec des treillis. Pour un code réaliste, ces treillis sont très
complexes à cause du nombre de mots de code à considérer. Cet article présente le principe d’un algorithme
de regroupement de mots de CLV en un nombre minimal de classes, ce qui permet de réduire
significativement la complexité des treillis utilisés pour le décodage souple de CLV. L’adaptation
des algorithmes de décodage tels que SOVA ou BCJR à ce type de treillis est détaillée. Une illustration sur les
CLV de la norme H.263+ est proposée ainsi qu’un exemple d’application à la localisation des frontières de
blocs de texture H.263+
One and Two Bit Message Passing for SC-LDPC Codes with Higher-Order Modulation
Low complexity decoding algorithms are necessary to meet data rate
requirements in excess of 1 Tbps. In this paper, we study one and two bit
message passing algorithms for belief propagation decoding of low-density
parity-check (LDPC) codes and analyze them by density evolution. The variable
nodes (VNs) exploit soft information from the channel output. To decrease the
data flow, the messages exchanged between check nodes (CNs) and VNs are
represented by one or two bits. The newly proposed quaternary message passing
(QMP) algorithm is compared asymptotically and in finite length simulations to
binary message passing (BMP) and ternary message passing (TMP) for spectrally
efficient communication with higher-order modulation and probabilistic
amplitude shaping (PAS). To showcase the potential for high throughput forward
error correction, spatially coupled LDPC codes and a target spectral efficiency
(SE) of 3 bits/QAM symbol are considered. Gains of about 0.7 dB and 0.1 dB are
observed compared to BMP and TMP, respectively. The gap to unquantized belief
propagation (BP) decoding is reduced to about 0.75 dB. For smaller code rates,
the gain of QMP compared to TMP is more pronounced and amounts to 0.24 dB in
the considered example.Comment: Accepted for IEEE/OSA Journal on Lightwave Technolog
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