Analytical tools for optimizing the error correction performance of arithmetic codes

International audienceIn joint source-channel arithmetic coding (JSCAC) schemes, additional redundancy may be introduced into an arithmetic source code in order to be more robust against transmission errors. The purpose of this work is to provide analytical tools to predict and evaluate the effectiveness of that redundancy. Integer binary Arithmetic Coding (AC) is modeled by a reduced-state automaton in order to obtain a bit-clock trellis describing the encoding process. Considering AC as a trellis code, distance spectra are then derived. In particular, an algorithm to compute the free distance of an arithmetic code is proposed. The obtained code properties allow to compute upper bounds on both bit error and symbol error probabilities and thus provide an objective criterion to analyze the behavior of JSCAC schemes when used on noisy channels. This criterion is then exploited to design efficient error-correcting arithmetic codes. Simulation results highlight the validity of the theoretical error bounds and show that for equivalent rate and complexity, a simple optimization yields JSCACs that outperform classical tandem schemes at low to medium SNR

Asymptotic errorcorrecting performance of joint source-channel schemes based on arithmetic coding

Abstract — In joint source-channel (JSC) schemes based on arithmetic coding (AC), additional redundancy may be introduced in order to reduce transmission errors. The purpose of this work is to provide analytical tools to predict and evaluate the effectiveness of that redundancy. Integer binary AC is modeled by a reduced-state automaton in order to obtain a bit-clock trellis of the AC. Considering AC as a trellis code, distance spectra are then derived. In particular, an algorithm to compute the free distance of an arithmetic code is proposed. The obtained code properties allow to compute upper bounds on both bit error and symbol error probabilities and thus provide an objective criterion to analyze the behavior of JSCAC schemes when used on noisy channels. I

Codage-décodage source-canal conjoint des codes arithmétiques (application au décodage robuste des vidéos codées H.264)

Cette thèse s'inscrit dans le contexte du codage/décodage source-canal conjoint (CSCC/DSCC) des codes arithmétiques (CA). Nous nous intéressons au décodage robuste des trames codées par CABAC, une version du CA adoptée dans des standards tels que le H.264. Nous proposons un estimateur au sens du maximum a posteriori, sans approximations, prenant compte les contraintes d'une implémentation réaliste du CA. Pour l'évaluation de cet estimateur, nous utilisons les techniques de décodage séquentiel, qui posent, en revanche, des problèmes de complexité. Nous développons alors un critère objectif de décision permettant d'ajuster le compromis complexité-efficacité. Ensuite, nous nous intéressons à la caractérisation analytique de la robustesse des schémas de CSCC utilisant le CA. Dans ce but, nous représentons le CA par une machine à états finis pour générer un treillis bi-dimensionnel adapté à un décodage Viterbi. Ce treillis permet de calculer la distance libre et le spectre des distances du CA. Les outils analytiques développés sont exploitées pour concevoir un schéma de CSCC optimisé de manière à minimiser asymptotiquement la probabilité d'erreur symbole.This thesis deals with joint source-channel coding and decoding (JSCC/JSCD) schemes involving arithmetic codes (AC). First, we develop a JSCD technique based on MAP estimation of CABAC encoded data. This estimator is considered to be exact as it is evaluated without approximations and with respect of the constraints imposed by an actual implementation of AC. The evaluation of the proposed MAP estimator is achieved using an improved sequential decoding technique, allowing to adjust the decoder complexity-efficiency trade-off. The purpose of the second part of this thesis is to provide analytical tools to predict and evaluate the effectiveness of the redundancy introduced by the JSCC schemes into AC. Integer binary AC is then modelled by a reduced-state automaton to obtain a bit-clock trellis. Distance spectra are then derived. The obtained distance properties provide an objective criteria that are then exploited to design efficient error-correcting arithmetic codes.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Robust decoding of h.264 encoded video transmitted over wireless channels

Abstract — Due to its high compression efficiency, the H.264 video coder is very sensitive to impairments due to transmission over noisy channels. Most error resilience/concealment techniques provided in the H.264 standard were dealing with packet losses. In wireless environments, the proportion of corrupted packets (and thus considered as lost) may become very high. This paper shows that the H.264 decoder may be seen as a parity check decoder able to detect erroneous packets. Combined with soft estimation techniques, it allows to correct transmission errors and to reduce significantly the number of packets deemed lost. The proposed solution is compatible with the error-resilience features of H.264. I