Joint Source-Channel Decoding of Variable-Length Codes with Soft Information: A Survey
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Joint Source-Channel Decoding of Variable-Length Codes with Soft Information: A Survey Christine Guillemot IRISA-INRIA, Campus de Beaulieu, 35042 Rennes Cedex, France Email: [email protected]
Pierre Siohan R&D Division, France Telecom, 35512 Rennes Cedex, France Email: [email protected] Received 13 October 2003; Revised 27 August 2004 Multimedia transmission over time-varying wireless channels presents a number of challenges beyond existing capabilities conceived so far for third-generation networks. Efficient quality-of-service (QoS) provisioning for multimedia on these channels may in particular require a loosening and a rethinking of the layer separation principle. In that context, joint source-channel decoding (JSCD) strategies have gained attention as viable alternatives to separate decoding of source and channel codes. A statistical framework based on hidden Markov models (HMMs) capturing dependencies between the source and channel coding components sets the foundation for optimal design of techniques of joint decoding of source and channel codes. The problem has been largely addressed in the research community, by considering both fixed-length codes (FLC) and variable-length source codes (VLC) widely used in compression standards. Joint source-channel decoding of VLC raises specific difficulties due to the fact that the segmentation of the received bitstream into source symbols is random. This paper makes a survey of recent theoretical and practical advances in the area of JSCD with soft information of VLC-encoded sources. It first describes the main paths followed for designing efficient estimators for VLC-encoded sources, the key component of the JSCD iterative structure. It then presents the main issues involved in the application of the turbo principle to JSCD of VLC-encoded sources as well as the main approaches to source-controlled channel decoding. This survey terminates by performance illustrations with real image and video decoding systems. Keywords and phrases: joint source-channel decoding, source-controlled decoding, turbo principle, variable-length codes.
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INTRODUCTION
The advent of wireless communications, ultimately in a global mobility context with highly varying channel characteristics, is creating challenging problems in the area of coding. Design principles prevailing so far and stemming from Shannon’s source and channel separation theorem are being reconsidered. The separation theorem, stating that source and channel optimum performance bounds can be approached as close as desired by designing independently source and channel coding strategies, holds only under asymptotic conditions where both codes are allowed infinite length and complexity, and under conditions of source stationarity. If the design of the system is heavily constrained in terms of complexity or delay, source and channel coders can be largely suboptimal, leading to residual channel error rates, which can be large and lead to dramatic source symbol error rates. The assumption prevailing so far was es
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