Iterative List Decoding of Concatenated Source-Channel Codes
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Iterative List Decoding of Concatenated Source-Channel Codes Ahmadreza Hedayat Multimedia Communications Laboratory, The University of Texas at Dallas, TX 75083-0688, USA Email: [email protected]
Aria Nosratinia Multimedia Communications Laboratory, The University of Texas at Dallas, TX 75083-0688, USA Email: [email protected] Received 6 October 2003; Revised 17 June 2004 Whenever variable-length entropy codes are used in the presence of a noisy channel, any channel errors will propagate and cause significant harm. Despite using channel codes, some residual errors always remain, whose effect will get magnified by error propagation. Mitigating this undesirable effect is of great practical interest. One approach is to use the residual redundancy of variable-length codes for joint source-channel decoding. In this paper, we improve the performance of residual redundancy source-channel decoding via an iterative list decoder made possible by a nonbinary outer CRC code. We show that the list decoding of VLCs is beneficial for entropy codes that contain redundancy. Such codes are used in state-of-the-art video coders, for example. The proposed list decoder improves the overall performance significantly in AWGN and fully interleaved Rayleigh fading channels. Keywords and phrases: joint source-channel coding, variable-length codes, list decoding, iterative decoding.
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INTRODUCTION
Variable-length codes (VLCs) for entropy coding are by now a central part of most data compression techniques, which are in turn essential for many communications applications, including text, voice, images, and video. While VLCs achieve significant compression, they also introduce dependencies in the data structure through their variable length, thus leading to error propagation in the decoded sequence. One of the techniques that has been used to combat this undesirable effect is joint source-channel decoding. It is known that even the most efficient symbol-by-symbol compression (Huffman code) does not always achieve the entropy limit, therefore redundancy often remains in compressed data. This redundancy can, in principle, be used to assist the decoder. Taking this argument one step further, it has been proposed to leave redundancy intentionally in entropy codes, for the purposes of resilience against channel noise. For example, the video coding standard H.263+ and its descendants use a reversible variable-length code (RVLC) [1] whose compression efficiency is less than Huffman codes. However, the RVLC allows bidirectional symbol-based decoding which is useful in the presence of channel errors. This approach
has been generalized by designing entropy codes with prespecified minimum distance [2, 3]. The error resilience of entropy codes can be used to “clean up” any residual errors from the traditional error control coding (see Figure 1). For example, in the case of RVLC, one may start decoding from the end of the sequence whenever an error is observed. This is a separable approach to decoding. However, we know today that serially concatenated codes
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