The error robustness of digital communication systems using source and channel coding can be improved by iterative source-channel decoding (ISCD). The turbo-like evaluation of natural residual source redundancy and of artificial channel coding redundancy makes step-wise quality gains possible by several iterations. The maximum number of profitable iterations is predictable by an EXIT chart analysis. In this contribution, we exploit the EXIT chart representation to improve the error correcting/concealing capabilities of ISCD schemes. We propose new design guidelines to select appropriate bit mappings and to design the channel coding component. A parametric source coding scheme with some residual redundancy is assumed. Applying both innovations, the new EXIT-optimized index assignment as well as the appropriately designed recursive nonsystematic convolutional (RNSC) code, allows to outperform known approaches to ISCD by far in the most relevant channel conditions.

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Iterative source-channel decoding (ISCD) exploits the residual redundancy of the source, e.g., codec parameters, for quality improvements. In contrast to the well-known convolutional coded ISCD systems, we propose in this paper an ISCD scheme which features a superior performance but is based solely on two short block codes. The block codes serve as highly redundant index assignment and rate-1 inner channel code respectively. The improved capabilities are confirmed by EXIT charts. Finally, the effects of imperfect knowledge of the receiver on the residual redundancy are analyzed. With the proposed flexible index assignment no feedback from the receiver to the transmitter is required and competitive results are obtained even for slightly erroneously estimated residual redundancy.

Whenever digital communication is subject to transmission errors, the utilization of reliability information at the receiver is heneficial. Soft-channel decoding techniques improve the ermr correcring capabilities and in addition, sofrbir source decoding performs error concealmenr. While the reliability gain of channel decoding is based on artificial diversities explicitly introduced at the transmitter, sofrbit sauce decoding exploits implicit redundancies remaining in the hitstream after state-of-the-art source encoding.In an iterative source-channel decoding scheme the reliability gains of both are exchanged iteratively in order to enhance the common error resistance. However, first investigations have shown that the number of profitable iterations is limited to small values.In this paper, the convergence behavior is analyzed using a modem tool called EXIT cham [l]. EXIT charts permit to predict the convereence behavior bv studvine the individual comoonents. As realized in systematic form, i.e., the single data bits zn,,(X) with X = 1 , . . . K , are p m of the code words E,. ~-..L(zx,T(X) I z w ( X ) , Z z a , . . . ) I ~ I -Figure 2: Receiver for iterative source-channel decoding; To sima novelty, EXIT characteristics are specified for sofrbit source decoding. The convergence analysis is confirmed by simulation. plify matters notation regards the deinterleaved domain 1. ITERATIVE SOURCE-CHANNEL DECODING At the receiver reliahilitv information about the transmission of At time instant r. a source encoder determines a set & , of M source codec parameters u~,~ with K = 1 , . . . A4 denoting the position within the set. The single components u -,~ in the set t~, are assumed to be statistically independent among each other. Each value U=,,, which is continuous in magnitude but discrete in time, is individually quantized by ZK" reproduction levels +It). The reproduction levels are invariant with respect to T and the whale set i s given by 0,. To each quantizer reproduction level iit' specified at time instant r a unique bit pattem x~,~ is assigned. Therein, K , denotes the length of x % ,~. The complete set of hit pattems specified at time instant r is given by x, (see Figure 1). Source EncoderFigure 1: Transmitter for iterative source-channel decoding scrambles the incoming data bit stream in a deterministic manner. Thereby, the interleavermight be sized such that several consecutive sets s7 are rearranged in common. If so, the interleaver introduces a delay, which might be unacceptable in practical applications. However, the interleaver has to be designed such that the diversities resulting either from source or from channel encoding allow independent reliability gains at the receiver. As diversity of source encoding is based on residual redundancy in source codec parameters U,,,, independence will be ensured when channel encoding is performed across uncorrelated bit patterns x~, ? .To simplify matters, channel encoding is assumed to be A bit interleaver single data bits Z*,~(X) is utilized. Such reliability info...

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