Iterative Source-Channel Decoding Using Short Block Codes

Abstract

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.

On redundant index assignments for iterative source-channel decoding

“…Such an index assignment with Q < 2 M can be considered as a (potentially non-linear) block code with the binary representation of the quantizer level index {ξ} 2 , ξ = 0, ... Q − 1, as input (see also [11], [12] for noniterative schemes). In case of an index assignment by a linear block code with generator matrix G Γ we get [8] …”

“…NB stands for natural binary, EO for EXIT optimized [6], and BC for block coded [8]. In the left EXIT chart we can observe that the EXIT characteristics T of conventional IAs are limited to a maximum extrinsic mutual information I…”

“…In contrast, the EXIT characteristics of redundant IAs can reach the point (1,1) in the EXIT chart, if the minimum Hamming distance d min between the bit patterns x of the IA is larger than 1, i.e., d min ≥ 2 [13], [8], [14], [9]. Note, for unequally distributed bits x actually only the point (H(x), H(x)) can be reached, with H(x) being the entropy of x.…”

“…The block coded IAs in [8] were chosen for a high d min , e.g., d min (G Γ BC(8,4) ) = 4. The EXIT optimized IAs of [9] (not to be confused with the EXIT optimized IAs of [6]) result actually in a single parity-check code (SPC) for G Γ ("Mapping 2" in [9], cmp.…”

“…In [7] the concept of redundant IAs is presented, which gives a significant performance improvement and enables a multi-mode ISCD system. More results on redundant IAs can be found in [8], [9]. In this letter we analyze the redundant IAs and optimization criteria in [7], [8], [9], propose a novel optimization concept, and demonstrate by simulation that the new simple redundant IAs outperform the previously known ones.…”