Catherine Lamy

Reduced complexity maximum a posteriori decoding of variable-length codes

Previously, joint source channel techniques mostly focused on systems using fixed-length coding, even though variable-length coding (VLC) is widely used, particularly in video coding. Typically, VLC bit streams are made channel-robust through packetization and standard forward-error correction (FEC). However, when the channel conditions are fairly mild, FEC can reveal itself bandwidth-inefficient. A variable-rate extension of joint source channel decoding could thus potentially replace FEC under mild conditions or, for noisier channels, could be used together with FEC to ameliorate the coding rate, extending in both cases the range of situations under which the bit stream is adequately protected. We propose two reduced-complexity VLC soft-input decoding techniques, as well as a comparison with existing algorithms. Experimental results of a new proposed VLC decoding algorithm show very good performance and low complexity.

Optimised constructions for variable-length error correcting codes

An optimised construction of variable-length error-correcting (VLEC) codes is proposed. Compared to the state-of-the-art, the three main improvements consist in: (1) improving the codewords search algorithm complexity/efficiency trade-off; (2) loosening the codewords deletion rule; (3) taking advantage of eventual previous searches. It is shown that the optimised algorithm can find good VLEC codes for alphabets of up to 200 symbols, and outperforms existing algorithms.