Reduced complexity maximum a posteriori decoding of variable-length codes

Until very recently, joint source channel techniques mostly focused on systems using fixed-length coding, eventhough 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 variablerate 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 here 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. Keywords—joint source channel decoding, MAP estimation, soft-input soft-output decoding, variable length code.