MAP decoding of variable length codes based on a new ´bit-level´ trellis

Soft information associated with each bit in the output of variable length codes (VLC) can be used to improve the performance of VLC decoding, especially in a joint source-channel decoding structure. But the main problem of the decoding in bit level is the huge number of states, which is linear with the value of the trellis bit step parameter. In this paper, according to tree presentation of VLC, a new ´bit-level´ trellis is presented, which states are less than the number of nodes in the VLC tree. Moreover, it allows the application of the maximum a posteriori (MAP) algorithm, and is more convenient to construct than previous represented we have found in literatures.