Heuristic Search Based Soft-Input Soft-Output Decoding of Arithmetic Codes

Recent research (e.g. [1]) has shown that by introducing a “forbidden symbol ” (FS), one can significantly improve the error resilience of the decoding of arithmetic codes (ACs) when the bitstream is transmitted over error prone channels. The FS was used to facilitate continuous pruning of possible source paths and deemed necessary because the number of such paths can grow exponentially. However, the FS is an extra overhead whose introduction in many applications is prohibited or highly undesirable. In this paper we took a different approach to error resilient AC decoding. Instead of an FS, we utilized heuristic search algorithms (HSAs) in artificial intelligence ([2]) for finding the minimal-weight path through directed and non-negatively-weighted graphs. HSAs were designed to efficiently deal with exponential growth of paths without the assistance of an embedded “error ” flag such as the FS. Instead, an HSA maintains a list (Open) of nodes that might need to be examined, and, starting from only the root node (r) in Open, removes the most “promising ” (based on a pre-determined metric) node n from Open, and checks if n is a solution. If yes, the algorithm exits; otherwise, it adds all children of n that are not currently in Open to Open, and records the concatenation of th