A soft-output decoding algorithm for concatenated systems

In order to fully utilize the SDD (soft-decision decoding) capacity of the outer codes in a concatenated system, reliability information on the inner decoder outputs (called soft outputs) needs to be provided to the outer decoder. This paper shows that a modif ied MAP algorithm can be effectively and accurately used to generate such information. In the course of the presentation, a metric based on the reliability information is proposed for the outer decoder. This metric has the Eucl idean metric on AWGN channels as its special case, which leads to the concept of general ized SDD (GSDD). Several practical concerns regarding the proposed soft-output decoder are addressed through theoretical analysis and simulation: the effect of finite decoding depth, computat ional complexity, range overflow, and scaling. Compar isons to previous work on soft-output decoders are made.