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 t o the outer decoder, This paper shows that a modified MAP algorithm ca n be effectively and accurately used to generate such information, In the course of the presentation, a metric based on the reliability info rmation is proposed for the outer decoder, This metric has the Euclide an metric on AWGN channels as its special case, which leads to the con cept of generalized SDD (GSDD), Several practical concerns regarding t he proposed soft-output decoder are addressed through theoretical anal ysis and simulation: the effect of finite decoding depth, computationa l complexity, range overflow, and scaling, Comparisons to previous wor k on soft-output decoders are made.