Low complexity stopping criteria for UMTS turbo-decoders

Turbo-codes are part of the third generation wireless communications system (UMTS). A turbo-decoder consists of two soft-in soft-out component decoders, which exchange information (soft-values) in an iterative process. The number of iterations for decoding strongly depends on the channel characteristic which can change from block to block due to fading. In this paper, we present two new stopping criteria which can be implemented on dedicated hardware or DSP with negligible overhead. The new criteria operate on the sum of the absolute soft output values, calculated after each component decoder and is referred to as sum reliability. We compare the communications performance and average number of iterations of our proposed criteria to other criteria in literature using a fixed-point 8-state turbo-decoder implementation in an UMTS FDD-downlink chain. An analysis of the arithmetic complexity and memory demand yields minimal overhead with excellent performance compared to other stopping criteria.