Low-power log-MAP decoding based on reduced metric memory access

Abstract—Due to the powerful error correcting performance, turbo codes have been adopted in many wireless communication standards such as W-CDMA and CDMA2000. Although several low-power techniques have been proposed, power consumption is still a major issue to be solved in practical implementations. Since turbo decoding is classified as a memory-intensive algorithm, re-ducing memory accesses is crucial to achieve a low power design. To reduce the number of memory accesses for maximum a poste-riori (MAP) decoding, this paper proposes an approximate reverse calculation method that can be implemented with simple arith-metic operations such as addition and comparison. Simulation re-sults show that the proposed method applied to the W-CDMA stan-dard reduces the access rate of the backward metric memory by 87 % without degrading error-correcting performance. A proto-type log-MAP decoder based on the proposed reverse calculation achieves 29 % power reduction compared to a conventional decoder that does not use the reverse calculation. Index Terms—Low power design, maximum a posteriori (MAP) algorithm, memory optimization, reverse calculation, turbo codes, turbo decoding. I