Linearly Approximated Log-MAP Algorithms for Turbo Decoding

Different floating-point and fixed-point implementation issues of the Log-MAP algorithm for decoding turbo codes are studied. A theoretical framework is proposed to explain why the Log-MAP turbo decoders are more tolerant of overestimation of SNR than underestimation. A linear approximation (the Log-Lin algorithm) that allows for efficient ASIC implementations and results in negligble performance loss in floating-point models is proposed. It is further shown that a fixed-point implementation of this Log-Lin algorithm, including clipping and quantization of the received samples to 6 bits, results in a loss of performance of less than 0.05 dB.