Modified Max-Log-MAP turbo decoding algorithm using optimized scaling factor

The Max-Log-MAP is a Soft Input Soft Output (SISO) algorithm, which determines the probability of most likely path through the trellis and hence it gives sub optimal performance compared to Log-MAP algorithm. A simple but effective technique to improve the performance of Max-Log-MAP (MLMAP) algorithm is to scale the extrinsic information exchanged between two decoders using appropriate Scaling Factor (SF). Modified Max-Log-MAP (M-MLMAP) algorithm is achieved by fixing an arbitrary SF for inner decoder S₂ and an optimized SF for the outer decoder S₁. This paper presents the performance of the Modified Max-Log-MAP decoding algorithm by reducing the over estimation of reliability values to achieve low Bit Error Rate (BER). Appropriate mathematical relationship between SF and E_b/N₀ is also proposed. The numerical results show that M-MLMAP algorithm improved the performance of turbo decoding over Additive White Gaussian Noise (AWGN) and Rayleigh fading channels. The proposed M-MLMAP algorithm showed a gain of 0.75dB over MLMAP algorithm at BER of 2×10^-5 for Rayleigh fading channel.