Performance Enhancement of Modified Turbo Decoding Algorithms for Mobile WiMAX

Turbo coding has been adopted as a channel coding scheme for several 3G mobile systems, in particular 3GPP (Third Generation Partnership Project) and 4G standards for high data rates. Turbo decoder uses any one of the decoding algorithm, maximum a posteriori probability (MAP) or soft output Viterbi algorithm (SOVA). A simple but effective technique to improve the performance of the decoding algorithms is to scale the extrinsic information exchanged between two decoders. Modified log MAP (MMAP) and modified SOVA (MSOVA) algorithms are achieved by fixing an arbitrary value for inner decoder (S2) and an optimised value for the outer decoder (S1). We propose to enhance the performance of MMAP and MSOVA by optimising both the scaling factors S1 and S 2 , thus achieving low bit error rate (BER). A comprehensive analysis of the selection of scaling factors for enhanced log MAP and enhanced SOVA (ESOVA) are presented. The performance of various scaling factors is compared for various E b /N 0 and optimised scaling factor is obtained, which is an empirical value. Appropriate mathematical relationship between scaling factor and E b /N 0 are also proposed. The performance of mobile WiMAX (worldwide interoperability for microwave access) physical layer is also investigated and a suitable decoding algorithm is proposed. Different forward error correction codes are considered for this purpose.