A soft multiuser demodulation for non-binary LDPC coded FFH MA/MFSK systems

Abstract

A multiuser detection scheme is proposed for non- binary LDPC coded FFH MA/MFSK system to reduce the bit error rate. To mitigate the soft ambiguity in the multiuser detection, a three-level hard-limiter on each chip is proposed with the detection thresholds determined by maximum-likelihood criterion. Based on the evaluation of PDFs of chip corresponding to different number of users hit on it, soft output is derived. Since the channel is non-binary, non-binary LDPC code make there no binary-symbol conversion loss between multiuser detection and channel decoder. Those users whose soft reliability values of the estimated coded symbols exceed a predefined threshold are defined as reliable users. As iteration proceeds, the number of reliable users is increased and multiuser detection process is carried out. Numerical results show that in both AWGN and Rayleigh fading channels, the performances of three-level systems outperform two-level systems and performances in AWGN channels are better than that in Rayleigh channels. However, for two-level systems, the performance in AWGN channel takes no benefit over Rayleigh channel due to useful information is lost.