No abstract

A multiuser detection scheme is proposed for nonbinary 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. Loop Outer data Estimated Code Address Code AddressΠ Fig. 1. FFH/MFSK system models: (a) The transmitter. (b) The receiver.

No abstract

This paper studies an iterative multiuser detection and decoding for nonbinary low density parity check (LDPC) coded multicarrier M-ary frequency shift keying (MC MFSK) multiple access system to increase system spectral efficiency under specific signal-to-noise ratio. At multiuser detection side, soft metric for each symbol is quantized to 2 levels (2L) or 3 levels (3L) and then pass to the decoder. When the soft output of a symbol from decoder exceeds a threshold, the user is declared as a reliable symbol. The reliable information is fed back to the detection side and updates the symbol soft output when iterative processing is carried out. Numerical results show that system spectral efficiency is improved for 3L system over Rayleigh fading.

A turbo principle based iterative multiuser detection is proposed to reduce the BER in LDPC coded FFH/MFSK MA systems. First, a three-level hard-limiter on each chip with maximum-likelihood decision is made to avoid soft output ambiguity. Secondly, the reliable information from channel decoder is applied to aide multiuser interference reduction. When the soft reliability information of coded bit exceeds a predefined threshold, these code bits are defined as reliable bits. A symbol is reliable when all its bit vectors are reliable. As iteration proceeds, the number of reliable users is counted which will change the detection probability of the estimated symbol. Numerical results show that three-level hard-limited systems outperform two-level systems and the systems with modified soft outputs of multiuser detection have better performance than that of conventional iterative decoding systems.