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.