Abstract：Sparse Code Multiple Access (SCMA) is a non-orthogonal multiple access technology based on multi-dimensional codebook, which can effectively address challenges in 5G such as massive connectivity, high spectral efficiency and millisecond delay. For the problem that threshold-based Message Passing Algorithm (MPA) has a high Bit Error Rate (BER) when the threshold is low, an improved SCMA multiuser detection scheme is proposed in this paper. Based on the threshold MPA scheme, the proposed scheme adds a judgment on the necessary conditions of user node stability. The users who not only satisfy the threshold criterion but also pass the judgment on the necessary conditions of user node stability can be decoded in advance. This improves the reliability of codeword which is judged in advance and reduces the loss of posterior soft information caused by the detection mechanism similar to the hard decision. Compared with the threshold-based MPA scheme, the proposed scheme allows the messages to be iterated more fully at low thresholds, which makes SCMA users achieve better BER performance at low thresholds. The simulation results show that better BER performance is achieved with the proposed scheme than that with the threshold-based MPA scheme for SCMA users.
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