Performance Analysis of Suboptimal Multiuser Detection Algorithms Based on MPA in Uplink SM-SCMA System

Sparse code multiple access (SCMA) combined with spatial modulation (SM) has recently been proposed for improving massive connectivity and high spectral efficiency (SE), respectively. Generally, the message passing algorithm (MPA) is implemented at the decoder in the uplink SM-SCMA systems in order to achieve a strong iterative decoding performance. Nevertheless, it suffers from high complexity due to the large number of involved exponential operations to compute the log likelihood ratio (LLR) and the large number of iterations for convergence performance. This paper provides three simplified suboptimal versions based on MPA known as Max-Log-MPA, S-MPA, and S-Max-Log-MPA for the SM-SCMA scenario to ensure a trade-off between performance and complexity. The simulation results demonstrate that the proposed algorithms can reduce computational complexity from 31 % to 55% compared with the conventional MPA algorithm while retaining negligible BER performance for the uplink SM-SCMA system.