Practical Soft Demapper-Based Decoder for Sparse Code Multiple Access

Existing SCMA systems presented for multiuser detection has high computational complexity. The complexity is a prime concern considering its implementation in a machine-to-machine (M2M) communication involving limited capacity devices. Since these devices have low processing power, so practical implementation of SCMA needs to be optimised for better performance. This paper proposes an SCMA decoder with a soft demapping technique to reduce the computational complexity, providing better performance. To improve the stability of SCMA system for M2M communication, this paper proposes the use of noise variance threshold ψ, to handle the system’s stability. Results confirmed that the proposed soft demapper-based SCMA reduced the execution time to 33% and the iteration number to 63% compared to the conventional SCMA scheme deploying message passing algorithm (MPA)-based decoding. The proposed SCMA provides an improvement of 2.5 dB at a BER of 10−3 under the block Rayleigh fading channels and an improvement of approximately 1.1 dB at a BER of 10−5 dB under the fast Rayleigh fading channels. The improvements come from the fast convergence of iterative decoding using a soft demapper supported by a variance threshold ψ that avoids the duality decision at the border of the decision area.