Optimal receiver design for SCMA system

Sparse code multiple access (SCMA) is a promising non-orthogonal multiple access scheme for 5G systems. In this paper, based on the sphere decoding (SD), we design an optimal receiver for SCMA, which provides much lower complexity than the maximum likelihood (ML) detection without sacrifice of the optimal performance. Regarding the coded SCMA system, we propose a list sphere decoding (LSD) method to output each user's bit soft decisions to the corresponding channel decoders. In addition, the existing lattice points in the list are employed to set the original radius so as to further reduce the complexity of LSD. The performance and complexity analyses indicate that the proposed optimal receiver provides notable gain compared to the most popular SCMA receiver equipped with the message passing algorithm.