An optimized design of SCMA codebook based on star-QAM signaling constellations

In this paper, an optimized codebook design for a non-orthogonal multiple access scheme, called sparse code multiple access (SCMA), is presented. Unlike the low density signature (LDS) systems, in SCMA systems, the procedure of bits to QAM symbol mapping and spreading are combined together, and the incoming bits are directly mapped to the codewords of the SCMA codebook sets. Each layer or user has its dedicated codebook and the codebooks are all different. An improved method based on star-QAM signaling constellations is proposed here for designing the SCMA codebooks. It is demonstrated that the new method can greatly improve the BER performance without sacrificing the low detection complexity, compared to the existing codebooks and LDS.