Low Complexity Techniques for SCMA Detection

Sparse code multiple access (SCMA) is a codebook- based non-orthogonal multiplexing technique. In SCMA, the procedure of bit to QAM symbol mapping and spreading of CDMA are combined together and incoming bits are directly mapped to multi-dimensional codewords of SCMA codebook sets. Due to the sparse nature of codewords, SCMA enjoys the low complexity reception, taking advantage of a near optimal message passing algorithm (MPA). This makes SCMA a candidate for supporting massive connectivity in future 5G networks, where the number of users can potentially be higher than the codeword length (spreading factor). To this end, more efficient reception techniques are needed on top of what MPA delivers. In this paper, some complexity reduction techniques are presented to further reduce the SCMA decoding complexity. These techniques are considered from two perspectives: i) transmitter-side technique, by designing SCMA codebooks with a specific structure providing low complexity of detections, and ii) low complexity decoding techniques taking advantage of the SCMA codebook structure. The proposed techniques are evaluated in terms of both complexity and performance. It is shown that significant amount of complexity reduction is possible using the proposed techniques with negligible performance penalty, which paves the way of supporting various applications in future 5G systems using SCMA.