Single Carrier SCMA Detection Schemes for Frequency Selective Channels

Sparse code multiple access (SCMA) is a candidate technology for future wireless communication systems to enhance network access capacity. Existing multi-carrier techniques, such as orthogonal frequency division multiplexing (OFDM) that are based on SCMA schemes, are well-researched and considered promising for future 6G mobile communication networks. However, the high peak-to-average power ratio (PAPR) issue stemmed from the multi-carrier schemes limits the applications of SCMA in long-distance transmission scenarios and low-cost sensor networks. To address this issue, single-carrier based SCMA transmission scheme (SC-SCMA) with low PAPR has been proposed. However multi-user signal detection remains a significant challenge for SC-SCMA, particularly with frequency-selective channels (FSC). In this paper, we establish the self-interference analysis model of the SC-SCMA system in FSC and construct an extended factor graph (EFG) for multi-user detection. Based on that, we propose the Gaussian approximate interference based belief propagation algorithm processing on EFG (E-GAIBP) to realize efficient detection over FSC with reduced complexity. Meanwhile, an extrinsic information transfer (EXIT) chart is introduced to analyze the detection performance and convergence properties of the E-GAIBP detector. Finally, to further improve performance gains, we propose two joint E-GAIBP detection and LDPC decoding schemes, E-GAIBP-JDD and layered E-GAIBP-JDD. Numerical results and complexity evaluations show that the layered E-GAIBP-JDD excels in detection performance and convergence speed, offering an effective receiver design strategy.