Joint Iterative Detection and Decoding for Coded SCMA-OTFS System

Abstract

Future beyond-5G (B5G) and 6G wireless networks are expected to support ultra-reliable and low-latency communications in high-mobility scenarios while meeting massive connectivity requirements. Orthogonal time-frequency space (OTFS) modulation has emerged as a promising technique to combat severe Doppler effects, whereas sparse code multiple access (SCMA) improves spectral efficiency through nonorthogonal resource sharing. In this paper, we investigate a low-density parity-check (LDPC)-coded SCMA-OTFS system and propose a joint iterative detection and decoding scheme based on a turbo equalization framework. Specifically, linear minimum mean square error (LMMSE) equalization is first performed in the delay-Doppler (DD) domain to suppress channel distortion, followed by message passing algorithm (MPA)-based SCMA detection under an equivalent AWGN approximation. The resulting soft information is iteratively exchanged between the SCMA detector and the LDPC decoder, where symbol-level a posteriori probabilities (APPs) are converted into bit-level log-likelihood ratios (LLRs) for decoding and the decoder outputs are remapped into symbol-level a priori probabilities for subsequent detection. Simulation results show that the proposed iterative scheme achieves notable bit error rate (BER) performance gains over the non-iterative baseline under the considered high-mobility channel conditions.