Pattern-Domain NOMA-Based Orthogonal Time Frequency Space Modulation

Orthogonal time frequency space (OTFS) is envisioned as a highly promising modulation technique due to its superior performance in high-mobility scenarios. Meanwhile, non-orthogonal multiple access (NOMA) stands out as a key contender for enhancing spectral efficiency and reducing transmission delay. In this paper, we amalgamate the pattern division multiple access (PDMA), recognized as an appealing NOMA scheme, with OTFS, coined as OTFS-PDMA, to leverage the benefits of both technologies and address the low-latency and massive connectivity demands of sixth-generation (6G) networks. Specifically, we first reformulate the input-output relations in the delay-Doppler domain for localized and distributed-mapping-based OTFS-PDMA systems featuring various PDMA pattern allocation schemes. Then, we propose a low-complexity power vector expectation propagation detection algorithm (PV-EPA) for OTFS-PDMA schemes operating in spatially correlated channels. Simulation results demonstrate that the proposed OTFS-PDMA scheme offers substantial performance improvements over the conventional OTFS-OMA scheme. For irregular PDMA patterns, localized mapping-based OTFS-PDMA schemes exhibit superior performance relative to their distributed counterparts. Additionally, the proposed PV-EPA receiver shows remarkable robustness against channel correlation and achieves a favorable trade-off between performance and complexity.