OTFS-OCC Based Multi-User Multiplexing for Non-Terrestrial Networks

Non-terrestrial networks (NTNs) are envisioned as a key enabler of ubiquitous connectivity for future sixth generation (6 G) communication systems. However, limited spectral resources, combined with large Doppler frequency offsets and extended delay spreads, pose significant challenges to efficient multi-user access. To address these issues, we propose OTFS-OCC, an uplink multi-user multiplexing scheme that integrates orthogonal time frequency space (OTFS) modulation with orthogonal covering codes (OCCs). In the proposed scheme, users are grouped and multiplexed within a shared delay-Doppler (DD) domain region, where their signals are repeatedly transmitted with distinct delay-domain placements. By leveraging the orthogonality of OCCs, intra-group users can be transmitted in a mutually orthogonal manner, while inter-group interference can be mitigated by inserting guard intervals only after repetitions are completed. Furthermore, a dedicated multi-user detection algorithm is designed to support the proposed OTFS-OCC scheme. Both theoretical analysis and simulation results demonstrate that the OTFS-OCC scheme can significantly improve transmission reliability and spectral efficiency compared to the conventional OTFS-orthogonal multiple access (OMA) scheme.