Cell-Free Macro-Diversity Schemes in LEO Non-Terrestrial Networks With OTFS and OFDM Modulations

Satellite-based Non-Terrestrial Networks, particularly those using large constellations of Low Earth Orbit (LEO) satellites, are expected to play a critical role in enabling global 6G connectivity. While offering promising coverage and capacity, LEO systems face challenges such as intermittent link blockages and high Doppler shifts, especially in mobile or obstructed environments. To mitigate these issues, this paper investigates advanced macro-diversity techniques tailored for LEO satellite systems. Inspired by user-centric cell-free massive MIMO architectures in terrestrial networks, we propose a comprehensive end-to-end transceiver model that captures the complete signal chain, from symbol generation and continuous-time waveform transmission to receiver-side sampling and data detection. Crucially, we account for satellite-specific Doppler shifts and phase offsets, often overlooked in simplified models. We analyze and compare two modulation schemes: traditional OFDM and OTFS, the latter offering enhanced resilience to time-varying channels. Furthermore, we consider the case of multi-antenna user terminals (UTs) and demonstrate how receive beamforming can effectively mitigate inter-satellite phase misalignment, significantly improving system robustness and performance. The obtained numerical results show the effectiveness of the proposed schemes, confirm the superiority of the OTFS modulation w.r.t. OFDM, and provide evidence that multiple antennas at the UT can be exploited to overcome the phase misalignment effects of downlink signals coming from different serving LEO satellites. Finally, results also show that satellite-UT association schemes may have a considerable impact on system performance.