OTFS-Based Spatial Modulation for Multi-Satellite Cooperative Communication Systems

Low earth orbit (LEO) satellite constellations are the key enablers to achieve the ubiquitous communication in the 6G era, but the performance is hindered by the Doppler effects and the underutilization of spatial diversity. By leveraging the quasi-static properties of the delay-Doppler (DD) domain, orthogonal time-frequency space (OTFS) demonstrates robustness against the Doppler shifts. Moreover, spatial modulation offers the potential to improve spectrum efficiency and energy efficiency. This paper proposes an OTFS-based spatial modulation (OTFS-SM) paradigm for multi-satellite cooperative transmissions, aiming to enhance system reliability. In order to further exploit the multi-satellite diversity, a satellite selection (SS) strategy based on channel capacity maximization criterion is proposed. Additionally, the average bit error rate (ABER) upper bound of the proposed OTFS-SM system in multi-satellite scenarios is derived using minimum mean square error (MMSE) detection. Numerical results verify the theoretical analysis of the ABER and demonstrate that the proposed strategy significantly outperforms both the single-satellite scenario and the OFDM-SM system in terms of ABER performance.