Joint Precoding and Link Scheduling for OTFS-Based Multi-Satellite Cooperative Transmission

Multi-satellite cooperative transmission (MSCT) is a promising paradigm to enhance spectral efficiency in Low Earth Orbit constellations. However, heterogeneous Doppler shifts and coupled multi-satellite interference hinder the capacity improvement of conventional precoding in multiple-input multiple-output (MIMO). Although orthogonal time-frequency space (OTFS) modulation exhibits robustness against doubly-selective channels, its direct application in massive MIMO faces prohibitive computational complexity. To address these challenges, this paper proposes a joint precoding and link scheduling (JPL) design for OTFS-based MSCT systems. Specifically, we formulate a sum-rate maximization problem that couples continuous precoding matrices with discrete link indicators, and decompose it into two tractable subproblems. For precoder design, based on regularized zero-forcing (RZF)-criterion, we present single-satellite precoding (SSP) and multi-satellite precoding (MSP) in the delay-Doppler domain for different MSCT modes, and optimize per-satellite regularization coefficients to maximize the sum-rate. Leveraging the quasi-banded MIMO-OTFS structure, a low-complexity RZF algorithm is developed to reduce the cubic complexity to quadratic order without performance loss. Based on random matrix theory, we conduct asymptotic analysis and derive the deterministic equivalents of sum-rate for SSP and MSP. For link scheduling, a two-stage heuristic algorithm is designed based on tabu search to iteratively optimize link indicators for maximizing the sum-rate. By alternately optimizing precoding matrices with each scheduling iteration, the JPL algorithm is proposed to achieve near-optimal sum-rate performance with polynomial computational complexity. Numerical results demonstrate the proposed schemes significantly improve the sum-rate performance compared to existing works.