Reconfigurable Intelligent Surface-Aided Cooperative Multi-Satellite System for Energy-Efficient Multi-User Uplink Transmission

Satellite network is envisioned as a key enabler for wide-area coverage and seamless connectivity. Nonetheless, satellite-terrestrial communications are confronted with critical challenges, including significant signal attenuation and complex inter-satellite interference, particularly in ensuring uplink performance for power-constrained ground users. Reconfigurable intelligent surface (RIS) is considered as a promising solution to compensate for the severe path loss, owing to their high-gain and dynamic beamforming capabilities. In addition, with the deployment of ultra-dense satellite constellations, multi-satellite cooperation is expected to effectively mitigate inter-satellite interference and enhance channel gains. In this paper, a novel RIS-aided multi-satellite cooperative reception scheme is proposed for multi-user uplink transmission. We first analyze five types of multi-satellite cooperative reception framework within cell-free paradigm and formulate a multi-user total energy efficiency (EE) maximization problem. Based on block coordinate descent method, this nonconvex optimization problem is decomposed into three subproblems: receiver detection vector design, RIS phase shift optimization, and multi-user transmit power control. Maximum ratio combining, zero forcing, and minimum mean square error are adopted for linear detection vector designs, respectively. Then the RIS phase shift is optimized based on Riemannian conjugate gradient algorithm. Furthermore, multi-user transmit power control is designed using Dinkelbach’s algorithm. Finally, the total EE optimization problem is solved in an iterative manner. Extensive simulation results validate the effectiveness of the proposed reception scheme and algorithm.