RIS-Enabled Multi-Satellite Cooperative Communication for Remote UAV Fleet

The space-air-ground-sea integrated network (SAGSIN) has emerged as a multi-domain solution to enable ubiquitous coverage in remote regions. As essential aerial nodes in SAGSIN, the remote unmanned aerial vehicles (UAVs) confront severe communication challenges as the deficiency of terrestrial support leads to inefficiency, while communication with satellites inherently suffers from substantial signal attenuation caused by ultra-long-distance propagation. This paper proposes a reconfigurable intelligent surface (RIS)-enabled multi-satellite cooperative system to ensure efficient downlink communication for remote UAV fleets. RIS is deployed on an unmanned aerial aircraft carrier (UAAC) to intelligently reconfigure propagation environments and dynamically follow the subordinate UAVs, without burdening task-executing UAVs. A joint optimization framework is formulated to maximize the overall system sum rate by jointly designing the satellite beamforming vectors, the RIS phase shift matrix, and the horizontal position of the UAAC. To address the intricate coupling among the optimization variables, an alternating optimization (AO) algorithm is proposed, decomposing into three subproblems: beamforming optimization via the weighted minimum mean square error (WMMSE) algorithm, phase shift optimization via the manifold optimization (MO) method, and UAAC position optimization via the projected gradient ascent (PGA) method. Comprehensive simulations demonstrate significant performance improvements compared to both the single-satellite systems and the RIS-disabled systems.