Manifold-Based Robust Beamforming Design for RIS-Assisted LEO Satellite in SAGIN-ISAC Systems

Abstract

To enable ubiquitous global connectivity and target detection within a unified platform, this paper presents a space-air-ground integrated network (SAGIN) that simultaneously supports communication and radar functionalities. Specifically, the air segment of the SAGIN system is realized through a group of high-altitude platforms (HAPs), each carrying a reconfigurable intelligent surface (RIS). However, the realization of such integrated sensing and communication (ISAC) systems presents significant challenges due to the severe interference. This motivates us to propose a novel robust beamforming design for RIS-assisted LEO satellite in SAGIN-ISAC systems, in which the radar receiver is decoupled from the satellite to mitigate severe echo path loss and reduce the satellite’s power consumption burden. The robust design takes into account the channel state information (CSI) imperfection of both direct and indirect communication channels by considering Gaussian CSI errors, in which the outage probability constraint is adopted. To efficiently coordinate the dual-functional signal design, we develop a Riemannian manifold-based optimization algorithm that jointly designs the active and passive beamforming strategies. Simulation results demonstrate that the proposed framework effectively enhances the radar signal-to-interference-plus-noise ratio (SINR) subject to communication outage as well as radar and communication power constraints.