A Hybrid Framework of RIS-Assisted Robust Secure Transmission Design for Multibeam Satellite Communications

In this paper, we investigate the hybrid secure transmission strategy for reconfigurable intelligent surface (RIS)-assisted multibeam satellite communications. In particular, two scenarios are considered based on actual transmission conditions and information theory. One is that when RIS has proper signal links with the satellite, it operates as the signal-enhancing RIS (SER). Otherwise, RIS operates as the signal-jamming RIS (SJR). In detail, two scenarios are presented as follows. I) For the SER scenario, a secure optimization problem is established by jointly designing the satellite and RIS beamforming under the constraints of total transmit power and quality of service. When the imperfect channel state information (CSI) of the RIS-eavesdropper (Eve) is obtained, a robust algorithm is proposed, and the \mathcal S-procedure-based successive convex approximation as well as the alternating optimization (AO) method are adopted. The case without Eve's CSI is also studied, and the maximum ratio transmission-based scheme is adopted. II) For the SJR scenario, different algorithms of zero-forcing (ZF) and ZF-based AO algorithms are given to obtain the artificial noise beamforming when considering the imperfect CSI of RIS-Eve and the CSI of no Eve, respectively, and both avoid the self-interference with legitimate user. Then, an AO-based algorithm for satellite beamforming design is presented. Numerical simulations are performed to evaluate the proposed approaches, which show superior performance.