Self-powered absorptive reconfigurable intelligent surfaces for securing satellite-terrestrial integrated networks

Satellite communications have attracted significant interests due to its advantages of large footprint and massive access. However, the commonly used onboard beamforming is hard to achieve reliable security because of the highly correlated legitimate and wiretap downlink channels. We exploit the benefits of satellite-terrestrial integrated network (STIN) and a novel absorptive reconfigurable intelligent surface (RIS) for improving the security of satellite downlink communications (SDC) in the presence of eavesdroppers (Eves). This paper aims to maximize the achievable secrecy rate of the earth station (ES) while satisfying the signal reception constraints, harvested power threshold at the RIS, and total transmit power budget. To solve this nonconvex problem, we propose a penalty-function based dual decomposition scheme, which firstly transforms the original problem into a two-layer optimization problem. Then, the outer layer and inner problems are solved by utilizing the successive convex approximation, Lagrange-dual and Rayleigh quotient methods to obtain the beamforming weight vectors and the reflective coefficient matrix. Finally, simulation results verify the effectiveness of the proposed scheme for enhancing the SDC security.