Secrecy Rate Optimization for Active-Ris Assisted LEO Satellite Communications

The low earth orbit (LEO) satellite communications (SatComs) have been viewed as a flexible and effective solution for global connectivity and reliable data delivery. However, signals of the satellite-terrestrial links may be intercepted or eavesdropped, resulting in compromised secrecy performance of LEO SatComs. In this paper, the active-reconfigurable intelligent surface (RIS) is introduced into LEO SatComs, and the secrecy rate (SR) is optimized by means of terrestrial user signals to interfere with eavesdroppers to ensure information security for users served by satellites. Firstly, an active-RIS assisted LEO satellite network is presented, and reflection channels are modeled by introducing azimuth and elevation angles to distinguish signals from the satellite and terrestrial networks. Following that, aiming at the vulnerability of signal eavesdropped, the maximized SR function is constructed by jointly considering total ground power consumption, transmission power, and reliable communication threshold. After that, an alternating optimization (AO) algorithm is proposed to decouple the above non-convex problem via optimizing the beamforming vector at the base station (BS), satellite transmit power, and the reflection coefficient matrix of active-RIS. Simulation results show that the proposed active-RIS assisted LEO SatComs scheme can mitigate the impact of "double fading" effect, and enhance the security performance compared with passive-RIS and without RIS designs.