Active RIS-Assisted Secure Transmission for Cognitive Satellite Terrestrial Networks

This correspondence develops a physical-layer security scheme for a cognitive-satellite terrestrial network, where the satellite and base station (BS) share the spectrum resource, and multiple eavesdroppers attempt to intercept the private signal from the BS to the mobile user. Different from the commonly used passive reconfigurable intelligent surface (RIS), the active RIS, whose reflecting elements can control both the amplitude and phase of the incident signal, is deployed to cooperatively enhance the secure transmission from the BS to the mobile user, and suppress the interference imposed to the earth station. We attempt to maximize the achievable secrecy rate subject to the transmit power constraint and the interference threshold. To address the above non-convex problem, we propose an effective alternating optimization scheme to jointly optimize the beamformer and artificial noise at the BS, and the reflecting coefficient at the RIS. Simulation results indicate that the impact of the “double fading” can be effectively relieved by using active RIS, thus leading to an apparently enhanced secrecy performance gain compared to those with the passive RIS and no RIS designs.