Active Reconfigurable Intelligent Surface Assisted Secure and Robust Cooperative Beamforming for Cognitive Satellite-Terrestrial Networks

This paper proposes a novel active reconfigurable intelligent surface (RIS) assisted secure and robust cooperative beamforming for cognitive satellite-terrestrial networks. Specifically, an active RIS enhances interference at an eavesdropper and reduces interference at a satellite user both from a ground base station (BS), aiming to ensure the secure transmission performance of satellite users in a primary network. Meanwhile, the transmission performance of a terrestrial user in a secondary network is expected to be enhanced without affecting the satellite user. Furthermore, the impact of imperfect channel state information is also reconsidered due to the amplifying effect of active RIS. Accordingly, we focus on minimizing the transmit power at the ground BS while guaranteeing the achievable secrecy rate of the satellite user and the achievable rate of the terrestrial user. Based on the non-convex cooperative beamforming problem, an efficient alternating optimization scheme is developed. Finally, the numerical results demonstrate the potential of active RIS on power saving under secure and robust cognitive satellite-terrestrial transmission.