Join active and passive beamforming optimization for double-RIS aided CR networks

Intelligent Reflector Surface (IRS), also known as Reconfigurable Intelligent Surface (RIS), is a promising method to improve the energy efficiency of wireless communication system by intelligently reconfiguring the channel environment in recent years. Cognitive radio (CR) is an effective solution to improve the spectrum efficiency of wireless communication, which allows secondary users (SU) to share the spectrum with primary users (PU). In this paper, double-RIS are introduced into the downlink multi-input single-output (MISO) CR system to assist communication. Our design goal is to maximize the sum of the achievable rates of SU and PUs by jointly optimizing the beamforming of cognitive base station and main base station and the reflection coefficients of the double-RIS under the constraints of base station transmission power and interference threshold. In order to achieve this goal, we use semi-positive definite relaxation method (SDR) and alternate optimization method to get the optimal solutions of beamforming vector and reflection coefficient. The simulation results show that in the double-RIS aided CR networks, the proposed scheme can significantly improve the reachable rate of all users.