Abstract : This paper exploits reconfigurable intelligent surfaces (RIS) to enhance physical layer security (PLS) in a challenging radio environment. By adjusting the reflecting coefficients, RIS can provide a programmable wireless environment so that the electromagnetic wave can propagate in the desired way. Specifically, we consider a scenario where multiple user pairs communicate simultaneously over the same channel in a multiuser MISO interference channel and each pair of transceivers keeps their confidential information secret from other receivers. We investigate the secrecy rate balance of the system aided by multiple RISs and exploit the additional design degrees of freedom provided by the coordinated RISs to increase the secrecy capacity, achieving secrecy transmission. In particular, we adopt cooperative beamforming among the collaborative transmitters to maximize all users’ minimum secrecy rate, achieving the secrecy rate balance. Due to the non-convexity of the formulated optimization problem, we propose alternating optimization (AO) to solve it iteratively and optimize active the transmit beamforming at the transmitters and passive phase shifts at the RISs based on positive semi-definite relaxation (SDR) and successive convex approximation (SCA). In each iteration, we solve a semi-definite programming (SDP) problem and finally get the optimal local solution to the optimization problem. Finally, the simulation results validate that the proposed algorithm is effective and the introduced RISs can significantly enhance the system secrecy performance compared to conventional baseline schemes.
Index terms : Physical layer security, reconfigurable intelligent surface, secrecy rate, semi-definite relaxation, successive convex approximation.