Manifold Optimization for Physical Layer Security in Double-RIS-Assisted Communications

Reconfigurable intelligent surfaces (RISs) are an emerging wireless communication technology that has attracted significant attention, particularly in the field of physical layer security (PLS). This paper proposes a novel double-RIS-aided PLS communication system in a scenario where the direct links between the access point (AP) and the legitimate user/eavesdropper are blocked, based on an alternating optimization algorithm based on the manifold optimization (MOAO) algorithm, which jointly optimizes the transmit beamforming and phase-shift matrices of the RISs to enhance the system’s secrecy rate performance. The maximum ratio transmission method is adopted to optimize the beamforming vector, and the manifold optimization-based algorithm is utilized to simultaneously optimize the phase-shift matrices of the two RISs. Meanwhile, we also propose a successive convex approximation (SCA)-based algorithm as a benchmark scheme for comparison with the MOAO algorithm. Simulation results show that the MOAO algorithm achieves a significantly improved secrecy rate while exhibiting a reduced computational complexity on the order of O(N12+N22) compared with the SCA-based benchmark.