Hybrid Active-Passive RIS Empowered Secure Communications: Joint Architecture Design and Beamforming Optimization

Reconfigurable intelligent surface (RIS) has recently emerged as a promising solution to significantly enhance the security of wireless communication systems. By combining the advantages of the conventional fully-active and fully-passive RISs, a novel hybrid active-passive RIS has been anticipated to achieve the excellent communication performance at a low cost. In this paper, we aim to maximize the secrecy rate in a hybrid active-passive RIS assisted multi-input single-output multi-antenna Eve (MISOME) system, where both fixed and dynamic hybrid RIS architectures are considered. To tackle this intractable problem, we jointly optimize the transmit covariance matrix at the base station (BS), the reflection matrices of active and passive sub-RISs, as well as the element allocation matrix for the dynamic hybrid RIS. Specifically, we firstly explore the rank-1 structure of the optimal BS transmit covariance matrix. Then, for the fixed hybrid RIS, we develop an efficient two-loop successive convex approximation (SCA) based iterative algorithm, where the optimal semi-closed-form solution to each subproblem can be obtained. For the dynamic RIS, this proposed algorithm is still applicable by relaxing the binary active/passive elements allocation variables into exponent-based continuous ones. Simulation results validate the superior secrecy performance of the proposed designs over the existing fully-active and fully-passive RIS designs. Moreover, it is demonstrated that the dynamic hybrid RIS is able to strike a good balance between the passive beamforming gain and the power amplification gain to adapt to the varying propagation environment.