Hybrid Active-Passive RIS-Enhanced Physical Layer Security for MIMO Communications

Reconfigurable intelligent surfaces (RIS) critically enhance physical layer security (PLS), safeguarding next-generation MIMO communications against eavesdropping. Passive RIS are limited by multiplicative path loss, while active RIS implementations introduce uncontrolled noise amplification and significant power consumption. To overcome these dual constraints, we propose a hybrid active-passive RIS (HRIS) architecture. The proposed design synergistically incorporates a minimal number of active amplifying elements within a passive structure to maximize the secrecy rate (SR). The problem of SR maximization is tackled by jointly optimizing the base station beamforming vectors and the HRIS reflection coefficients. To address the non-convexity of the optimization problem, we decompose it into three sub-problems of optimizing the transmitter beamforming, phase shift coefficient and amplification coefficient. These sub-problems are solved iteratively within an alternating optimization framework. Simulation results demonstrate that the proposed design effectively balances secrecy performance and power consumption, providing a resource-efficient solution for secure MIMO communications.