Secure mmWave MIMO Networks Employing Hybrid Active-Passive RIS

By combining the benefits of passive reflecting and active relay, the hybrid active-passive reconfigurable intelligent surfaces (RIS) (HRIS) architecture can overcome the double path loss and limited performance gains of the conventional passive RIS. In this work, we investigate the secrecy performance of a millimeter wave multiple-input-multiple-output (MIMO) network in the presence of a multiantenna eavesdropper with imperfect channel state information (CSI). To maximize the secrecy rate, we propose a joint design of the transmit beamformer and HRIS coefficients utilizing the combined tools of block coordinate ascent (BCA), stationary solution to Rayleigh quotient, Dinkelbach method, and concave-convex procedure (CCCP). The design problem encompasses both fixed and dynamic HRIS architectures. For the former, a pre-manufactured set of active elements is considered. For the latter, the set of active elements is adaptively updated, and their coefficients are optimized via a stochastic game-based algorithm. Simulation results demonstrate that the proposed HRIS schemes outperform the conventional passive RIS, particularly when high transmit power and many active coefficients are deployed at the HRIS. Additionally, a dynamic HRIS can offer 60% improvement in the secrecy rate compared to the passive RIS scheme.