Robust and Low-Complexity Secure Beamforming for Distributed RIS-Aided mmWave Systems with Limited Feedback

This paper presents a robust and low-complexity beamforming design to enhance physical layer security in mmWave systems with distributed RIS under imperfect CSI. This design jointly optimizes feedback bit allocation with active and passive beamforming to adapt to varying channel conditions. To address this complex and non-convex optimization, we propose an alternating optimization algorithm that decouples the problem into subproblems, applying appropriate methods iteratively at each level to obtain the optimal solution. Simulations demonstrate the robustness of the proposed algorithm in mitigating the effects of imperfect CSI, providing improved security performance and resource efficiency compared to benchmarks, particularly with limited feedback bit resources or in challenging spatial conditions.