Secure Downlink Transmission of Active RIS Aided Cell-Free System

This letter considers an active reconfigurable intelligent surfaces (RIS) aided secure cell-free system, and focuses on enhancing the secure transmission performance of legitimate users by assuming imperfect channel state information of malicious eavesdropper. To achieve this goal, an efficient alternating optimization (AO) algorithm is proposed to maximize the worst case downlink signal to interference plus noise ratio of users under eavesdropping attack. In particular, a Dinkelbach and successive approximation combined algorithm is proposed to optimize the phase shift and amplification coefficients at RIS with detailed proof of convergence. Simulation results have verified the significant performance gain returned by the proposed algorithm compared with benchmark schemes.