Joint Beamforming Design for Active Intelligent Reflecting Surface-Aided Physical Layer Secure Transmission with Inaccessible Eavesdroppers’ CSI

To address the limited secrecy performance improvement of passive intelligent reflecting surface (IRS)-aided systems caused by the “multiplicative fading” effect and the challenges in solving optimization problems arising from unavailable complete channel state information (CSI) of multiple eavesdroppers in practical systems, this study investigates the physical layer security performance of active IRS (AIRS)-aided wireless networks in the presence of multiple eavesdroppers with inaccessible CSI. The optimization problem of maximizing user secrecy rate (SR) is formulated, subject to the secrecy outage probability (SOP) constraint. By leveraging the exponentially distributed nature of the signal strength over Rayleigh fading links, the probabilistic constraint is transformed into an approximate deterministic one. An approximate closed-form expression for the SOP is derived and incorporated into the objective function to reformulate the optimization problem. An alternating optimization (AO)-based SR maximization scheme for AIRS-aided wireless communication systems (AO-SRMSAIRS) is presented, where the original problem is decomposed into two sub-problems. These sub-problems are tackled using the semi-definite relaxation and minorization–maximization algorithms, respectively, to jointly optimize the active beamforming at the transmitter and the reflection coefficient matrix of AIRS. Simulation results indicate that AIRS effectively alleviates the “multiplicative fading” effect, and the AO-SRMSAIRS scheme delivers enhanced secrecy performance gains over existing solutions.