Outage-Constrained Energy-Efficient Resource Allocation for IRS-Assisted Secure Communications

In this paper, we investigate the resource allocation problem of a downlink system from a multi-antenna base station (BS) to multiple single-antenna legitimate users, where an intelligent reflecting surface (IRS) is employed to guarantee the physical layer security from multiple single-antenna eavesdroppers. Considering the passive characteristic of eavesdroppers, only statistic channel state information (CSI) can be obtained for all channels related to eavesdroppers and artificial noise (AN) is applied to handle this challenging issue. Aiming to maximize the secrecy energy efficiency (SEE), we jointly optimize the beamformers and AN at the BS and phase shifts at the IRS subject to secrecy outage probability (SOP) constraints. To tackle the non-convexity of the formulated problem, an alternating optimization (AO) scheme based on Dinkelbach's algorithm and successive convex approximation (SCA) approach is proposed. Simulation results verify the convergence of the proposed scheme and demonstrate that both IRS and AN can significantly improve the SEE. Meanwhile, it is illustrated that with sufficient transmit power budget, the AN-assisted scheme even outperforms the IRS-assisted scheme.