Optimal Semi-quasi-static Design for An IRS-aided PLS Wireless Communication System

The costs of channel estimation, phase shift adjustment, and computation have severe impacts on an intelligent reflective surface (IRS)-aided physical layer security (PLS) wireless communication system in practice but are usually overlooked for simplicity in most existing works. In this paper, a multi-antenna base station communicating with a single-antenna legitimate user with the aid of a multi-element IRS under the surveillance of a single-antenna eavesdropper is considered. The partial-instantaneous channel state information (ICSI) case with the legitimate user's and eavesdropper's statistical CSI (SCSI) and the legitimate user's ICSI is investigated. We present a partial-ICSI-adaptive beamforming design and a SCSI-adaptive (also termed quasi-static) phase shift design, together called a semi-quasi-static design, which has low computation and phase shift adjustment costs. First, the maximization of the achievable ergodic secrecy rate (ESR) with respect to (w.r.t.) the semi-quasi-static design is formulated as a two-timescale stochastic non-convex problem. Then, a stochastic iterative algorithm is proposed to obtain a stationary point, and its computational complexity is analyzed. Next, it is shown that the semi-quasi-static design achieves lower computation and phase shift adjustment costs than the existing design in the partial-ICSI case. Lastly, we numerically demonstrate the proposed semi-quasi-static design's notable gains over existing designs.