Intelligent Reflecting Surface Aided Secure Transmission with Energy Harvesting Eavesdropper

In this paper, we investigate secure transmission assisted by intelligent reflecting surface (IRS) in multiple-input single-output (MISO) wireless communication system, where an untrusted energy harvesting node that can eavesdrop on legitimate signals. Particularly, we consider the use of discrete phases at the IRS. The aim of this paper is to maximize the secrecy rate under the constraints of maximum transmit power of legitimate transmitter (LT) and the minimum energy demand in untrusted node (UN). By jointly optimizing the transmit beamforming and IRS phase coefficients, the largest secrecy rate is obtained. To address the formulated nonconvex problem, we first obtain the transmit beamforming vector under the given IRS phase shifts. Then, we propose a low-complexity optimization algorithm based on fractional programming (FP) and alternating direction method of multipliers (ADMM) to obtain near optimal IRS phase shifts. Finally, numerical simulation shows that our proposed methods can effectively improve secure transmission rate of IRS system.