Secure transmission for intelligent reflecting surface-aided communication system

Intelligent reflecting surface (IRS) is convinced to have a wide range of applications in the future wireless networks by dynamically configuring signal propagation in order to enhance communication performance. In this paper, we focus on the IRS-assisted communication system, where a multiple-antennas base station communicates with a single-antenna receiver in the presence of a single-antenna eavesdropper. The base station can control the phase shifts of the IRS, and further enhance the secrecy rate. Under the assumption that the eavesdropper's channel statistical information is known, we derive a lower bound of secrecy rate and aim to maximize it. To solve the resulting non-convex optimization problem, an alternating optimization approach is proposed. In particularly, the coordinate descent method (CDM) is invoked in the process of designing the phase shifts at the IRS, which can achieve a similar performance with the semi-definite relaxation (SDR) approach, but requires a lower computational complexity. Simulation results show that our proposed algorithm can improve the secure performance of the considered system, and deploying a 3-bit quantizer at IRS is enough to obtain a good performance as compared to the continuous quantizer.