IRS-Aided Secure Communications over an Untrusted AF Relay System

In this paper, an intelligent reflecting surface (IRS) assisted wireless secure communication system in the presence of an untrusted relay is studied. A multi-antenna access point (AP) intends to send confidential information to a single-antenna user in the presence of an amplify-and-forward (AF) untrusted relay, which may eavesdrop the message when helping relay the signal. It is assumed that the direct link between the AP and the user is blocked by the obstacles. The achievable secrecy rate maximization problem is then formulated. To overcome the non-convexity of the formulated problem, an alternating iteration algorithm is proposed to jointly optimize the active and passive beamforming. Specifically, the transmit beamforming vector at the AP, the phase shift matrix at the IRS, and the relay beamforming matrix are jointly optimized to maximize the secrecy rate. Moreover, the asymptotic expressions for the maximum secrecy rates of the proposed scheme in the high and low signal-to-noise ratio (SNR) regimes of the AP-relay and relay-user links are derived as well. Finally, numerical evaluations demonstrate the superiority of the proposed scheme compared with other benchmark schemes, and validate the theoretical analysis. Some useful insights are also obtained, including the number of antennas at the AP, the scale selection and the phase shift settings at the IRS, and the potential cooperation gain of the untrusted relay.