Joint Beamforming Design for Secure Communications over An IRS-Aided Untrusted Relay Network

In this paper, a secure wireless communication system, where a multi-antenna access point (AP) sends confidential information to a single-antenna user with the help of an untrusted relay adopting amplify-and-forward (AF) protocol and an IRS, is investigated. It is assumed that the link between the IRS and the untrusted relay is present. A secrecy rate maximization problem subject to the resource constraints at the AP and the untrusted relay is formulated. Then, an alternating iteration algorithm jointly optimizing the transmit beamforming matrix at the AP, the phase shifts of the IRS in two time slots and the relay beamforming matrix is proposed. Afterwards, the asymptotic expressions for the maximum secrecy rates of the proposed scheme in the high and low transmitted signal-to-noise ratio (SNR) regimes are derived as well. Finally, numerical evaluations demonstrate the superiority of the proposed scheme compared with other benchmark schemes, highlight the importance of properly designed phase shifts at the IRS, and validate the theoretical analysis.