IRS-Aided Secure Communications Over An Untrusted AF Relay System

Intelligent Reflecting Surface(IRS)is a meta-surface controlled by the software,usually consisting of a large number of low-cost passive reflection elements.IRS is both spectral and energy efficient technique with low hardware cost,which is beneficial for realizing intelligent reconfigurable wireless transmission environment.In this paper,an IRS assisted wireless secure communication system is studied.It is assumed that the direct link between the access point(AP)and the user is blocked by the obstacles.The multi-antenna 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.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 AP transmit beamforming vector,the IRS phase shift matrix,and the relay beamforming matrix 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 are derived as well.We consider a fading environment,all the channels follow the Rayleigh fading models for small-scale fading,meanwhile the path loss model is also considered for large-scale fading.Numerical evaluations validate the theoretical analysis,demonstrate the superiority of the proposed scheme compared with other benchmark schemes,and highlight the importance of properly designed phase shifts at the IRS.It is also demonstrated that the number of antennas at the AP should be strictly larger than the number of antennas at the relay to harvest the benefits of increasing the transmit power at the AP in increasing the secrecy rate.