Research On Beamforming Of IRS-Assisted Secure Communication System

Physical layer security technology has become an effective solution to ensure wireless communication security.As a green and economical technology,Intelligent reflecting surface(IRS)can change the wireless propagation environment of signals,making it conducive to signal transmission.By optimizing the beamforming at the base station and the reflecting beamforming at the IRS,the performance of the communication system can be further improved under limited cost conditions,providing a new idea for physical layer secure communication.In this context,this paper mainly studies the beamforming problem of IRS-assisted secure communication systems.To fully utilize the advantages of passive IRS in a secure communication system,this paper considers the cooperation link between double passive IRSs,providing a new transmission path for wireless communication.Under the constraints of the base station transmission power and the coefficients of the two passive IRSs,the system’s secrecy rate is maximized through alternating iterations of the base station’s transmission beamforming and the two passive IRSs’ reflecting beamforming.To solve this problem with multiple non-convex constraints,an alternating iteration algorithm based on semi-definite relaxation(SDR)method is proposed.Firstly,given the reflecting beamforming of IRS1 and IRS2,the base station’s transmission beamforming is designed using the property of Rayleigh entropy.Secondly,given the base station’s transmission beamforming and IRS2’s reflecting beamforming,SDR method with the CVX toolbox is used to design IRS1’s reflecting beamforming.Finally,given the base station’s transmission beamforming and IRS1’s reflecting beamforming,SDR method with the CVX toolbox is used to design IRS2’s reflecting beamforming.Simulation results show that the proposed algorithm can converge well and achieve high secrecy performance gains.For a secure communication system assisted by an active IRS,both the user and the eavesdropper receive signals reflected by the active IRS.The goal is to maximize the system’s secrecy rate by optimizing the base station’s transmission beamforming and the active IRS’s reflecting beamforming,which increases the information rate at the user end and decreases the information rate at the eavesdropper end.To solve the non-convex secrecy rate optimization problem based on this system,the Dinkelbach method is first used to transform the problem into the form of subtracting auxiliary variables.Then,an alternating iteration method is used to solve the problem.When optimizing the base station’s transmission beamforming,the SDR method is used in conjunction with the CVX toolbox.When optimizing the active IRS’s reflecting beamforming,the quadratic transform(QT)technique and the CVX toolbox are used.Simulation results show that with the help of an active IRS design,the "double-fading" effect can be effectively alleviated,and higher secrecy performance gains can be obtained compared to existing passive IRS solutions.