Active Intelligent Reflecting Surface Assisted Beamforming Design of Secure Communication System

‍ ‍Intelligent Reflecting Surface （IRS） has attracted great attention from academia and industry due to its passivity and low power consumption. IRS has been used for secure transmission at the physical layer， it is a potential innovative technology that can reflect signal by adjusting the phase shift and create additional link for transmission. Compared with the situation without the assistance of IRS， the existence of IRS helps to obtain limited security performance gains. However， due to the “double fading” effect in the reflection channel link between the transmitter and the user， in order to compensate for the “double fading” effect in the IRS assisted link， a large number of reflecting elements （REs） are traditionally arranged at the IRS， which leads to a large surface size and considerable circuit loss. On this basis， we propose a new design scheme of active IRS. Compared with passive IRS， active IRS can not only adjust the phase shift of the reflection unit， but also control beamforming by adjusting the amplitude， thus improving the security of wireless transmission. In this work， we consider the active IRS assisted security communication system in the case of multi-antenna base station， single-antenna user and eavesdropper. The user and eavesdropper receive the signal reflected by the IRS at the same time. Our goal is to maximize the security rate of the system by optimizing the transmit beamforming at the base station and the reflection beamforming of the active IRS. In order to solve the non-convex security rate optimization problem based on the system， the Dinkelbach method is first used to change the problem into the form of auxiliary variable subtraction and then the semidefinite relaxation （SDR） method is used to solve the problem with CVX toolbox when optimizing the transmit beamforming at the base station through the alternate optimization method； Quadratic transform （QT） technology and CVX toolbox are used to optimize the reflection beamforming of active IRS. The simulation results show that with the help of active IRS design， the influence of “double fading” effect can be effectively alleviated， and higher security performance gain can be obtained compared with the existing passive IRS solutions.