Research On Physical Layer Security Technologies Of UAV Communication Systems Based On Reconfigurable Intelligent Surface

In recent years,wireless communications have been transforming all aspects of people’s lives,making them more convenient and smarter.In particular,the emergence of unmanned aerial vehicles(UAV)-assisted communications has expanded wireless network coverage,enhanced network deployment flexibility,and provided a new dimension of communication,but also brought new challenges.First,the broadcast nature of radio propagation,malicious eavesdropping users in communication networks can steal private information from other users,bringing wireless information security issues.Second,obstacle blockage in complex communication scenarios makes the UAV and ground user communication link loss serious and user service quality cannot be guaranteed.To address this,this thesis focuses on the study of physical layer security technologies for UAV communication systems based on reconfigurable intelligent surface,aiming to improve transmission security performance by using reconfigurable smart surfaces.On the other hand,reconfigurable intelligent surface,which is composed of a large number of passive reflective elements,can reflect the signal beam emitted by the source to improve the UAV secure communication propagation environment and enhance information security.The main research of this thesis are as follows.(1)For future wireless network communication security coverage issues,the aerial reconfigurable intelligent surface(ARIS)assisted security communication system is proposed,which uses RIS deployed on UAV to enhance security communication links.First,a closed-form expression for the average secrecy rate of all time slots in flight on ARIS is derived to characterize the information transmission security performance.Then,the transmit beamforming of the base station,the phase shift matrix and trajectory of the ARIS are jointly designed with the aim of maximizing the average secrecy rate performance.To solve the non-convex problem,it is proposed to decompose the original optimization problem into three subproblems and solve them using an iterative algorithm.In particular,the optimal beamforming design scheme is proposed based on the generalized eigenvalue optimization method,and a Minorization-Maximization(MM)-based algorithm and a deep deterministic policy gradient(DDPG)-based method are proposed to derive the solutions of phase shift matrix and trajectory,respectively.Simulation results show that ARIS assistance can obtain nearly twice the secrecy performance of terrestrial intelligent reflecting surface(TRIS)systems.(2)A secure communication scheme based on Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface(STAR-RIS)is proposed for the UAV-ground security communication system.In this scheme,the UAV as an aerial mobile base station serves multiple users simultaneously with the help of the transmitting surface and reflecting surface of the reconfigurable intelligent surface,and satisfies the quality of service and security communication requirements of different users.On the basis of satisfying the overall service requirements of multiple users,a joint optimized design scheme of transmission phase shift,reflection phase shift and the information beamforming is proposed based on the time-division communication mode of the RIS.The optimal solution of the information beamforming is derived based on the generalized eigenvalue optimization method,and the transmitting and reflecting phase shift matrices are jointly optimized with the help of the semidefinite relaxation technique and the minimizationmaximization algorithm.The results show that the UAV secure communication scheme based on reconfigurable intelligent surfaces with simultaneous transmission and reflection maximizes the multi-user weighting and rate of the UAV security communication system,while guaranteeing the information transmission for users on the reflective surface and security communication for users on the reflective surface.