Rate Optimization Of Reconfigurable Intelligent Surface-Assisted UAV Communication System

Due to their low cost and flexible deployment,unmanned aerial vehicles(UAVs)have been widely used in existing networks to enhance coverage and alleviate network congestion.However,the complex environment poses challenges for UAV communication systems,such as blocked links and limited energy consumption.As a new technology for reconstructing communication environments,reconfigurable intelligent surfaces(RISs)can be applied to UAV communication systems.By adding controllable power amplifiers to the reflecting units,RISs can further improve system gain,which is of great significance for solving the problem of blocked links.However,the existing research on the performance of RIS-assisted UAV communication system is insufficient.In this thesis,an active RIS-assisted UAV communication system is established and the transmission rate and secrecy rate of the system are studied and analyzed,and the optimization scheme is proposed to improve the system performance.The main contents and innovations of this thesis are listed as follows:A novel scheme to optimize the transmission rate of active RISassisted UAV communication system is proposed.To improve communication quality in harsh communication environments in urban scenarios,UAVs act as mobile base stations to provide downlink transmission services to ground user,maximizing the average downlink transmission rate by jointly optimizing the beamforming,UAV trajectory and reflection parameters.To solve the proposed optimization problem,a joint optimization algorithm based on block coordinate descent(BCD)is proposed,and the simulation results show that the proposed scheme has a significant gain in the downlink rate of the system.An active RIS-assisted rate optimization scheme for secure UAV communication is proposed.To further address the physical layer security transmission problem in urban scenarios,in the constructed model,the UAV actively selects the served legitimate users based on the current channel state.In order to maximize the average secrecy rate of the system,an iterative optimization algorithm based on BCD algorithm,continuous convex approximation algorithm and semi-positive definite relaxation algorithm was proposed.And the user scheduling,UAV trajectory,beamforming and reflection parameters were jointly optimized to create a safer communication environment.Finally,the convergence and complexity of the algorithm were analyzed.Through simulation and performance comparison with other benchmark schemes,it is proved that the proposed scheme can effectively improve the average secrecy rate of the system.