Research On Intelligent Reflecting Surface Assisted UAV Communication

With the further development of wireless network,more and more devices will access the network in the future and lead to extensive network traffic growth.So far,a number of innovative wireless technologies have been proposed to improve network capacity,enhance signal quality,reduce network delay,improve network coverage and so on.Thanks to the line-of-sight(Lo S)transmission and flexible deployment,UAV can effectively improve the throughput of wireless network,improve the coverage of network and alleviate the network load.However,due to the complex propagation environment,especially in urban areas,affected by the buildings,vehicles and other scatterers,the channel state changes rapidly,and it is difficult to be accurately characterized.Thus,the communication quality is prone to severe deterioration.Recently,intelligent reflecting surface(IRS)is widely studied as a promising technology.By intelligently reflecting the incident electromagnetic wave,it constructs a controllable electromagnetic transmission environment,which can significantly improve the communication environment and improve the communication quality.Motivated by this,this paper attempts to integrate UAV and IRS techniques,uses their respective characteristics to achieve complementary advantages,and then study the potential of IRS-assisted UAV communication systems,so as to provide the corresponding theoretical basis and guidance for the construction of future network.In this paper,an IRS-assisted UAV communication system with single user is studied,which aims to maximize the average achievable rate of the system by the joint UAV trajectory and passive beamforming.To tackle the formulated non-convex problem,we divide it into two subproblems,namely,passive beamforming and trajectory optimization.We first derive a closed-form phase-shift solution for any given UAV trajectory to achieve the phase alignment of the received signals from different transmission paths.Then,with the optimal phase-shift solution,we obtain a suboptimal trajectory solution by using the successive convex approximation(SCA)method.Numerical results demonstrate that the proposed algorithm can considerably improve the average achievable rate of the system.Based on the above study,we further investigate a novel unmanned aerial vehicles(UAVs)secure communication system with the assistance of reconfigurable intelligent surfaces(RISs),where an UAV and a ground user communicate with each other,while an eavesdropper tends to wiretap their information.Due to the limited capacity of UAVs,a RIS is applied to further improve the quality of the secure communications.The time division multiple access(TDMA)protocol is applied for the communications between the UAV and the ground user,namely,the downlink(DL)and the uplink(UL)communications.In particular,the channel state information(CSI)of the eavesdropping channels is assumed to be imperfect.We aim to maximize the average worst-case secrecy rate by the robust joint design of the UAV’s trajectory,RIS’s passive beamforming,and transmit power of the legitimate transmitters.It is challenging to solve the Joint UL/DL optimization problem due to its non-convexity.To this end,we develop an efficient algorithm based on the alternating optimization(AO)technique.Specifically,the formulated problem is divided into three sub-problems,and the successive convex approximation(SCA),-Procedure,and semidefinite relaxation(SDR)are applied to tackle these non-convex sub-problems.Numerical results demonstrate that the proposed algorithm can considerably improve the average secrecy rate compared with the benchmark algorithms,and also confirm the robustness of the proposed algorithm.