Research On Physical Layer Security Performance Of UAV Communications Assisted By Reconfigurable Intelligent Surface Under Cooperative Jamming

With the gradual acceleration of 5G network construction,people’s demand for communication networks is increasing.Unmanned aerial vehicles(UAVs)are very popular in modern mobile communication research because of their advantages such as low cost,strong mobility,wide coverage,and short-range line-of-sight communication with users.However,in real life,the signal propagation environment is too complex,the signal fading is serious,and the existence of obstacles and eavesdroppers will greatly reduce the communication quality and security rate of the UAV communication network,which limits the wide use of UAVs.With the rise of reconfigurable intelligent surface(RIS)technology,it is possible to fully utilize the characteristics of RIS to intelligently regulate the incident signals and construct new transmission paths,the UAV combined with RIS to improve the transmission performance of UAV communication network.Due to the broadcast characteristics of wireless channels,there is a potential leakage of information during transmission,which makes the secure transmission of information in the UAV communication network become a key issue.Although traditional encryption techniques can effectively achieve the secure transmission of information,the cumbersome encryption and decryption process will consume more energy for UAVs that are inherently limited in power consumption.Based on this,to improve the security of communication networks,this thesis adopts physical layer security technology to study the physical layer security performance of RIS-assisted UAV communication under cooperative jamming from the perspective of the physical characteristics of wireless channels.The main contributions are as follows:(1)The physical layer security performance of the UAV communication network assisted by fixed RIS is studied.In this communication network,a UAV acts as an aerial base station,which can communicate directly with the ground user or reflect signals to the ground user with the help of RIS fixed on the surface of a building.Considering the presence of an eavesdropper on the ground,in order to improve the security performance of the communication network,another UAV,which has a potential cooperation relationship with the user,acts as a friendly jammer to send jamming signals to confuse the eavesdropper.A non-convex optimization problem with the objective of maximizing the average secrecy rate of communication networks is constructed.By jointly optimizing the flight trajectory and transmit power of the base station UAV,the transmit power of the jammer,and the RIS phase shift,an iterative optimization algorithm using block coordinate descent and successive convex approximation(SCA)method are proposed to obtain a suboptimal solution to the optimization problem.Simulation results validate the effectiveness of this algorithm and significantly improve security performance compared to other schemes.(2)Based on the research on the physical security performance of UAV communication assisted by fixed RIS,in order to better adapt to complex communication scenarios in real life and enhance the flexibility of communication network,the physical security performance of UAV cognitive communication network assisted by mobile RIS is studied by changing the deployment status of RIS.In the secondary network,due to the presence of tall obstacles,the base station and the user cannot communicate directly.Therefore,the RIS is mounted on a UAV,which acts as the passive relay in the air.The mobility of the UAV and the characteristics of the reflected signal from the RIS are utilized to assist the base station in achieving communication with the secondary user.At the same time,considering the existence of an eavesdropper and primary network users,a friendly jammer on the ground that has a cooperative relationship with the secondary user is used to deliberately send jamming signals to interfere with the eavesdropper.Under the constraints of satisfying primary user interference restrictions and other restrictions,the average security rate of the secondary communication network is maximized by jointly optimizing the RIS phase shift,UAV flight trajectory,base station transmission power,and jammer transmission power.Because the problem is non convex and difficult to solve directly,an optimization algorithm combining block coordinate descent and SCA technology was proposed to successfully obtain the suboptimal solution of the problem.Simulation results show that this algorithm not only can converge quickly,but also can better improve the security performance of communication networks compared to other schemes.