Research On The Techniques Of Physical Layer Secure Transmission In UAV Communications

Unmanned Aerial Vehicle(UAV)with its simple operation,low cost and flexibility,will play an important role in air communication networks.UAV provides the possibility to further improve the communication rate,network capacity and area coverage of mobile communication networks.However,due to the broadcasting characteristics of wireless communication,it makes the UAV communication system extremely vulnerable to be detected and eavesdropped from eavesdroppers.The secure transmission of wireless signals has become a bottleneck problem that restricts the application and development of UAV communication.The existing secure communication mechanism is based on computational cryptography,but limited by the characteristics of the UAV communication system itself.On the one hand,the UAV platform is limited by its own computing power and energy constraints,and high computational complexity in running encryption and decryption algorithms will increase the operational burden.On the other hand,the high mobility of the UAV platform makes the online distribution and management of keys more and more difficult.This thesis focuses on the major demand of secure transmission of UAV communication.The physical layer security technology is used to prevent the detection and eavesdropping of eavesdroppers.By effectively optimizing the location of UAV flight trajectory and transmit power,we propose a physical layer security transmission scheme based on channel difference,which means actively enhancing the legitimate channel gain while attenuating the eavesdropping channel gain.The theoretical and technical supports benefit for the deployment and application of UAV communication networks.The main contents of the thesis are as follows:First,the channel of UAV communication system has been measured,analyzed and modeled.The experiment demonstrated the necessity of joint optimization of UAV flight trajectory and resources.The Xlinx-based ZYNQ hardware platform has been built to analyze the influence of the trajectory of UAV on physical layer signal transmission security.The results show that the greater the distance between the legitimate receiving node and the eavesdropping node in the same application scenario,the greater the difference between the legitimate channel and the eavesdropping channel.So,it is feasible to improve the physical layer security transmission performance by optimizing the flight trajectory of UAV.Secondly,a single UAV communication scenario under the condition of estimation error of the eavesdropper’s position in the urban environment has been established,in this thesis.This thesis designed a secure transmission scheme that jointly optimizes the UAV flight trajectory and signal transmitting power.Since this optimization problem is nonconvex,this thesis proposed an algorithm which based on successive convex approximation(SCA)and block coordinate descent(BCD)to solve the problem of UAV flight trajectory and signal transmitting power control strategies separately,thus transforming them into convex problem.The scheme is applicable to both Line of Sight(LOS)and Not Line of Sight(NLOS)scenarios.Simulation results show that the proposed algorithm can not only increase the maximum secrecy rate by about 2 bps/Hz,but also increase the transmission time at a more stable secrecy rate by about two times compared with the scheme without trajectory optimization and power allocation,which effectively improves the robustness of the UAV communication system.Thirdly,a collaborative secure transmission scheme with joint optimization of multiUAV flight trajectory and transmit power is designed for the scenario of two UAVs collaboratively serving ground nodes in urban environment.The scheme considers a more general air-ground model.By jointly optimize the flight trajectory and power control of two UAVs,the legitimate channel gain is enhanced while the eavesdropping channel gain is further weakened.The maximum secrecy transmission rate is achieved by joint UAV signal transmission power allocation.Simulation results show that the robustness of the proposed scheme is significantly improved compared to the transmission scheme without optimized deployment.The total secrecy transmission rate of the proposed scheme is improved by 50% compared to the single UAV communication system.Finally,aiming at the urban environment where UAVs and ground nodes cannot establish line-of-sight links,this thesis designed a secure UAV transmission scheme assisted by Intelligent Reflecting Surface(IRS).By deploying the IRS on the surface of buildings,using the passive beamforming property of IRS to increase the difference between the legitimate channel and the eavesdropping channel.By combining the UAV trajectory planning,phase,and signal transmitting power to enhance the secrecy rate.The simulation results show that under the assist of IRS,the secrecy rate can effectively avoid degradation due to blocked line-of-sight links.Using its reflection property,the secrecy rate can be increased with the increase of channel differentiation between legitimate and eavesdropping nodes.