| To meet the great demand of growing emerging wireless services for network capacity and coverage,space-air-ground integrated network is expected to be the research directions in the 6th generation(6G)mobile communication system.As an important part of the space-air-ground integrated network,Unmanned Aerial Vehicle(UAV)communication system benefiting from the low cost,flexible deployment,and strong line of sight(LoS)transmission link has been extensively applied in improving wireless coverage and capacity.However,the broadcast nature of wireless channel between UAV and ground terminals makes air-to-ground(A2G)channel more prone to various attacks.UAV communications face severe security challenges.Most of exsiting literatures on secure UAV communications assumes perfect channel station information(CSI),which ignores channel estimation errors caused by the unique jitter feature of UAV platform.Due to the absence of fixed infrastructure,the inherent randomness of airflow,and the vibration caused by body motion,the A2G channel faces unpredicted estimation errors to CSI,which causes non-negligible influence to the establishment of robust A2G communication link.This thesis aims to study robust and efficient secure transmission schemes for A2G communications with focusing on eavesdropping and anti-eavesdropping problem considering the impact of UAV jitter.The main contributions are summarized as follows.System secrecy rate maximization problem in the downlink A2G system with passive eavesdropper is firstly studied.Considering the impact of UAV jitter on antenna array response and secrecy performance,a robust artificial noise aided beamforming scheme is proposed to maximize the system secrecy rate.The worst jitter scenario with minimum legitimate data rate and maximum eavesdropping data rate is considered.In order to obtain the optimal solutions,successive convex approximation(SCA)and Cauchy’s inequality are applied to approximate the non-convex optimization problem to a convex problem.Further,an iterative algorithm is proposed to obtain the optimal solution.Simulations are provided to demonstrate the effectiveness and robustness of the proposed scheme.Maximum ratio transmission(MRT)scheme and transmission without artificial noise scheme are also provided.Results show that when the UAV transmission power is 5 w,the proposed scheme can achieve about 98.6%and 157.9%improvement of the secrecy rate respectively compared with MRT scheme and secure transmission without artificial noise.Moreover,when the AOD estimation error between the UAV and the legitimate user is 0.1,the proposed scheme can achieve about 115.2%and 85.5%improvement of the secrecy rate respectively,which proves the robustness of the proposed scheme.Transmission power minimization problem in the downlink A2G system with passive eavesdropper is studied.Considering the constraints for the minimum data rate of the legitimate channel and the maximum eavesdropping rate of the eavesdropping channel in the scenario with UAV jitter,joint design of confidential signal beamform and artificial noise vector is investigated to minimize the transmission power of UAV.S-procedure and semidefinite relaxation(SDR)technique are used to transform the non-convex optimization problem into a semidefinite program(SDP)problem.Numerical results demonstrate the effectiveness of the proposed scheme and analyze the impact of key system parameters,including AOD estimation error,security rate threshold,number of UAV antennas,and the height of UAV on the energy saving of UAV.Results show that the transmission power of UAV increases with increasing AOD estimation error.Moreover,when the AOD estimation error between the UAV and the legitimate user is 0.1,the transmission power of UAV is saved about 52%with six antennas equipped at the UAV compared with five antennas equipped at the UAV.Finally,strategy interaction of the eavesdropping and anti-eavesdropping problem in downlink A2G system with active eavesdropper is studied.An artificial noise aided beamforming scheme is applied at the UAV to combat the eavesdropping of downlink signal.For the eavesdropper,it aims to actively adjust its jamming strategy to minimize the system secrecy rate.For the UAV,it aims to minimize the transmission power against active eavesdropping attacks in the worst scenario.To study the interactions and competitions between the UAV and active eavesdropper,a Stackelberg game model is formulated.Existence and uniqueness of the Stackelberg equilibrium are proved.To obtain the equilibrium solutions,an iterative algorithm is proposed.Simulations are provided to demonstrate the effectiveness of the proposed transmission scheme based on the Stackelberg game model.MRT scheme is also provided as a comparison scheme.Results show that when the AOD estimation error between the UAV and the legitimate user is 0.04,the proposed scheme can save about 15.3%of transmission power at UAV compared with MRT scheme.In addition,the proposed scheme can consume more power on the full-duplex active eavesdropper compared with MRT scheme. |
