| Currently,the increase of 5G wireless communication services has led to increasingly scarce spectrum resources.In order to improve spectrum efficiency(SE),cognitive radio(CR)technology came into being,which allows secondary users(SUs)and primary users(PUs)to share spectrum.At the same time,unmanned aerial vehicles(UAVs)are widely used in wireless communication due to their advantages such as high mobility and line of sight(Lo S)links.Furthermore,some scholars combined cognitive radio technology with UAV and proposed UAV cognitive radio system to improve the communication performances of traditional cognitive radio system.However,due to the openness of wireless communication and the existence of good air-to-ground Lo S link characteristics,the UAV cognitive radio system is vulnerable to be wiretapped by potential malicious users on the ground,and the Lo S transmission characteristics between the UAV and the good users make the PU on the ground more vulnerable to interference form UAV.At present,intelligent reflecting surface(IRS)is considered one of the effective methods to solve the above-mentioned security communication problems.By controlling the phases of the incident electromagnetic wave to change the propagation channels,signal stacking is performed at the receiver to improve the received signal strength,and signal cancellation is performed at the eavesdropper to reduce information leakage.Based on the above background,this paper mainly studies the secure communication problem of IRS assisted UAV cognitive radio system.Aiming at the two situations of perfectly known and imperfect known channel state information(CSI)of wiretap channels.This paper makes full use of the reconfigurable channel characteristics of IRS and the high flexibility of UAV to maximize the average secrecy rate of SU.The main work is as follows:(1)IRS assisted safe communication of the UAV cognitive radio system.When CSI of the wiretap channels is perfectly known,the UAV is used as a secondary transmitter for communication transmission with SU on the ground.Under the constraint condition of meeting the interference temperature of the PU,the phase shift matrices of IRS,UAV flight trajectory and transmission power are jointly optimized to maximize the average secrecy rate of SU.To solve the proposed non-convex optimization problem,an effective iterative algorithm is proposed based on the alternating optimization(AO)and successive convex approximation(SCA)techniques.Simulation results show that the proposed scheme can significantly improve the safety communication performances of UAV cognitive radio system compared with the benchmark scheme.(2)IRS assisted robust and secure communication of UAV cognitive radio system.A deterministic model is used to describe the uncertainty of CSI of the wiretapped channels when the CSI of the wiretapped channels is not perfectly known.In order to improve the safe communication performances between the UAV and the SU on the ground,the phase shift matrices of IRS,the flight trajectory and the transmitting power of the UAV are jointly optimized to maximize the average secrecy rate of SU in the worst case,where meeting constraints of PU interference temperature,mobility and power of the UAV.However,the optimization problem is non-convex and difficult to be solved directly.Based on alternate optimization,successive convex approximation,S-Procedure and semidefinite relaxation(SDR)method,an effective three-stage iterative algorithm is proposed,in which the original problem was divided into three subproblems and solved successively to obtain the suboptimal solution of the original problem.The simulation results show that compared with the non-robust scheme,the proposed robust optimization scheme can significantly improve the worst average secrecy rate of SU. |
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