Research On Physical Layer Secure Transmission Strategy Of Untrusted UAV Relay System

Unmanned aerial vehicle(UAV)as relay auxiliary wireless communication has been widely used in various coverage enhancement scenarios.Due to the broadcast characteristics of wireless communication itself,information will face threats such as eavesdropping and tampering in the process of transmission.Compared with traditional cryptography,low complexity physical layer security technology has attracted more attention in wireless communication field.For UAV relay network,the existing physical layer security research is mainly to reduce the eavesdropping rate of external eavesdroppers.In fact,UAV relay network is usually a heterogeneous network,and there are differences between UAV and the original user terminal in terms of communication ability and security level.This means that the UAV may steal some information from the user’s terminal while forwarding the information.For the ground terminal,the UAV relay is not fully trusted.Therefore,it is necessary to design corresponding communication protocol for untrusted relay to avoid information leakage when completing information relay.In this paper,two different untrusted UAV relay scenarios are studied,and combined with the UAV’s high mobility and cooperative jamming technology,the channel characteristics are fully utilized.The secure communication is maximized by the UAV’s trajectory design and the user’s resource allocation.Specifically,the research contents and innovations of this paper are as follows:Aiming at the Physical Layer security scenario of multi-user pair bidirectional untrusted relay,the UAV can cache the received information first and select the appropriate time slot before forwarding.In this paper,a time slot pairing transmission strategy based on physical-Layer Network Coding(PNC)protocol is proposed.The strategy aims to obtain more time slots with good channel conditions in the whole communication process by using the mobility of UAV,and to select the appropriate time slots for the matching of user information.The problem is described as a nonconvex joint optimization problem with trajectory,power and time slot pairing.The optimal solution is obtained by introducing relaxation variables and applying continuous convex approximation techniques.In order to verify the superiority of the proposed relay protocol,performance analysis and comparison with several other common relay protocols are carried out in this paper.Finally,simulation results are given to verify the effectiveness of the proposed design.Aiming at the data acquisition scenario of untrusted UAV relay under base station cooperative interference,in this scenario,UAV is used to collect confidential information of multiple user nodes and forward it to base station.To improve the system’s security,the base station also acts as a friendly jammer,helping to send jamming signals to untrusted UAV to reduce eavesdropping rates.We focus on the influence of 3D path design of untrusted UAV on improving the secrecy rate under the probabilistic line-of-sight channel model.The problem is described as a joint optimization problem of user scheduling,source transmitting power,UAV interference power and UAV horizontal and vertical trajectory.By using the block coordinate method,the original problem is decoupled from the alternate solution of multiple subproblems,since each individual subproblem is still non-convex,especially the 3D trajectory optimization problem.In this paper,the successive convex approximation method is used to simplify the problem to a local optimal solution based on Taylor expansion.In order to accelerate the convergence efficiency,an improved greedy iterative optimization algorithm is designed on the basis of alternate iteration.Finally,numerical simulation results are given.Compared with other benchmark algorithms,the convergence speed and security performance of the proposed algorithm are improved significantly.