Enhancing Security in Multi-Hop UAV Relaying via Joint Bandwidth, Power, and Trajectory Optimization

Unmanned Aerial Vehicle (UAV) communication has many advantages due to its high mobility, low cost, and flexibility of use. However, the transmission and line-of-sight characteristics of the air-to-ground radio channel present a new challenge to achieving secure UAV communication with ground users. This paper addresses this challenge through the application of security techniques at the physical layer. We consider uplink and downlink communications between base station to UAV and UAV to ground users, i.e., base station to UAV(BS-UAV1) and UAV to ground users (UAV2-User) communications, respectively, and the presence of potential eavesdroppers on the ground. We use the high mobility of UAVs to actively create channels for both secure data links and interception paths through their trajectory design. We pose the novel problem of maximising the secure communication threshold enhancement minimum transmission by simultaneously adjusting the bandwidth and energy regulation for legitimate transmitters and the trajectory of the UAVs. Although these proposed problems are non-convex, we use an iterative algorithm for the proposed problems in order to solve them in an efficient way through the application of block coordinate descent and iterative convex optimisation strategies. Specifically, the UAV's flight path is used to optimally adjust the bandwidth and radiant power. One is fixed and the other one is optimised until the algorithm converges. Experimental validation shows that compared to conventional methods without bandwidth and power control or trajectory optimisation, the integrated optimisation methodology is able to improve the secrecy rate of communication.