Toward Secrecy-Energy-Efficiency Optimization for UAV-Assisted Bidirectional Systems With Active Eavesdroppers

Ensuring secrecy information transmission with enhanced energy efficiency is one of the critical issues in unmanned aerial vehicles (UAVs)-assisted wireless systems owing to the open nature of wireless channels and the limited battery power. In this article, the secrecy energy efficiency (SEE) is investigated for a more critical scenario, where an UAV acts as a bidirectional relay exchanging information between the two ground users and a malicious terminal attempts to implement eavesdropping and interfering on the confidential information simultaneously. Multiple parameters, such as time scheduling, power allocation, UAV trajectory within a given flight cycle, and both multiple access (MA) and broadcast (BC) phases are optimized jointly to maximize the system SEE. To address this nonconvex optimization problem involved with coupled variables tightly, we employ the block coordinate descent (BCD) method and develop an iterative algorithm that leverages successive convex approximation (SCA) and the Dinkelbach algorithm to obtain the optimal solution. Meanwhile, simulation results are presented to demonstrate the superiority of our proposed scheme in terms of SEE.