I. Introduction
Unmanned aerial vehicle (UAV) aided communication as a promising technique has been extensively used in both military and civilian applications [1]. In particular, the system performance can be significantly improved in UAV-assisted wireless networks by properly designing the UAV's trajectory [2]. In addition, the UAV's characteristics of flexible deployment can be utilized to actively monitor suspicious communications in vicinity [3]. However, the utilization of UAV brings new challenges for wireless communication networks. Specifically, the air-ground channel link is dominated by line-of-sight (LoS), which is conducive for the illegitimate receivers to detect the signal transmitted by UAV and then intercept it. Besides, given the several limitations of on-board power level and the ability to adapt to changes in the environment, the UAV may not be fully autonomous and can only operate for short flight-durations.