  Unmanned aerial vehicle (UAV) wireless communications have experienced an
upsurge of interest in both military and civilian applications, due to its high
mobility, low cost, on-demand deployment, and inherent line-of-sight (LoS)
air-to-ground channels. However, these benefits also make UAV wireless
communication systems vulnerable to malicious eavesdropping attacks. In this
article, we aim to examine the physical layer security issues in UAV systems.
In particular, passive and active eavesdroppings are two primary attacks in UAV
systems. We provide an overview on emerging techniques, such as trajectory
design, resource allocation, and cooperative UAVs, to fight against both types
of eavesdroppings in UAV wireless communication systems. Moreover, the
applications of non-orthogonal multiple access, multiple-input and
multiple-output, and millimeter wave in UAV systems are also proposed to
improve the system spectral efficiency and to guarantee security
simultaneously. Finally, we discuss some potential research directions and
challenges in terms of physical layer security in UAV systems.
