  Due to their flexible deployment and on-demand mobility, small-scale unmanned
aerial vehicles (UAVs) are anticipated to be involved in widespread
communication applications in the forthcoming fifth-generation (5G) networks.
However, the confidentiality of UAV communication applications is vulnerable to
security threats due to the broadcast nature and dominant line-of-sight (LoS)
channel conditions, and physical-layer security (PLS) technique can be applied
for secrecy performance enhancement in such a context. On the other hand, it is
also promising to exploit UAVs to cooperatively protect secure communications.
This article provides an overview of the recent research efforts on
UAV-involved secure communications at the physical layer. We focus on the
design of secure transmission schemes according to different roles of UAVs and
the optimization of introduced degrees of freedom (DoFs) by the unique
characteristics of UAVs. We also propose some future research directions on
this topic.
