  With the assistance of device-to-device (D2D) communications, unmanned aerial
vehicle (UAV) networks are anticipated to support widespread applications in
the fifth generation (5G) and beyond wireless systems, by providing seamless
coverage, flexible deployment, and high channel rate. However, the networks
face significant security threats from malicious eavesdroppers due to the
inherent broadcast and openness nature of wireless channels. To ensure secure
communications of such networks, physical layer security is a promising
technique, which utilizes the randomness and noise of wireless channels to
enhance secrecy performance. This article investigates physical layer security
performance via spectrum sharing in D2D-enabled UAV networks. We first present
two typical network architectures where each UAV serves as either a flying base
station or an aerial user equipment. Then, we propose a spectrum sharing
strategy to fully exploit interference incurred by spectrum reuse for improving
secrecy performance. We further conduct two case studies to evaluate the
spectrum sharing strategy in these two typical network architectures, and also
show secrecy performance gains compared to traditional spectrum sharing
strategy. Finally, we discuss some future research directions in D2D-enabled
UAV networks.
