Physical layer security for UAV communications: A comprehensive survey

Due to its high mobility and flexible deployment, unmanned aerial vehicle (UAV) is drawing unprecedented interest in both military and civil applications to enable agile and ubiquitous connectivity. Mainly operating in an open environment, UAV communications benefit from dominant line-of-sight links; however, this on the other hand renders the communications more vulnerable to malicious attacks. Recently, physical layer security (PLS) has been introduced to UAV systems as an important complement to the conventional cryptography-based approaches. In this paper, a comprehensive survey on the current achievements of UAV-PLS is conducted. We first introduce the basic concepts including typical static/mobile UAV deployment scenarios, the unique air-to-ground channel and aerial nodes distribution models, as well as various roles that a UAV may act when PLS is concerned. Then, we start by reviewing the secrecy performance analysis and enhancing techniques for statically deployed UAV systems, and extend the discussion to the more general scenario where the UAVs' mobility is further exploited. For both cases, respectively, we summarize the commonly adopted methodologies, then describe important works in the literature in detail. Finally, potential research directions and challenges are discussed to provide an outlook for future works in the area of UAV-PLS.