Secure 3D Mobile UAV Relaying for Hybrid Satellite-Terrestrial Networks

In this paper, we propose a novel decode-and-forward (DF)-based secure 3D mobile unmanned aerial vehicle (UAV) relaying for hybrid satellite-terrestrial networks (HSTNs) in the presence of an aerial eavesdropper lying around a serving UAV relay in a circular plane. Herein, we adopt a stochastic mixed mobility (MM) model for mobilizing the UAV relays in a 3D cylindrical cell with a ground user equipment (UE). We consider the deployment of eavesdropper under the cases: (a) the eavesdropper is located at certain fixed distance around a serving UAV relay; (b) the eavesdropper is located uniformly random around the relay. By considering the opportunistic closest-, uniform-, and maximum-signal-to-noise ratio (maximum-SNR) UAV relay selection (URS) strategies, we analyze the secrecy performance of the considered HSTN in terms of probability of non-zero secrecy capacity (PNZSC) and secrecy outage probability (SOP). In particular, we derive exact PNZSC and SOP for the closest-URS (CURS) and uniform-URS (UURS). While, we derive analytical lower and upper bounds on PNZSC and SOP for maximum-SNR URS (MURS). We further carry out the corresponding asymptotic SOP analysis to assess the secrecy diversity order and coding gain of aforementioned URS strategies. Simulations are performed to corroborate the analysis.