Opportunistic relaying for low-altitude UAV swarm secure communications with multiple eavesdroppers

Abstract

In this study, we investigate the secrecy outage performance achieved by opportunistic relaying for a low-altitude unmanned aerial vehicle (UAV) swarm secure communication system in the presence of multiple UAV-eavesdroppers. In the primary channel, multiple UAV-transmitters are connected to a ground station via the ground-to-air (G2A) wireless backhaul and a UAV-transmitter and UAV-relay are selected to transmit the source signal to a far ground destination under air-to-air and air-to-ground Nakagami-m fading links. In the eavesdropping channel, maximum ratio combining is applied across multiple UAV-eavesdroppers for intercepting the legitimate transmissions from both the selected UAV-transmitter and UAV-relay. The backhaul reliability and eavesdropping probability are introduced to reflect the practical constraints on the G2A wireless backhaul and UAV-eavesdropper cooperation, respectively. The closed-form expression for the secrecy outage probability is derived in terms of the UAV cooperation, backhaul reliability, eavesdropping probability, and Nakagami-m fading parameters. In the high signal-to-noise ratio region, the asymptotic secrecy outage probability is also derived. It is shown that the secrecy diversity gain achieved by opportunistic relaying is jointly determined by the UAV cooperation and shape factors of Nakagami-m fading links in the primary channel. The analytical secrecy outage metrics achieved by opportunistic relaying are verified by Monte Carlo simulation results.