Secrecy Outage Performance Analysis of UAV-Assisted Relay Communication Systems With Multiple Aerial and Ground Eavesdroppers

In this article, we study the secrecy performance of an unmanned aerial vehicle (UAV)-assisted relay communication system, where a ground base station (GBS) intends to send confidential information to the ground legitimate user Bob with the help of a UAV relay in the presence of multiple aerial and ground eavesdroppers. We consider the height-dependent probabilistic line-of-sight and the general
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shadowed channel fading model in the system. To enhance the secrecy performance, the GBS and UAV relay apply directional beamforming transmission while implementing protection zones around their intended receiving destinations. We then derive the approximate closed-form expressions of the secrecy outage probability (SOP) under different aerial and ground eavesdropperscolluding strategies. To get insights, we also obtain the SOP expression over Rayleigh fading as a special case. After discussing the impacts of transmitting and multiple aerial and ground eavesdropping parameters on the SOP, we show that the optimal altitude of UAV relay exists with the minimal SOP value under different urban environments. We also show that UAV eavesdroppers have a larger effect on the system performance than ground eavesdroppers.