On Secure Downlink NOMA Systems with Aerial Eavesdroppers

In this paper, we study the secure communication performance of nonorthogonal multiple access (NOMA) communication systems in presence of aerial eavesdroppers. The security of ground-to-air NOMA communication systems is investigated wherein, utilizing stochastic geometry, the near user is uniformly distributed in an internal three-dimensional (3D) spherical cap and the far user is uniformly distributed in an external 3D spherical cap. To enhance secrecy performance, a protection zone and directional antenna are introduced. The scenarios of both a single eavesdropper and multiple eavesdroppers are considered and new exact expressions for the secrecy outage probability (SOP) under both scenarios are derived. Monte Carlo simulation results are presented to verify the correctness of the derived analytical expressions and the effects of system parameters on SOP of the considered system are investigated.