  Unmanned aerial vehicles (UAVs) are set to feature heavily in upcoming fifth
generation (5G) networks. Yet, the adoption of multi-UAV networks means that
spectrum scarcity in UAV communications is an issue in need of urgent
solutions. Towards this end, downlink non-orthogonal multiple access (NOMA) is
investigated in this paper for multi-UAV networks to improve spectrum
utilization. Using the bivariate Rician shadowed fading model, closed-form
expressions for the joint probability density function (PDF), marginal
cumulative distribution functions (CDFs), and outage probability expressions
are derived. Under a stochastic geometry framework for downlink NOMA at the
UAVs, an outage probability analysis of the multi-UAV network is conducted,
where it is shown that downlink NOMA attains lower outage probability than
orthogonal multiple access (OMA). Furthermore, it is shown that NOMA is less
susceptible to shadowing than OMA.
