This paper advocates a pair of strategies in non-orthogonal multiple access
(NOMA) in unmanned aerial vehicles (UAVs) communications, where multiple UAVs
play as new aerial communications platforms for serving terrestrial NOMA users.
A new multiple UAVs framework with invoking stochastic geometry technique is
proposed, in which a pair of practical strategies are considered: 1)
UAV-Centric strategy for offloading actions and 2) User-Centric strategy for
providing emergency communications. In order to provide practical insights for
the proposed NOMA assisted UAV framework, an imperfect successive interference
cancelation (ipSIC) scenario is taken into account. For both UAV-Centric
strategy and User-Centric strategy, we derive new exact expressions for the
coverage probability. We also derive new analytical results for orthogonal
multiple access (OMA) for providing a benchmark scheme. The derived analytical
results in both User-Centric strategy and UAV-Centric strategy explicitly
indicate that the ipSIC coefficient is a dominant component in terms of
coverage probability. Numerical results are provided to confirm that i) for
both User-Centric strategy and UAV-Centric strategy, NOMA assisted multi-UAV
networks is capable of outperforming OMA by setting power allocation factors
and targeted rate properly; and ii) the coverage probability of NOMA assisted
multi-UAV framework is affected to a large extent by ipSIC coefficient, target
rates and power allocations factors of paired NOMA users.