  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) the
UAV-centric strategy for offloading actions and 2) the 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 UAV cellular
networks is capable of outperforming OMA by setting power allocation factors
and targeted rate properly; and ii) the coverage probability of NOMA assisted
UAV cellular framework is affected to a large extent by ipSIC coefficient,
target rates and power allocations factors of paired NOMA users.
