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.
Index TermsNon-orthogonal multiple access, stochastic geometry, unmanned aerial vehicles. Regarding the literature of UAV networks, early research contributions have studied the performance of single UAV or multiple UAVs networks. Mozaffari et al. [3] proposed a UAV assisted underlaid D2D network with LoS probability, which depends on the height of the UAV, the horizontal distance between the UAV and users, the carrier frequency and type of environment. In the case that LoS exists, a fixed LoS coefficient, e.g., an extra 20dB attenuation, is the dominant component of small-scale fading channels. Note that the proposed model in [3], [4] is a practical model for implementation. For mathematically tractable, the distinctive channel characteristics for UAV networks were investigated in [13], where different types of small-scale fading channels, i.e., Loo model, Rayleigh model, Nakagami-m model, Rician model and Werbull model, were summarized to demonstrate the channel propagation of UAV networks. The air-to-air channel characterization in [14], studied the influence of the altitude-dependent Rician K factor. This work indicated that the impact of the ground reflected multi-path fading reduces with increasing UAV altitude. Jiang et al. [15] proposed a UAV assisted ground-to-air network, where Rician channels are used for evaluating strong LoS components between UAV and ground users. It is also worth noting that Rayleigh fading channel, which is a well-known model in scattering environment, can be also used to model the UAV ch...