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In this paper, we consider an unmanned aerial vehicle (UAV)-enabled downlink wireless system wherein a UAV serves as flying base station to communicate with two ground users using non- orthogonal multiple access (NOMA). Specifically, we deploy a fixed-wing type UAV which moves in a circular trajectory around the centre of a macrocell to provide ubiquitous coverage to the ground users located outside an offloaded BS coverage region within the cell. By adopting Rician fading model for the line-of-sight (LoS) UAV-to-ground links, we investigate outage probability (OP) of both ground users with NOMA. For comparison with NOMA, we also analyze the outage performance of ground users with orthogonal multiple access (OMA). We highlight the relative performance between NOMA and OMA schemes for the considered system. Further, to manage the fairness among ground users outage performance, we derive the dominant conditions which guarantee the outage performance of both ground users with NOMA transmission mode is better than that with OMA. Based on these conditions, at high signal-to-noise ratio (SNR), we observe that the power allocation factor for far ground user from UAV is independent of channel and trajectory parameters for better performance of NOMA. Whereas the dominant condition for near ground user strongly affected by the channel and trajectory parameters.
Sharma et al. (Fri,) studied this question.