  Unmanned aerial vehicle (UAV) base stations (BSs) can be a promising solution
to provide connectivity and quality of service (QoS) guarantees during
temporary events and after disasters. In this paper, we consider a scenario
where UAV-BSs are serving large number of mobile users in a hot spot area
(e.g., in a stadium). We introduce non-orthogonal multiple access (NOMA)
transmission at UAV-BSs to serve more users simultaneously considering user
distances as the available feedback for user ordering during NOMA formulation.
With millimeter-wave (mmWave) transmission and multi-antenna techniques, we
assume UAV-BS generates directional beams and multiple users are served
simultaneously within the same beam. However, due to the limitations of
physical vertical beamwidth of the UAV-BS beam, it may not be possible to cover
the entire user region at UAV altitudes of practical relevance. During such
situations, a beam scanning approach is proposed to maximize the achievable sum
rates. We develop a comprehensive framework over which outage probabilities and
respective sum rates are derived rigorously and we investigate the optimal
operational altitude of UAV-BS to maximize the sum rates using our analytical
framework. Our analysis shows that NOMA with distance feedback can provide
better outage sum rates compared to orthogonal multiple access.
