  In this letter, we study multiuser communication systems enabled by an
unmanned aerial vehicle (UAV) that is equipped with a directional antenna of
adjustable beamwidth. We propose a fly-hover-and-communicate protocol where the
ground terminals (GTs) are partitioned into disjoint clusters that are
sequentially served by the UAV as it hovers above the corresponding cluster
centers. We jointly optimize the UAV's flying altitude and antenna beamwidth
for throughput optimization in three fundamental multiuser communication
models, namely UAV-enabled downlink multicasting (MC), downlink broadcasting
(BC), and uplink multiple access (MAC). Our results show that the optimal UAV
altitude and antenna beamwidth critically depend on the communication model
considered.
