text: abstract
  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-hoverand-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.