  This letter proposes an analytical framework to evaluate the coverage
performance of a cellular-connected unmanned aerial vehicle (UAV) network in
which UAV user equipments (UAV-UEs) are equipped with directional antennas and
move according to a three-dimensional (3D) mobility model. The ground base
stations (GBSs) equipped with practical down-tilted antennas are distributed
according to a Poisson point process (PPP). With tools from stochastic
geometry, we derive the handover probability and coverage probability of a
random UAV-UE under the strongest average received signal strength (RSS)
association strategy. The proposed analytical framework allows to investigate
the effect of UAV-UE antenna beamwidth, mobility speed, cell association, and
vertical motions on both the handover probability and coverage probability. We
conclude that the optimal UAV-UE antenna beamwidth decreases with the GBS
density, and the omnidirectional antenna model is preferred in the sparse
network scenario. What's more, the superiority of the strongest average RSS
association over the nearest association diminishes with the increment of GBS
density.
