  This letter analyzes the handover probability in a drone cellular network
where the initial positions of drone base stations (DBSs) serving a set of user
equipment (UE) on the ground are modeled by a homogeneous Poisson point process
(PPP). Inspired by the mobility model considered in the third generation
partnership project (3GPP) studies, we assume that all the DBSs move along
straight lines in random directions. We further consider two different
scenarios for the DBS speeds: (i) same speed model (SSM), and (ii) different
speed model (DSM). Assuming nearest-neighbor association policy for the UEs on
the ground, we characterize the handover probability of this network for both
mobility scenarios. For the SSM, we compute the exact handover probability by
establishing equivalence with a single-tier terrestrial cellular network, in
which the base stations (BSs) are static while the UEs are mobile. We then
derive a lower bound for the handover probability in the DSM by characterizing
the evolution of the spatial distribution of the DBSs over time.
