  The past few years have witnessed a tremendous increase on the use of
unmanned aerial vehicles (UAVs) in civilian applications, which call for
high-performance communication between UAVs and their ground clients,
especially when they are densely deployed. To achieve this goal, cellular base
stations (BSs) can be leveraged to provide a new and promising solution to
support massive UAV communications simultaneously in a cost-effective way.
However, different from terrestrial communication channels, UAV-to-BS channels
are usually dominated by the light-of-sight (LoS) link, which aggravates the
co-channel interference and renders the spatial frequency reuse in existing
cellular networks ineffective. In this paper, we consider the use of a
directional antenna at each UAV to confine the interference to/from other UAV
users within a limited region and hence improve the spatial reuse of the
spectrum. Under this model, a UAV user may be temporarily blocked from
communication if it cannot find any BS in its antenna main-lobe, or it finds
that all BSs under its main-lobe are simultaneously covered by those of some
other UAVs and hence suffer from strong co-channel interference. Assuming
independent homogeneous Poisson point processes (HPPPs) for the UAVs' and
ground BSs' locations respectively, we first analytically derive a closed-form
upper bound for the UAV blocking probability and then characterize the
achievable average spatial throughput of the cellular-enabled UAV communication
network, in terms of various key parameters including the BS/UAV densities as
well as the UAV's flying altitude and antenna beamwidth. Simulation results
verify that the derived bound is practically tight, and further show that
adaptively adjusting the UAV altitude and/or beamwidth with different BS/UAV
densities can significantly reduce the UAV blocking probability and hence
improve the network spatial throughput.
