  Unmanned aerial vehicles (UAVs) can be users that support new applications,
or be communication access points that serve terrestrial and/or aerial users.
In this paper, we focus on the connectivity problem of aerial users when they
are exclusively served by aerial base stations (BS), i.e., UAVBSs.
Specifically, the 3D placement problem of a directional antenna equipped
UAV-BS, aiming to maximize the number of covered aerial users under a spectrum
sharing policy with terrestrial networks, is investigated. Given a known
spectrum sharing policy between the aerial and terrestrial networks, we propose
a 3D placement algorithm that achieves optimality. Simulation results show the
performance of our approach, in terms of number of covered aerial users, for
different configurations and parameters, such as the spectrum sharing policy,
antenna beamwidth, transmit power, and aerial users density. These results
represent novel guidelines for exclusive aerial networks deployment and
applications, distinctively for orthogonal and non-orthogonal spectrum sharing
policies with terrestrial networks.
