  On-demand deployments of millimeter-wave (mmWave) access points (APs) carried
by unmanned aerial vehicles (UAVs) are considered today as a potential solution
to enhance the performance of 5G+ networks. The battery lifetime of modern
UAVs, though, limits the flight times in such systems. In this letter, we
evaluate a feasible deployment alternative for temporary capacity boost in the
areas with highly fluctuating user demands. The approach is to land UAV-based
mmWave APs on the nearby buildings instead of hovering over the area. Within
the developed mathematical framework, we compare the system-level performance
of airborne and landed deployments by taking into account the full operation
cycle of the employed drones. Our numerical results demonstrate that the choice
of the UAV deployment option is determined by an interplay of the separation
distance between the service area and the UAV charging station, drone battery
lifetime, and the number of aerial APs in use. The presented methodology and
results can support efficient on-demand deployments of UAV-based mmWave APs in
prospective 5G+ networks.
