  One of the major challenges slowing down the use of unmanned aerial vehicles
(UAVs) as aerial base stations (ABSs) is the limited on-board power supply
which reduces the UAV's flight time. Using a tether to provide UAVs with power
can be considered a reasonable compromise that will enhance the flight time
while limiting the UAV's mobility. In this work, we propose a system where ABSs
are deployed at the centers of user hotspots to offload the traffic and assist
terrestrial base stations (TBSs). Firstly, given the location of the ground
station in the user hotspot (user cluster) and the users spatial distribution,
we compute the optimal inclination angle and length of the tether. Using these
results, we compute the densities of the tethered UAVs deployed at different
altitudes, which enables tractable analysis of the interference in the
considered setup. Next, using tools from stochastic geometry and an approach of
dividing user clusters into finite frames, we analyze the coverage probability
as a function of the maximum tether length, the density of accessible rooftops
for UAV ground station deployment, and the density of clusters. We verify our
findings using Monte-Carlo simulations and draw multiple useful insights. For
instance, we show that it is actually better to deploy UAVs at a fraction of
the clusters, not all of them as it is usually assumed in literature.
