  Unmanned aerial vehicles (UAVs) can be used as aerial wireless base stations
when cellular networks are not operational due to natural disasters. They can
also be used to supplement the ground base station in order to provide better
coverage and higher data rates for the users. Prior studies on UAV based
wireless coverage typically consider an Air-to-Ground path loss model, which
assumes that the users are outdoor and located on a 2D plane. In this paper, we
propose using UAVs to provide wireless coverage for indoor users inside a
high-rise building. First, we present realistic Outdoor-Indoor path loss models
and describe the tradeoff introduced by these models. Then we study the problem
of efficient placement of a single UAV, where the objective is to minimize the
total transmit power required to cover the entire high-rise building. The
formulated problem is non-convex and is generally difficult to solve. To that
end, we consider three cases of practical interest and provide efficient
solutions to the formulated problem under these cases. Then we study the
problem of minimizing the number of UAVs required to provide wireless coverage
to high rise buildings and prove that this problem is NP-complete. Due to the
intractability of the problem, we use clustering to minimize the number of UAVs
required to cover the indoor users. We demonstrate through simulations that the
method that clusters the building into regular structures and places the UAVs
in each cluster requires 80% more number of UAVs relative to our clustering
algorithm.
