  The use of unmanned aerial vehicles (UAVs) operating as aerial base stations
(BSs) has emerged as a promising solution especially in scenarios requiring
rapid deployments (e.g., in the cases of crowded hotspots, sporting events,
emergencies, natural disasters) in order to assist the ground BSs. In this
paper, an analytical framework is provided to analyze the
signal-to-interference-plus-noise ratio (SINR) coverage probability of unmanned
aerial vehicle (UAV) assisted cellular networks with clustered user equipments
(UEs). Locations of UAVs and ground BSs are modeled as Poison point processes
(PPPs), and UEs are assumed to be distributed according to a Poisson cluster
process (PCP) around the projections of UAVs on the ground. Initially, the
complementary cumulative distribution function (CCDF) and probability density
function (PDF) of path losses for both UAV and ground BS tiers are derived.
Subsequently, association probabilities with each tier are obtained. SINR
coverage probability is derived for the entire network using tools from
stochastic geometry. Finally, area spectral efficiency (ASE) of the entire
network is determined, and SINR coverage probability expression for a more
general model is presented by considering that UAVs are located at different
heights. Via numerical results, we have shown that UAV height and path-loss
exponents play important roles on the coverage performance. Moreover, coverage
probability can be improved with smaller number of UAVs, while better area
spectral efficiency is achieved by employing more UAVs and having UEs more
compactly clustered around the UAVs.
