  In this paper, an analytical framework is provided to analyze the energy
coverage performance of unmanned aerial vehicle (UAV) energy harvesting
networks with clustered user equipments (UEs). Locations of UEs are modeled as
a Poisson Cluster Process (PCP), and UAVs are assumed to be located at a
certain height above the center of user clusters. Hence, user-centric UAV
deployments are addressed. Two different models are considered for the
line-of-sight (LOS) probability function to compare their effects on the
network performance. Moreover, antennas with doughnut-shaped radiation patterns
are employed at both UAVs and UEs, and the impact of practical 3D antenna
radiation patterns on the network performance is also investigated. Initially,
the path loss of each tier is statistically described by deriving the
complementary cumulative distribution function and probability density
function. Following this, association probabilities with each tier are
determined, and energy coverage probability of the UAV network is characterized
in terms of key system and network parameters for UAV deployments both at a
single height level and more generally at multiple heights. Through numerical
results, we have shown that cluster size and UAV height play crucial roles on
the energy coverage performance. Furthermore, energy coverage probability is
significantly affected by the antenna orientation and number of UAVs in the
network.
