  Unmanned aerial vehicles (UAVs) have recently found abundant applications in
the public and civil domains. To ensure reliable control and navigation,
connecting UAVs to controllers via existing cellular network infrastructure,
i.e., ground base stations (GBSs), has been proposed as a promising solution.
Nevertheless, it is highly challenging to characterize the communication
performance of cellular-connected UAVs, due to their unique propagation
conditions. This paper proposes a tractable framework for the coverage analysis
of cellular-connected UAV networks, which consists of a new blockage model and
an effective approach to handle general fading channels. In particular, a
line-of-sight (LoS) ball model is proposed to capture the probabilistic
propagation in UAV communication systems, and a tractable expression is derived
for the Laplace transform of the aggregate interference with general Nakagami
fading. This framework leads to a tractable expression for the coverage
probability, which in turn helps to investigate the impact of the GBS density.
Specifically, a tight lower bound on the optimal density that maximizes the
coverage probability is derived. Numerical results show that the proposed LoS
ball model is accurate, and the optimal GBS density decreases when the UAV
altitude increases.
