  Path probability prediction is essential to describe the dynamic birth and
death of propagation paths, and build the accurate channel model for
air-to-ground (A2G) communications. The occurrence probability of each path is
complex and time-variant due to fast changeable altitudes of UAVs and
scattering environments. Considering the A2G channels under urban scenarios,
this paper presents three novel stochastic probability models for the
line-of-sight (LoS) path, ground specular (GS) path, and building-scattering
(BS) path, respectively. By analyzing the geometric stochastic information of
three-dimensional (3D) scattering environments, the proposed models are derived
with respect to the width, height, and distribution of buildings. The effect of
Fresnel zone and altitudes of transceivers are also taken into account.
Simulation results show that the proposed LoS path probability model has good
performance at different frequencies and altitudes, and is also consistent with
existing models at the low or high altitude. Moreover, the proposed LoS and
NLoS path probability models show good agreement with the ray-tracing (RT)
simulation method.
