  Based on the three-dimensional propagation characteristic under built-up
scenarios, a height-dependent line-of-sight (LoS) probability model for
air-to-ground (A2G) millimeter wave (mmWave) communications is proposed in this
paper. With comprehensive considerations of scenario factors, i.e., building
height distribution, building width, building space, and the heights of
transceivers, this paper upgrades the prediction method of International
Telecommunication Union-Radio (ITU-R) standard to both low altitude and high
altitude cases. In order to speed up the LoS probability prediction, an
approximate parametric model is also developed based on the theoretical
expression. The simulation results based on ray-tracing (RT) method show that
the proposed model has good consistency with existing models at the low
altitude. However, it has better performance at the high altitude. The new
model can be used for the A2G channel modeling and performance analysis such as
cell coverage, outage probability, and bit error rate of A2G communication
systems.
