  Unmanned aerial vehicle (UAV) has the advantages of large coverage and
flexibility, which could be applied in disaster management to provide wireless
services to the rescuers and victims. When UAVs forms an aerial mesh network,
line-of-sight (LoS) air-to-air (A2A) communications have long transmission
distance, which extends the coverage of multiple UAVs. However, the capacity of
UAV is constrained due to the multiple hop transmissions in aerial mesh
networks. In this paper, spectrum sharing between UAV enabled wireless mesh
networks and ground networks is studied to improve the capacity of UAV
networks. Considering two-dimensional (2D) and three-dimensional (3D)
homogeneous Poisson point process (PPP) modeling for the distribution of UAVs
within a vertical range {\Delta}h, stochastic geometry is applied to analyze
the impact of the height of UAVs, the transmit power of UAVs, the density of
UAVs and the vertical range, etc., on the coverage probability of ground
network user and UAV network user. Besides, performance improvement of spectrum
sharing with directional antenna is verified. With the object function of
maximizing the transmission capacity, the optimal altitude of UAVs is obtained.
This paper provides a theoretical guideline for the spectrum sharing of UAV
enabled wireless mesh networks, which may contribute significant value to the
study of spectrum sharing mechanisms for UAV enabled wireless mesh networks.
