  The utilization of existing terrestrial infrastructures to provide coverage
for aerial users is a potentially low-cost solution. However, the already
deployed terrestrial base stations (TBSs) result in weak ground-to-air (G2A)
coverage due to the down-tilted antennas. Furthermore, achieving optimal
coverage across the entire airspace through antenna adjustment is challenging
due to the complex signal coverage requirements in three-dimensional space,
especially in the vertical direction. In this paper, we propose a cooperative
tri-point (CoTP) model-based method that utilizes cooperative beams to enhance
the G2A coverage extension. To utilize existing TBSs for establishing effective
cooperation, we prove that the cooperation among three TBSs can ensure G2A
coverage with a minimum coverage overlap, and design the CoTP model to analyze
the G2A coverage extension. Using the model, a cooperative coverage structure
based on Delaunay triangulation is designed to divide triangular prism-shaped
subspaces and corresponding TBS cooperation sets. To enable TBSs in the
cooperation set to cover different height subspaces while maintaining ground
coverage, we design a cooperative beam generation algorithm to maximize the
coverage in the triangular prism-shaped airspace. The simulation results and
field trials demonstrate that the proposed method can efficiently enhance the
G2A coverage extension while guaranteeing ground coverage.
