Connectivity of LEO Satellite Mega Constellations: An Application of Percolation Theory on a Sphere

With the advent of the 6G era, global connectivity has become a common goal in the evolution of communications, aiming to bring Internet services to more unconnected regions. In addition, the rise of applications such as the Internet of Everything and remote education also requires global connectivity. Nonterrestrial networks, particularly low Earth orbit (LEO) satellites, play a crucial role in this future vision. Although some literature already analyzed the coverage performance using stochastic geometry, the ability of generating large-scale continuous service area is still expected to analyze. Therefore, in this article, we mainly investigate the necessary conditions of LEO satellite deployment for large-scale continuous service coverage on the Earth. First, we apply percolation theory to a closed spherical surface and define the percolation on a sphere for the first time. We introduce the subcritical and supercritical cases to prove the existence of the phase transition of percolation probability. Then, through stereographic projection, we introduce the tight bounds and closed-form expression of the critical number of LEO satellites on the same constellation. In addition, we also investigate how the altitude and maximum slant range of LEO satellites affect percolation probability, and derive the critical values of them. Based on our findings, we provide useful recommendations for companies planning to deploy LEO satellite networks to enhance connectivity.