“…Currently, communication services are largely limited to terrestrial mobile communications, while remote areas and special scenarios are still blind zones. In order to achieve ubiquitous global coverage, 6G will expand from terrestrial communications to space-air-ground-sea communications, integrating satellite communications [62], unmanned aerial vehicle (UAV) communications [63], terrestrial ultra-dense com-munication networks, maritime communications [64], underwater communications [65], and underground communications [66]. By integrating various communication networks, 6G will achieve seamless three-dimensional (3D) ubiquitous coverage and connectivity, providing multiple communication services.…”