HAPS are radio stations located on an object that floats at the stratosphere layer and at a specified, nominal, fixed point relative to the Earth, providing telecommunication services coverage over a wide geographical range, with high quality signal.
Air-to-Ground is a Two-way communication between aircraft and stations or locations on the surface of the earth.
The ground network consists of A2G base stations with skywards-pointing antennas optimized for airspace coverage, which create very large cells in the sky. Given that, unlike on the ground, radio signals can freely propagate.
A LAP is an object (may be a UAV or balloon) that have the capability of flying at low altitudes (e.g., a few hundreds of meters) for a sufficient endurance for completing a mission.