In terms of constellations, only a few GSO satellites are required for global coverage whereas LEO requires a swarm – of course LEO can also provide discontinuous global coverage with just a single satellite in polar orbit. The Doppler and delay variations are a challenge in LEO at the link-level and at the system-level, there is the challenge of tracking UEs in tracking-areas (TAs), which in conventional cellular are coupled to specific base stations. In GSO, the delay is a challenge along with the path-loss due to the distance to earth.