Doppler, timing advance, and large round-trip handling

Why Satellite Links Are Hard

A satellite link breaks several assumptions baked into terrestrial cellular standards. First, the propagation delay is enormous — round-trip 24 milliseconds for LEO, 240 milliseconds for GEO, compared to typical terrestrial 1 millisecond. Second, the satellite is moving at orbital velocity, producing Doppler shifts of tens of kilohertz at gigahertz carrier frequencies. Third, the satellite footprint sweeps across the ground, so a stationary UE sees its serving cell change every few minutes (LEO) or stays put (GEO). Fourth, link budget is constrained — the device must reach a satellite…