How does non-terrestrial network integration work in 3GPP standards for satellite and high altitude platforms?

Non-terrestrial network (NTN) integration in 3GPP standards provides a framework for incorporating satellite and high-altitude platform stations (HAPS) into the 5G and beyond cellular network architecture, enabling seamless connectivity between terrestrial and space/airborne base stations. The 3GPP NTN standards (introduced in Release 17 for NR, Release 18 for IoT-NTN) define: the NTN architecture (the NTN base station (gNodeB) operates on a satellite (LEO, MEO, GEO) or HAPS; the user equipment (UE) on the ground communicates with the NTN gNodeB using standard 5G NR waveforms (OFDM) adapted for the satellite link), the key technical adaptations (timing advance (the round-trip propagation delay to a GEO satellite is approximately 540 ms, far exceeding terrestrial 5G timing; the standard defines: GNSS-based pre-compensation of the timing advance at the UE, and enhanced timing advance mechanisms that handle the large and varying delay), Doppler pre-compensation (LEO satellites move at 7+ km/s, creating Doppler shifts of ±25 kHz at 2 GHz; the UE pre-compensates using GNSS-derived velocity and satellite ephemeris), and HARQ adaptation (the long round-trip time makes traditional HARQ (Hybrid Automatic Repeat Request) impractical for GEO; the standard disables HARQ for GEO or uses multiple parallel HARQ processes)), the frequency bands (S-band (around 2 GHz) for handheld UE (direct-to-satellite), Ka-band (20/30 GHz) for VSAT and fixed terminals, and L-band (1.5/1.6 GHz) for IoT-NTN), and the deployment scenarios (transparent (bent-pipe) satellite: the satellite simply amplifies and frequency-converts the signal; all processing is done at the ground gateway. Regenerative satellite: the satellite includes an on-board gNodeB (full baseband processing on the satellite)).