Non-Terrestrial Networks (NTN) in 3GPP
Non-Terrestrial Networks bring satellites and high-altitude platforms into the mobile system, so a phone or IoT device can connect even where there is no ground tower. This page explains what NTN is, how 3GPP structures it, the challenges it solves, and how it has grown across releases.
Status: NTN became normative in Release 17 (building on the TR 38.821 study), with enhancements in Release 18 and 19. It is a foundational building block for 6G's goal of everywhere-coverage.
What NTN is
NTN extends the 5G NR (and IoT) air interface to spaceborne or airborne platforms rather than inventing a new radio. Two platform classes are covered:
- Satellites in low, medium, or geostationary orbit (LEO, MEO, GEO).
- High-Altitude Platform Stations (HAPS) flying at roughly 8–50 km.
The key point: an NTN-capable device is a standard NR device (with GNSS) using the same protocols, adapted to cope with the satellite environment. The 5G core network is essentially unaware whether access came via a ground tower or a satellite.
The architecture
NTN has three parts joined by two radio links:
- NTN payload — the radio equipment on the satellite or HAPS.
- Service link — the radio link between the payload and the user's device.
- Feeder link — the radio link between the payload and an NTN gateway on the ground.
- The NTN gateway connects onward to the base station and core network.
A single gateway can serve several payloads, and a payload can be reached by several gateways.
Transparent vs regenerative payloads
A crucial distinction:
- Transparent payload — a "bent-pipe" relay. The satellite just forwards the radio signal; all the base-station intelligence stays on the ground. This was the primary mode in the early releases.
- Regenerative payload — the satellite hosts an actual base station (or part of one), terminating the radio protocols on board. This enables inter-satellite links between satellites and is the direction of travel toward a true space network.
Orbits
| Orbit | Altitude | Character |
|---|---|---|
| LEO | ~300–1,500 km | Fast-moving, low latency, short pass overhead |
| MEO | ~7,000–25,000 km | Intermediate |
| GEO | ~35,786 km | Appears fixed over one spot, high latency |
Because satellites (except GEO) move, 3GPP ties a tracking area to a fixed patch of ground rather than to a moving beam, and uses stable "mapped" cell identities so the device sees a consistent cell even as satellites pass overhead.
The hard problems NTN solves
- Long delay. A GEO round trip can be around half a second — far beyond what terrestrial timers and retransmission (HARQ) assume. 3GPP extended timers and adapted HARQ for NTN.
- Timing advance. The device uses its GNSS position plus the satellite's broadcast ephemeris to work out the propagation delay itself and pre-compensate its uplink timing. A valid GNSS fix is required before it may transmit.
- Doppler shift. Fast-moving LEO satellites cause large frequency shifts; the device pre-compensates the service-link Doppler, while the network handles the feeder-link side.
- Mobility. As a satellite moves out of view, the network performs a service-link switch to the next satellite, and feeder-link switching as ground coverage changes.
NR-NTN vs IoT-NTN
3GPP standardised two tracks:
- NR-NTN — 5G NR over satellite, for smartphones, broadband, and direct-to-device.
- IoT-NTN — NB-IoT and eMTC over satellite, for low-power sensors and asset tracking, including store-and-forward for intermittent coverage.
How it grew across releases
- Rel-15/16 study — TR 38.821 analysed scenarios and solutions.
- Rel-17 — first normative NTN for both NR and IoT: timing/Doppler compensation, GEO and LEO support, the SIB19 system information for NTN.
- Rel-18 — enhancements: coverage for handheld devices, discontinuous-coverage handling, and store-and-forward extended toward NR.
- Rel-19 onward — regenerative payloads, inter-satellite links, and TN-NTN multi-connectivity (see the companion article on TN-NTN integration).
Reference points (3GPP)
- TR 38.821 — the foundational NR-for-NTN study.
- TS 38.300 §16.14 — the NR-NTN architecture and procedures.
- TS 38.331 — NTN RRC and the SIB19 system information.
- TS 38.101-5 / TS 38.521-5 — NTN device RF and conformance.
- TS 36.300 — the IoT-NTN side.
In short
NTN lets the mobile network reach beyond ground towers by adapting the same NR air interface to satellites and high platforms — handling delay, Doppler, and mobility — and it is steadily evolving from simple relays toward a genuine space-ground network.