How it works
NTN reuses the 5G system and the NR protocol stack; the differences are concentrated in the radio and in a small number of architecture roles.
Key specifications: 3GPP TR 38.811 (Study on NR access to non-terrestrial networks: the foundational NTN study item), TR 38.821 (Solutions for NR to support NTN), TS 38.300 (NR overall description, NTN aspects), TR 38.863 (NTN related RF and co-existence aspects). For satellite broadcast delivery, see also ETSI TS 103 720 (5G Broadcast system for linear TV and radio), which is relevant to GEO-based NTN broadcast scenarios.
NTN system model and terminology
- Feeder link / service link. The service link is the wireless link between the NTN payload and the UE; the feeder link is the wireless transport link between the NTN Gateway and the NTN payload (TS 38.300 clause 3.1). To accommodate the propagation delay, several timing relationships are enhanced by a Common Timing Advance and configured offsets (TS 38.300 clause 16.14.2.1).
- Transparent vs regenerative payload. The NTN payload, on board a satellite or high altitude platform station, is a TNL node (transparent payload) or a gNB (regenerative payload) (TS 38.300 clause 3.1). The transparent payload forwards the radio protocol between the service link and the feeder link; the regenerative payload terminates the Uu, NG and Xn interfaces, and may terminate inter-satellite links towards other regenerative payloads (TS 38.300 clause 16.14.1).
- Cells and service links. NTN can provide Earth-moving, quasi-Earth-fixed and Earth-fixed cells, supported by three types of service link: Earth-fixed beams cover the same area all the time (e.g. GSO satellites); quasi-Earth-fixed beams cover one area for a limited period and another area later (e.g. NGSO satellites with steerable beams); Earth-moving beams slide over the Earth surface (e.g. NGSO satellites with fixed beams). A Tracking Area corresponds to a fixed geographical area, and a Mapped Cell ID corresponds to a fixed geographical area (TS 38.300 clauses 3.1 and 16.14.1). These cell types drive the handover models on the Mobility Aspects for NTN page.
- GNSS at the UE. In this release, the UE supporting NTN is GNSS-capable, and NTN is only applicable to FDD (TS 38.300 clause 16.14.1).
Radio-layer adaptations for NTN
The long, variable delay and the large Doppler shift on fast NGSO links are handled mainly at the physical and MAC layers, with assistance signalled in system information: a large, UE-specific timing advance computed from GNSS position and satellite ephemeris (plus a network-broadcast common component, anchored to an epoch time with a validity window); Doppler pre-compensation on the service link, computed by the device from geometry; a k-offset scheduling offset so uplink grants and other timing relationships remain valid despite the long round trip; and HARQ/RRC/MAC timers adapted for round trips that can far exceed terrestrial values, with some HARQ processes disabled and reliability moved to higher layers.
NTN system information for media delivery
Two system information blocks are central to media over NTN:
- SIB19 carries the NTN assistance information a device needs to acquire and track a satellite cell: serving-cell (and optionally neighbour-cell) ephemeris, common timing advance parameters, the k-offset, the epoch time and its validity duration, and the cell reference location. SIB19 was introduced with the Release 17 NTN work in TS 38.331.
- SIB27, introduced in Release 19 of TS 38.331, contains the intended service areas (ISA) of MBS broadcast services in an NTN cell, each given as a polygon or a circle; see MBS Broadcast NTN.
Carrying MBS over NTN
TS 23.247 Release 19 states that transmission of a Broadcast MBS Session is supported over NR non-terrestrial networks:
- Broadcast MBS session. Delivered with 5GC Shared MBS traffic delivery and PTM delivery only, and receivable in RRC_IDLE, RRC_INACTIVE and RRC_CONNECTED. Over NR NTN, Release 19 adds the NR NTN Intended Service Area, carried from the 5G Core to NG-RAN in the MBS service area and to the UE in SIB27, which can restrict the MBS service area to an area smaller than a cell.
- Multicast MBS session. NG-RAN is the decision point for switching between PTP and PTM, and the UE receives it in RRC_CONNECTED, or in RRC_INACTIVE with PTM. TS 23.247 has no NTN clause for the multicast MBS session; TS 38.321 has, since Release 17, a multicast DRX HARQ timer for non-terrestrial networks that adds the UE-gNB round trip.
What each analysis page adds
- Mobility Aspects for NTN sets out three deployment models (single NTN operator; common NTN/TN operator; independent operators) and what Release 19 specifies for mobility in each: Earth-fixed, quasi-Earth-fixed and Earth-moving cells, time- and location-based conditional handover, the satellite switch with re-synchronization and the feeder link switchover.
- MBS Multicast over NTN sets out requirements for multicast over NTN, with the gNB switching each UE between PTP and PTM, against what Release 19 specifies.
- Mobility for MBS Multicast over NTN sets out what Release 19 provides, UE by UE, when multicast receivers move over NTN (it has no group handover), distinguishing mobility triggered by satellite motion from mobility triggered by the UEs.
- MBS Broadcast over NTN follows the Intended Service Area of a broadcast MBS session from the 5G Core to SIB27, the MCCH and the UE.
Related: 5G Multicast Broadcast Services (MBS) · 5G Broadcast - TV and Radio Services · Standards: 5G Multicast & Broadcast Services · Standards: 5G Broadcast - TV and Radio Services
Sources for this page (NTN terms)
- Feeder link, service link, NTN payload, Mapped Cell ID: TS 38.300 V19.4.0, clause 3.1.
- Transparent and regenerative payload, cells and service links, Tracking Area, GNSS, FDD: TS 38.300 V19.4.0, clause 16.14.1.
- Timing: TS 38.300 V19.4.0, clause 16.14.2.1.