NTN

NR-NTN mobility and satellite switch

Satellite assistance in SIB19 and how NR uses it for synchronisation, measurement initiation, conditional handover and satellite switch.

5GNTN

Overview

SIB19 carries satellite assistance information for NTN access. Its NTN-Config includes epochTime, ntn-UlSyncValidityDuration, cellSpecificKoffset, kmac, ta-Info, polarization, ephemerisInfo and ta-Report. SIB19 also carries t-Service, referenceLocation, distanceThresh and a list of neighbour-cell NTN configurations.

The UE restarts T430 for the serving cell with the value ntn-UlSyncValidityDuration, counted from the subframe indicated by epochTime. TS 38.331 notes that the UE should try to re-acquire SIB19 before that duration ends.

t-Service indicates when a cell is going to stop serving the area, and applies to service link switches in quasi-Earth-fixed cells and to feeder link switches. UEs can use it to start measurements early (TS 38.304) and in satellite switch with re-synchronisation (TS 38.331). Cells can be Earth-fixed, quasi-Earth-fixed or Earth-moving.

Call flow

1
Satellite celltoUE
SIB19
Ephemeris, epoch time, Koffset, t-Service, UL sync validity, location thresholds.
2
UE
Use GNSS location and SIB19
Pre-compensation and location-based measurement initiation.
3
UE
T430 running
Valid for ntn-UlSyncValidityDuration from epochTime.
4
gNBtoUE
RRCReconfiguration (CHO with D1, D2 or T1 condition)
5
gNBtoUE
satSwitchWithReSync in SIB19
Satellite switch with re-synchronisation. Rel-18.

Timers and parameters

Name	Where	Role
T430	UE	Runs for ntn-UlSyncValidityDuration (5 s to 900 s) from epochTime for the serving cell.
t-Service	Broadcast	Time at which the cell stops serving the area.
cellSpecificKoffset	Broadcast	Integer 1 to 1023 in NTN-Config (-r17).

Full parameter and timer lookup →

Specifications

Spec	What it covers
TS 38.300	NTN. Mobility is 16.14.3, satellite switch with re-synchronisation is 16.14.3.2.3, switchover is 16.14.4.(see clause 16.14)
TS 38.331	SIB19 and NTN-Config, and conditional events D1, D2 and T1.
TS 38.304	Location-based and time-based measurement initiation in idle and inactive mode.

Changes by release

Release	Change
Rel-17	SIB19-r17, NTN-Config-r17 (epochTime, ntn-UlSyncValidityDuration, cellSpecificKoffset, kmac, ta-Info, ephemerisInfo), condEventD1-r17 and condEventT1-r17.
Rel-18	satSwitchWithReSync-r18, t-ServiceStart-r18, movingReferenceLocation-r18, ntn-CovEnh-r18 and condEventD2-r18.
Rel-19	refLocList-r19 and NTN-Config-r19 in TS 38.331.