Content for  TS 38.300  Word version:  18.1.0

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16.14  Non-Terrestrial Networks |R17|p. 206

16.14.1  Overviewp. 206

Figure 16.14.1-1 below illustrates an example of a Non-Terrestrial Network (NTN) providing non-terrestrial NR access to the UE by means of an NTN payload and an NTN Gateway, depicting a service link between the NTN payload and a UE, and a feeder link between the NTN Gateway and the NTN payload.
Reproduction of 3GPP TS 38.300, Fig. 16.14.1-1: Overall illustration of an NTN
The NTN payload transparently forwards the radio protocol received from the UE (via the service link) to the NTN Gateway (via the feeder link) and vice-versa. The following connectivity is supported by the NTN payload:
  • An NTN gateway may serve multiple NTN payloads;
  • An NTN payload may be served by multiple NTN gateways.
For NTN, the following applies in addition to Network Identities as described in clause 8.2:
  • A Tracking Area corresponds to a fixed geographical area. Any respective mapping is configured in the RAN;
  • A Mapped Cell ID as specified in clause 16.14.5.
Three types of service links are supported:
  • Earth-fixed: provisioned by beam(s) continuously covering the same geographical areas all the time (e.g., the case of GSO satellites);
  • Quasi-Earth-fixed: provisioned by beam(s) covering one geographic area for a limited period and a different geographic area during another period (e.g., the case of NGSO satellites generating steerable beams);
  • Earth-moving: provisioned by beam(s) whose coverage area slides over the Earth surface (e.g., the case of NGSO satellites generating fixed or non-steerable beams).
With NGSO satellites, the gNB can provide either quasi-Earth-fixed service link or Earth-moving service link, while gNB operating with GSO satellite can provide Earth fixed service link or quasi-Earth-fixed service link.
In this release, the UE supporting NTN is GNSS-capable.
In NTN, the distance refers to Euclidean distance.

16.14.2  Timing and Synchronizationp. 208  Scheduling and Timingp. 208

DL and UL are frame aligned at the uplink time synchronization reference point (RP) with an offset given by NTA,offset (see clause 4.2 of TS 38.213).
To accommodate the propagation delay in NTNs, several timing relationships are enhanced by a Common Timing Advance (Common TA) and two offsets Koffset and kmac:
  • Common TA is a configured timing offset that is equal to the RTT between the RP and the NTN payload.
  • Koffset is a configured scheduling offset that needs to be larger or equal to the sum of the service link RTT and the Common TA.
  • kmac is a configured offset that is approximately equal to the RTT between the RP and the gNB.
The scheduling offset Koffset is used to allow the UE sufficient processing time between a downlink reception and an uplink transmission, see TS 38.213.
The offset kmac is used to delay the application of a downlink configuration indicated by a MAC CE command on PDSCH, see TS 38.213, and in estimation of UE-gNB RTT, see TS 38.321. It may be provided by the network when downlink and uplink frame timing are not aligned at gNB. The kmac is also used in the random access procedure, to determine the start time of RAR window/MsgB window after a Msg1/MsgA transmission (see TS 38.213).
The Service link RTT, Feeder link RTT, RP, Common TA, kmac and TTA (see clause are illustrated in Figure
Reproduction of 3GPP TS 38.300, Fig. Illustration of timing relationship (for collocated gNB and NTN Gateway)
The network may configure the HARQ operation as follows:
  • For downlink, HARQ feedback can be enabled or disabled per HARQ process (as specified in clause and clause 5.7 of TS 38.321). Disabling HARQ feedback allows scheduling a HARQ process before one HARQ RTT has elapsed since last scheduled.
  • For uplink, HARQ mode (i.e. HARQ mode A or HARQ mode B) can be configured per HARQ process (as specified in clause and clause 5.7 of TS 38.321). HARQ mode B allows scheduling a HARQ process before one HARQ RTT has elapsed since last scheduled.
Up  Timing Advance and Frequency Pre-compensationp. 209

For the serving cell, the network broadcast valid ephemeris information and Common TA parameters. The UE shall have valid GNSS position as well as ephemeris and Common TA before connecting to an NTN cell. To achieve synchronisation, before and during connection to an NTN cell, the UE shall compute the RTT between UE and the RP based on the GNSS position, the ephemeris, and the Common TA parameters (see clause 4.2 of TS 38.213), and autonomously pre-compensate the TTA for the RTT between the UE and the RP as illustrated in Figure (see clause 4.3 of TS 38.211).
The UE shall compute the frequency Doppler shift of the service link, and autonomously pre-compensate for it in the uplink transmissions, by considering UE position and the ephemeris. If the UE does not have a valid GNSS position and/or valid ephemeris and Common TA, it shall not transmit until both are regained.
In connected mode, the UE shall be able to continuously update the Timing Advance and frequency pre-compensation.
The UE may be configured to report Timing Advance during Random Access procedures or in connected mode. In connected mode, event-triggered reporting of the Timing Advance is supported.
(no figure)
While the pre-compensation of the instantaneous Doppler shift experienced on the service link is to be performed by the UE, the management of Doppler shift experienced over the feeder link and transponder frequency error is left to the network implementation.

16.14.3  Mobility and State transitionp. 209  Mobility in RRC_IDLE and RRC_INACTIVEp. 209

The same principles as described in clause 9.2.1 apply to mobility in RRC_IDLE for NTN and the same principles as described in clause 9.2.2 apply to mobility in RRC_INACTIVE for NTN unless hereunder specified.
The network may broadcast multiple Tracking Area Codes (TACs) per PLMN in an NR NTN cell. A TAC change in the System Information is under network control, i.e. it may not be exactly synchronised with real-time illumination of beams on ground.
For the NTN-TN mobility, the network may broadcast cell information on NR TN and EUTRA TN coverage areas in SIB25. This is supported for Earth-Fixed, Quasi-Earth-fixed and Earth-Moving cells. The coverage information consists in a list of geographical TN areas, with associated frequency information also indicated. UE can skip TN measurement based on the broadcast TN coverage information.
The UE can determine the network type (terrestrial or non-terrestrial) implicitly by the existence of cellBarredNTN in SIB1.
The NTN ephemeris is provided in SIB19. In an NTN cell, it includes serving cell's NTN payload ephemeris and optionally neighbouring cell's NTN payload ephemeris.
Up  Mobility in RRC_CONNECTEDp. 210  Handoverp. 210
The same principle as described in clause applies unless hereunder specified:
During mobility between NTN and Terrestrial Network (TN), a UE is not required to connect to both NTN and TN at the same time.
DAPS handover is not supported for NTN in this release of the specification.
UE may support mobility between gNBs operating with NTN payloads in different orbits (e.g., GSO, NGSO at different altitudes).
RACH-less handover as specified in TS 38.321 and TS 38.331 is supported in NTNs. The RRCReconfiguration message triggering the RACH-less handover includes a timing adjustment indication and either a configured grant or a beam indication for accessing the target cell. UE synchronizes to the target cell by applying the timing adjustment indication and transmits the RRCReconfigurationComplete message using the configured uplink grant if included. UE can fallback to RACH when there is no valid configured uplink grant. If the configured uplink grant is not included, UE receives an uplink grant by monitoring PDCCH according to the beam indication.
Up  Conditional Handoverp. 210
The same principle as described in clause applies to NTN unless hereunder specified.
NTN supports the following additional trigger conditions upon which UE may execute CHO to a candidate cell, as defined in TS 38.331:
  • The RRM measurement-based event A4;
  • A time-based trigger condition;
  • A location-based trigger condition.
Time-based or location-based trigger conditions may be configured independently from the measurement condition for CHO in NTN in at least hard satellite switch case where the service discontinuity gap time length is assumed to be zero or negligible. However, if the service discontinuity gap time length is not zero or negligible, then a time-based or a location-based trigger condition is always configured together with one of the measurement-based trigger conditions (CHO events A3/A4/A5) as defined in TS 38.331.
It is up to UE implementation how the UE evaluates the time- or location-based trigger condition together with the RRM measurement-based event.
When a time-based trigger condition is used, the source gNB may signal the corresponding parameters to a single target gNB via the Source NG-RAN Node to Target NG-RAN Node Transparent Container in an NG-C based handover, see TS 23.502, or Xn based handover. The source gNB signals the corresponding CHO configuration to the UE in the RRC Reconfiguration message during handover.
When time-based trigger condition is used, the source NG-RAN node should consider the time indicated to the UE to decide when to start the early data forwarding to the target NG-RAN node.
Time-based CHO can be performed via RACH-less.
Up  Satellite switch with re-sync |R18|p. 210
Upon both hard and soft satellite switch over in the quasi-Earth fixed scenario with the same SSB frequency and the same gNB, the satellite switch with re-sync procedure is supported. The satellite switch with re-sync avoids a L3 mobility for UEs in the cell by maintaining the same PCI on the geographical area covered by quasi-Earth fixed beam. CHO can be configured simultaneously with the satellite switch with re-sync procedure.
For soft satellite switch over, the UE can start synchronizing with the target satellite before the source satellite ends to serve the cell. It is not required for the UE to be connected to source satellite when the UE switches to target satellite.
For hard satellite switch over, the UE can only start synchronizing with the target satellite after the switch to the target satellite is initiated.
Up  Measurementsp. 211

The same principle as described in clause 9.2.4 applies to measurements in NTN unless hereunder specified.
The network can configure:
  • multiple SMTCs in parallel per carrier and for a given set of cells depending on UE capabilities;
  • measurement gaps based on multiple SMTCs;
  • assistance information (e.g., ephemeris, Common TA parameters, kmac) provided in SIB19 for UE to perform measurement on neighbour cells in RRC_IDLE/RRC_INACTIVE/RRC_CONNECTED.
NW-controlled adjustment of SMTCs can be based on UE assistance information reported in RRC_CONNECTED. A UE in RRC_IDLE/RRC_INACTIVE can adjust SMTCs based on its location and assistance information in SIB19.
UE assistance information consists of the service link propagation delay difference(s) between serving the cell and neighbour cell(s).
For a UE in Idle/Inactive mode it's up to UE implementation whether to perform NTN neighbour cell measurements on a cell indicated in SIB3/SIB4 but not included in SIB19.
For a UE in Connected mode, it's up to UE implementation whether to perform NTN neighbour cell measurements on a cell included in the measurement configuration but not included in SIB19.
UE can perform time-based and location-based measurements on neighbour cells in RRC_IDLE/RRC_INACTIVE:
  • The timing and location information associated to the serving cell is provided in SIB19;
  • Timing information refers to the UTC time when the serving cell stops serving the current geographical area;
  • Location information refers:
    • In the quasi-Earth fixed cell scenario, to the reference location of the serving cell and a distance threshold to the reference location.
    • In the Earth moving cell scenario, to the reference location of the serving cell at the epoch time and a distance threshold to the reference location.
The time-based measurement initiation may be applicable for the feeder link switchover case for cell (re)selection.
Measurement rules for cell re-selection based on timing information and location information are specified in clause in TS 38.304.

16.14.4  Switchoverp. 211  Definitionsp. 211

A feeder link switchover is the procedure where the feeder link is changed from a source NTN Gateway to a target NTN Gateway for a specific NTN payload. The feeder link switchover is a Transport Network Layer procedure. Service link switch refers to a change of the serving NTN payload.
Both hard and soft feeder link switchover are supported in NTN.  Assumptionsp. 211

A feeder link switch over may result in transferring the established connection for the affected UEs between two gNBs.
For soft feeder link switch over, an NTN payload is able to connect to more than one NTN Gateway during a given period, i.e. a temporary overlap can be ensured during the transition between the feeder links.
For hard feeder link switch over, an NTN payload connects to only one NTN Gateway at any given time, i.e. a radio link interruption may occur during the transition between the feeder links.
Up  Proceduresp. 212

The NTN Control function (see Annex B.4) determines the point in time when the feeder link switch over between two gNBs is performed. The transfer of the affected UE(s)' context between the two gNBs at feeder link switch over is performed by means of either NG based or Xn based handover, and it depends on the gNBs' implementation and configuration information provided to the gNBs by the NTN Control function.

16.14.5  NG-RAN signallingp. 212

The Cell Identity, as defined in TS 38.413 and TS 38.423, used in following cases corresponds to a Mapped Cell ID, irrespective of the orbit of the NTN payload or the types of service links supported:
  • The Cell Identity indicated by the gNB to the Core Network as part of the User Location Information;
  • The Cell Identity used for Paging Optimization in NG interface;
  • The Cell Identity used for Area of Interest;
  • The Cell Identity used for PWS.
The Cell Identity included within the target identification of the handover messages allows identifying the correct target cell. The cell identity used in the NG and Xn handover messages, Xn Setup and Xn NG-RAN Node Configuration Update procedures is expected to be Uu Cell ID.
The Cell Identities used in the RAN Paging Area during Xn RAN paging allow the identification of the correct target cells for RAN paging.
The mapping between Mapped Cell IDs and geographical areas is configured in the RAN and Core Network.
The gNB is responsible for constructing the Mapped Cell ID based on the UE location information received from the UE, if available. The mapping may be pre-configured (e.g., up to operator's policy) or up to implementation.
The gNB reports the broadcasted TAC(s) of the selected PLMN to the AMF as part of ULI. In case the gNB knows the UE's location information, the gNB may determine the TAI the UE is currently located in and provide that TAI to the AMF as part of ULI.

16.14.6  AMF (Re-)Selectionp. 212

The gNB implements the NAS Node Selection Function specified in TS 38.410.
For an RRC_CONNECTED UE, when the gNB is configured to ensure that the UE connects to an AMF that serves the country in which the UE is located, if the gNB detects that the UE is in a different country to that served by the serving AMF, then it should perform an NG handover to change to an appropriate AMF, or initiate an UE Context Release Request procedure towards the serving AMF (in which case the AMF may decide to de-register the UE).
For the purpose of selecting an appropriate AMF, the 5GC may verify the UE location according to TS 23.501 and TS 38.305 after the UE has attached to the network.

16.14.7  O&M Requirementsp. 213

The following NTN related parameters shall be provided by O&M to the gNB providing NTN access:
  • Ephemeris information describing the orbital trajectory information or coordinates for the NTN payload. This information is provided on a regular basis or upon demand to the gNB;
  • Two different sets of ephemeris format shall be supported:
    • Set 1: NTN payload position and velocity state vectors:
      • Position;
      • Velocity.
    • Set 2: At least the following parameters in orbital parameter ephemeris format, as specified in NIMA TR 8350.2 [51]:
      • Semi-major axis;
      • Eccentricity;
      • Argument of periapsis;
      • Longitude of ascending node;
      • Inclination;
      • Mean anomaly at epoch time.
  • The explicit epoch time associated to ephemeris data;
  • The location of the NTN Gateways;
  • Additional information to enable gNB operation for feeder/service link switch overs.

16.14.8  Coarse UE location reportingp. 213

Upon network request, after AS security is established in connected mode, a UE should report its coarse UE location information (most significant bits of the GNSS coordinates, ensuring an accuracy in the order of 2 km) to the NG-RAN if available.

16.14.9  Support for NR NTN coverage enhancements |R18|p. 213

To improve NR uplink coverage in NTN, the following enhancements are supported:
  • PUCCH repetition for Msg4 HARQ-ACK configured in system information or dynamically in DCI for Msg4 when multiple repetition factors are configured in the system information:
    • UEs reports the capability of PUCCH repetition for Msg4 HARQ-ACK in Msg3 PUSCH;
    • When Msg4 HARQ-ACK is repeated, PUCCH repetition is applied for all PUCCH transmission before dedicated PUCCH resource is provided.
  • Improved channel estimation by NTN-specific PUSCH DMRS bundling enhancement that enables DMRS bundling in presence of timing drift, where the UE maintains phase continuity by considering effects of transmission delay variation between the UE and the uplink time synchronization reference point.

16.14.10  Verification of UE location |R18|p. 214

The core network may trigger a network verification procedure for a UE in RRC_CONNECTED to verify it is consistent with the network-based assessed location. It is up to network implementation how to handle UEs which does not support the location verification.
For UE location verification based on multi-RTT with single satellite in NTN, at least the following UE and gNB measurements specified in TS 38.215 are reported: gNB receive-transmit time difference at the uplink time synchronization reference point, UE receive-transmit time difference, UE receive-transmit time difference subframe offset and DL timing drift.
The assistance information provided to the CN may include ephemeris information including accurate satellite position and velocity at the time of multi-RTT measurement, and common TA parameters (ta-Common, ta-CommonDrift, ta-CommonDriftVariant), and Epoch time.

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