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.
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;
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.
In this release, the UE supporting NTN is GNSS-capable.
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 22.214.171.124) are illustrated in Figure 126.96.36.199-1.
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 188.8.131.52 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 184.108.40.206 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.
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 220.127.116.11-1 (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.
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.
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.
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. It includes serving cell's NTN payload ephemeris and optionally neighbouring cell's NTN payload ephemeris.
The same principle as described in clause 18.104.22.168 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).
The same principle as described in clause 22.214.171.124 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.
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.
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.
In the quasi-earth fixed cell scenario, UE can perform time-based and location-based measurements on neighbour cells in RRC_IDLE/RRC_INACTIVE:
In the quasi-earth fixed cell scenario, UE can perform time-based and location-based measurement 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 to the reference location of the serving cell and a distance threshold to the reference location.
Measurement rules for cell re-selection based on timing information and location information are specified in clause 126.96.36.199 in TS 38.304.
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.
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.
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.
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 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.
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).
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.