16.14 Non-Terrestrial Networks

38.3003GPPNRNR and NG-RAN Overall descriptionRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

16.14.1 Overview

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.

Figure 16.14.1-1: Overall illustration of an NTN

NOTE 1: Figure 16.14.1-1 illustrates an NTN; RAN4 aspects are out of scope.

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 gateway.

NOTE 2: In this release, the NTN-payload may change the carrier frequency, before re-transmitting it on the service link, and vice versa (respectively on the feeder link).

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.

In this release, the UE supporting NTN is GNSS-capable.

The support for Non-Terrestrial Networks (NTNs) is facilitated by the mechanisms described in the following clauses.

16.14.2 Timing and Synchronization

16.14.2.1 Scheduling and Timing

To accommodate the propagation delay in NTNs, several timing relationships are enhanced by a Common Timing Advance (Common TA) and two scheduling offsets and illustrated in Figure 16.14.2.1-1:

– is a configured offset that corresponds to the RTT between the Reference Point (RP) and the NTN payload.

– is a configured scheduling offset that need to be larger or equal to the sum of the service link RTT and the common TA.

– is a configured offset that need to be larger or equal to the RTT between the RP and the gNB.

Figure 16.14.2.1-1: Illustration of timing relationship

DL and UL are frame aligned at the uplink time synchronization reference point (RP) with an offset given by N_TA,offset(see clause 4.3 of TS 38.211 [52]).

is a scheduling offset supported in NTN for MAC CE timing relationships enhancement. It is provided by the network if downlink and uplink frame timing are not aligned at gNB. It is needed for UE timing of downlink configuration change indicated by a MAC-CE command in PDSCH. The is also used in the beam failure recovery, where after a PRACH transmission in uplink slot n the UE monitors the corresponding PDCCH starting from downlink slot "n + + 4" within a corresponding RAR window.

16.14.2.2 Pre-compensation by the UE

For the serving cell, the network broadcast ephemeris information and common TA parameters. The UE shall have valid GNSS position as well as the satellite ephemeris and common TA before connecting to an NTN cell. To achieve synchronisation, before and during connection to an NTN cell, the UE computes the service link RTT based on the GNSS position and the satellite ephemeris and autonomously pre-compensates the T_TA (see clause 4.3 of TS 38.211 [52]).

As illustrated in the Figure 16.14.2.2-1, the UE computes the frequency Doppler shift by considering UE position and the satellite ephemeris. If the UE does not have a valid GNSS position and/or valid satellite ephemeris, it does not communicate with the network until both are regained.

In connected mode, the UE should be able to continuously update the Timing Advance and frequency pre-compensation.

The UEs 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.

Figure 16.14.2.2-1: Illustration of Uplink/Downlink Radio Frame Timing at the UE

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 satellite network implementation.

16.14.3 Mobility and State transition

16.14.3.1 Mobility in RRC_IDLE and RRC_INACTIVE

The same principles as described in 9.2.1 apply to mobility in RRC_IDLE for NTN and the same principles as described in 9.2.2 apply to mobility in RRC_INACTIVE for NTN unless hereunder specified.

The network may broadcast multiple Tracking Area Codes (TAC) per PLMN in a 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 provisioned. It includes serving cell’s satellite ephemeris and neighbouring cell’s satellite ephemeris.

16.14.3.2 Mobility in RRC_CONNECTED

16.14.3.2.1 Handover

The same principle as described in 9.2.3.2 applies unless hereunder specified:

During mobility between NTN and Terrestrial Network, a UE is not required to connect to both NTN and Terrestrial Network at the same time.

NOTE: NTN-Terrestrial Network hand-over refers to mobility in both directions, i.e. from NTN to Terrestrial Network (hand-in) and from Terrestrial Network to NTN (hand-out).

DAPS handover is not supported for NTN in this release of the specification.

UE may support mobility between radio access technologies each based on different orbit (GSO, NGSO at different altitude).

16.14.3.2.2 Conditional Handover

The same principle as described in 9.2.3.4 applies to NTN unless hereunder specified.

NTN supports the following additional triggering conditions upon which UE may execute CHO to a candidate cell, as defined in TS 38.331 [12]:

– 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). Location is defined by the distance between UE and a reference location.

16.14.3.3 Measurements

The same principle as described in 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 using propagation delay difference calculated by UE;

– measurement gaps based on multiple SMTCs.

NW-controlled adjustment of SMTCs can be based on UE assistance information reported in Connected mode. For Idle/Inactive mode UE can adjust SMTCs based on its location and satellite assistance information (e.g. ephemeris, common TA parameters).

UE assistance information is in the form of a service link propagation delay difference between serving cell and neighbour cells.

In the quasi-earth fixed cell scenario, UE can perform time-based and location-based measurements on neighbour cells in RRC_IDLE/RRC_INACTIVE:

– The timing and location information associated to a cell are provided via system information;

– Timing information refers to the time when the serving cell is going to stop serving a geographical area;

– Location information refers to the reference location of serving cell and a distance threshold.

Measurement rules for cell re-selection with timing information and location information are specified in clause 5.2.4.2 in TS 38.304 [10].

16.14.4 Switchover

16.14.4.1 Definitions

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 satellite.

Both hard and soft feeder link switchover are supported in NTN.

16.14.4.2 Assumptions

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.

16.14.4.3 Procedures

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 signalling

The Cell Identity, as defined in TS 38.413 [26] and TS 38.423 [50], 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.

NOTE 1: The Cell Identity used for RAN Paging is assumed to typically represent a Uu Cell ID.

The mapping between Mapped Cell IDs and geographical areas is configured in the RAN and Core Network.

NOTE 2: A specific geographical location may be mapped to multiple Mapped Cell ID(s), and such Mapped Cell IDs may be configured to indicate differerent geographical areas (e.g. overlapping and/or with different dimensions).

The gNB is responsible for constructing the Mapped Cell ID based on the UE location info received from the UE, if available. The mapping may be pre-configured (e.g., up to operator’s policy) or up to implementation.

NOTE 3: As described in TS 23.501 [3], the User Location Information may enable the AMF to determine whether the UE is allowed to operate at its present location. Special Mapped Cell IDs or TACs may be used to indicate areas outside the serving PLMN’s country.

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-)Selection by gNB

The gNB implements the NAS Node Selection Function specified in TS 38.410 [16].

For a 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).

16.14.7 O&M Requirements

The following NTN related parameters shall be provided by O&M to the gNB providing non-terrestrial NR access:

– Ephemeris information describing the orbital trajectory information or coordinates for the NTN vehicles. 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: Satellite 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 to.

– The explicit epoch time associated to ephemeris data;

– The location of the NTN-Gateways;

NOTE 1: The ephemeris of the satellites and the location of the NTN-Gateways, are used at least for the Uplink timing and frequency synchronization. It may also be used for the random access and the mobility management purposes.

– Additional information to enable gNB operation for feeder/service link switch overs.

NOTE 2: The NTN related parameters provided by O&M to the gNB may depend on the type of supported service links e.g. earth fixed beams, quasi earth fixed beams, earth moving beams, etc.

16.14.8 Coarse UE location reporting

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.