The 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:
A gNB may serve multiple NTN payloads;
An NTN payload may be served by multiple gNBs.
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;
Non-Geosynchronous orbit (NGSO) includes Low Earth Orbit at altitude approximately between 300 km and 1500 km and Medium Earth Orbit at altitude approximately between 7000 km and 25000 km.
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 cell coverage or Earth-moving cell coverage, while gNB operating with GSO satellite can provide Earth fixed cell coverage.
In this release, the UE supporting NTN is GNSS-capable.
In the case of NGSO, service link switch refers to a change of serving satellite.
The support for Non-Terrestrial Networks (NTNs) is facilitated by the mechanisms described in the following clauses.
The UE may be configured to report the UE's Timing Advance:
during Random Access procedure in Idle/Inactive state;
in connected mode:
using event-triggered reporting;
for RRC re-establishment procedure, if an indication is broadcasted by the target cell's SI;
for handover, the UE should trigger TA report if the target cell indicates this in the handover command.
To accommodate the long propagation delay, Use Plane procedures are adapted as follow:
For downlink, HARQ feedback can be enabled or disabled per HARQ process;
For uplink, the UE can be configured with a HARQ mode A or B per HARQ process;
Maximum number of HARQ processes is extended to 32;
The value ranges of MAC (i.e. sr-ProhibitTimer and configuredGrantTimer), RLC (i.e. t-Reassembly) and PDCP (i.e. discardTimer and t-reordering) layer timers are extended.
If a logical channel is configured with allowedHARQ-mode, it can only be mapped to a HARQ process with the same HARQ mode.
Impact on timing aspects:
To accommodate the long propagation delays, several NR timings involving DL-UL timing interaction are enhanced by the support of two scheduling offsets: K_offset and k_mac.
The timing relationships that need to be modified for NTN using Koffset are summarized as follows:
The transmission timing of DCI scheduled PUSCH, including channel state information (CSI) transmission on PUSCH;
The transmission timing of random access response (RAR) grant or fallbackRAR grant scheduled PUSCH;
The timing of the first PUSCH transmission opportunity in type-2 configured grant;
The transmission timing of HARQ-ACK on physical uplink control channel (PUCCH), including HARQ-ACK on PUCCH to message B (MsgB) in 2-step random access;
The transmission timing of PDCCH ordered physical random access channel (PRACH);
The timing of the adjustment of uplink transmission timing upon reception of a corresponding timing advance command;
The transmission timing of aperiodic sounding reference signal (SRS);
The CSI reference resource timing.
Figure 16.14.2-1 is an illustration of the transmission timing of DCI scheduled PUSCH, the slot allocated for the PUSCH can be modified to be n+K_2+K_offset. Note for this example the subcarrier spacing (SCS) value of the downlink is supposed to be the same as that of the uplink.
For initial access, the information of K_offset is carried in system information. Update of K_offset after initial access is supported. The UE-specific K_offset can be provided and updated by the network with MAC CE.
k_mac 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 action and assumption on downlink configuration indicated by a MAC-CE command in PDSCH. The K_mac 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 + k_mac + 4" within a corresponding RAR window.
If a UE is provided with a k_mac value, when the UE would transmit a PUCCH with HARQ-ACK information in uplink slot n corresponding to a PDSCH carrying a MAC CE command on a downlink configuration, the UE action and assumption on the downlink configuration shall be applied starting from the first slot that is after slot n+〖3N〗_slot^(subframe,μ)+k_mac, where μ is the SCS configuration for the PUCCH. MAC CE timing relationship enhancement with k_mac is illustrated in Figure 16.14.2-2.
Timing pre-compensation at the UE:
To accommodate the long propagation delays experienced in NTN on both service link and feeder link, the UE should be able to perform time pre-compensation for all its uplink transmissions; including PRACH preamble transmissions and uplink transmissions during the RRC_CONNECTED state. To do such pre-compensation, the UE is assisted by its GNSS and by the network which periodically broadcasts assistance information including serving satellite ephemeris as well as higher layer Common-TA-related parameters, where the latter may be used to calculate the common RTD e.g. delay on the feeder link.
Release-17 specified the following formula for TA calculation that shall be applied by NTN UEs for PRACH preamble transmission and in RRC_CONNECTED state:
T_"TA" =(N_"TA" +N_"TA,offset" +N_"TA,adj" ^"common" +N_"TA,adj" ^"UE" )×T_"c"
N_"TA" and N_"TA,offset" were already specified in TS 38.213TS 38.211 as part of the existing TA Control;
N_"TA,adj" ^"common" is network-controlled common TA, and may include any timing offset considered necessary by the network (e.g. feeder link delay). It is derived from the higher-layer parameters TACommon, TACommonDrift, and TACommonDriftVariation if configured, otherwise N_"TA,adj" ^"common" =0;
N_"TA,adj" ^"UE" is UE self-estimated TA to pre-compensate for the service link delay. It is computed by the UE based on UE position and serving satellite-ephemeris-related higher-layers parameters if configured, otherwise N_"TA,adj" ^"UE" =0;
Frequency pre-compensation at the UE:
The UE shall be capable of using its acquired GNSS position and serving satellite ephemeris information (when provided by the network) to calculate frequency pre-compensation to counter shift the instantaneous Doppler shift experienced on the service link.
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 as well as any transponder frequency error whether it is introduced in Downlink or Uplink is left to the network implementation without any specification impacts in Release 17.
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 (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 scheduling information of SIB19 in SIB1.
The NTN ephemeris is divided into serving cell's satellite ephemeris and neighbouring cell's satellite ephemeris.
At least in the quasi-earth fixed cell scenario, UE can perform time-based and location-based cell selection /reselection:
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 or neighbouring cells.
Location information may be used to assist cell reselection in NTN with for example a condition based on the distance between UE and the reference location of the serving cell and/or neighbour cells.
UE may support mobility between radio access technologies based on different orbit (GSO, NGSO at different altitude).
The same principle as described in clause 188.8.131.52 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.
DAPS handover is not supported for NTN in this release of the specification.
UE may support mobility between radio access technologies based on different orbit (GSO, NGSO at different altitude).
NTN supports the following additional triggering conditions upon which UE may execute CHO to a candidate cell, as defined in TS 38.331:
A time-based trigger condition;
A location-based trigger condition.
A time-based or a location-based trigger condition is always configured together with the measurement-based trigger conditions (CHO events A4). Location is defined by the distance between UE and a reference location. Time is defined by the time between T1 and T2, where T1 is an absolute time value and T2 is a duration started at T1.
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 using propagation delay difference, feeder link delay as well as serving/neighbour satellite cell ephemeris;
measurement gaps using the same propagation delay difference as computed for SMTC.
The adjustment of SMTCs is possible under network control for connected mode and under UE control based on UE location information and ephemeris for idle/inactive modes.
A feeder link switch over 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 switch over is a Transport Network Layer procedure.
Both hard and soft feeder link switch over are applicable to 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 only connect to 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 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.
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 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).