Content for  TS 38.300  Word version:  18.4.0

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 K_offset and K_mac:

• Common TA is a configured timing offset that is equal to the RTT between the RP and the NTN payload.
• K_offset is a configured scheduling offset that needs to be larger or equal to the sum of the service link RTT and the Common TA.
• K_mac is a configured offset (see TS 38.331) that is approximately equal to the RTT between the RP and the gNB.

The scheduling offset K_offset is used to allow the UE sufficient processing time between a downlink reception and an uplink transmission, see TS 38.213. The offset K_mac 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 K_mac 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, K_mac and TTA (see clause 16.14.2.2) are illustrated in Figure 16.14.2.1-1.

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 5.3.2.2 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 5.4.3.1 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 16.14.2.1-1 (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. 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.

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, k_mac) 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 is 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 is up to UE implementation whether to perform NTN neighbour cell measurements on a cell included in the measurement configuration, but without corresponding satellite information in measurement configuration or 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 5.2.4.2 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.