16.14.2.2 Timing Advance and Frequency Pre-compensation

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 in TS 38.213 [38]), 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 [52]).

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.

Figure 16.14.2.2-1: Void

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.