9.2.3.6 RACH-less handover

RACH-less handover is supported for an intra-gNB handover procedure in TN and NTN. RACH-less handover is also supported for an inter-gNB handover procedure in the case of NTN or mobile IAB-DU migration procedure. The RACH-less handover procedure applies the following functionality:

• The UE uses the same timing advance value (i.e., NTA value) at the target cell as in the source cell or timing advance value (i.e., NTA value) of 0.
• The handover command for the UE may contain a beam identifier for the beam to be used by the UE at the target cell. The beam may be determined based on a UE measurement report and/or left up to gNB implementation, e.g., using the target cell's knowledge about the beam(s) used by the UE at the co-located source cell.
• The handover command may include a configured UL grant. UE can fallback to RACH when there is no valid configured uplink grant. Alternatively, an UL grant is dynamically signalled by the target cell.
• The UE transmits the RRCReconfigurationComplete message using the configured or dynamically signalled UL grant. Successful UL data reception on the target cell terminates the RACH-less handover execution.