Section: 8.2.3.2 Intra-CU topology adaptation procedure in NSA using MCG SRB
Content: This procedure is used when the migrating IAB-MT moves from source parent node to target parent node within the same IAB-donor-CU while only MCG SRB is available for IAB-node during EN-DC operation. The target parent node may use a different IAB-donor-DU than the one used by the source parent node. The source path may have common nodes with the target path. Figure 8.2.3.2-1 shows the topology adaptation procedure using MCG SRB of IAB-MT in EN-DC, where the target parent node uses a different IAB-donor-DU than the source parent node. Figure 8.2.3.2-1: IAB intra-CU topology adaptation procedure using MCG SRB in EN-DC 1. The migrating IAB-MT sends an ULInformationTransferMRDC message to the MeNB 2. The MeNB sends RRC TRANSFER message to the IAB-donor-CU. 3. The IAB-donor-CU may send UE CONTEXT MODIFICATION REQUEST message to the source parent node IAB-DU, to query the latest SCG configuration. 4. The source parent node IAB-DU responds with a UE CONTEXT MODIFICATION RESPONSE message that includes full configuration information. 5. The IAB-donor-CU sends a UE CONTEXT SETUP REQUEST message to the target parent node IAB-DU, to create a UE context for migrating IAB-MT and set up one or more bearers. These bearers can be used by the migrating IAB-MT for its own signalling, and, optionally, data traffic. The UE CONTEXT SETUP REQUEST message includes CG-ConfigInfo. 6. The target parent node IAB-DU responds to the IAB-donor-CU with a UE CONTEXT SETUP RESPONSE message. 7. The IAB-donor-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source parent node IAB-DU, this message includes the Transmission Action Indicator IE, which instructs the source parent node IAB-DU to stop the data transmission to the migrating IAB-node. The source parent node IAB-DU also sends a Downlink Data Delivery Status frame to inform the IAB-donor-CU about the unsuccessfully transmitted downlink data to the migrating IAB-node. 8. The source parent node IAB-DU responds to the IAB-donor-CU with a UE CONTEXT MODIFICATION RESPONSE message. 9. The IAB-donor-CU sends an SGNB MODIFICATION REQUIRED message to the MeNB. 10/11. The MeNB initiated SgNB Modification procedure may be triggered by the SgNB initiated SgNB Modification procedure (e.g. to provide information such as data forwarding addresses, new SN security key, measurement gap, etc.). 12. The MeNB and the migrating IAB-MT perform RRC Connection Reconfiguration procedure. The RRCConnectionReconfiguration message includes information as described for the intra-CU topology adaptation procedure in SA in clause 8.2.3.1. 13. The MeNB sends an SGNB MODIFICATION CONFIRM message to the IAB-donor-CU. 14. The migrating IAB-MT performs Random Access procedure at the target parent node IAB-DU. 15-19. The remaining steps of the procedure follow the steps 11-15 of the intra-CU topology adaptation procedure in SA scenario, as defined in clause 8.2.3.1. The main difference is that the RRC message for the migrating IAB-node, if involved, will be transmitted using the MCG SRB.