Section: 8.23.3 Mobile IAB-DU migration procedure
Content: To support the mobile IAB-DU migration procedure, the mobile IAB-node concurrently supports two logical mobile IAB-DUs, which have F1 connections set up with the source F1-terminating IAB-donor-CU and target F1-terminating IAB-donor-CU, respectively. The mobile IAB-MT’s IAB-donor-CU may be same as either the source F1-termainting IAB-donor CU or the target F1-terminating IAB-donor-CU, or it may be different from both source and target F1-terminating IAB-donor-CUs. The UE(s) connected to the mobile IAB-node are handed over from the cell(s) of the source logical mobile IAB-DU that have F1 set up with the source F1-terminating IAB-donor-CU to the cell(s) of the target logical mobile IAB-DU that have F1 set up with the target F1-terminating IAB-donor-CU. After the UE(s) are handed over, the F1 between the source logical mobile IAB-DU’s and the source F1-terminating IAB-donor-CU may be removed. Figure 8.23.3-1 shows an example of the mobile IAB-DU migration procedure. In this example, the source and the target F1-terminating IAB-donor-CUs are different from the RRC-terminating IAB-donor-CU. Figure 8.23.3-1: Mobile IAB-DU inter-CU migration procedure 1. The source F1-terminating IAB-donor-CU may send an MIAB F1 SETUP TRIGGERING message to the source logical mobile IAB-DU to initialize the F1 Setup procedure towards the target F1-terminating IAB-donor-CU. The MIAB F1 SETUP TRIGGERING message includes the gNB ID of the target F1-terminating IAB-donor-CU and the information needed to establish the TNL connection with the target F1-terminating IAB-donor-CU for F1-C. NOTE: The mobile IAB-DU migration can also be triggered by the OAM. In this case, the OAM provides the mobile IAB-node with all information to initiate the F1 Setup procedure towards the target F1-terminating IAB-donor-CU, and step 1 is omitted. 2. The target logical mobile IAB-DU initiates TNL establishment and F1 setup (as defined in clause 8.5) with the target F1-terminating IAB-donor-CU. During the F1 Setup procedure, the target logical mobile IAB-DU includes the gNB ID of the RRC-terminating IAB-donor-CU, and the BAP address of the co-located mobile IAB-MT in the F1 SETUP REQUEST message. 3. The target F1-terminating IAB-donor-CU responds to the target logical mobile IAB-DU with an F1 SETUP RESPONSE message. After F1 setup with the target F1-terminating IAB-donor-CU, the target logical mobile IAB-DU can serve UEs via the target mobile IAB-DU’s activated cell(s). 4. By sending the MIAB F1 SETUP OUTCOME NOTIFICATION message, the source logical mobile IAB-DU informs the source F1-terminating IAB-donor-CU about the outcome of the F1 interface setup between the co-located target logical mobile IAB-DU and the target F1-terminating IAB-donor-CU. The source logical mobile IAB-DU may provide the source F1-terminating IAB-donor-CU with a mapping between activated cells of the source logical mobile IAB-DU and activated cells of the target logical mobile IAB-DU. If the mobile IAB-DU migration is triggered by the OAM, the gNB-ID of the target F1-terminating IAB-donor-CU is included in this message. 5. The source F1-terminating IAB-donor-CU hands over the UE from a source cell served by the source logical mobile IAB-DU to a target cell served by the target logical mobile IAB-DU. The target F1-terminating IAB-donor-CU initiates the IAB Transport Migration Management procedure towards the RRC-terminating IAB-donor-CU for offloading the UE’s traffic during this step. In case the IAB Transport Migration Management procedure is the first procedure for the mobile IAB-MT, it includes the mobile IAB-MT’s BAP address in the IAB TRANSPORT MIGRATION MANAGEMENT REQUEST message. After the completion of the UE handover, the source F1-terminating IAB-donor-CU requests the release of the UE’s traffic offloaded to the RRC-terminating IAB-donor-CU by initiating IAB Transport Migration Management procedure. NOTE 1: In step 5, the sequence of procedures for UE Handover and the IAB Transport Migration Management procedure initiated by the target F1-terminating IAB-donor-CU is up to implementation. NOTE 2: It is up to RRC-terminating IAB-donor-CU’s implementation to set up new backhaul resources or reuse the existing backhaul resources for the UE’s traffic. NOTE 3: How to perform the IAB Transport Migration Management/Modification procedures, and the IAB Resource Coordination procedure between the target F1-terminating IAB-donor-CU and the RRC-terminating IAB-donor-CU without Xn interface is up to implementation. 6. After all the UEs are handed over, the source F1-terminating IAB-donor-CU may initiate the removal of the F1 interface towards the source logical mobile IAB-DU.