Section: 5.35A Support for Mobile Base Station Relay (MBSR) 5.35A.1 General
Content: The MBSR uses the IAB architecture as defined in clause 5.35, and operates as an IAB node (with an IAB-UE and gNB-DU) with mobility when integrated with the serving PLMN. The architecture described in clause 5.35 applies unless specific handling is specified in clause 5.35A. Additionally, the following limitations apply to the MBSR: - the MBSR has a single hop to the IAB-donor node; - NR Uu is used for the radio link between a MBSR and served UEs, and between MBSR and IAB-donor node. Regulatory requirements (e.g. emergency services, priority services) are supported when UEs access 5GS via a MBSR. LCS framework as defined in TS 23.273[ 5G System (5GS) Location Services (LCS); Stage 2 ]  [87] is used for providing the location service to the served UEs, with additional enhancements described in clause 5.35A.5. Roaming of the MBSR is supported, i.e. a MBSR can integrated with a VPLMN's IAB-donor node. The corresponding enhancements to support MBSR roaming are described in clause 5.35A.4. CAG mechanism as defined in clause 5.30 can be used for the control of UE's access to the MBSR. Optional enhancements to the CAG mechanism for MBSR use are described in clause 5.35A.6. For a MBSR node to operate as a MBSR, it provides a mobile IAB-indication to the IAB-donor-CU when the RRC connection is established as defined in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [28]. When the mobile IAB-indication is received, the IAB-donor-CU selects an AMF that supports mobile IAB-node and includes the mobile IAB-indication in the N2 INITIAL UE MESSAGE as defined in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [34] so that the AMF can perform MBSR authorization as described in clause 5.35A.4. If the MBSR node is not authorized, e.g. due to the MBSR authorization indication from AMF, it also provides the mobile IAB-indication when establishing new RRC connection so that the AMF supporting mobile IAB-node will be selected by the IAB-donor-CU, to ensure that the operation related to MBSR authorization status change for a registered MBSR node can be performed as described in clause 5.35A.4. If the MBSR receives MBSR authorized indication from AMF, it provides the information about the authorization result to its IAB-DU component based on non-standardized interface as described in clause 5.35A.4. After the IAB-UE performs registration procedure in 5GS, further mobility procedure can be performed to change the IAB-donor-DU, the IAB-donor-CU as specified in TS 38.401[ NG-RAN; Architecture description ]  [42]. The mobility support of UEs served by the MBSR is specified in clause 5.35A.3