5.35A Support for Mobile Base Station Relay (MBSR)

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5. 35A.1 General

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 5.35.1 applies. 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 [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, it provides a mobile IAB-indication to the IAB-donor-CU when the RRC connection is established as defined in TS 38.331 [28]. When the mobile IAB-indication is received, the IAB-donor-CU selects an AMF that supports IAB-node with mobility and includes the mobile IAB-indication in the N2 INITIAL UE MESSAGE as defined in TS 38.413 [34] so that the AMF can perform mobile IAB authorization.

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 [42].

Editor’s note: Description of RRC indication and full migration procedure will be updated after coordination with RAN WGs.

5.35A.2 Configuration of the MBSR

Editor’s note: The description of the MBSR configuration will be added based on updated conclusion of clause 8 of TR 23.700-051 [147].

5.35A.3 Mobility support of UEs served by MBSR

5.35A.3.1 UE mobility between a fixed cell and MBSR cell

The procedure of Inter-gNB-DU Mobility as defined in TS 38.401 [42] or the handover procedure using the Xn/N2 reference points as defined in TS 23.502 [3] can be used.

For UEs in RRC_IDLE and RRC_INACTIVE state when a MBSR goes out-of-service, procedure for cell (re-)selection as specified in TS 38.304 [50] for RRC_IDLE and RRC_INACTIVE is used.

For UEs in RRC Connected state, if the MBSR goes out-of-service due to e.g. MBSR moves to an area where the MBSR is not allowed to provide the relay service, the procedure for IAB node release as specified in TS 38.401 [42] is used.

Editor’s note: Handling of UEs accessing emergency service while served by a MBSR, including when the MBSR becomes unable/not allowed to provide the relay service, is FFS.

5.35A.3.2 UE mobility between MBSR cells

Similar to the behaviours described in clause 5.35A.3.1, UEs use existing procedures defined in TS 38.401 [42], TS 23.502 [3], or TS 38.304 [50] to handle the mobility between MBSR cells.

5.35A.3.3 UE mobility when moving together with a MBSR cell

Editor’s note: The description of the support of enhancements for UE mobility when moving together with a MBSR will be added based on updated conclusion of clause 8.3 of TR 23.700-05 [147].

5.35A.4 MBSR authorization

For a MBSR, the subscription information stored in the HPLMN indicates whether it is authorized to operate as MBSR, and the corresponding location and time periods.

Editor’s note: The subscription information for the MBSR will be further specified.

When MBSR roaming is supported, a roaming agreement between VPLMN and HPLMN regarding MBSR operation is in place, and the 5GC can make use of it for authorization of MBSR in VPLMN. MBSR (IAB-DU) can use IAB-node integration procedure or inter-IAB-donor gNB mobility procedure to integrate into VPLMN to provide service.

The MBSR(IAB-UE) is assumed to be configured with preferred PLMN lists and forbidden PLMNs by the HPLMN for the MBSR operation.

Editor’s note: Configuration mechanism will be further described based on development of clause 5.35A.2.1.

When the MBSR (IAB-UE) performs initial registration with the serving PLMN, it indicates the request to operate as a MBSR as described in clause 5.35A.1. The AMF authorizes the MBSR based on the subscription information, and provides MBSR authorized indication to NG-RAN. The MBSR establishes the connection to OAM system using the configuration information for MBSR operation.

Editor’s note: It is FFS whether the MBSR indication is a new IE or part of UE 5G MM capabilities.

Editor’s note: Whether existing IAB-Operation allowed indication and IAB authorized indication can be reused for MBSR will be determined.

NOTE 1: How the MBSR obtains the configuration information for MBSR operation is described in clause 5.35A.2.1.

The AMF of the MBSR can indicate to the MBSR that it is not allowed to act as an IAB node as part of registration procedure, and in this case does not include MBSR authorization indication to donor-gNB.

Editor’s note: Which NAS message(s) (registration accept, registration reject or/and deregistration request) is used by the AMF of the MBSR for indicating the MBSR is not allowed to act as MBSR is FFS. Whether the NAS layer includes indication of the MBSR (IAB-UE) supports specific feature is FFS.

NOTE 2: The mechanism applies to both roaming and non-roaming MBSR operations.

5.35A.5 Location Service Support of UEs served by MBSR

When a UE accesses 5GS via a MBSR, it can use the location service as defined in TS 23.273 [87]. However, in order to provide accurate estimation of the UE location, LMF needs to take the location of the MBSR into account. Enhancements to the LCS framework for MBSR support is described in clause 5.9 of TS 23.273 [87].

5.35A.6 Providing cell ID/TAC of MBSR for services

Editor’s note: The description of how to provide the MBSR cell ID/TAC for certain services will be added based on RAN WG feedbacks and updated conclusion of clause 8.6 of TR 23.700-05 [147].

5.35A.7 Control of UE access to MBSR

CAG Identifier is used to control the access of UE via MBSR (i.e. mobile IAB-node) and existing CAG mechanism defined in clause 5.30.3 can be used for managing UE’s access to MBSR, with the following additional considerations:

– When the MBSR is allowed to operate as an IAB node for a PLMN, the MBSR is configured, either during the communication with the serving PLMN OAM or (pre-)configuration mechanism, with a CAG identifier which is unique within the scope of this PLMN. If the MBSR is (pre-)configured with the PLMN list in which the MBSR is allowed to operate as MBSR, the corresponding CAG Identifier per PLMN is also configured in the MBSR.

– NG-RAN and 5GC support the UE access control based on the CAG identifier associated with the MBSR cell and the allowed CAG identifiers for the UE that supports CAG functionality.

– For the UE that does not support CAG functionality, NG-RAN and 5GC are allowed to use not only CAG mechanism but also the other existing mechanism e.g. forbidden Tracking Area, to manage its access to MBSR.

NOTE 1: If CAG for both MBSR and fixed private network are supported in a PLMN, the CAG identifier value of the MBSR cell(s) need to be different from the CAG identifier(S) of fixed cell(s) and unique within the scope of a PLMN ID.

– Time duration restriction and location restriction information may be provided together with the CAG Identifier(s) for the MBSR(s) that the UE can access. The enhanced Allowed CAG list will be provided to UE and AMF for enforcement, to make sure that UE not accessing the MBSR cell outside of the time duration or location restriction area. For example, if the time when a certain CAG is allowed for a UE is up or UE is out of the geographic area, the CAG for the UE is revoked from the network.

Editor’s note: Further description on the efficient UE configuration approach based on time and location based information may be added.

NOTE 2: Control of the MBSR access to the serving network is based on normal mobility restriction management based on subscription data form MBSR (i.e. IAB-UE).

5.35.6 IAB operation involving EPC

When the IAB-donor gNB has connection to both EPC and 5GC, based on PLMN configuration, there are two possible operation modes:

– the IAB-node connects to a 5GC via the IAB-donor gNB, while the UEs served by the IAB-node connect to EPC with Dual Connectivity as defined in TS 37.340 [31]. In this operation mode, the IAB-donor gNB has connection to an eNB, and the 5GC is restricted for IAB-node access only; and

– the IAB-node connects to an EPC via the IAB-donor gNB with Dual Connectivity as defined in TS 37.340 [31], while the UEs served by the IAB-node connect to the 5GC. In this operation mode, the EPC is restricted for IAB-node access only.

To support the above operation modes, the IAB-UE shall be configured to select only a specific PLMN (as defined in TS 23.122 [17]) and whether it needs to connect to 5GC or EPC.

NOTE: For a particular PLMN, it is expected that only one of the modes would be deployed in a known region.