5.35.2 5G System enhancements to support IAB

In IAB operation, the IAB-UE interacts with the 5GC using procedures defined for UE. The IAB-node gNB-DU only interacts with the IAB-donor-CU and follows the CU/DU design defined in TS 38.401 [42].

For the IAB-UE operation, the existing UE authentication methods as defined in TS 33.501 [29] applies. Both USIM based methods and EAP based methods are allowed and NAI based SUPIs can be used.

The following aspects are enhanced to support the IAB operation in the Registration procedure as defined in clause 4.2.2.2 of TS 23.502 [3]:

• The IAB-node provides an IAB-indication to the IAB-donor-CU when the RRC connection is established as defined in TS 38.331 [28]. When the IAB-indication is received, the IAB-donor-CU selects an AMF that supports IAB and includes the IAB-indication in the N2 INITIAL UE MESSAGE as defined in TS 38.413 [34] so that the AMF can perform IAB authorization;
• the UE Subscription data as defined in clause 5.2.3 of TS 23.502 [3] is enhanced to include the authorization information for the IAB operation;
• Authorization procedure during the UE Registration procedure is enhanced to perform verification of IAB subscription information;
• If the IAB operation is not authorized and IAB-UE is not allowed to be registered, the AMF rejects the IAB-UE's registration or de-register the IAB-UE. The AMF initiates UE Context setup/modification procedure by providing IAB authorized indication with the value set to "not authorized" to the NG-RAN, if the IAB-UE is still allowed to be registered;
• If the IAB operation is authorized, UE Context setup/modification procedure is enhanced to provide IAB authorized indication with the value set to "authorized" to NG-RAN.

After registered to the 5G system, the IAB-node remains in CM-CONNECTED state if the IAB operation is authorized. In the case of radio link failure, the IAB-UE uses existing UE procedure to restore the connection with the network. The IAB-UE uses Deregistration Procedure as defined in clause 4.2.2.3 of TS 23.502 [3] to disconnect from the network. In the case of controlled IAB-node release as specified in clause 8.9.10 of TS 38.401 [42] (including the case when authorization state of the IAB-node is changed from authorized to non-authorized), after UE Context Modification message to NG-RAN with authorization indication as not authorized and after a certain period (e.g. based on the expiration of a timer configured on the AMF), the AMF may trigger the IAB-UE Deregistration.