4.3.32 Support for Integrated access and backhaul (IAB)

23.4013GPPGeneral Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) accessRelease 18TS

4.3.32.1 IAB architecture and functional entities

Integrated access and backhaul (IAB) enables wireless in-band and out-of-band relaying of NR Uu access traffic via NR Uu backhaul links.

At high level, IAB has the following characteristics:

– IAB uses the CU/DU architecture defined in TS 38.401 [90], and the IAB operation via F1 (between IAB-donor and IAB-node) is invisible to the EPC.

– IAB performs relaying at layer-2, and therefore does not require a local S/P-GW;

– IAB supports multi-hop backhauling;

– IAB supports dynamic topology update, i.e. the IAB-node can change the parent node, e.g. another IAB-node, or the IAB-donor, during operation, for example in response to backhaul link failure or blockage.

Figure 4.3.32.1-1 shows the IAB reference architecture with two backhaul hops, when connected to EPC, where both the IAB-node and the UE connect to the EPC with Dual Connectivity as defined in TS 37.340 [85].

Figure 4.3.32.1-1: IAB architecture for EPS, with IAB-node and UE both connected to EPC

Each relay, referred to as IAB-node, consists of a gNB-DU function and a UE function (referred to as IAB-UE). The gNB-DU in the IAB-node is responsible for providing NR Uu access to UEs and child IAB-nodes. The corresponding gNB-CU function resides on the IAB-donor gNB (referred to as IAB-donor-CU), which controls IAB-node gNB-DU via the F1 interface. When a UE connects to EPC via an IAB-node, the gNB-DU of the IAB-node appears as a normal secondary gNB to the UE. When the IAB-UE of another IAB-node connects to EPC via the IAB-node, the gNB-DU of the IAB-node also appears as a secondary gNB to the IAB-UE.

The IAB-UE function reuses UE procedures to connect to:

– the gNB-DU on a parent IAB-node or IAB-donor for access and backhauling;

– the gNB-CU on the IAB-donor via RRC for control of the access and backhaul link;

– EPC, e.g. MME,

– OAM system via a PDN connection (based on implementation).

NOTE: The EPC, e.g. MME, may detect that a PDN connection for the IAB-UE is for the OAM system access, e.g. by checking the APN and/or configuration. It is up to the operator configuration to choose whether to use 1 or multiple EPS bearers for OAM traffic and the appropriate QoS parameters, e.g. using QCI=6 for software downloading, and QCI=80 or a pre-configured QCI for alarm or control traffic.

4.3.32.2 System enhancements to support IAB

In IAB operation, the IAB-UE interacts with the EPC 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 [90].

For the IAB-UE operation, the existing UE authentication methods as defined in TS 33.401 [41] apply. For EPC, only USIM based methods are allowed.

Editor’s note: Security aspect is being studied by SA WG3, and the above will be revised and aligned after the conclusion of the study.

The following aspects of EPS are enhanced to support the IAB operation:

– the Attach procedure as defined in clause 5.3.2 is enhanced to indicate IAB-node’s capability to the MME:

– the IAB-node provides an IAB-indication to the eNodeB when the IAB-node establishes the RRC connection. If the IAB-indication is received, the eNodeB selects an MME that supports IAB, and includes the IAB-indication in the INITIAL UE MESSAGE so that the MME can perform IAB authorization;

– the UE Subscription data as defined in clause 5.7.1 is enhanced to include the authorization information for the IAB operation;

– Authorization procedure during the UE attach procedure is enhanced to perform verification of IAB subscription information;

– UE Context setup/modification procedure is enhanced to provide IAB authorized indication to eNodeB and IAB-donor gNB.

After attached or registered, the IAB-node remains in ECM-CONNECTED state. 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 the Detach procedure defined in clause 5.3.8 to disconnect from the network.

4.3.32.3 Data handling and QoS support with IAB

Editor’s note: The protocol stack for the support of IAB EN-DC operation is still being discussed in RAN WGs. Reference to RAN specifications can be added later to reflect the protocol design.

4.3.32.4 Mobility support with IAB

For UEs, all existing NR intra-RAT mobility and dual-connectivity procedures are supported with IAB. During UE mobility or dual-connectivity procedures, backhaul channels including IAB layer may be updated. This update is carried out by the IAB-donor-CU and signalled to the IAB-nodes via RRC and/or F1-C. The update is transparent to the UE.

4.3.32.5 Charging support with IAB

IAB-donor has all the information regarding the UE and the IAB-node and corresponding mapping of the bearers. The PDN connection for the UE and IAB-node are separate from IAB-node onwards to the core network. Therefore, the existing charging mechanism as defined in clause 5.7A can be used to support IAB.