8.12 IAB-node Integration Procedure

38.4013GPPArchitecture descriptionNG-RANRelease 17TS

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

8.12.1 Standalone IAB integration

A high-level flow chart for SA-based IAB integration is shown in the Figure 8.12.1-1:

Figure 8.12.1-1: The integration procedure for IAB-node in SA

Phase 1: IAB-MT setup. In this phase, the IAB-MT of the new IAB-node (e.g. IAB-node 2 in Figure 8.12.1-1) connects to the network in the same way as a UE, by performing RRC connection setup procedure with IAB-donor-CU, authentication with the core network, IAB-node 2-related context management, IAB-node 2’s access traffic-related radio bearer configuration at the RAN side (SRBs and optionally DRBs), and, optionally, OAM connectivity establishment by using the IAB-MT’s PDU session. The IAB-node can select the parent node for access based on an over-the-air indication from potential parent node IAB-DU (transmitted in SIB1). To indicate its IAB capability, the IAB-MT includes the IAB-node indication in RRCSetupComplete message, to assist the IAB-donor to select an AMF supporting IAB.

NOTE: The signalling flow for UE initial access procedure as shown in Figure 8.1-1/Figure 8.9.1-1 is used for the setup of the IAB-MT.

Phase 2-1: BH RLC channel establishment. During the bootstrapping procedure, one default BH RLC channel for non-UP traffic e.g. carrying F1-C traffic/non-F1 traffic to and from the IAB-node 2 in the integration phase, is established. This may require the setup of a new BH RLC channel or modification of an existing BH RLC channel between IAB-node 1 and IAB-donor-DU. The IAB-donor-CU may establish additional (non-default) BH RLC channels. This phase also includes configuring the BAP Address of the IAB-node 2 and default BAP Routing ID for the upstream direction.

NOTE: If the OAM connectivity is supported via backhaul IP layer by implementation, one or more BH RLC channels used for OAM traffic can also be established.

Phase 2-2: Routing update. In this phase, the BAP sublayer is updated to support routing between the new IAB-node 2 and the IAB-donor-DU. For the downstream direction, the IAB-donor-CU initiates F1AP procedure to configure the IAB-donor-DU with the mapping from IP header field(s) to the BAP Routing ID related to IAB-node 2. The routing tables are updated on all ancestor IAB-nodes (e.g. IAB-node 1 in Figure 8.12.1-1) and on the IAB-donor-DU, with routing entries for the new BAP Routing ID(s). This phase may also include the IP address allocation procedure for IAB-node 2. IAB-node 2 may request one or more IP addresses from the IAB-donor-CU via RRC. The IAB-donor-CU may send the IP address(es) to the IAB-node 2 via RRC. The IAB-donor-CU may obtain the IP address(es) from the IAB-donor-DU via F1-AP or by other means (e.g. OAM, DHCP). IP address allocation procedure may occur at any time after RRC connection has been established.

Phase 3: IAB-DU part setup. In this phase, the IAB-DU of IAB-node 2 is configured via OAM. The IAB-DU of IAB-node 2 initiates the TNL establishment, and F1 setup (as defined in clause 8.5) with the IAB-donor-CU using the allocated IP address(es). The IAB-donor-CU discovers collocation of IAB-MT and IAB-DU from the IAB-node’s BAP Address included in the F1 SETUP REQUEST message. After the F1 is set up, the IAB-node 2 can start serving the UEs.

NOTE: The IAB-DU can discover the IAB-donor-CU’s IP address in the same manner as a non-IAB gNB-DU.

NOTE: If the IAB-node establishes NR-DC before the establishment of F1-C connection, the MN decides whether the MN or the SN becomes the F1-terminating IAB-donor. In case it decides that the SN becomes the F1-terminating IAB-donor, it notifies the SN via Xn. The IAB-node can implicitly derive whether the MN or the SN is the F1-terminating IAB-donor, e.g., based on the entity which provides the default BAP configuration.

NOTE: For OAM-based IAB-donor selection, if the IAB-node establishes NR-DC before the establishment of F1-C connection, the IAB-node indicates the F1-terminating IAB-donor by signaling its IP address(es) to this IAB-donor via RRC signaling.

8.12.2 NSA IAB Integration procedure

The IAB integration procedure for NSA is shown in Figure 8.12.2-1.

Figure 8.12.2-1: Signalling flow for IAB integration procedure in NSA

Phase 1-1. IAB-MT part setup with E-UTRAN. In this phase, the IAB-MT part connects to the LTE network as a UE, by performing RRC connection setup procedure with an eNB, authentication with the EPC, IAB-node’s access traffic-related radio bearer configuration at the E-UTRAN side, and optionally, OAM connectivity establishment by using the IAB-MT’s PDN connection. The IAB-node can select the IAB-supporting eNB based on an over-the-air indication from eNB (transmitted in SIB1). To indicate its IAB capability, the IAB-MT includes the IAB-node indication in RRCConnectionSetupComplete message, to assist the eNB to select an MME supporting IAB. The eNB then configures the IAB-MT part with an NR measurement configuration in order to perform discovery, measurement and measurement reporting of candidate gNBs. To enable the eNB choose an en-gNB which supports IAB function, the IAB capability of neighbour gNBs can be pre-configured in the eNB (e.g. by OAM).

NOTE: Other ways to enable the eNB know the IAB capability of neighbour gNBs are not precluded.

Phase 1-2. SgNB addition. In this phase, the IAB-MT part connects to the parent node IAB-DU and IAB-donor-CU via the EN-DC SgNB Addition procedure. The procedure defined in clause 8.4.1 is reused. The eNB includes “IAB Node Indication” in SGNB ADDITION REQUEST message to inform the IAB-donor-CU that the request is for an IAB-node. In addition, SRB3 can be set up for the IAB-MT, to transmit RRC message between the IAB-MT and the IAB-donor-CU via the NR links directly.

Phase 2-1: BH RLC channel establishment. This phase is the same as Phase 2-1 in the standalone IAB integration procedure (refer to the Phase 2-1 in clause 8.12.1). This step may occur in Phase 1-2.

Phase 2-2: Routing update. This phase is the same as Phase 2-2 in the standalone IAB integration procedure (refer to the Phase 2-2 in clause 8.12.1), except that the IP traffic on the F1-C interface may be transmitted via the MeNB.

Phase 3. IAB-DU part setup. This phase is the same as Phase 3 in the standalone IAB integration procedure (refer to the Phase 3 in clause 8.12.1), except that the IP traffic on the F1-C interface may be transmitted via the MeNB.

The IAB-donor-CU decides to only configure LTE leg, or only to configure NR leg, or to configure both LTE leg and NR leg, to be used for F1-C traffic transfer. The configuration may be performed before IAB-DU part setup. IAB-donor-CU may also change the configuration after IAB-DU part setup. In case the configuration is not performed before IAB-DU part setup, the IAB node uses the NR leg as the default one. When both LTE leg and NR leg are configured, it is up to the implementation to select the leg for F1-C traffic transfer.