Integrated Access and Backhaul (IAB) in 5G

Introduction

Building of new 5G networks, especially on new high-frequency mmWave bands, requires much denser cell sites than in lower-frequency bands typical of 4G networks, since the attenuation of the radio signal is much higher at high frequencies. Furthermore, mmWave deployments typically require almost line-of-sight (LOS) propagation since the high frequencies do not penetrate through buildings and other obstacles. Therefore, the initial 5G mmWave deployments will require new fibre or wireless backhaul solutions to meet the required smaller intersite distances. Here, wireless backhauling via IAB can provide a faster and more cost-efficient way for (initial) 5G deployments.

In 3GPP Release 10, relay operation was specified for LTE-Advanced but LTE relays were

not deployed widely. LTE relays were L3 relays where the relay node implemented the full 3GPP radio protocol stacks and only a single relay hop was supported. IAB uses L2 relaying where only the lower layers (PHY, MAC, RLC) of the radio protocol stack are used for backhauling. Furthermore, multi-hop relaying is supported by IAB.

IAB is specified in 3GPP Release 16 and is described in 3GPP TS 38.300 and 3GPP TS38.401

IAB Architecture

IAB supports wireless relaying for 5G. The relay node is called the IAB-node and it supports both access and backhaul using NR radio access. The network node

terminating the wireless backhauling on the network side is called the IAB-donor gNB.

• The IAB-donor gNB is a gNB with added functionality to support IAB. IAB wireless backhauling supports both single and multiple hops.

• The IAB architecture takes advantage of the split gNB architecture with the CU in the IAB-donor and the DU in the IAB-node.

• For the IAB-node, the F1 interface specified between gNB-CU and gNB-DU is extended over the wireless backhaul connecting the DU in the IAB-node (IAB-DU) and the CU in the IAB-donor (IAB-donor-CU).

• The core network interface (NG interface) terminates at the IAB-donor-CU. Thus, the IAB-node is a radio access network node with limited visibility to the core network.

The IAB-node also supports UE functionality, which is referred to as IAB-MT (Mobile Termination).  IAB-MT supports, for example, a physical layer, layer-2, RRC (Radio Resource Control) and NAS (Non- Access Stratum) functionality, and IAB-MT connects the IAB-node to IAB-DU of another IAB-node (multi-hop) or to an IAB-donor.

Deployment Scenarios and Use Cases

IAB supports relaying for both SA (i.e. NR only) and non-SA (NSA, i.e. LTE and NR dual

connectivity) deployments.  In the SA case, both access UEs and IAB-MTs

use NR only and both connect to the 5G core. For NSA, there are two variants, which are

both supported in Release 16:

• Access UEs are NSA UEs and connected to the EPC (Evolved Packet Core), whereas IAB-MTs are NR only (SA) and connected to the NGC (Next Generation Core).

• Both access UEs and IAB-MTs support NSA and both are connected to the EPC.

Due to mmWave blockage, the backhaul link may sometimes deteriorate and, therefore, IAB supports topology adaptation where the backhaul connection of an IAB-node can be switched from a parent to another parent.

IAB Protocol Stacks

The user plane protocol stack for IAB is shown below.

• Above shows L2 relaying, where backhauling uses NR PHY, MAC and RLC layers as well as a new adaptation layer called Backhaul Adaptation Protocol (BAP).

• F1-U between the IAB-DU and the IABdonor-CU is carried over IP

• The full F1-U stack (GTP-U/UDP/IP) is carried on top of the BAP layer on the wireless backhaul.

• The UE bearers terminate in the IAB-donor-CU (i.e. SDAP –and PDCP are between the access UE and the IAB-donor-CU).

Control-plane protocol stacks for IAB are shown below,

• A full F1-C stack (F1AP/SCTP/IP) is carried on the BAP layer over the wireless backhaul links.

• The IAB-MT connects first as a UE and therefore it establishes its own Signalling Radio Bearers (SRBs) for RRC and NAS signalling, as well as Data Radio Bearers (DRBs)

• These SRBs and DRBs are transported to the parent-node DU in the same way as normal UE traffic (i.e. there is no BAP layer over the first wireless hop). The parent node encapsulates these bearers into F1-U and F1-C to be carried over the following backhaul hops.

IAB-Node Integration

There are four major steps when a new IAB-node connects to an IAB-donor or another

IAB-node.

1.      An IAB-MT connects to the network as a normal UE and indicates to the network that it is part of an IAB-node. During the IAB-MT connection setup, SRBs are set up for RRC and NAS signalling and a DRB and PDU session for OAM. A new SIB1 parameter is introduced to tell the IAB-MTs which cells support IAB. Also, at least one backhaul RLC channel is set up during this phase.

2.      The IAB-donor-CU updates the routing information both downstream and upstream in all related IAB-nodes and IAB-donor-DU via F1AP signalling. The newly added IAB node is given a BAP address and IP address via RRC signalling, thus creating IP connectivity to the IAB-node.

3.      The IAB-DU performs the F1AP setup procedure. Before F1AP signalling can start, an SCTP and optionally IPsec are set up between the IAB-node and the IAB-donor-CU.

4. The IAB-DU is activated and the IAB-node starts serving UEs and other IAB-nodes.

IAB in 3GPP Release 17 and Beyond

In 3GPP, IAB work will continue in the coming releases. For IAB Release 17, enhancements in the following areas will be specified:

• Duplexing enhancements: Release 16 IAB only supports Time-Division Multiplexing (TDM) of radio resources between IAB-MT and IAB-DU. In Release 17, Space-Division Multiplexing (SDM) and Frequency-Division Multiplexing (FDM) support and full duplex capability will be added to PHY specs.

• Resource management enhancements to have full support for dual connectivity in PHY specs.

• Procedures for IAB-node partial migration between IAB-donor-CUs where only the  IAB-MT is migrated to another IAB-donor-CU (Release 16 limits this to single IABdonor-CU scenarios).

• Enhancements to reduce service interruptions due to IAB-node migration and BH RLF recovery (e.g. conditional handover for topology adaptation). Mobile IAB-nodes were considered for Release 17 but due to the high workload of 3GPP working groups, were postponed to later releases.

References

• 5G Backhaul and Fronthaul, First Edition. Edited by Esa Markus Metsälä and Juha T.T. Salmelin. © 2023 John Wiley & Sons Ltd. Published 2023 by John Wiley & Sons Ltd.

• 3GPP TR 38.874 V16.0.0 Study on Integrated Access and Backhaul;(Release 16)