Integrated Access and Backhaul Node

Description

The IAB-node is a fundamental component of the Integrated Access and Backhaul (IAB) architecture introduced in 3GPP Release 16 for 5G NR. It acts as a relay node that simultaneously serves UEs (access) and connects to the network wirelessly (backhaul), using the same NR radio interface. This eliminates the need for a dedicated fiber backhaul to every base station, significantly reducing deployment costs and time, especially for dense urban or hard-to-reach areas.

Architecturally, an IAB-node comprises two logical functions: a UE function (IAB-UE) and a Distributed Unit (DU) function (IAB-DU). The IAB-UE connects to a parent node (either a donor or another IAB-node) via the NR Uu interface, while the IAB-DU serves child nodes and UEs. The IAB-node is managed by a Central Unit (CU) located at the IAB-donor, which controls the DU via the F1 interface. The CU/DU split allows centralized scheduling and mobility management, while the DU handles real-time functions.

The IAB-node supports both in-band and out-of-band relaying. In-band operation means the access and backhaul links share the same frequency band, requiring careful resource coordination to avoid interference. Out-of-band uses separate frequencies, simplifying interference management. The backhaul link can be established via NR Uu, and the IAB-node can be multi-hop, meaning it can connect through other IAB-nodes to reach the donor. This creates a tree topology rooted at the IAB-donor, which has a wired connection to the core network.

Key procedures for IAB-node operation include initial integration, where the IAB-UE performs cell search and random access to a parent node, followed by backhaul link setup and F1-AP establishment with the CU. The IAB-node also supports mobility (e.g., handover between parent nodes) and topology adaptation to maintain connectivity. Resource allocation for backhaul and access links is managed by the CU through centralized scheduling or distributed coordination, depending on the configuration.

The IAB-node is specified across multiple 3GPP specifications: TS 38.401 defines the overall architecture, TS 38.321 covers MAC procedures, TS 37.340 addresses multi-connectivity, and TS 23.501/23.401 detail core network aspects. The term has remained stable across Releases 16 to 20, with only minor editorial updates (e.g., reference number change in Rel-20). This consistency reflects the maturity of the IAB concept and its continued relevance in 5G-Advanced and beyond.

Purpose & Motivation

The IAB-node was created to address the challenge of providing ubiquitous high-capacity coverage in 5G networks without the prohibitive cost and logistical complexity of deploying fiber backhaul to every small cell or relay site. Traditional backhaul solutions, such as fiber or microwave, are expensive and time-consuming to install, especially in dense urban environments, historical sites, or rural areas. The IAB-node solves this by enabling wireless self-backhauling using the same NR spectrum, allowing rapid and flexible network densification.

Before IAB, relay nodes in LTE (e.g., Rel-10 relays) used separate backhaul links and were limited in flexibility and spectral efficiency. They often required dedicated resources and could not easily adapt to dynamic traffic conditions. The IAB architecture, with its CU/DU split and centralized control, allows for more efficient resource utilization, multi-hop topologies, and seamless integration with the 5G core. It also supports both in-band and out-of-band operation, giving operators deployment flexibility.

The IAB-node is particularly motivated by the need for cost-effective coverage extension in millimeter wave (mmWave) bands, where propagation is challenging and cell sizes are small. By using wireless backhaul, operators can deploy IAB-nodes on street furniture, buildings, or vehicles without trenching fiber. This accelerates 5G rollout and enables new use cases like temporary events, disaster recovery, and industrial IoT. The IAB-node thus represents a key enabler for scalable, high-performance 5G networks.