Integrated Access and Backhaul

Description

Integrated Access and Backhaul (IAB) is a key architectural innovation in 5G New Radio (NR) that allows a single radio node to simultaneously function as an access point for User Equipment (UE) and as a relay node for backhaul traffic. An IAB node consists of two logical components: a Mobile Termination (IAB-MT) and a Distributed Unit (IAB-DU). The IAB-MT acts as a UE towards its parent node (another IAB node or an IAB donor), establishing a wireless backhaul link. The IAB-DU functions as a gNB-DU towards its child nodes (which could be UEs or other downstream IAB nodes), providing the radio access interface. The IAB donor is a gNB that provides network access to the IAB nodes and contains a Central Unit (CU) that controls the entire IAB topology.

The operation relies on sophisticated resource partitioning and multiplexing. Time, frequency, and spatial resources are dynamically allocated between access and backhaul links to prevent self-interference and optimize capacity. This is managed by the IAB donor CU through F1 application protocol (F1-AP) over the backhaul RLC channels. Key protocols are adapted: the IAB-MT uses the NR Uu interface stack (SDAP, PDCP, RLC, MAC, PHY) to connect upstream, while the IAB-DU uses the NR F1 interface stack to connect to the donor CU, with the backhaul RLC channel serving as the transport network layer. Routing in the IAB network is topology-aware, with the donor CU managing BH RLC channel establishment and BAP (Backhaul Adaptation Protocol) routing for efficient hop-by-hop forwarding of user plane data.

Deployment scenarios are flexible, supporting multi-hop topologies (tree, mesh) to extend coverage deep indoors or into remote areas. The IAB node uses the same 5G NR spectrum and waveforms for both functions, ensuring a unified, spectrum-efficient air interface. Key technical challenges addressed include topology management, route selection, discovery and integration of new IAB nodes, and robust resource management to handle the compounded latency and capacity constraints of multi-hop wireless paths. IAB is a cornerstone for rapid, cost-effective 5G network densification.

Purpose & Motivation

IAB was created to solve the critical economic and logistical challenge of providing high-capacity fiber backhaul to every small cell in a dense 5G network. Deploying fiber to every streetlight, traffic light, or building facade for ultra-dense networks (UDN) is prohibitively expensive and time-consuming. IAB provides a wireless self-backhauling solution, allowing operators to deploy nodes rapidly where only power and a mounting location are available, using the already-licensed radio spectrum for connectivity.

Historically, microwave links were used for wireless backhaul, but they operated in dedicated, often expensive spectrum bands with separate equipment. IAB integrates this function directly into the 5G NR standard, using the same baseband and RF hardware for access and backhaul. This dramatically reduces deployment costs, site acquisition complexity, and time-to-market. It is particularly motivated by the need for enhanced Mobile Broadband (eMBB) and massive IoT deployments in urban canyons, stadiums, factories, and temporary event sites.

Furthermore, IAB enables flexible network topologies that can self-organize and self-heal. In a mesh configuration, it provides resilience against link failures by offering alternative paths. It addresses the limitations of previous relay technologies in LTE, which were often less integrated and inefficient. By being a native part of the 5G NR standard from Release 16 onwards, IAB ensures vendor interoperability, efficient spectrum use through dynamic resource sharing, and seamless integration with the 5G core network, making network densification economically viable for the 5G era.