IAB Distributed Unit

Description

The IAB Distributed Unit (IAB-DU) is one of the two key functional components of an Integrated Access and Backhaul (IAB) node, the other being the IAB-MT (Mobile Termination). The IAB-DU is responsible for the radio access network functionality towards its 'child' nodes. From the perspective of a User Equipment (UE) or a downstream IAB node's IAB-MT, the IAB-DU appears and operates identically to a standard gNB Distributed Unit in a 5G network. It terminates the radio interface layers up to the Radio Link Control (RLC) layer, handling physical layer processing, Medium Access Control (MAC) scheduling, and radio resource management for its served cell(s).

Architecturally, the IAB-DU is controlled by a Central Unit (CU) located in the IAB donor node. This control relationship is established over the F1 interface, specifically the F1-C (Control plane) and F1-U (User plane). However, unlike a traditional fronthaul link, this F1 connection is not carried over a direct fiber or microwave link. Instead, it is tunneled through the wireless backhaul path established by the IAB-MT component of the same IAB node and potentially other upstream IAB nodes. The F1-AP messages and user plane data are encapsulated and routed over Backhaul RLC channels, using the Backhaul Adaptation Protocol (BAP) for hop-by-hop forwarding.

The IAB-DU's operation is tightly coordinated with the IAB-MT to avoid self-interference and to optimize the overall capacity of the node. The IAB donor CU provides a resource configuration that dictates when the IAB-DU can transmit/receive to its children (access links) and when the IAB-MT must transmit/receive to its parent (backhaul link). This configuration defines a pattern of 'soft' cells, where the IAB-DU's cell is available only during specific time resources if TDM (Time Division Multiplexing) is used. The IAB-DU performs all standard gNB-DU functions like broadcasting system information, random access channel (RACH) handling, scheduling of uplink and downlink data, and HARQ processes, but within the constraints of the resources allocated to it by the donor CU.

Purpose & Motivation

The IAB-DU exists to provide the essential radio access point functionality within a wireless IAB node, enabling the creation of a multi-hop, self-backhauling network without requiring a separate, dedicated access radio unit. Its purpose is to decouple the radio access function from the need for a direct, wired connection to the core network. By integrating a standard-compliant gNB-DU into the IAB node, the network can present a normal 5G radio interface to end-user devices, ensuring full UE compatibility and leveraging all existing NR features for access.

This design solves the problem of how to make a relay node appear as a native part of the 5G RAN to the devices it serves. Previous relay solutions sometimes required specialized UE behavior or created non-transparent links. The IAB-DU, by conforming to the 3GPP-defined gNB-DU specification, ensures transparency and interoperability. The motivation stems from the need for cost-effective network densification; the IAB-DU allows an operator to drop a node that immediately starts serving traffic, using its wireless backhaul (via the co-located IAB-MT) to reach the network core.

Furthermore, separating the DU function from the CU (in the donor) allows for centralized coordination and efficient resource pooling across the entire IAB topology. The donor CU can dynamically adjust resources between the IAB-DU (for access) and the IAB-MT (for backhaul) of multiple nodes based on traffic demand, optimizing the capacity of the entire wireless mesh. The IAB-DU is thus a critical enabler for the flexible, rapid, and cost-efficient deployment of 5G small cells envisioned for urban environments and industrial IoT campuses.