Integrated Access and Backhaul Mobile Termination

Description

The IAB-MT (Integrated Access and Backhaul Mobile Termination) is a fundamental component of the 3GPP-defined IAB architecture introduced in Release 16. It represents the 'user equipment' side of an IAB node. Functionally, the IAB-MT is the logical entity that establishes and maintains a wireless connection to a parent node, which can be another IAB node or a donor gNB (gNB-DU). This connection forms the wireless backhaul link, carrying both control plane signaling and user plane data for the IAB node itself and for any downstream nodes or UEs it serves. The IAB-MT operates according to the standard NR Uu interface specifications, meaning it uses the same physical layer (as defined in 38.211), layer 2 protocols, and RRC procedures (as in 38.331) as a conventional 5G UE. This design choice maximizes reuse of existing UE functionality and ensures reliable, standardized connectivity for the backhaul.

Architecturally, an IAB node comprises two main functional parts: the IAB-MT and the IAB-DU (Distributed Unit). The IAB-MT is responsible for the wireless backhaul uplink, while the IAB-DU provides the access network functionality to serve end-user UEs or child IAB nodes via the NR Uu interface. The IAB-MT and IAB-DU within the same node are coordinated by an IAB-specific adaptation layer and are managed by a central IAB-donor CU (Centralized Unit). This split architecture allows for efficient resource allocation and topology management. The IAB-MT's operation is tightly integrated with the IAB-DU's scheduling to manage time-division multiplexing (TDM) or spatial-division multiplexing between backhaul and access links, preventing self-interference and optimizing spectral efficiency.

The role of the IAB-MT is critical for enabling self-backhauling networks. It allows an operator to deploy a network node where fiber is unavailable or too costly, by having the node 'wire itself in' to the network. The IAB-MT supports carrier aggregation, dual connectivity, and operation in FR1 and FR2 frequency ranges, as specified in the relevant RF performance specs (38.174, 38.176). Its performance directly impacts the capacity and latency of the multi-hop backhaul chain. Key procedures involving the IAB-MT include initial access and connection setup to a parent node, RRC reconfiguration for mobility (e.g., parent cell change), and the handling of backhaul adaptation layer (BAP) routing identifiers to correctly steer packets through the IAB topology.

Purpose & Motivation

IAB-MT was created to address the challenge of deploying dense 5G networks, especially in millimeter-wave (mmWave) bands where signal propagation is limited and fiber installation is prohibitively expensive for every cell site. Traditional networks require a fiber connection to each base station for backhaul, which is a major bottleneck for rapid and cost-effective deployment. The purpose of the IAB-MT is to turn a network node into a relay, allowing it to connect to the core network wirelessly through a multi-hop mesh of similar nodes. This enables operators to extend coverage and capacity into new areas—like urban canyons, indoor venues, or temporary event sites—by building upon an initial fiber-connected anchor point (the donor).

Historically, similar concepts existed in LTE (Relay Nodes), but they were not fully integrated into the RAN architecture. The 5G IAB architecture, with the IAB-MT as a core component, is a native, more flexible, and scalable solution. It solves the problem of network densification by decoupling the need for fiber from the need for radio coverage. The IAB-MT specifically solves the problem of how a relay node integrates into the network as a peer. By behaving as a standard UE for its upstream connection, it leverages all the existing mobility, security, and radio resource management mechanisms of 5G NR, ensuring robust and manageable backhaul links. This design was motivated by the industry's push towards more agile and deployable networks for 5G and beyond.