Mobile Integrated Access and Backhaul Distributed Unit

Description

Mobile IAB-DU refers to the Distributed Unit (DU) function of an Integrated Access and Backhaul (IAB) node that is mobile. IAB is a 3GPP technology introduced in Release 16 that allows a base station (called IAB-node) to wirelessly backhaul its traffic to the core network via a parent node, rather than requiring a fiber connection. The IAB-node consists of two logical parts: the IAB-DU, which serves UEs (access) and child IAB-nodes (backhaul), and the IAB-MT (Mobile Termination), which connects to the parent node for backhaul. In a mobile IAB scenario, the entire IAB-node (including its DU and MT) can move, such as being mounted on a vehicle (e.g., bus, train) or a drone. The Mobile IAB-DU is thus the DU function on such a mobile node, as defined in 3GPP TS 38.300.

The architecture of Mobile IAB-DU involves the DU providing NR access to UEs and backhaul to child nodes, while the MT maintains the wireless backhaul link to the parent node. The DU is responsible for the physical layer, MAC, RLC, and part of the PDCP layers, similar to a conventional gNB-DU. However, because the node is mobile, the DU must handle dynamic changes in the radio environment, including frequent handovers of the backhaul link and adjustments to timing and synchronization. The Mobile IAB-DU may also need to coordinate with the parent node for resource allocation and mobility management. The IAB-node's mobility is managed by the network, which may treat the mobile IAB-node as a UE for the purpose of backhaul link management.

Mobile IAB-DU is particularly useful for providing coverage in moving scenarios, such as public transportation (buses, trains), temporary events, or emergency situations. It allows the network to extend coverage and capacity dynamically without the need for fixed backhaul infrastructure. For example, a bus equipped with a mobile IAB-node can provide connectivity to passengers while maintaining a wireless backhaul link to a donor gNB. As the bus moves, the IAB-node's MT performs handovers to maintain the backhaul link, while the DU continues to serve the passengers' UEs. This requires seamless mobility and robust backhaul link management to avoid service interruptions.

Key technical challenges for Mobile IAB-DU include maintaining synchronization and timing alignment as the node moves, managing interference, and ensuring seamless handover of the backhaul link. The DU must also support the same features as a fixed IAB-DU, such as scheduling, power control, and beam management. In 3GPP Release 19, enhancements for mobile IAB were introduced, including improved mobility procedures and support for mobile IAB-nodes. The Mobile IAB-DU is defined in TS 38.174 (IAB) and TS 38.175 (mobile IAB), which specify the requirements and procedures for mobile IAB operation.

Overall, Mobile IAB-DU represents an evolution of IAB technology to support mobility, enabling flexible and cost-effective network deployment in dynamic environments. It leverages the same principles as fixed IAB but adds mobility management to handle the moving node. This is expected to play a role in 5G-Advanced and future networks, particularly for vehicular and drone-based communication.

Purpose & Motivation

Mobile IAB-DU was created to address the need for flexible, mobile network infrastructure that can provide coverage and capacity in scenarios where fixed backhaul is impractical or unavailable. Traditional IAB nodes are fixed, meaning they are installed at a specific location and provide coverage to a static area. However, there are many use cases where a mobile relay node is beneficial, such as providing connectivity to passengers on moving vehicles (buses, trains), supporting temporary events (sports games, festivals), or enabling emergency communications in disaster areas. In these scenarios, a mobile IAB-node can move with the users, ensuring continuous service.

The motivation for Mobile IAB-DU stems from the limitations of fixed IAB and the need for dynamic network deployment. Fixed IAB requires careful planning and installation, and it cannot adapt to changing traffic patterns or user mobility. Mobile IAB allows the network to follow the users, improving coverage and capacity where it is needed most. It also reduces the cost of deploying fiber backhaul to every location, as the backhaul link is wireless and can be maintained while moving.

Furthermore, Mobile IAB-DU enables new business models and services, such as providing connectivity on public transportation, which can be challenging due to the high mobility and the need for seamless handovers. By integrating the DU function into a mobile node, 3GPP provides a standardized way to support such deployments, ensuring interoperability and performance. This is particularly important for 5G-Advanced and beyond, where ubiquitous connectivity is expected.