Integrated Access and Backhaul Donor

Description

The IAB-donor is a fundamental component of the Integrated Access and Backhaul (IAB) architecture in 5G NR. It is an NG-RAN node that supports the IAB feature and provides connectivity to the core network (5GC or EPC) for IAB-nodes. In the IAB architecture, the IAB-donor hosts the Central Unit (CU) function of the CU/DU split defined in TS 38.401. The CU is responsible for the Radio Resource Control (RRC), Service Data Adaptation Protocol (SDAP), and Packet Data Convergence Protocol (PDCP) layers, while the Distributed Units (DUs) handle RLC, MAC, and PHY layers. The IAB-donor's CU controls the IAB-nodes' DUs and the UEs attached to them.

The IAB-donor connects to the core network via the NG interface (for 5GC) or S1 interface (for EPC). It also communicates with other NG-RAN nodes via the Xn interface. In the IAB context, the IAB-donor acts as a parent node for IAB-nodes, which can be either directly attached (one hop) or via multiple hops. The IAB-donor's CU manages the backhaul links by configuring the IAB-MT of child nodes and the IAB-DU of the donor itself. The donor also provides the necessary backhaul transport for user plane and control plane data between the core network and the IAB-nodes.

The IAB-donor can be deployed in different configurations: as a standalone gNB (SA mode) or as an en-gNB (NSA mode) in EN-DC. In Rel-16, the IAB-donor was defined as an NR Secondary RAN node in the context of EN-DC, supporting the CU function for IAB. In Rel-20, the definition was updated to a more general NG-RAN node, reflecting the evolution towards standalone and multi-connectivity scenarios. The IAB-donor may also support dual connectivity with other RAN nodes to enhance reliability and throughput.

Key functions of the IAB-donor include: terminating the backhaul links from IAB-nodes, managing the IAB topology (including admission control, route selection, and resource allocation), and providing the interface to the core network. The donor's CU also handles the F1 interface towards the IAB-DUs, which may be located in the donor itself or in child IAB-nodes. The F1 application protocol (F1AP) is used for control plane signaling, while the F1 user plane (F1-U) carries data. The IAB-donor may also support the Xn interface to other gNBs for mobility and coordination.

In summary, the IAB-donor is the anchor node of the IAB architecture, providing core network connectivity and managing the IAB topology. It hosts the CU function and ensures that IAB-nodes can operate as relay nodes, extending network coverage and capacity without additional fiber. The IAB-donor is essential for the deployment of IAB in 5G networks, enabling flexible and cost-effective backhaul solutions.

Purpose & Motivation

The IAB-donor exists to solve the problem of providing backhaul connectivity to IAB-nodes in a wireless manner. Traditional cellular networks rely on fiber backhaul for base stations, which is costly and time-consuming to deploy, especially in dense urban areas or hard-to-reach locations. IAB enables wireless backhaul by using the same NR radio interface for both access and backhaul, allowing IAB-nodes to relay traffic to a donor node that has core network connectivity. The IAB-donor is the anchor point that connects the IAB network to the core network, making this possible.

Historically, relay nodes in earlier generations (e.g., LTE relays) had limitations such as half-duplex operation, separate backhaul links, and limited scalability. IAB in 5G NR was designed to overcome these limitations by integrating access and backhaul in a unified framework, supporting multi-hop, multi-beam, and dynamic resource allocation. The IAB-donor centralizes the control and management of the IAB topology, simplifying operations and enabling efficient resource utilization. It also supports network slicing and QoS differentiation, ensuring that backhaul traffic is treated appropriately.

The motivation for the IAB-donor also includes the need for flexible deployment in millimeter wave and sub-6 GHz bands. By allowing wireless backhaul, operators can quickly extend coverage and capacity without the need for new fiber runs. The IAB-donor's CU/DU split architecture allows for centralized scheduling and interference management, which is critical for dense deployments. Furthermore, the IAB-donor can be integrated with other NG-RAN nodes, enabling seamless mobility and load balancing. Thus, the IAB-donor is a key enabler for cost-effective and scalable 5G deployments.