Wireless Access Backhaul Node

Description

A WAB-node is a novel NG-RAN node introduced in 3GPP Release 19 to support Integrated Access and Backhaul (IAB) evolution, specifically the Wireless Access Backhaul (WAB) architecture. It comprises two logical entities: the WAB-MT (Mobile Termination), which acts as a UE-like endpoint towards a parent node, and the WAB-gNB, which provides NR access to UEs and potentially backhaul to child nodes. This dual functionality allows the node to simultaneously serve user traffic and relay backhaul traffic wirelessly, eliminating the need for fiber or other wired backhaul to every base station.

The WAB-node connects to the core network via a parent node (which could be a gNB or another WAB-node) using the NR Uu interface for the backhaul link. The WAB-MT handles the radio protocols for the backhaul connection, including PHY, MAC, RLC, PDCP, and RRC, similar to a UE. The WAB-gNB, on the other hand, manages the access link for UEs and the backhaul link for child nodes, performing functions such as scheduling, radio resource management, and mobility support. The node operates under the control of the parent node and the core network, with signaling over the NG interface (N2 for control plane, N3 for user plane) for the gNB part.

Architecturally, the WAB-node is part of the NG-RAN and communicates with the 5G Core (5GC) through the parent node's NG interface. The WAB-MT establishes a connection to the parent node's gNB, which then relays the WAB-node's NG signaling to the AMF and UPF. This creates a multi-hop topology where the WAB-node can extend coverage and capacity in a flexible manner. The WAB-node can also serve as a parent for other WAB-nodes, forming a tree or mesh topology. The 3GPP specifications 38.401 (NG-RAN architecture) and 38.413 (NGAP) define the procedures and interfaces for WAB-node operation, including setup, configuration, and mobility.

Key procedures for WAB-nodes include integration (initial access and backhaul establishment), backhaul link management (e.g., link adaptation, handover), and route management (path selection and optimization). The WAB-node supports both standalone (SA) and non-standalone (NSA) deployments, though Rel-19 focuses on SA. It also supports network slicing, QoS enforcement, and security mechanisms inherited from IAB. The WAB-node can be deployed in various scenarios, such as dense urban areas, indoor environments, and temporary events, where wired backhaul is impractical or costly.

In summary, the WAB-node is a key enabler for flexible and cost-effective 5G network densification, leveraging wireless backhaul to reduce deployment costs and improve coverage. Its introduction in Rel-19 builds on the IAB framework from Rel-16/17, enhancing capabilities for multi-hop and multi-connectivity scenarios.

Purpose & Motivation

The WAB-node was created to address the growing need for flexible and cost-efficient backhaul solutions in 5G networks, particularly in scenarios where deploying fiber or other wired backhaul is challenging or expensive. Traditional RAN deployments rely on wired backhaul for each base station, which limits scalability and increases deployment time and cost. The WAB-node enables wireless self-backhauling, allowing base stations to connect to the core network via wireless links, thus simplifying network rollout and reducing operational expenditure.

Prior to Rel-19, 3GPP introduced IAB in Rel-16 to provide wireless backhaul for NR. However, IAB had limitations in terms of multi-hop support, mobility, and integration with the core network. The WAB-node in Rel-19 represents an evolution of IAB, incorporating enhancements such as improved mobility, better resource efficiency, and support for more flexible topologies. It also aligns with the broader 5G vision of ultra-dense networks and ubiquitous coverage.

The WAB-node solves the problem of backhaul bottleneck in dense deployments by allowing dynamic and adaptive backhaul links. It also enables rapid deployment in emergency situations or temporary events where wired infrastructure is unavailable. By integrating the WAB-MT and WAB-gNB into a single node, it simplifies the architecture and reduces the number of network elements required.