Wireless Access Backhaul gNB

Description

WAB-gNB is a term introduced in 3GPP Release 19 to describe the gNB functionality of a Wireless Access Backhaul (WAB) node. A WAB-node is a relay node that provides both access and backhaul using wireless links. The WAB-gNB specifically handles the NR access interface towards User Equipment (UE), meaning it acts as a base station for UEs connected to it. This functionality is defined in TS 38.300, which specifies the overall NR architecture. The WAB-gNB is part of a larger WAB architecture that includes a backhaul gNB (BH-gNB) and a WAB-MT (Mobile Termination) function.

In the WAB architecture, the WAB-node connects to the core network via a wireless backhaul link to a donor gNB (the BH-gNB). The WAB-gNB provides the NR Uu interface to UEs, allowing them to access network services. The WAB-MT terminates the backhaul link towards the BH-gNB, using procedures similar to those of a UE. This dual functionality allows the WAB-node to act as both a relay and a base station. The WAB-gNB is responsible for radio resource management, scheduling, and other gNB functions for the UEs it serves. It communicates with the BH-gNB over the backhaul link, which may use the same NR technology (integrated access and backhaul, IAB) or a different technology.

The introduction of WAB-gNB in Rel-19 reflects the ongoing evolution of 5G towards more flexible and dense deployments. Wireless backhaul is particularly useful in scenarios where fiber is unavailable or too costly, such as in rural areas, temporary deployments, or dense urban environments. By integrating access and backhaul, the WAB-node can be deployed quickly and at lower cost. The WAB-gNB is a logical entity that can be implemented in various ways, including as part of a distributed unit (DU) or a centralized unit (CU) in a disaggregated RAN architecture.

From a protocol perspective, the WAB-gNB interacts with the BH-gNB via the F1 interface (if the WAB-node is a DU) or via other interfaces. The specifications TS 38.401, 38.413, and 38.455 define the architecture, NGAP, and F1AP aspects relevant to WAB. The WAB-gNB must support handover, mobility, and QoS management for UEs, just like a regular gNB. However, it also must manage the backhaul link, which may have different characteristics (e.g., higher latency, lower reliability) than a fiber backhaul. This requires enhancements to scheduling and resource management.

The WAB-gNB is a critical component for enabling self-backhauling and multi-hop relaying in 5G. It allows network operators to extend coverage and capacity without deploying additional fiber. As 5G evolves towards 6G, wireless backhaul will become even more important, and the WAB-gNB concept will likely be further refined. Understanding WAB-gNB is essential for engineers working on advanced RAN deployments and relay technologies.

Purpose & Motivation

The WAB-gNB exists to enable wireless backhaul in 5G networks, addressing the need for flexible and cost-effective deployment of small cells and relay nodes. Traditional cellular networks rely on fiber backhaul for base stations, which is expensive and time-consuming to deploy, especially in areas with difficult terrain or temporary events. Wireless backhaul allows a base station to connect to the core network via a wireless link, eliminating the need for fiber. However, early wireless backhaul solutions used separate technologies for access and backhaul, leading to inefficiencies and higher costs.

The WAB-gNB is part of an integrated access and backhaul (IAB) approach, where the same NR technology is used for both access and backhaul. This integration simplifies deployment and improves spectral efficiency. The WAB-gNB specifically handles the access link to UEs, while the WAB-MT handles the backhaul link. This separation of functions allows the WAB-node to act as a relay without requiring a full gNB implementation for backhaul. The purpose is to provide a standardized way for relay nodes to operate within the 5G NR framework, ensuring interoperability and enabling multi-vendor deployments.

By defining WAB-gNB in Rel-19, 3GPP provides a clear architectural component that can be used in various deployment scenarios, from fixed wireless access to mobile relays. This supports the broader goal of 5G to provide ubiquitous coverage and high capacity, even in challenging environments.