Wireless Access Backhaul Mobile Termination

Description

WAB-MT is a term introduced in 3GPP Release 19 to describe the function within a Wireless Access Backhaul (WAB) node that terminates the Uu interface towards the backhaul gNB (BH-gNB). It uses the procedures and behaviors specified for User Equipment (UE), effectively acting as a UE for the backhaul link. This function corresponds to the MWAB-UE function defined in TS 23.501. The WAB-MT is a critical component of the WAB architecture, enabling the WAB-node to connect to the network via a wireless backhaul link.

In the WAB architecture, a WAB-node serves dual roles: it provides access to UEs via the WAB-gNB function and connects to the core network via the WAB-MT function. The WAB-MT establishes a radio connection with the BH-gNB, which is a donor gNB that provides backhaul connectivity. This connection uses the same NR Uu interface as a regular UE, but with enhancements to support the backhaul requirements. The WAB-MT handles the protocols for the backhaul link, including radio resource control (RRC), packet data convergence protocol (PDCP), and others, as if it were a UE. However, it may have additional capabilities, such as support for higher data rates and lower latency, to meet backhaul demands.

The WAB-MT is defined in TS 38.401 and TS 38.455, which specify the NG-RAN architecture and F1AP protocol. The WAB-MT communicates with the BH-gNB over the Uu interface, and the BH-gNB forwards traffic to the core network. The WAB-MT may support multiple backhaul links for redundancy or load balancing. It also handles mobility procedures, such as handover between BH-gNBs, to maintain connectivity as the WAB-node moves or as network conditions change.

From a protocol stack perspective, the WAB-MT implements the UE side of the NR protocols. It includes the physical layer, MAC, RLC, PDCP, and RRC layers. The WAB-MT may also support dual connectivity or carrier aggregation to increase backhaul capacity. The BH-gNB treats the WAB-MT as a special UE, with additional context for backhaul management. The WAB-MT is responsible for reporting backhaul link quality, requesting resources, and managing the backhaul connection.

The WAB-MT is essential for enabling self-backhauling and multi-hop relaying in 5G. It allows a WAB-node to be deployed without a wired backhaul, making it suitable for temporary or hard-to-reach locations. As 5G evolves, the WAB-MT will continue to be refined to support higher capacities and more advanced features. Understanding WAB-MT is crucial for engineers working on relay and backhaul solutions.

Purpose & Motivation

The WAB-MT exists to provide a standardized way for a relay node to connect to the network via a wireless backhaul link. In traditional cellular networks, base stations are connected to the core network via fiber or microwave links, which are expensive and require careful planning. Wireless backhaul using the same radio technology as access (integrated access and backhaul) offers a more flexible and cost-effective alternative. The WAB-MT is the component that makes this possible by acting as a UE for the backhaul link.

Before the introduction of WAB-MT, relay nodes used proprietary or non-standardized backhaul mechanisms, which limited interoperability and increased complexity. By defining WAB-MT in 3GPP specifications, the industry can develop multi-vendor solutions that integrate seamlessly into 5G networks. The WAB-MT leverages the existing UE procedures, which are well-defined and widely implemented, reducing development effort and ensuring compatibility.

The WAB-MT also enables advanced features such as multi-hop relaying and dynamic backhaul reconfiguration. It allows the network to treat the backhaul link as a special UE connection, simplifying resource management and mobility. This is particularly important for 5G deployments in millimeter wave bands, where coverage is limited and relays are needed to extend range. The WAB-MT is thus a key enabler for the flexible and scalable 5G networks envisioned by 3GPP.