Wireless Access Backhaul

Description

Wireless Access Backhaul (WAB) is a foundational architectural concept within 3GPP's Integrated Access and Backhaul (IAB) framework. It refers to the principle of utilizing the same New Radio (NR) air interface technology to serve two distinct functions: providing access links to end-user devices (UEs) and establishing wireless backhaul links between network nodes. This dual-use of the spectrum and radio resources is a key enabler for IAB, where an IAB node acts as a relay. The IAB node has a Mobile Termination (MT) function that connects it upstream to a parent node (another IAB node or an IAB donor) via a wireless backhaul link, and a Distributed Unit (DU) function that provides access links downstream to UEs or potentially further IAB nodes. The WAB concept is realized through sophisticated time-division multiplexing (TDM) of radio resources, where specific time slots or symbols are allocated for backhaul transmissions and others for access transmissions, preventing self-interference. This requires tight coordination and scheduling, managed by the IAB donor's Central Unit (CU), which uses modified F1 application protocol (F1-AP) messages over the RRC and RLC channels to configure the IAB node's MT and DU. The architecture is detailed in specifications like 38.300 and 38.401, while the specific control plane (38.413) and user plane (38.423) protocols ensure reliable data routing and topology management across the multi-hop wireless backhaul network. WAB is crucial for enabling rapid, cost-efficient deployment of dense 5G networks, especially in areas where laying fiber is impractical or prohibitively expensive.

Purpose & Motivation

The primary purpose of WAB is to solve the economic and logistical 'backhaul bottleneck' associated with network densification for 5G and beyond. Traditional small cell deployments require a dedicated, high-capacity wired (usually fiber) connection for backhaul, which is often the most costly and time-consuming part of the rollout, particularly in urban canyons, temporary event sites, or rural areas. WAB, as the enabling principle behind IAB, addresses this by making the backhaul wireless and self-using the deployed NR infrastructure. This eliminates the dependency on fiber availability at every node location. The motivation stems from the need for ultra-dense networks to meet the capacity and coverage demands of enhanced Mobile Broadband (eMBB) and support for high-frequency bands (mmWave) with limited propagation range. Historically, separate microwave links were used for wireless backhaul, but they operated on different spectrum and required distinct, often proprietary, equipment. WAB integrates backhaul seamlessly into the 3GPP NR standard, allowing for unified spectrum management, simplified network planning, and dynamic resource allocation between access and backhaul based on real-time demand, thereby future-proofing network deployments.