Backhaul Radio Link Control Channel

Description

The Backhaul (BH) RLC channel is a fundamental transport channel in 3GPP Integrated Access and Backhaul (IAB) networks, introduced in Release 16. In an IAB network, a node (IAB-node) can act as a relay, providing both access to User Equipment (UE) and backhaul connectivity to the core network via one or more parent nodes. The BH RLC channel is the logical channel established between two IAB nodes (or between an IAB node and an IAB donor) over the radio interface, specifically for the purpose of carrying backhaul packets. These packets may include user plane data (e.g., IP packets) and control plane signaling (e.g., RRC, F1AP, etc.) that need to be forwarded across multiple hops.

Architecturally, the BH RLC channel operates at Layer 2 of the radio protocol stack, utilizing the Radio Link Control (RLC) protocol. It is configured and managed by the controlling IAB donor, which is responsible for the overall topology and resource management. The BH RLC channel is mapped onto logical channels, which are then multiplexed onto transport channels and physical channels for transmission over the air. The RLC mode used (AM, UM, or TM) can be configured based on the required reliability and latency for the backhaul traffic. Typically, for control plane and high-reliability data, RLC AM is used, while for delay-sensitive traffic, RLC UM may be employed.

The BH RLC channel is identified by a BH RLC Channel ID, which is unique within the scope of the two nodes. This identifier is used in the adaptation layer (Backhaul Adaptation Protocol, BAP) to route packets correctly. The BAP layer, also introduced in Rel-16, sits above the RLC and is responsible for routing and bearer mapping. When a packet arrives at an IAB node, the BAP layer determines the next hop and the corresponding BH RLC channel based on the configured routing table and bearer mapping rules. The packet is then passed to the RLC entity associated with that BH RLC channel for transmission.

The configuration of BH RLC channels is performed via RRC signaling (as specified in 38.331) and F1AP signaling (38.473) between the IAB donor and IAB nodes. The donor can establish, modify, or release BH RLC channels as needed to adapt to changing traffic patterns or network topology. The BH RLC channel can be associated with specific QoS requirements, such as priority and packet delay budget, to ensure proper handling of different traffic types. This is particularly important for supporting differentiated services over the backhaul.

In summary, the BH RLC channel is a key enabler for IAB, allowing flexible and efficient wireless backhaul in 5G networks. It provides a standardized mechanism to transport backhaul packets over the air interface, supporting multi-hop topologies and dynamic reconfiguration. Its design ensures that backhaul traffic can be treated with appropriate QoS and reliability, making IAB a viable solution for dense deployments and coverage extension.

Purpose & Motivation

The BH RLC channel was created to address the need for wireless backhaul in 5G networks, particularly for Integrated Access and Backhaul (IAB). Traditional backhaul relied on fiber or dedicated microwave links, which are costly and time-consuming to deploy, especially in dense urban areas or hard-to-reach locations. IAB allows a network node to use the same spectrum for both access and backhaul, reducing deployment costs and increasing flexibility. The BH RLC channel is the transport conduit for backhaul packets over the radio interface, enabling multi-hop relaying without the need for a wired connection at every node.

Before Rel-16, there was no standardized way to carry backhaul traffic over the NR air interface in a multi-hop fashion. The BH RLC channel, along with the BAP layer, provides a complete solution for routing and transporting backhaul packets. It solves the problem of how to multiplex backhaul traffic with access traffic, how to ensure QoS for different backhaul flows, and how to dynamically configure and reconfigure the backhaul topology. This is essential for network operators to extend coverage and capacity in a cost-effective manner, especially for mmWave deployments where propagation is challenging.

Moreover, the BH RLC channel supports the separation of control plane and user plane backhaul traffic, allowing for independent scaling and optimization. It also enables the use of different RLC modes to match the reliability and latency requirements of various backhaul services. The introduction of the BH RLC channel in Rel-16 marked a significant step towards making IAB a practical and standardized technology, which continues to evolve in later releases with enhancements for mobile IAB, dual connectivity, and more efficient resource utilization.