Integrated Access and Backhaul Distributed Unit Radio Frequency Bandwidth

Description

The IAB-DU RF Bandwidth is a fundamental radio-frequency (RF) parameter defined in 3GPP specifications 38.174 and 38.176 for Integrated Access and Backhaul (IAB) nodes. An IAB node consists of two logical entities: the IAB-DU (Distributed Unit), which serves as a base station for user equipment (UE) and child IAB nodes, and the IAB-MT (Mobile Termination), which acts as a UE towards a parent IAB node or donor. The IAB-DU RF Bandwidth specifies the range of frequencies over which the IAB-DU can transmit and/or receive one or multiple carriers within a supported operating band. This parameter is essential for defining the spectral occupancy of the IAB-DU and ensuring compliance with regulatory and 3GPP RF requirements.

In the IAB architecture, the IAB-DU provides NR Uu access to UEs and also supports the backhaul link to child IAB nodes via the same radio interface. The RF bandwidth of the IAB-DU must be carefully managed to avoid interference with the IAB-MT and other network elements. The IAB-DU RF Bandwidth can encompass multiple component carriers (CCs) in carrier aggregation scenarios, and it may be configured to operate in different frequency ranges (FR1 or FR2). The definition explicitly allows for single or multiple carriers, reflecting the flexibility needed for IAB deployments where the DU may aggregate carriers to increase capacity or coverage.

The IAB-DU RF Bandwidth is characterized by its edges, known as IAB-DU RF Bandwidth edges, which are the frequencies at the lower and upper boundaries of the bandwidth. These edges are used in RF requirement definitions, such as out-of-band emissions and spurious emissions, to specify the transition between the operating band and the unwanted emissions domain. The bandwidth itself is typically defined by the network operator during deployment and must align with the supported operating bands of the IAB node, as specified in 3GPP TS 38.104 for NR base stations and adapted for IAB in TS 38.174 and TS 38.176.

From a conformance perspective, the IAB-DU RF Bandwidth determines the test configurations for RF performance, including transmitter power, unwanted emissions, and receiver sensitivity. It also impacts coexistence with other systems, as the bandwidth edges define the boundary for adjacent channel interference. In multi-carrier operation, the IAB-DU RF Bandwidth may span the aggregated bandwidth of all carriers, and the edges are then the outermost frequencies of the aggregated set. This is particularly important for IAB nodes that use wide bandwidths in mmWave frequencies to provide high-capacity backhaul.

The parameter is introduced in Release 19 and carried forward to Release 20, reflecting the ongoing standardization of IAB in 3GPP. It is part of a broader set of RF parameters for IAB nodes, including IAB-MT RF Bandwidth and their respective edges, which together ensure that both the access and backhaul parts of the IAB node meet the necessary RF requirements. Understanding the IAB-DU RF Bandwidth is crucial for engineers designing and deploying IAB networks, as it directly affects spectrum utilization, interference management, and overall network performance.

Purpose & Motivation

The IAB-DU RF Bandwidth parameter exists to provide a clear and standardized definition of the spectral operating range of the distributed unit within an IAB node. In traditional RAN deployments, base station RF bandwidth is well-defined, but IAB introduces a new node type that combines base station and UE functionalities. This dual role creates unique RF challenges, as the IAB-DU and IAB-MT may operate on the same or adjacent frequencies, potentially causing self-interference. By defining the IAB-DU RF Bandwidth, 3GPP enables consistent RF requirement specifications, ensuring that IAB nodes can coexist with each other and with other network elements without causing harmful interference.

The parameter also addresses the need for flexible spectrum usage in IAB deployments. IAB nodes are expected to be deployed in various scenarios, including dense urban environments and hard-to-reach areas, where spectrum may be fragmented or shared. The ability to define a bandwidth that encompasses single or multiple carriers allows operators to tailor the IAB-DU operation to the available spectrum, maximizing efficiency. Furthermore, the definition of bandwidth edges facilitates the specification of out-of-band emission limits, which are critical for regulatory compliance and coexistence with services in adjacent bands.

Historically, IAB was introduced in Release 16 as a key enabler for 5G millimeter wave and sub-6 GHz deployments, providing wireless backhaul without the need for fiber. However, early releases focused on basic functionality, and detailed RF parameters for IAB nodes were further refined in later releases. The introduction of IAB-DU RF Bandwidth in Release 19 represents a maturation of the standard, providing precise definitions needed for conformance testing and global deployment. It solves the problem of ambiguous RF boundaries in IAB nodes, which could lead to inconsistent testing and deployment challenges. By clearly defining the bandwidth and its edges, 3GPP ensures that IAB equipment from different vendors can interoperate and meet regulatory requirements.