Integrated Access and Backhaul Distributed Unit Radio Frequency Bandwidth Edge

Description

The IAB-DU RF Bandwidth edge is a specific frequency point that defines the lower or upper boundary of the IAB-DU RF Bandwidth. As defined in 3GPP specifications 38.174 and 38.176, it is 'the frequency of one of the edges of the IAB-DU RF Bandwidth.' This parameter is crucial for RF requirement specifications, particularly for defining out-of-band emissions, spurious emissions, and adjacent channel leakage ratio (ACLR). The edges serve as reference points for separating the in-band domain, where the IAB-DU is intended to transmit or receive, from the out-of-band domain, where emissions must be limited to avoid interference with other systems.

In the context of IAB, the IAB-DU RF Bandwidth edge is used in conformance testing to verify that the IAB-DU meets regulatory and 3GPP requirements for unwanted emissions. For example, the 3GPP TS 38.174 specifies that the out-of-band emissions are measured from the edge of the RF bandwidth, and the edge frequency determines the start of the transition region. Similarly, the ACLR is defined as the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency, with the edges defining the boundaries of these channels. Without precise definition of the edges, these measurements would be ambiguous, leading to inconsistent testing and potential non-compliance.

The IAB-DU RF Bandwidth edge is also important for coexistence with other radio services. In multi-operator deployments, the edges of the IAB-DU RF Bandwidth must be aligned with the allocated spectrum blocks to minimize interference. The edges are typically determined by the network operator during deployment and must fall within the supported operating band of the IAB node. In carrier aggregation scenarios, the IAB-DU RF Bandwidth may encompass multiple carriers, and the edges are then the outermost frequencies of the aggregated bandwidth. This ensures that the entire aggregated spectrum is considered for emission limits.

From an implementation perspective, the IAB-DU RF Bandwidth edge is a parameter that the IAB node's RF front-end must be aware of to apply appropriate filtering and power control. The edges may be signaled to the IAB node via operations, administration, and maintenance (OAM) or configured during initial setup. The 3GPP specifications define the edges in relation to the operating band, and the IAB node must ensure that its transmissions do not exceed the specified emission limits at these edges. This is particularly challenging in mmWave frequencies, where wider bandwidths and higher path losses require careful design of the RF chain.

The parameter was introduced in Release 19 and carried forward to Release 20 without changes. It is part of a family of RF parameters for IAB nodes, including the IAB-MT RF Bandwidth edge, which together provide a complete framework for RF conformance and coexistence. Understanding the IAB-DU RF Bandwidth edge is essential for engineers involved in IAB deployment, RF planning, and regulatory compliance, as it directly impacts the ability to deploy IAB nodes in shared or adjacent spectrum.

Purpose & Motivation

The IAB-DU RF Bandwidth edge exists to provide a precise frequency reference for defining the boundaries of the IAB-DU's operating bandwidth. In RF engineering, the edges of a transmission bandwidth are critical for specifying emission limits, as they determine where the in-band domain ends and the out-of-band domain begins. Without a clear definition of these edges, it would be impossible to consistently test and enforce out-of-band emission requirements, leading to potential interference with adjacent channels and other radio services. The IAB-DU RF Bandwidth edge solves this problem by establishing a standardized definition that can be used across different vendors and deployments.

The parameter also addresses the unique challenges of IAB nodes, which combine base station and UE functionalities in a single node. The IAB-DU transmits to UEs and child IAB nodes, while the IAB-MT communicates with parent nodes. These two entities may operate on the same or adjacent frequencies, and their RF bandwidths must be carefully managed to avoid self-interference. By defining the edges of the IAB-DU RF Bandwidth, 3GPP enables the specification of guard bands and filtering requirements that prevent the IAB-DU from interfering with the IAB-MT and vice versa. This is essential for the reliable operation of IAB nodes, especially in deployments where spectrum is scarce.

Historically, the concept of bandwidth edges has been used in 3GPP specifications for base stations and UEs for many years. However, the introduction of IAB required adapting these concepts to the new node architecture. The IAB-DU RF Bandwidth edge was introduced in Release 19 as part of the effort to mature IAB specifications and provide detailed RF requirements. It builds on the existing framework for NR base stations (TS 38.104) but is tailored to the specific characteristics of IAB. By clearly defining the edges, 3GPP ensures that IAB equipment can be tested consistently and can coexist with other systems, facilitating global deployment.