Integrated Access and Backhaul Mobile Termination Radio Frequency Bandwidth Edge

Description

The IAB-MT RF Bandwidth edge is a frequency parameter that specifies the lower or upper boundary of the IAB-MT RF Bandwidth, as defined in 3GPP specifications 38.174 and 38.176. The IAB-MT (Mobile Termination) is the component of an IAB node that acts as a UE towards a parent IAB node or donor, enabling wireless backhaul. The RF bandwidth of the IAB-MT is the range of frequencies over which it transmits and/or receives signals, and its edges are the frequencies at the boundaries of this range. The definition states that it is 'the frequency of one of the edges of the IAB-MT RF Bandwidth.' This parameter is essential for defining RF requirements such as out-of-band emissions, spurious emissions, and adjacent channel leakage ratio (ACLR) for the backhaul link.

In the IAB architecture, the IAB-MT communicates with the parent node using the same NR radio interface as UEs, but with additional capabilities for backhaul. The IAB-MT RF Bandwidth edge is used in conformance testing to ensure that the IAB-MT meets the necessary emission limits and does not interfere with other transmissions. For example, 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 relative to the edges of the assigned channel. Without precise definition of these edges, testing would be inconsistent, and IAB nodes might cause harmful interference.

The IAB-MT RF Bandwidth edge is also important for coexistence with the IAB-DU and other network elements. Since the IAB-MT and IAB-DU may operate on the same or adjacent frequencies, their RF bandwidths must be coordinated to avoid self-interference. The edges of the IAB-MT RF Bandwidth define the spectral range that the IAB-MT occupies, and the network must ensure that this range does not overlap with the IAB-DU's operating range in a way that causes interference. This is particularly challenging in in-band IAB deployments, where the access and backhaul links share the same frequency band. In such cases, the edges are used to define guard bands or to schedule transmissions in a time-division multiplexed manner.

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

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-DU RF Bandwidth edge, which together provide a complete framework for RF conformance and coexistence. Understanding the IAB-MT 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-MT RF Bandwidth edge exists to provide a precise frequency reference for defining the boundaries of the IAB-MT'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 for the backhaul link, leading to potential interference with adjacent channels and other radio services. The IAB-MT 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-MT communicates with parent nodes, while the IAB-DU serves UEs and child 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-MT RF Bandwidth, 3GPP enables the specification of guard bands and filtering requirements that prevent the IAB-MT from interfering with the IAB-DU 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 UEs and base stations for many years. However, the introduction of IAB required adapting these concepts to the new node architecture. The IAB-MT 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 UEs (TS 38.101) but is tailored to the specific characteristics of IAB-MT. By clearly defining the edges, 3GPP ensures that IAB equipment can be tested consistently and can coexist with other systems, facilitating global deployment.