Integrated Access and Backhaul Type 2-O (FR2 OTA Requirements)

Description

IAB type 2-O is a 3GPP Rel-19 classification for Integrated Access and Backhaul (IAB) nodes operating in Frequency Range 2 (FR2, 24.25 GHz – 71 GHz). It specifies that both the IAB Mobile Termination (IAB-MT) and the IAB Distributed Unit (IAB-DU) are subject to a requirement set consisting only of Over-The-Air (OTA) requirements. These OTA requirements are defined at the respective Radiated Interface Boundary (RIB), which is the physical boundary where radiated measurements are performed. This classification is introduced in TS 38.174 and TS 38.175, which cover RF requirements for IAB nodes.

The IAB architecture enables wireless backhauling for NR cells, allowing dense deployment without a proportional increase in fiber infrastructure. An IAB node consists of two logical entities: the IAB-MT, which functions like a UE towards a parent node (IAB-donor or another IAB node), and the IAB-DU, which serves UEs and child IAB nodes. In type 2-O, both entities operate at FR2 and are tested exclusively via OTA methods. This means that all RF performance requirements—such as output power, unwanted emissions, and sensitivity—are verified at the RIB rather than at conducted antenna ports. The RIB is typically the far-field boundary of the radiated near-field, ensuring that measurements capture the true radiated characteristics of the integrated antenna array.

The shift to OTA-only requirements for IAB type 2-O is even more critical than for FR1 because at millimeter wave frequencies, conducted testing is extremely challenging due to high losses and the impracticality of providing test ports. Integrated antenna arrays are standard at FR2, and OTA testing is the only viable method to evaluate performance. OTA testing at the RIB allows verification of the composite radiated performance, including the effects of the antenna array, beamforming, and mutual coupling. This approach is consistent with the broader industry trend towards OTA testing for AAS base stations, as seen in TS 37.105 and TS 38.104.

Type 2-O is distinguished from other IAB types, such as type 1-O (FR1 OTA-only) and type 2-H (FR2 hybrid: conducted and OTA requirements). The '2' denotes FR2 operation, and the 'O' denotes OTA-only requirements. The respective RIB is defined in TS 38.174 for the IAB-DU and in TS 38.175 for the IAB-MT. These specifications detail the exact test setups, measurement distances, and uncertainty budgets required to ensure repeatable and reproducible OTA measurements. The requirements cover both the transmitter and receiver characteristics of the IAB-MT and IAB-DU, ensuring that the node meets regulatory and performance standards when deployed in a network.

In summary, IAB type 2-O is a critical classification for FR2 IAB nodes that are tested exclusively over the air. It simplifies testing for integrated antenna systems, ensures consistent performance evaluation, and supports the flexible deployment of IAB in 5G networks. As IAB technology matures, type 2-O will facilitate the certification and deployment of a wide range of FR2 IAB products.

Purpose & Motivation

The primary purpose of IAB type 2-O is to provide a clear regulatory and testing framework for FR2 IAB nodes that cannot be tested via conducted methods due to their integrated antenna design. At millimeter wave frequencies, conducted testing is impractical because of high losses and the lack of physical ports. OTA testing at the RIB solves this by evaluating the radiated performance directly, which is more representative of actual deployment conditions. This classification ensures that IAB nodes meet the necessary RF requirements without compromising on design flexibility.

Historically, base station RF requirements were defined at conducted ports, assuming separable antenna and radio units. However, with the advent of massive MIMO and AAS, the antenna became an integral part of the radio, making conducted testing impractical. 3GPP introduced OTA requirements for AAS base stations in earlier releases, but IAB nodes required specific consideration because they combine both UE-like (IAB-MT) and base-station-like (IAB-DU) functionalities in one unit. Type 2-O addresses this by defining OTA requirements for both entities at FR2, ensuring that the unique characteristics of IAB nodes are properly evaluated.

The motivation for type 2-O also stems from the need to streamline certification and reduce test complexity. By allowing OTA-only testing, manufacturers can avoid the cost and complexity of providing conducted test ports, which may not be feasible for compact integrated nodes. This accelerates product development and deployment. Furthermore, OTA testing at the RIB provides a more accurate assessment of spatial performance, including beamforming gains and sidelobe suppression, which are critical for IAB operation in dense networks. Thus, type 2-O enables efficient and reliable deployment of FR2 IAB nodes.