Base Station Type 2-O (OTA-Only FR2 Base Station)

Description

BS type 2-O is a 3GPP-defined base station category introduced in Release 15 for NR. It designates an NR base station operating in Frequency Range 2 (FR2, 24250 MHz – 52600 MHz, i.e., millimeter wave) whose entire radio requirement set consists of Over-The-Air (OTA) requirements defined at the RIB (Radiated Interface Boundary). The '2' indicates FR2 operation, and the 'O' indicates OTA-only requirements.

FR2 operation brings unique challenges: very high path loss, small antenna elements, and highly integrated phased-array designs. At millimeter-wave frequencies, base stations are invariably built as active antenna systems with integrated arrays and no accessible conducted test ports. Consequently, all requirements for BS type 2-O are defined and measured at the RIB, in a radiated manner. There are no conducted requirements at TAB connectors, unlike the hybrid BS type 1-H.

The architecture of BS type 2-O typically involves large phased arrays with beamforming, integrated RF front-ends, and dense packaging. Because the wavelength is small, many elements can be packed into a compact aperture, enabling high-gain beams. Conformance testing must therefore be performed in OTA environments such as anechoic chambers or compact antenna test ranges, measuring parameters like EIRP, TRP, beam characteristics, and unwanted emissions. The RIB serves as the reference boundary for these measurements.

In the 3GPP base station taxonomy, BS type 2-O is the FR2 OTA-only category, complementing BS type 1-O (FR1 OTA-only) and BS type 1-H (FR1 hybrid). The classification determines the applicable requirements in TS 38.104, emission limits in TS 38.113, and test methods in TS 38.141. Notably, BS type 2-O is not listed in TS 37.114 in the provided specification set, reflecting that FR2 requirements are handled primarily in the NR-specific specifications.

BS type 2-O is central to 5G millimeter-wave deployments, including high-band spectrum for enhanced mobile broadband and fixed wireless access. Its definition reflects the reality that FR2 base stations are inherently integrated and must be characterized over the air. The specifications covering it include TS 38.104, 38.113, 38.141, 38.174, 38.817, and 38.820.

Purpose & Motivation

BS type 2-O was created to provide a standardized framework for millimeter-wave (FR2) base stations, which are inherently integrated and cannot be tested via conducted methods. At FR2 frequencies, antenna arrays are small and tightly integrated with RF circuitry, making external test ports impractical. A dedicated OTA-only category was necessary to define requirements and conformance testing for these designs.

The term solves the problem of specifying radio performance for FR2 base stations where the only meaningful interface is the radiated one. By defining all requirements at the RIB, 3GPP enabled consistent, rigorous testing of millimeter-wave base stations, which is critical for spectrum regulation, interoperability, and deployment confidence.

Historically, BS type 2-O emerged from 3GPP's Release 15 work on NR, which for the first time defined FR2 as a distinct frequency range with its own requirements. It builds on earlier AAS and OTA concepts but is tailored to the millimeter-wave regime. It addresses the limitations of conducted testing at FR2 and provides the basis for OTA test methodologies that are now standard practice for 5G mmWave.