Reference Beam Direction Pair

Description

The Reference Beam Direction Pair is a concept in 3GPP specifications that defines a pair of directions used as a reference for characterizing beamforming antennas. It consists of the reference beam centre direction and the reference beam peak direction. The reference beam centre direction represents the geometric center of the beam's main lobe, while the reference beam peak direction is the direction of the intended maximum EIRP within the OTA peak directions set. This pair provides a more complete description of a beam's orientation compared to a single direction, enabling more accurate testing and analysis of beamforming performance.

The concept was introduced in Rel-13 as an evolution of the earlier 'Reference beam direction' term. The need for a pair arose from the recognition that a beam's main lobe is not a single point but has a spatial extent. The centre direction gives an overall orientation, while the peak direction indicates where the maximum power is radiated. This distinction is important for OTA testing, where the measurement antenna must be aligned with the beam to capture the maximum EIRP. The OTA peak directions set is a defined set of directions over which the EIRP is measured and must meet certain requirements. The reference beam peak direction is the direction within this set where the intended maximum EIRP occurs.

In practice, the reference beam direction pair is declared by the equipment manufacturer and used in test setups. For example, in a test chamber, the device under test is positioned so that the reference beam centre direction aligns with the measurement antenna. The EIRP is then measured in the reference beam peak direction to verify compliance with limits. The pair is also used in beam management procedures, where the network and UE need to align their beams for optimal communication. By defining a reference pair, 3GPP ensures that both parties have a common understanding of the beam orientation.

The definition of reference beam direction pair has evolved over releases. In Rel-13, it was defined as 'declared beam direction pair, including reference beam centre direction and reference beam peak direction where the reference beam peak direction is the direction for the intended maximum EIRP within the EIRP accuracy directions set compliance.' In Rel-15, the wording was slightly rephrased to 'within the EIRP accuracy compliance directions set.' In Rel-16, the definition changed to refer to the 'OTA peak directions set' instead of the 'EIRP accuracy compliance directions set.' This change reflects the increasing emphasis on OTA testing in 5G NR, especially for millimeter-wave frequencies. The OTA peak directions set is a more specific concept that defines the directions over which the EIRP is measured in an OTA test environment.

The reference beam direction pair is used in a wide range of specifications, including TS 36.108, TS 37.105, TS 37.145, TS 37.842, TS 37.843, TS 38.104, TS 38.108, TS 38.141, TS 38.174, and TS 38.817. These specifications cover base station and UE radio transmission and reception, as well as OTA testing methodologies. The concept is essential for ensuring that beamforming devices comply with regulatory requirements and interoperate seamlessly. As beamforming technology advances, the reference beam direction pair will continue to be a fundamental tool for testing and certification.

Purpose & Motivation

The Reference Beam Direction Pair was created to provide a more precise and comprehensive reference for beamforming antenna characterization. While the earlier 'Reference beam direction' provided a single direction, it became clear that a beam's main lobe has both a centre and a peak, and that these may not coincide. The pair allows manufacturers and test labs to specify both the geometric center and the direction of maximum EIRP, leading to more accurate and repeatable measurements. This is particularly important for OTA testing, where the alignment of the measurement antenna with the beam is critical. The concept also supports beam management in live networks, where the network and UE must align their beams. By defining a reference pair, 3GPP provides a common language for describing beam orientation, facilitating interoperability and compliance. The evolution to the OTA peak directions set in Rel-16 reflects the growing importance of OTA testing in 5G NR, especially at millimeter-wave frequencies where conducted testing is impractical.