Directional Requirement

Description

A directional requirement is a type of radio performance requirement defined in 3GPP specifications for Over-The-Air (OTA) testing of base stations and user equipment. Unlike traditional conducted requirements, which are measured at antenna ports, directional requirements are specified for a particular direction within the OTA coverage range. This is particularly relevant for advanced antenna systems (AAS) and millimeter-wave (FR2) deployments, where beamforming and spatial filtering are used. The requirement applies when the angle of arrival (AoA) of the incident wave of a received signal falls within a specified reference angle range, such as the FR1 OTA REFSENS RoAoA (Reference Sensitivity Range of Angles of Arrival) or FR2 OTA REFSENS RoAoA, or the minSENS RoAoA for the receiver.

The concept of directional requirements emerged with the introduction of 5G NR and the need to test devices and base stations that use active antenna arrays. In such systems, performance can vary significantly with direction due to beamforming gains and nulls. Therefore, 3GPP defined OTA test methodologies and corresponding requirements that account for spatial characteristics. The directional requirement ensures that the equipment meets performance criteria when the signal arrives from a specific direction, which is critical for evaluating beamforming capabilities and spatial multiplexing.

The specifications that define directional requirements include TS 38.104 (BS radio transmission and reception), TS 38.108 (satellite access node radio transmission and reception), TS 38.141 (BS conformance testing), and TS 38.174 (integrated access and backhaul). These documents specify the test procedures, reference angles, and limits for various parameters such as reference sensitivity, blocking, and adjacent channel selectivity. The requirements are typically expressed as a set of directions or a range of angles (RoAoA) within which the equipment must meet the specified performance. The RoAoA is defined as the range of angles of arrival for which the requirement is applicable, and it may differ for different frequency ranges (FR1 and FR2).

Directional requirements are essential for ensuring that 5G equipment can operate effectively in real-world scenarios where signals arrive from various directions. They complement other OTA requirements, such as isotropic requirements, which apply uniformly in all directions. The distinction between directional and isotropic requirements allows for more accurate and realistic testing of beamforming systems. As 5G evolves towards higher frequencies and more advanced antenna systems, directional requirements will continue to play a crucial role in standardizing performance and ensuring interoperability.

Purpose & Motivation

Directional requirements exist to address the unique challenges of testing and specifying performance for radio equipment that employs beamforming and advanced antenna systems. In traditional cellular systems, conducted testing at antenna ports was sufficient because antennas were passive and performance was largely direction-independent. However, with the advent of 5G NR and the use of active antenna arrays, especially in millimeter-wave bands, the performance of a device or base station can vary dramatically with direction. This necessitated the development of OTA testing and directional requirements to ensure that equipment meets performance criteria in specific directions.

The motivation for directional requirements came from the need to accurately characterize beamforming gains and spatial filtering. In a beamforming system, the effective isotropic radiated power (EIRP) and sensitivity depend on the direction of the beam. Therefore, a single conducted measurement cannot capture the overall performance. Directional requirements allow operators and vendors to verify that the equipment can achieve the required performance when the signal arrives from a direction within the intended coverage area. This is particularly important for ensuring reliable connectivity and mobility in 5G networks.

Historically, the lack of directional requirements would have led to either overly conservative designs or inadequate performance in certain directions. The 3GPP specifications introduced directional requirements in Rel-15, aligning with the first release of 5G NR. These requirements are defined in conjunction with OTA test methods, which specify how to measure performance in an anechoic chamber or using near-field to far-field transformations. The concept continues to evolve with new releases, addressing higher frequency bands and more complex antenna systems. Directional requirements are a key enabler for the successful deployment of 5G and beyond, ensuring that beamforming technology delivers its promised benefits.