Over-The-Air Peak Directions Set

Description

The OTA peak directions set is a fundamental concept in over-the-air (OTA) testing of advanced antenna systems (AAS) and mmWave base stations. It specifies the set(s) of beam peak directions, typically expressed in spherical coordinates (azimuth and elevation), within which certain transmitter (TX) OTA requirements—such as maximum EIRP, TRP, and beam pointing accuracy—must be satisfied. These sets are always subsets of the overall OTA coverage range, which represents the full angular extent over which the base station is designed to communicate. The primary purpose is to provide a well-defined and testable framework for verifying that a base station can meet its radiated performance targets when its beams are steered to specific directions. Without such a definition, OTA conformance testing would be ambiguous because beamforming systems can produce vastly different performance depending on the direction of transmission.

The concept emerged with the commercialization of 5G NR in Release 15, where large antenna arrays and beamforming became essential, especially in frequency range 2 (FR2) from 24.25 GHz to 52.6 GHz. In these bands, traditional conducted testing is impractical due to the integrated nature of antenna arrays; OTA testing became mandatory. The OTA peak directions set defines the specific directions where the peak EIRP and other TX requirements are evaluated. Typically, for a base station with a planar array, the peak directions might include broadside (boresight) and a range of scan angles. The set is declared by the equipment manufacturer and approved through test specifications like 38.104 (base station radio transmission and reception), 38.141-1/2 (conducted and radiated conformance testing), and 37.145-1/2 (active antenna system base station radiated testing).

Technically, the OTA peak directions set is defined as a collection of directions, each specified as a pair of angles (θ, φ) in the spherical coordinate system. For each direction in the set, the base station must meet TX requirements such as maximum output power, power spectral density, unwanted emissions, and modulation quality. The set is bounded by the OTA coverage range, which is the full set of directions the base station supports for transmission and reception. The peak directions set is usually a subset because certain requirements are only guaranteed when the beam is steered to directions where the array gain is near its maximum. As the beam is steered away from broadside, the effective isotropic radiated power (EIRP) decreases due to array factor roll-off and element pattern, so the peak directions set captures the directions where the system still meets the stringent peak EIRP requirement. While the peak directions set focuses on TX, it has a counterpart on the receiver side (OTA REFSENS RoAoA) that defines the directions where reference sensitivity is met.

The evolution of the definition across releases has been minimal, with only editorial and formatting changes. This stability indicates that the fundamental concept has been well-understood and consistently applied since Release 15. The set is used in test campaigns to reduce test time: instead of testing all possible directions, the test lab selects the declared peak directions set and verifies compliance at those specific angles. This approach balances test coverage with efficiency. The OTA peak directions set is particularly important for mmWave base stations, where beamforming is mandatory and conducted testing is impossible. It also applies to some sub-6 GHz massive MIMO systems that are tested OTA.

In summary, the OTA peak directions set is a key enabler for OTA conformance testing of 5G NR base stations, ensuring that beamformed transmissions meet regulatory and performance requirements in the directions where they are intended to operate. It is defined in multiple 3GPP specifications, including 38.104, 38.141, and 37.145, and its consistent definition across releases underscores its importance and maturity.

Purpose & Motivation

The OTA peak directions set exists to solve the problem of testing beamforming base stations in a repeatable and standardized manner. Traditional conducted testing, where signals are measured at an antenna connector, is not feasible for AAS and mmWave base stations because the antenna array is integrated with the radio and cannot be disconnected. OTA testing measures the radiated performance, but without a defined set of directions, it would be unclear where to point the test antenna and what performance to expect. The peak directions set provides a common understanding between manufacturers, test labs, and regulators about the specific angular regions where the base station must meet peak EIRP and other TX requirements. It thus enables consistent certification and market access.

Historically, before 5G, base station testing relied on conducted measurements at the antenna port, and OTA was used only for small cells or specific cases. With the advent of 5G NR in Release 15, especially in FR2, OTA became the primary method for base station conformance. The OTA peak directions set was introduced as a foundational concept to define the testable directions for TX requirements. It addresses the limitations of previous approaches by providing a flexible yet rigorous framework: the set can be small (e.g., only broadside) or large (e.g., covering a wide scan range), depending on the manufacturer's declaration and the base station's capabilities. This flexibility allows a trade-off between test complexity and performance guarantees. The concept has been adopted in test specifications for both base station (BS) and user equipment (UE) OTA testing, though the UE definitions may vary slightly. Overall, it ensures that 5G beamforming systems are evaluated fairly and that their peak radiated performance is verifiable.