mmWave 5G OTA Measurements Fundamentals

2020-08-27 | 6 min read

5G millimeter-wave (mmWave) devices bring over-the-air (OTA) challenges to design and test engineers across the device workflow. OTA is the only viable approach for all radio test cases at mmWave frequencies.

OTA replaces the cable of a connected test setup with an over-the-air link. The device under test (DUT) uses this link to communicate directly to an antenna in the test setup.

Using an anechoic chamber to manage the OTA connection helps ensure a well-behaved RF environment. The chamber includes the following key components:

Enclosure with appropriate RF isolation and internal treatment to reduce internal signal reflections
Measurement or “probe” antenna to provide the primary RF measurement link to the DUT
Positioner to change the DUT’s orientation or position
Software to control the positioner and measurement equipment

Electromagnetic field behavior and characteristics vary depending on the distance from the antenna. You need to consider the characteristics of the region in which you are making measurements carefully:

Reactive near-field (NF) is the closest region to the DUT antenna. Non-propagating evanescent fields predominate. The probe antenna reacts with the DUT antenna and becomes a part of the DUT radiating apparatus. This phenomenon puts significant limits on the types of measurements you can make.

Radiated NF is next. The probe antenna no longer reacts with the DUT antenna. The field behavior and phase front are less predictable and well-behaved. Making measurements require access to phase recovery in both the transmit and receive paths for the compensation algorithm.
Radiated far-field (FF) is the furthest region from the DUT antenna. The phase front approximates a plane. This area is ideal for both phase and amplitude measurements, but greater path loss and larger distances between the DUT and the probe antenna can be a challenge.

Figure 1. Wave propagation diagram

Defining an OTA measurement setup also requires considering several technical aspects including range length and DUT configuration.

Range length is the distance between the probe and your DUT. It must be optimized to enable stable and accurate measurements. If you are measuring in the far-field region, the range length needs to be greater than 𝑅=2𝐷²/λ. Using this formula has a direct impact on the size of the chamber. The far field at 28 GHz is about 50 cm for a 5-cm antenna, 190 cm for a 10-cm module, and over 4 m for a 15-cm device.

A DUT can range from being a radiating element to an entire device. In handsets, the DUT includes the mechanical size of the antenna and the coupling to the radiating elements. There are three DUT antenna configurations defined by the 3^rd Generation Partnership Project (3GPP):

Configuration 1 has at most one antenna panel with maximum aperture equal to or less than 5 cm active at any time.
Configuration 2 has more than one antenna panel with maximum aperture equal to or less than 5 cm each active at any time but without coherence so you can treat the panels independently.
Configuration 3 has multiple antenna panels and there is phase/amplitude coherence between them. In this case, you cannot treat the panels independently from the others, and the aperture of the antenna (D) has to enclose all of them.

Figure 2. 3GPP DUT configurations

In addition to range length and DUT configuration, you also need to keep the following concepts in mind:

Black box testing, a 3GPP-mandated concept for device conformance testing in which engineers must treat the position and number of antennas as unknown. You need to test the DUT as a “black box” and assume that the aperture of the antenna (D) is the same size as the entire DUT.
The quiet zone, the area where RF propagation is predictable and well-behaved. It needs to be large enough to contain the key item being tested. The size of the device or antenna being tested determines the size of the quiet zone. The larger the quiet zone required, the larger the chamber needs to be.
Compact antenna test range (CATR), an indirect far field (IFF) OTA test method that uses a shaped reflector to perform a physical near-field-to-far-field transformation. This transformation results in shorter range length and a larger quiet zone, reducing the size of the chamber depending on a given DUT size, aperture size, and frequency. The shorter range length also means less path loss between your DUT and the probe, allowing for better measurement dynamic range.

Figure 3. CATR concept

mmWave 5G is making OTA testing much more mainstream in the wireless communications industry. The challenges of these types of measurements are new to much of the commercial segment, but not to Keysight. For more information on Keysight 5G OTA chambers, click here.