Prior to 5G, most tests for wireless devices have been performed using conducted test methods. This includes the testing of modem chipsets, radio frequency (RF) parameter tests, and full device functional and performance verification. Over-the-air (OTA) test methods have been used mostly for antenna performance testing, and device multiple input multiple output (MIMO) performance measurements. 5G millimeter-wave (mmWave) devices represent a disruptive shift for the wireless industry because OTA is the only viable approach for test methods for all radio test cases.

At mmWave frequencies, the higher path loss and the shorter wavelengths require directional antennas (gain) that are electrically steerable— typically phased arrays. Many 5G devices will also require multiple sets of mmWave antennas in addition to traditional LTE and frequency range 1 (FR1) monopole antennas. Since the mmWave antennas must be bonded directly to the RF front-end (RFFE) amplifiers, devices cannot be accessed and tested the way they are at lower frequencies. Radiated test methods are needed.

Traditional conducted RF test methods use a well-behaved, predictable transmission line between the measurement solution and the device under test (DUT)— typically a coaxial cable. OTA replaces this cable with an over-the-air link through which the DUT communicates directly to an antenna that is part of the test solution. To ensure a well-behaved RF environment (i.e. a predictable transmission line plus elimination of external interference), the OTA connection is best managed inside of an anechoic chamber.

A typical OTA measurement solution therefore includes the RF measurement equipment and a chamber. There are several fundamental components associated with the chamber:

  • The enclosure itself, with appropriate RF isolation and internal treatment to reduce internal reflections of signals to an absolute minimum
  • The measurement antenna or “probe” antenna that provides the primary RF measurement link to the DUT 
  • A positioner that can change the DUT’s orientation or position
  • Software to control the positioner and measurement equipment.

Engineers need to consider several factors when choosing the right setup for the required measurements— but first, a quick review of electromagnetic field parlance and rules of thumb.

How does wave propagation work?

Figure 1 Wave propagation

As the electromagnetic distance from an antenna increases, the electromagnetic field’s behavior and characteristics will vary. A simplified model above shows three regions of concern: reactive near-field (reactive NF), radiated near-field (radiated NF), and radiated far-field (radiated FF). When making OTA measurements, the distance between the DUT and the probe antenna must be considered given the characteristics of each region. For example, making measurements in NF requires transformation techniques to convert the NF results to FF equivalents. This requires phase recovery or control of the input phase to the DUT. In this figure, R is the radial distance from the antenna, D is the diameter of the smallest sphere that could surround the radiating antenna aperture, and lambda is the wavelength (Figure 1).

  • Reactive NF is the region that is the closest to the DUT antenna. Not only do non-propagating evanescent fields predominate in this region, but a probe antenna in this region will also react with the DUT antenna, effectively becoming part of the DUT radiating apparatus. This puts significant limits on the types of measurements that can be made.
  • Radiated NF is a region in which the probe antenna will no longer react with the DUT antenna, but the field behavior and phase front are less predictable and well-behaved. Measurements in this region also require access to phase recovery in both the transmit and receive paths for the compensation algorithm.
  • Radiated FF is an area in which the phase front can be estimated as approximating a plane. This area is ideal for the measurement of both phase and amplitude, but has the drawback of greater path loss and larger (and sometimes unwieldy) distances between the DUT and the probe antenna.

So, what are the key considerations for an engineer to define an OTA measurement setup?

Range length: Distance between the probe and the DUT

The range length must be optimized for stable and accurate measurement results. As stated above, if measuring in the FF is required, the range length is best kept to a distance greater than R = 2D2/λ.

Hence, the size of the chamber is directly impacted by both the wavelength (frequency) in question and the size of the device antenna. For example, the far field for a 5-cm antenna at 28 GHz is about 50 cm. It increases to 190 cm for a 10-cm module at the same frequency, and grows to over 4 m for a 15-cm device (Figure 2).

Figure 2 Range length

DUT: device characteristics in a mmWave OTA test setup

The DUT can range from being a radiating element to an entire device. In handsets, the DUT will create a “D” that includes the mechanical size of the antenna plus the coupling to the radiating elements. The 3rd Generation Partnership Project (3GPP) has defined three DUT antenna configurations including (Figure 3):

  • Configuration 1: the DUT has at the most one antenna panel with maximum aperture equal to or less than 5 cm active at any time.
  • Configuration 2: the DUT has more than one antenna panel with maximum aperture equal to or less than 5 cm each active at any time but without coherence, which means that they can be treated as independent panels
  • Configuration 3: the DUT has multiple antenna panels and there is phase/amplitude coherence between those panels, which means that they cannot be treated as independent panels and D has to enclose all of them.

Figure 3 DUT antenna configurations

Black box testing

Black box testing is a 3GPP mandated concept for device conformance testing in which engineers must treat the position and number of antennas as being unknown. The DUT is tested as a “black box” and the aperture of the antenna (D) must be assumed to be the same size as the entire DUT. Thus, the device configuration has an impact on the required range length for FF measurements (Figure 4).

Figure 4 Black box testing

Quiet zone

The quiet zone is the area where RF propagation is predictable and well-behaved. This is important for accuracy and repeatability, especially for the testing of RF parameters or when low amplitude and phase variations are needed. The quiet zone needs to be large enough to contain the key item being tested—whether that is an entire device, or just the antenna. The size of the device or antenna being tested determines the requirements for the size of the quiet zone. And, of course, the larger the quiet zone required, the larger the chamber needs to be (Figure 5).

Figure 5 Quiet zone

CATR: An alternative approach to DFF OTA testing

A compact antenna test range (CATR) is an indirect far field (IFF) OTA test method. A CATR uses a shaped reflector to perform a physical near-field-to-far-field transformation. This results in a shorter range length and a larger quiet zone, and thus reduces the size of a chamber depending on a given DUT size, aperture size, and frequency. Beams reflected from a parabolic mirror become collimated beams. This transformation from a spherical wave front to a planar wave front enables a large quiet zone with very little amplitude and phase ripple. The resulting shorter range length also means less path loss between the DUT and the probe, which allows for better measurement dynamic range and better signal to noise (SNR) (Figure 6).

Figure 6 Compact antenna test range (CATR)

5G means that mmWave OTA testing is becoming a much more mainstream requirement. The challenges of these types of measurements are new to much of the commercial wireless industry. It is important to partner with a mmWave and OTA test expert who is also involved in 3GPP specifications for early knowledge and influence of the requirements. Keysight has been shipping commercial mmWave test capability for decades and has established the world’s leading family of mmWave OTA test solutions.

For more information on 5G device challenges, click here.

Jessy Cavazos is part of Keysight's Industry Solutions Marketing team.

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