LitePoint’s Khushboo Kalyani is the author of this three-part blog series. Throughout this series, you’ll learn why over-the-air (OTA) testing is crucial at millimeter wave (mmWave) frequencies, key concepts that influence OTA test chamber, measurement decisions and test methods as defined by 3GPP.

DFF vs. CATR OTA Chamber Test Methods as Defined by 3GPP

In my previous post, I talked about some of the key concepts involved in 5G over-the-air (OTA) test and measurement decisions and how each of these elements is critical in ensuring accurate and reliable 5G OTA measurements. Today I want to explore the permitted test methods for user equipment (UE) RF testing, including direct far field (DFF) and indirect far field (IFF), which is also referred to as compact antenna test range (CATR).

DFF Test Methodology

The direct far field (DFF) test method makes uses of a relatively simple OTA chamber design to perform antenna radiation pattern measurements. In this method, the device under test (DUT) is placed inside the anechoic chamber, such that the radiating antenna module on the DUT is in the direct line of sight (LOS) of the measurement horn antenna. 

To achieve comprehensive antenna pattern measurements, the DUT is generally mounted on a software-controlled positioner that can rotate in two independent axes (azimuth and elevation) to facilitate measurements over the entire 3D sphere.

As discussed in my previous blog, for accurate and repeatable measurements the DUT is positioned at a far field distance from the measurement horn (calculated using the Fraunhofer equation below) which, in effect, dictates and determines the overall dimension of the OTA chamber. 

‘R’ is the far field distance at which the spherical waves appear planar

‘D’ is the dimension of the antenna module

‘ƛ’ is the wavelength

As per 3GPP TR 38.810, this test methodology must be used when the location of the antenna module within the DUT is well known and the dimensions of the radiating antenna aperture is ≤ 5 cm. 

Challenges of DFF

Although the DFF test methodology is simple to implement, there are limitations associated with it. 

First and foremost is the higher OTA path loss for D> 5cm. As discussed already, the far field distance is directly proportional to the dimensions of the radiating antenna aperture. Thus, the larger the antenna aperture, the higher the OTA path loss owing to the increased far field distance.

Second is the higher capital equipment cost for D>5cm. An increase in the antenna aperture dimension would necessitate the use of a bigger chamber to provide the appropriate far field distance, thus increasing the overall cost and footprint of the equipment.

Third, is the tedious repositioning of the DUT in case of multiple mmWave antenna modules. To avoid measurement uncertainties, the antenna module on the DUT must be well aligned with the aperture of the measurement horn. Hence if the device is embedded with multiple antenna modules, it will need to be positioned differently to accurately characterize the performance of each mmWave antenna module.

Last, is the increased complexity and uncertainty when measuring a DUT with dimensions and location of the antenna module unknown. In such a scenario, determining the right size of the quiet zone with enough offset to accommodate for the entire device might be challenging. In addition, this would also lead to a larger chamber footprint and OTA path loss.

IFF Test Methodology

The indirect far field (IFF) test methodology is free of the limitations associated with the DFF test methodology. The technique allows large antenna arrays to be measured in a significantly shorter footprint than the DFF approach.

The test method is based on the compact antenna test range (CATR) that creates a far field environment using a parabolic reflector that helps collimate the spherical waves received from a feed antenna into planar waves to illuminate the aperture of the DUT.

In this methodology, the size and the termination of the reflector influence the operational frequency and accuracy of measurements – with edge configuration limiting the low frequency range and surface roughness affecting the upper frequencies. 

Unlike DFF, the far field is not the distance between the DUT and measurement horn but is the focal length; that is, the distance between the feed antenna and the parabolic reflector. It’s calculated using the following equation:

R = 3.5 × size of reflector = 3.5 × (2D)

As an example, for D = 5 cm, the CATR far-field distance, or focal length, is 3.5 × 2 × 5 = 35 cm, which allows for a more compact OTA chamber at the expense of a high-precision parabolic reflector.

This technique proves advantageous for making measurements on a device with unknown dimensions and location of the mmWave antenna module on the DUT. This is because the large quiet zone illuminates the entire form factor of the DUT eliminating the need for repositioning. Furthermore, for devices with D>=5cm, CATR creates a far field environment in a much shorter distance than the DFF chamber minimizing a significant increase in the OTA path loss and ensuring better signal to noise (SNR) ratio.

Although, the technique may sound easy to implement, the design of the chamber and precision of the parabolic reflector can be challenging to achieve.

Choosing Between a DFF or CATR Chamber

The choice between a DFF and CATR chamber depends on the DUT power class and antenna configuration and can significantly impact the OTA path loss and capital cost. As can be seen below, for a particular frequency the aperture dimension of 5cm is the inflection point beyond which the path loss observed in a DFF chamber is much larger than that seen in a CATR chamber.

This implies the DFF OTA chamber is more suitable for smaller handheld products, like a mobile phone or laptop, whereas a CATR chamber is appropriate for larger antenna arrays or DUT form factors.

Please visit our 5G page to learn more about LitePoint’s 5G OTA test and measurement solutions, and I invite you to view the full replay of my webinar on this topic.