With 5G networks expected to hit the market in 2019/2020, it is important for manufacturers to understand the methodology involved with radio frequency (RF) testing for 5G mmWave Over The Air (OTA) standards for mobile devices.

This will allow the development of mobile devices that are acceptable onto the expected network parameters.

5G is the next evolution in mobile wireless technology, moving beyond the current 4G mobile internet and allowing a world of the internet of things (IoT). It is expected that, in addition to greater speed, the new wireless networks will allow communication between a wide range of smart devices, creating a massive IoT ecosystem.

Manufacturers need to be certain that their products are compatible with the requirements for target frequencies above 24 GHz and the RF requirements as specified in the OTA standard, and these need to use the applicable metrics (EIRP: Effective Isotropic Radiated Power, TRP: Total Radiated Power, and EIS: Effective Isotropic Sensitivity).

## RF Testing Methodology

For RF testing methodology at high frequency (f > 24 GHz), the following general aspects apply:

• OTA measurement is the testing methodology for UE RF at high frequency (f > 24 GHz)
• Permitted test methods are Direct Far Field (DFF), Indirect Far Field (IFF), Near Field to Far-field transform (NFTF), that meets the equivalence criteria to the far field environment in an anechoic chamber

We will now look at the different measurement setups for the different permitted test methods.

## DFF Measurement Setup

The DFF measurement setup of RF characteristics for f > 24 GHz is capable of center and off-center beam measurements. The DFF setup can be simplified for center of the beam measurements.The measurement antenna and the link antenna can be combined so that the single antenna is used to steer the beam and to perform RF measurements.

The minimum far field distance R for a traditional far field anechoic chamber can be calculated based on the following equation: 

 R > 2D2 --------- λ

where D is the diameter of the smallest sphere that encloses the radiating parts of the Device Under Test (DUT). The distance can be very large for larger antenna sizes and higher frequencies. This could lead to the need for very large chambers, which may be prohibitively expensive. Methods may, therefore, be required to reduce the measurement distances for the compact antenna testing range and near field testing range.

## IFF Measurement Setup

The IFF measurement setup of RF characteristics for f > 24 GHz is capable of center and off-center beam measurements. The IFF method creates the far field environment using a transformation with a parabolic reflector. This is also known as the Compact Antenna Test Range (CATR). The CATR is a collimator system in which the spherical wave is transformed into a plane wave (uniform amplitude and phase), within the desired quiet zone.

Quiet zone size would mainly depend on the reflector, feed taper, and anechoic chamber design. Quiet zone quality can be impacted by amplitude uniformity, phase planarity and polarization purity. The CATR system does not require a measurement distance of $R >2D$2/λ to achieve a plane wave as in a standard far field range. The far field distance R is seen as the focal length, distance between the feed and reflector, which can be calculated as shown below (as a rule of thumb although it can vary depending on system implementation):

• D = X [m]
• Size of reflector = 2 x D
• R = focal length = 3.5 x size of reflector = 3.5 x (2xD)

There is a plane wave with no space loss from the reflector to the quiet zone.

NFTF Measurement Setup

The NFTF measurement setup of RF characteristics for f > 24 GHz is capable of center and off-center beam measurements. The NFTF system measures the amplitude and phase on a surface (spherical in this case) around the DUT. A circular probe array can measure the full 3D pattern with a rotation in azimuth only. Through use of electronic switching between the probe array elements the points in elevation can be measured without rotating the DUT in the elevation plane. The 3D far field pattern is obtained by using a modal spherical wave expansion.

The NFTF is based on the Huygen’s principle. A direct solution to the Helmholtz equations is found by applying boundary conditions on the surface at an infinite distance away from the DUT. From the tangential fields over the surface, the modal coefficients can be determined using the orthogonality of the modal expansion. The NFTF method computes the metrics defined in far field by using the near field to far field transformation. Radiated near field beam patterns are measured and based on the near field to far field mathematical transform, the final metric such as EIRP is the same as the metric for the baseline setup.

With the introduction of 5G wireless technology expected in 2019/2020, manufacturers of wireless phone devices must be ready to introduce compliant products that conform to OTA standards. Understanding the methodology associated with RF Testing for 5G mmWave OTA will help them take advantage of the opportunities associated with the introduction of this new technology.