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.

Why Over-the-Air (OTA) Testing is Needed at mmWave Frequencies

In my previous blog post series, I touched on the basics of 5G physical layer testing and the associated radio changes and how those brought about device test and measurement challenges at both sub 6 GHz and millimeter wave (mmWave) frequencies. Today I want to talk about why over-the-air (OTA) testing is necessary for devices operating in the mmWave frequency range.

Simply put, over-the-air (OTA) testing is needed to characterize the performance of the transmit and receive antennas. Millimeter wave frequency ranges, as defined by 3GPP, lie anywhere between 24.25 GHz to 52.6 GHz. It is well known that mmWave frequencies are exceptionally prone to path loss resulting in a signal-to-noise (SNR) ratio which falls between the mid to low SNR range, making them highly sensitive to RF impairments. To overcome this effect, mmWave makes use of beamforming.

Beamforming

Beamforming is a special massive MIMO technique which makes use of multiple, small, closely spaced antenna elements to generate a highly directional beam. Each of these elements is fed with a signal phase and amplitude adjusted in a way that steers the radio energy only in the direction of interest. Not only does this increase the SNR but improves the spectral efficiency and ensures a reliable coverage.  However, unlike previous generations of technologies, which focused more on generating a static beam, dynamic beam shaping and directionality makes it crucial to characterize the radiation pattern and antenna performance.

Why Only OTA Test?

As effective as beamforming may sound, its hardware implementation is slightly challenging at millimeter waves, as the higher the frequency, the smaller the antenna aperture, necessitating more of the antenna elements to be integrated in an array to achieve a certain output power and gain capability. Furthermore, to minimize the signal path loss attenuation, this highly compact antenna array is tightly integrated with the active radio circuitry forming a mmWave antenna module, precluding the use of probes or connectors for any conducted mode measurements.

Beamforming Testing

At mmWave frequencies, the antenna radiation pattern is fairly sensitive and can easily get altered by final product casing, user’s hand or due to reflections or losses from the surrounding environment. Hence, we must carefully characterize the performance of the transmit and receive antennas and validate the performance of the radiation pattern.

There are two kinds of beamforming tests that are performed – beamforming characterization and beamforming verification. Beamforming characterization is an R&D procedure that aims to find the proper phase angle and gain for each antenna element in order to generate a desired pattern in a desired direction specific to the device under test (DUT) design. These values are then stored in the form of code books and are later used by the DUT to generate beams.

Beamforming verification is mostly performed at the design verification test (DVT) stage, where the DUT uses the pre-defined code books and sets the specific phase and gain for each of the antenna elements to generate a beam in a certain direction with a certain phase and magnitude. Thereafter, the device behavior is verified for specific or corner case scenarios using over-the-air (OTA) testing and measurements.

Over-the-Air (OTA) Testing

In addition to 5G NR Sub-6GHz, even legacy cellular technologies have supported verification of transmit and receive characteristics in conducted mode and antenna pattern measurements in radiated mode. However, at mmWave frequencies, all measurements must be made using over-the-air (OTA) test methodology. This includes antenna performance measurements such as the gain of the beam, effective isotropic radiated power (EIRP), effective isotropic sensitivity (EIS), RF parametric measurements such as error vector magnitude (EVM), spectrum emissions mask (SEM), adjacent channel leakage ratio (ACLR) and signaling or end-user test functionality testing.

Figure 1: Typical mmWave OTA Test Setup

Figure 1 above shows a typical mmWave over-the-air (OTA) test setup. The setup would include a test and measurement equipment consisting of a fully integrated VSG and VSA capable of generating and analyzing signals at mmWave frequencies, a DUT housed in a shielded anechoic chamber for uninterrupted OTA testing and a measurement horn antenna well aligned with the antenna module under verification to avoid any measurement inaccuracies. 

In my next blog post, I’ll explore key concepts that influence over-the-air (OTA) testing chamber and measurement decisions. In the meantime, I invite you to visit the full replay of my webinar on this topic.