5G Over-the-Air Test Needs Calibration Standard

The move to 5G imposes many new challenges on wireless test engineers. Arguably the most onerous is the requirement for over-the-air (OTA) test.

OTA measurement, also known as radiated measurement, has been used in some applications for many years, but it is relatively new to the cellular space. OTA was first introduced to wireless communications to test antennas for multiple-input/multiple-output (MIMO) with LTE. 5G, however, makes far greater use of OTA testing because of its utilization of massive MIMO and millimeter-wave (mmWave) spectrum.

OTA test is especially relevant for 5G’s Frequency Range 2 (FR2), which occupies the spectrum between 24.25 GHz and 52.6 GHz. The vast majority of FR2 resides in the mmWave spectrum, the band of spectrum between 30 GHz and 300 GHz. The highly integrated packages of mmWave antennas and radio frequency integrated circuits (RFICs) typically lack the probe points needed to make conducted measurements possible, making OTA test a must. Some Frequency Range 1 (spectrum between 450 MHz and 6 GHz) conformance test cases mandate OTA testing, as do all FR2 conformance tests.

So far there has not been a standard developed for calibrating a measurement system for OTA test. Such a standard would go a long way toward helping metrology labs and test engineers throughout the world in the process of conducting accurate, repeatable measurements of 5G signals.

In the most recent episode of All Things 5G, Keysight’s monthly podcast devoted to examining the technology, business, and policy aspects of the fifth-generation wireless communications technology, Dr. Robert Horansky of the National Institute for Standards and Technology (NIST) discusses the challenges of OTA measurement and calibration and what NIST is doing to help. Because NIST plays an integral role in helping to define and develop standards for measurement, its current projections in the area of OTA test may go a long way to helping to improve the theory, hardware, and analysis techniques for laboratory testing of 5G devices.

During the interview, Dr. Horansky discussed the possibility of creating a standard for OTA measurements that could potentially be developed using NIST’s SAMURAI (short for Synthetic Aperture Measurement of Uncertainty Angle of Incidence) system. SAMURAI — which is small enough to be deployed in a lab environment and also offers the portability to be used in field tests — boasts the ability to create a detailed portrait of directional 5G device and channel performance. It is considered the first system to offer 5G wireless measurements with accuracy that can be traced back to fundamental physical standards.

“Recently we have been thinking about how a device or some kind of UE — user equipment — that has been traceably measured over the air at NIST and can be used as a standard to characterize people’s chambers or setups or environments,” Dr. Horansky told All Things 5G.

NIST engineers (from left) Rodney Leonhardt, Alec Wiess, and Jeanne Quimby with NIST's SAMURAI portable measurement system. (Photo by M. Hammer/NIST)

He added that at 5G’s higher frequencies, there is a lot more to consider than just bit error rate, such as the performance of antenna steering in various environments. “The calibration is really, what are the metrics of the signal but also what are the metrics of the environment you are dealing with,” he said.

Another approach that NIST has been working on lately is devising a method for calibrating a test setup — a way to traceably characterize the environment and be able to compare that to what you are trying to achieve.

“At least to know traceably what is impinging on a device at a certain point in your chamber or in your set up,” Dr. Horansky said. “These are the aspects that we've been working on at NIST and we are approaching with the SAMURAI system.”

NIST’s SAMURAI system features two antennas to transmit and receive signals and a six-axis robotic arm to position the antenna, as well as instrumentation with precise timing synchronization to generate radio transmissions and analyze reception. Among other things, NIST expects SAMURAI to help shed light on the use of active antennas in 5G.

Listen to more of Dr. Horansky’s interview with All Things 5G, which can be found on the All Things 5G homepage. Also, visit Keysight’s Calibration Services page for more information on calibration, and the Keysight 5G Solutions page for more information and resources related to 5G.