5G Over-the-Air Testing: Overcoming Path Loss
2020-02-29 | 5 min read
After getting initial deployments by more than 20 mobile network operators in 2019, 5G will go mainstream in many markets in 2020. In the U.S., marketers are working overtime building consumer awareness of 5G technology, with major MNOs flooding the airwaves to tout the advantage of their network's 5G coverage, transmission speeds, and other benchmarks.
Consumers, for the most part, do not know — or care — much about the technical advances required to make 5G a reality. They know what they have read and heard about 5G offering orders of magnitude improvement over the speed, responsivity, and reliability of 4G, enabling entirely new use cases in virtual and augmented reality, vehicle-to-everything communications, autonomous driving, and the Internet of Things (IoT).
But, of course, behind the scenes, 5G has posed many challenges to engineers — and will continue to do so.
In the world of test and measurement, 5G necessitates many changes. Perhaps none is more disruptive than the move from conducted test methods to over-the-air (OTA) testing with the advent of mmWave. The highly integrated packages of mmWave antennas and radio frequency integrated circuits (RFICs) typically lack the probe points needed to make conducted measurements possible, mandating the need for OTA testing.
OTA testing is one of the most challenging aspects of 5G device development. The many new technologies described by the 5G NR standard — including mmWave frequencies, flexible numerologies, massive multiple-input / multiple-output (MIMO), and beamforming — introducing substantial new test challenges that further complicate OTA testing for validation. 3GPP-approved OTA tests are in a constant state of evolution, further complicating matters.
For example, path loss is one significant challenge to OTA testing that is not an issue with traditional cabled tests. Cabled test systems demonstrate well-behaved physical properties that need calibration to produce accurate and repeatable results. Calibrating OTA test methods is possible, but the process is more time-consuming and complex. With mmWave devices, excess path loss makes precise OTA measurements more difficult.
OTA tests are typically carried out in either the near-field or far-field regions of the antenna array. The characteristics of the transmitted electromagnetic wave change depending on the distance from the transmitter. As the signal propagates from the antenna array, the signal becomes more developed.
Infographic: Beam properties at different distances from the antenna array
5G cellular communication links require using far-field assumptions. Due to the nature of radiated waves, the far-field distance and associated path loss grows more significant with the frequency. For example, the far-field region of a 4G LTE 15 cm device operating at 2 GHz starts at 0.3 meters and has a path loss of 28 dB. The far-field region of a 5G NR device running at 28 GHz has a far-field distance of 4.2 meters and a path loss of 73 dB.
Using traditional methods would result in an excessively large far-field test chamber and a path loss that is too great to make accurate and repeatable measurements at mmWave frequencies. The distance also grows more significant as the source antenna grows bigger, further compounding the size and path loss challenge.
The physical arrangement of a CATR chamber
To overcome the path loss and excessive far-field distance issues, 3GPP approved an indirect far-field (IFF) test method based on a compact antenna test range (CATR). This IFF test method uses a CATR with a parabolic reflector to collimate the signals transmitted by the probe antenna and create a far-field test environment. While this method is limited to measuring a single signal, it provides a much shorter distance and with less path loss than the direct far-field method for measuring mmWave devices.
For more information on solutions for overcoming OTA test challenges for 5G, including path loss mitigation, please visit the following webpages: