Primer On Millimeter-wave Test Interconnect Challenges Including 5G

The rise of emerging millimeter-wave applications has begun to change the landscape of millimeter-wave device and system testing. In previous years, millimeter-wave applications were the purview of scientific research, military/defense, and satellite communications. These applications are typically comfortable with low-volume production, high single-unit costs, long lead times, custom solutions, and labor-intensive testing/quality/verification methods. Unlike previous millimeter-wave applications, the emerging applications are driven by commercial interests that are interested in bringing millimeter-wave device and system testing to commercial scale volumes and commercial time frames.

These new applications include satellite communications based on massive satellite constellations, or mega-constellations, and millimeter-wave terrestrial communications and backhaul (millimeter-wave 5G). Both of these applications are trying to produce and leverage technology and supply chains that have long operated well below 6-GHz, and adapt/upgrade it to operate at tens of gigahertz. For instance, New Space companies have been racing hard for the past several years to advance silicon semiconductor technology to operate at the power thresholds, frequencies, and with the sensitivity that gallium arsenide (GaAs), gallium nitride (GaN), and indium phosphide (InP) have typically been required for.

Along with active hardware innovations, these new applications are also attempting to innovate the millimeter-wave device and system testing process to better mesh with the volumes and quality requirements of the new regime of millimeter-wave technology. One of the ways this is being done is pioneering seamless over-the-air (OTA) testing for advanced antenna system (AAS) driven telecommunication devices. These efforts include 5G base stations, user premise equipment, and handsets. The concept is that an OTA test system will be able to wirelessly communicate with a 5G device-under-test (DUT) via an OTA protocol built into the device for a range of testing that was traditionally done with connectorized protocols. It is deemed necessary by many to use OTA testing for 5G devices and systems, as connectorized testing would require a high number of test ports and is less readily adapted for high-throughput testing methods. 

However, OTA testing is not without its own challenges. For instance, during OTA testing it isn’t possible to control the testing environment to the degree that traditional connectorized testing provides. This introduces a variability to test conditions that may significantly impact the test results, even if shielding precautions are taken. Moreover, OTA testing involves much weaker signal levels than connectorized testing, which requires much higher dynamic range test equipment to account for the much weaker received signals from OTA testing. There are also practical challenges of testing antenna array devices that will likely require new OTA testing methods that have yet to be established or proven effective.

Connectorized testing approaches are also not ideal, but connectorized test setups can also be advanced to address some of the new requirements of OTA and high-throughput commercial testing. For instance, massive switch matrices, matrix attenuators, and power dividers/combiners can be used to increase the number of effective ports of large and expensive millimeter-wave test equipment without the need to purchase additional test instruments. These components can also be used to produce a wide range of testing scenarios, such as fading and interference testing, that would otherwise be impractical to do with OTA test methods.

The next challenge is for test houses and device manufacturers to find a balance and innovate new methods of using OTA and connectorized testing to most efficiently test new millimeter-wave devices and systems in volumes never before seen by the industry. These new efforts will likely result in greater accessibility of millimeter-wave technology, and will eventually lead to millimeter-wave technology being as ubiquitous as current Sub-6 GHz technology is today.