Testing Millimeter Wave for 5G

By Susan Rambo and Ed Sperling

The telecommunications world is hurtling toward 5G, but there is no consistency about how this next-gen wireless technology will be rolled out across various regions and plenty of unknowns about how it will be tested and how reliable it will be initially.

A fair amount of confusion exists around what 5G constitutes in the first place. There is sub-6GHz 5G, which is sometimes referred to as 4.5G. And then there is the millimeter wave band, which radically changes the wireless communications paradigm because of improvements in speed.

There also are significant limitations on how far a millimeter wave signal can travel, and perhaps even more important, how far it can travel without any loss of signal integrity. The signal has to be line of sight, point-to-point. The signal doesn’t go around objects well; a person, a tree, a building will interfere with the signal. The solution is beamforming, which aims or bounces the beam off an object like billiard ball bound for a side pocket.

None of this seems to be impeding efforts to implement this technology. Case in point: Starting Nov. 14, the U.S. Federal Communications Commission (FCC) will auction off the millimeter-wave spectrum. The FCC is selling 28 GHz (27.5-28.35 GHz) Upper Microwave Flexible Use Service (UMFUS) licenses (Auction 101), and 24 GHz (24.25-24.45, 24.75-25.25 GHz) licenses (Auction 102) once Auction 101 concludes. More high-speed bands—37 GHz, 39 GHz, and 47 GHz bands—will be auctioned in 2019.

5G networks promise incredible speeds and low latency for 100 times more devices, but involves all radio spectrum bands, low to high. As the use cases pile up for what 5G will bring, the high-band millimeter wave will be important. In theory very fast because it is less congested than other established bands, mmWave is supposed to be able to carry a lot of data with low latency. But there are a number of issues that have to be resolved first.

Test challenges
One of the key challenges involves testing, and this gets progressively harder as the frequency increases for 5G.

“Millimeter wave is a very different animal,” said Adrian Kwan, business development manager at Advantest. “It’s been around for more than 50 years for military radar. Testing of the frequency began about 15 to 20 years ago. Testers can go up to 60 GHz, but there are not a lot of mainstream consumer applications for this. As this technology moves into the consumer space, the market is trying to figure out how this is going to be adopted. That includes everything from the service providers to how these devices are manufactured, packaged and tested.”

This isn’t just about 5G handsets, either. 5G consists of an entire ecosystem, and all of those have different testing requirements.

“There is an automotive element, and then there are small cells, micro cells like pico cells and femto cells, and then there are technologies ranging from multi-mode to multi-band,” Kwan said. “There are also smart adaptive antennas in phase arrays, and due to battery life, you have two beams to locate points or users and a third area that may be minimized where you are constantly searching. Those two signals are used to target a very directed signal to the user to give them maximum bandwidth.”

Rather than testing just the transceiver, the transceiver needs to be tested in conjunction with the antennas, and in some cases just the antennas will need to be tested. With millimeter wave, everything and more will need to be tested, including the package and the speed and integrity of the signal, which likely will require over-the-air testing. This is brand new for the commercial mobile world, and it has to be done in much higher volume than for things like radar.

“We’re are participating in field trials with millimeter wave,” said James Kimery, director of product marketing for RF and communications at National Instruments. “This includes profile, mobility, handoff, handover, range and capacity. There are phase array antennas on devices and base stations. There are also higher-gain antennas, so the more elements the more gain to steer beams. But you still can’t overcome physics. The frequency of 28 GHz does not propagate as far as 2.8 GHz.”

Kimery noted that one of the challenges is the requirement for more base stations. “Right now you have one base station, but you may need five or more for millimeter wave. The cost is the backhaul, not the base station, so you have integrated access backhaul (IAB) so that you only run fiber to one and communicate over the air to the others.”

That requires functional-level testing across the network topology, and that topology will be inconsistent because 5G will likely start in densely populated areas and work outward from there.

Materials and architectures
That isn’t the only thing changing. As the cost of designing chips at advanced nodes continues to rise, along with the impact of various types of noise—electromagnetic interference, crosstalk, power and substrate, among others—chipmakers in this sector are beginning to focus on different ways of building chips.

One such change involves using different materials being used for 5G devices, such as FD-SOI and RF-SOI. The advantage of those devices is that they are planar, so heat doesn’t get trapped in the fins of finFETs, and there is an insulating layer to minimize noise, heat, and various electrostatic effects.

“We’re seeing a strong pull on 12nm FD-SOI for millimeter wave, as well as augmented and virtual reality,” said Jamie Schaeffer, senior director of product line management at GlobalFoundries. “You’ll see this roll out in the second half of 2020 or the first half of 2021. The sub-6 GHz version of 5G will use 14/16mm finFETs. But with millimeter wave, that will move to 28nm HPC with high-k/metal gate and FDX (GlobalFoundries’ version of FD-SOI). The key there is integrating power amplifiers with lower power. With digital beam-forming, you get superior isolation for that.”

A second approach is advanced packaging. Apple led the way by introducing fan-out technology into its iPhone a couple years ago. Since then, the industry has been experimenting with a variety of packaging approaches, ranging from 2.5D to various types of fan-outs, bridges, monolithic 3D, system-in-package and even direct bond.

“We can predict some problems by learning from the transition in 3G to 4G applications,” said Anil Bhalla, senior manager for marketing and sales at Astronics. “But less latency in 5G will require careful consideration before deployments. Also, we are likely to see new problems emerge as a result of miniaturization, such as system-in-package implementations. And the connection between antennas, the front end module (FEM) and transceivers will continue to cause variability, especially if antennas become integrated with a FEM and transceivers. This last packaging technology trend will likely require advancement in over-the-air test methodologies to produce more repeatable results that reveal defects.”

This will evolve as 5G technology spreads and problems start cropping up.

“It will require lots of testing as we ‘follow the chips’ in order to verify that designs and manufacturing operations produce solutions that are defect-free and safe according to regulatory standards,” said Bhalla, noting this is particularly important as 5G involves connected and increasingly autonomous vehicle communication. “Automotive will require the use of much more proven technology than consumer devices, such as smart phones. This testing will happen both at the component and system-level.”

For autonomous systems that need continual testing in the field that includes BISTs, or built-in self tests, “the burden falls back on the design for test teams,” said Brady Benware, senior marketing director from the Tessent product group at Mentor, a Siemens Business. “It’s not just doing the test at the end of the manufacturing phase, it’s a continuous testing of that device in the end-user application.”

Continual in-field testing of parts for failure requires automating tests, says Benware. “The massive increase in bandwidth promised by 5G is creating a large set of new applications for cellular technology in safety critical markets like automotive, medical, and factory automation. Unlike the traditional consumer markets of 4G, these markets have rigorous functional safety standards like ISO 26262, IEC 60601, and IEC 61508, which mean more rigorous testing and new in-field failure mitigation strategies must be deployed for 5G. Mentor has been actively bringing together a comprehensive set of capabilities to automate meeting these functional safety requirements.”

“Functional safety is really all about ensuring that if a failure develops in the field—if these parts degrade in the field—that there is a safe way that those parts fail and that the broader system can respond to that failure,” said Benware, who says 90 percent of the defects in advanced node chips are now are within the cells. “That requires a lot of analysis in the design phase of these devices to understand what areas of the device and what functionalities are susceptible to failure. What are the effects of those failures if they do occur and then inserting additional circuitry within the devices to ensure that if there is a failure that that is tolerated or detected so that it can be responded to in the system.”

Supply chain issues
Another piece that needs to be addressed is the supply chain. As with any new technology, it takes time to build a supply chain. But with 5G, the difference is that this is starting out as an established, highly successful and enormous end market based upon technology that has never been used on a mass scale.

The opportunity is enormous, and dozens of startups are pouring into this market. But that’s also a potential problem from a supply chain standpoint.

“5G is growing fast because everyone realizes the value, but it’s moving at a speed where it’s difficult to stop and figure out how everything will go together,” said Ranjit Adhikary, vice president of marketing at ClioSoft. “The back end is getting things in place, but you’ve got a lot of companies here that are looking to be acquired. But they have no idea how much IP they have, what it is or how much it’s worth. Organization is a low priority for them. You also don’t know what interface they’re working with, under what conditions, and what are the issues across the IP hierarchy. And then someone leaves the company, and no one know what they were working on.”

Conclusion
The promise of 5G means using millimeter-wave high-band spectrum. “These airwaves offer greater capacity but travel shorter distances than low-band spectrum,” states the CTIA, a lobbying group representing the U.S. wireless industry. All the major telecom companies across the world are testing 5G technology now. The global rush to jump over the hurdles and claim first status in 5G plays into the theory that to the first go the 5G spoils. As the activity heats up, so does the need for test. Testing will need to be automated for certain 5G applications that require continual testing, such as self-driving cars and other safety-critical uses.

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