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Millimeter Wave: A Bridge Too Far?

5G is rolling out in two flavors, but only one of those is guaranteed to work.

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5G is here. It already is available in new mobile phones, and the infrastructure for extremely fast cellular communication is being built out at a rapid pace. The big question now is which parts of this technology will be successful, and there still is no consistency in those predictions.

5G comes in two flavors, sub-6 GHz and millimeter wave, and the sub-6 GHz version offers immediate performance improvements over 4G LTE. In fact, you probably won’t notice much of a difference except that access to mail and the Internet will be significantly faster.

Millimeter wave, in contrast, is like the shift to autonomous driving. It represents a giant leap forward, but only under certain conditions. Signals don’t travel far and they don’t penetrate objects, go through windows or turn corners in hallways. In fact, they are only guaranteed to work if the receiving device is in direct line of the transmitter, whether that’s a base station, a small cell, or another mobile device. The idea of triangulating signals into a beam that tracks these receiving devices is a feat of engineering, but its value is controversial.

Being able to move massive amounts of data using higher frequency signals has captured the imagination of all cellular users and engineers, which is why there is so much attention and investment capital focused on millimeter wave. But there are a number of key problems with this technology that do not have an immediate solution. First is the limitation of the signals themselves, which makes it hard to use in a mobile setting where there are lots of moving objects. Initially, the idea was that millimeter wave technology would guide autonomous vehicles down the road and around people or objects in the road, but that idea was discarded as far too slow, too unreliable, and far too expensive.

Second, even if millimeter wave technology can overcome those limitations with nearly ubiquitous repeaters and small cells, moving that amount of data around requires an enormous amount of power. It’s much more efficient to process data closer to the source and dump whatever is useless at the source. In addition, the idea that phones will be constantly searching for millimeter wave signals will eat up battery life even in good reception areas.

Third, there is a significant latency when moving large quantities of data back and forth to the cloud, or even to some edge servers. No matter how fast the processing can happen outside of a device, that data still needs to be partitioned and processed and sent back and forth, and all of that takes time.

Fourth, the form factors of mobile devices are making the prospect of millimeter wave communication more difficult and more expensive. To use this technology, a phone doesn’t just need a single antenna or even a single phased array. It needs multiple phased arrays, because a person’s hand will block millimeter wave signals. This takes up space, adds engineering time and money, and it begins to eat into the amount of real estate that will be available for other components, particularly if phone makers opt for folding-screen devices.

Putting all of the pieces together, this represents an enormous gamble. While sub-6 5G is a sure thing, millimeter wave is not. And that doesn’t even begin to address all of the infrastructure needed to make this work, including rental space for repeaters on buildings and the technology needed to account for drift and aging as these repeaters are exposed to harsh elements for extended periods of time. It’s certainly possible to make millimeter wave work, but circumventing the laws of physics won’t be easy or cheap.

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1 comments

Victor says:

One thing I’ve never heard addressed has to do with the fact that the U.S. government owns the sub-6 Ghtz part of the spectrum in question. The multi-billion dollar question is: Why can’t the U.S government release this band for commercial use? Is it being used as a key part of our nuclear arms launch capability or something? Why does no one talk about this? Pursuing horrendously difficult mm wave technology essentially caps American companies in the knees (knee caps) and is allowing China to move forward, getting giant leaps ahead of the U.S.

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