The 5G Frontier: Millimeter Wireless

Almost limitless bandwidth beckons—if we can tame a wild region of the spectrum

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There is an eternal quality to how technology evolves. As existing areas get overworked, new frontiers open up at the fringes. Then innovators rush in to occupy the new territory before it, in turn, becomes overworked. There is an example of such a frontier today in wireless communications. IEEE’s 5G wireless initiative has the goal of serving many more users with much higher transmission speeds. But with the existing cellular bands tightly packed, where does all the required additional network capacity come from?

In contrast with the traditional radio-spectrum management view of scarce capacity, where a finite amount of spectrum must be divided up among users, communication theorists see wireless capacity as virtually unlimited. Capacity can be increased indefinitely by going to ever smaller cells and higher frequencies that offer more bandwidth, while greater efficiency can be achieved with advanced signal processing and new spectrum-sharing policies. Among all these approaches, the greatest immediate impact would be achieved by moving to the higher frequencies in the millimeter range—the region of 30 to 300 gigahertz, where bandwidth is available and plentiful.

But in many ways, millimeter-wave wireless truly is a frontier. Today the millimeter band is largely uninhabited and inhospitable, as signals using these wavelengths run up against difficult propagation problems. Even when signals travel through free space, attenuation increases with frequency, so usable path lengths for millimeter waves are short, roughly 100 to 200 meters. Such distances could be accommodated with the smaller cell sizes envisioned in 5G, but there are numerous other impediments. Buildings and the objects in and around them, including people, block the signal. Rain and foliage further attenuate millimeter waves, and diffraction—which can bend longer wavelengths around occluding objects—is far less effective. Even surfaces that might be conveniently nicely reflective at longer wavelengths appear rougher to millimeter waves, and so diffuse the signal.

So there may be gold in that frontier, but it is going to be very difficult to mine. Nevertheless, you never know until you try. I’m reminded of Marconi’s successful transatlantic transmission in 1901, when physicists insisted that the signal would fly off into space. Recently a team at NYU has been experimenting with millimeter-wave transmission within the urban canyons of New York City. Like the physicists of yesteryear, I would have said that this would never work. But the data show otherwise. They demonstrated a surprising amount of coverage despite the buildings, pedestrian and vehicular traffic, and general chaos typical of dense cities. Granted, there are a number of holes in the coverage, but initial results are encouraging.

When I expressed some surprise at these findings, someone pointed out to me that in its line-of-sight dependence, millimeter-wave propagation might be likened to that of visible light, and that the nighttime world isn’t as dark as might be expected. Taking this analogy to heart, I prowled my house on a dark night, the lone source of illumination a weak light at the end of a long corridor. I discovered dim light in unexpected places. I wondered: How did the light get here? Even so, nearby rooms might be caves of darkness. All the while, I was conscious of the strong Wi-Fi signal that followed me everywhere I went.

So the millimeter-wave frontier is going to be a difficult one, but we engineers are good at this kind of challenge, and we’re not without tools. For one thing, at these small wavelengths, we can build postage-stamp-size phased-array antennas, and high-speed electronics allow us to use advanced techniques that have been pioneered at longer wavelengths.

All this sophisticated technology so we will be able to view 3D video of cats while walking down a busy city street. Or, hopefully, some other use.

This article appears in the March 2017 print issue as “The Millimeter Wireless Frontier.”

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