March/April 2006

Cognitive Radio

To avoid future wireless traffic jams, Heather "Haitao" Zheng is finding ways to exploit unused radio spectrum.

By Neil Savage

This article is the fourth in a series of 10 stories we're running over two weeks, covering today's most significant (and just plain cool) emerging technologies. It's part of our annual "10 Emerging Technologies" report, which appears in the March/April print issue of Technology Review.

Growing numbers of people are making a habit of toting their laptops into Starbuck's, ordering half-caf skim lattes, and plunking down in chairs to surf the Web wirelessly. That means more people are also getting used to being kicked off the Net as computers competing for bandwidth interfere with one another. It's a local effect -- within 30 to 60 meters of a transceiver -- but there's just no more space in the part of the radio spectrum designated for Wi-Fi.

Imagine, then, what happens as more devices go wireless -- not just laptops, or cell phones and BlackBerrys, but sensor networks that monitor everything from temperature in office buildings to moisture in cornfields, radio frequency ID tags that track merchandise at the local Wal-Mart, devices that monitor nursing-home patients. All these gadgets have to share a finite -- and increasingly crowded -- amount of radio spectrum.

Heather Zheng, an assistant professor of computer science at the University of California, Santa Barbara, is working on ways to allow wireless devices to more efficiently share the airwaves. The problem, she says, is not a dearth of radio spectrum; it's the way that spectrum is used.

The Federal Communications Commission in the United States, and its counterparts around the world, allocate the radio spectrum in swaths of frequency of varying widths. One band covers AM radio, another VHF television, still others cell phones, citizen's-band radio, pagers, and so on; now, just as wireless devices have begun proliferating, there's little left over to dole out.

But as anyone who has twirled a radio dial knows, not every channel in every band is always in use. In fact, the FCC has determined that, in some locations or at some times of day, 70 percent of the allocated spectrum may be sitting idle, even though it's officially spoken for.

Zheng thinks the solution lies with cognitive radios, devices that figure out which frequencies are quiet and pick one or more over which to transmit and receive data. Without careful planning, however, certain bands could still end up jammed. Zheng's answer is to teach cognitive radios to negotiate with other devices in their vicinity. In Zheng's scheme, the FCC-designated owner of the spectrum gets priority, but other devices can divvy up unused spectrum among themselves.

But negotiation between devices uses bandwidth in itself, so Zheng simplified the process. She selected a set of rules based on "game theory" -- a type of mathematical modeling often used to find the optimal solutions to economics problems -- and designed software that made the devices follow those rules. Instead of each radio's having to tell its neighbor what it's doing, it simply observes its neighbors to see if they are transmitting and makes its own decisions.