Ten years ago, wireless devices were uncommon. Today, Bluetooth headphones, GPS units, Wi-Fi hotspots, and laptop computers are everywhere. Smart phones connect routinely to the Internet, while wireless networks link devices as diverse as medical monitors, factory sensors, media centers, and printers.
Joseph Mitola IIIWireless devices are changing the way we communicate. Unfortunately, like army ants on the march, they are also devouring all the remaining space on the airwaves as they go. “Wireless applications are coming out of walls, and there’s just not enough broadband spectrum to go around,” said Joe Mitola, Vice President for the Research Enterprise at Stevens Institute of Technology.
The result is already visible in many cities. “You put 100 home networks in an apartment building where they can jam each other and forget it,” Mitola explained. “People have to shut down their systems. Others wait until later. Some return their ‘broken’ wireless routers to the store.”
Wireless is facing broadband congestion, especially in large cities. Mitola has a plan to deal with it. This is not surprising. Mitola is the father of cognitive radio, smart wireless devices that adapt to the amount of radio bandwidth available. His latest idea involves teaching cognitive devices how to cooperate to make smarter use of spectrum and avoid jamming one another.
Mitola’s proposal builds on the work of Stevens researchers Yingying (Jennifer) Chen, Yu-Dong Yao, Hong Man and Mouli Chandramouli.
It should come as welcome news to the Federal Communications Commission (FCC), the agency that regulates the use of spectrum in the United States. In late 2009, FFC asked for help identifying concrete steps it could take support wireless innovation. Among its key concerns: wireless spectrum availability and use.
To appreciate Mitola’s approach, consider how the FCC manages spectrum, the range of electromagnetic frequencies used to transmit voice and data over the airwaves.
To keep signals from interfering with one another, FCC divides the spectrum into bands for different applications. AM radio stations, for example, broadcast between 540 and 1630 kilohertz (thousands of oscillations per second) while FM radios transmit between 88 and 108 megahertz (millions of oscillations per second).
FCC assigns broadcasters a frequency within that band, so they do not interfere with one another. It also limits their broadcast power, so they cannot down out a station using the same frequency in a nearby city or suburb.
As wireless devices began entering the market, the FCC set aside narrow bands of spectrum for them. For example, it assigned garage door openers 40 MHz and cordless phones 900 MHz. It allowed other wireless applications, like wireless microphones, to share under-utilized frequencies, such as those set aside for ultra-high-frequency television. While the FCC did not license these devices, it limited their power (and range) to prevent interference.
Now the system is crumbling under the sheer number of new wireless devices. Today, Americans own more than 270 million cell phones and operate tens of millions of home networks and Wi-Fi hot spots.
This new generation of wireless devices differs greatly from older cordless phones and garage door openers. Not only are there more of them, but they send and receive far more data, including Web sites, photos, movies, and music. It takes high-capacity, or broadband, spectrum for them to handle all this data smoothly.
Every year, millions of new devices — from Internet-ready televisions and games to medical monitors and building control systems –are crowding into those same broadband frequencies. With so many new devices, just limiting broadcast power is not enough to prevent problems.
“But when you put home networks into apartment buildings, even low-powered signals can interfere with each other,” Mitola said. “They might not penetrate the concrete floors and brick walls, but radio signals move easily along heating and air conditioning ductwork. If you install a Wi-Fi network, you’ll be interfering with everyone 10 floors up and 10 floors down.”
Even though Wi-Fi networks and hotspots have limited range, they can interfere with distant networks. Signals from a wireless router shoot out in all directions. Place it near a window, and those signals will bounce down a street lined with tall buildings. Like an echo in a canyon, such multipath reflections can be “heard” by Wi-Fi systems up to a kilometer away.
Mitola’s cognitive radio, approved by the FCC in November 2008, provides one way of dealing with this problem. It makes devices smart enough to find and use empty spectrum, or white space.
In fact, the FCC will require applications with assigned frequencies, such as radio and television, to announce where and when they plan to use their assigned frequencies. This schedule will go into a database. Cognitive devices will then check the database to look for unused times and frequencies – white space – that they can use for their own broadcasts.
That will work in many locations, but not in others. Why? “It doesn’t account for signal reflections,” Mitola explained. “When someone installs a home network that uses TV white space, they will have to input its location. But the two-dimensional location by itself is not enough.
“For example, imagine a home wireless network sending signals on TV white space from a building’s basement. The soil around it absorbs any stray radio signals. Now imagine that same device near a window on the tenth floor. It would be echoing signals down the street a kilometer away. Meanwhile, dozens or even hundreds of TV white space systems would be doing exactly the same thing.
“So a 2D database might say that you are fine, but in fact you’re producing one thousand times more interference than the database shows. To deal with that problem, we need another type of database,” Mitola concluded.
Stevens’ FCC proposal envisions a three-dimensional database. Instead of the user entering a device location, cognitive devices would collaborate with one another to discover their locations. They would then use that information to shape their signals to share broadband without interference.
Surprisingly, the technology to shape signals is already available: MIMO (multiple input, multiple output) antennas used by many Wi-Fi networks and hotspots. Ordinarily, home wireless devices broadcast signals of equal strength in all directions. MIMO devices take advantage of those bouncing signals to increase the data rate with each different echo.
Cognitive MIMO TV whitespace networks would use the 3D database to pinpoint the location of other networks and devices. They would then use their antennas to aim their output only at their own computers and printers while suppressing signals directed towards neighboring TV white space devices on someone else’s network. This would them enable them to use spectrum up to 10 times more efficiently while minimizing interference, Mitola said.
Today, even sophisticated devices have trouble locating their position in 2D space. “If you take a TomTom GPS unit and walk into hotel lobby with thick walls, it has no clue where it is,” Mitola explained.
“But suppose you put it near a window. Now it has a clear pathway to receive signals, so it can find its location on the grid. Every few hours, it wakes up and communicates its location to the 3D database. It also tests the signal and direction needed to communicate with other wireless devices.
“So maybe it finds a cell phone above it and says, ‘You’re in room 404 or 405.’ Then a laptop says, ‘I’m in 404 and you’re closer to me, so you must be in 404.’ Through random interactions over time, the devices collaborate to build a 3D map of their locations and transmit that information to the 3D database.
You could build a model of the entire country, including how signals echo down the street,” said Mitola. “It would happen without any user intervention, and the more devices in the database, the more accurate it would become,” Mitola said.
Once devices submitted their location to the database, TV white space networks and hotspots could calculate the best way to share spectrum, improve reception, and reduce interference. “Ten years ago, those calculations would have required a room full of computers running over the weekend. Today, companies like Remcom Inc. can produce high- fidelity models on a laptop in real time,” Mitola said.
Stevens researchers are current working with the IEEE P1900.5 Cognitive Radio Policy Language standards committee to standardize such 3D coordination. They are also creating a policy language that would enable devices to decide how to allocate spectrum.
“The 3D models and collaborative control would enable high-rise buildings to run many more white space networks, without interfering with each other or the heart monitors at the hospital down the street. We could give everyone more of the spectrum they need, automatically and affordably,” Mitola said.
Alan S. Brown is a science writer and blogger who serves as associate editor of Mechanical Engineering magazine.