The first “white spaces” devices, which thread long-range wireless data signals through gaps in TV spectrum, will start to appear later this year. Microsoft is bidding to play a central role in how they operate.
The coming devices are expected to include home routers to bring Internet to the home and even mobile devices such as phones or tablets. To avoid interfering with TV broadcasts, they will check with a government-appoved online database to learn of available white spaces between channels in their area. Microsoft has applied to the FCC to become an approved administrator of such a system, built using technology developed by its research wing, dubbed SenseLess. This would give the company an influential stake in the world’s first attempt to find a new way to free up the airwaves—an approach that is likely to be adopted worldwide. Google and eight other companies have already been granted permission to operate white spaces databases, but they have revealed little of their technology.
Microsoft’s system was recently demonstrated in Las Vegas, where it enabled an Xbox games console to get online using a prototype white spaces device made by startup Adaptrum.
TV spectrum signals have a longer wavelength than Wi-Fi or cellular signals, which means TV spectrum can support longer-range data connections. Microsoft’s trial white spaces network, on its Redmond, Washington, campus, can provide high-speed Internet at a range of over a mile.
To use the system, a device first supplies its location to the database, using a frequency that is known to be permanently free in that area. The system then tells the device which other chunks of spectrum are available to use at that time. SenseLess combines knowledge of every licensed TV signal in the U.S. with detailed topographic maps and models to determine how signals dissipate over distance and terrain.
Development of SenseLess has been led by Microsoft researcher Ranveer Chandra with colleagues Thomas Moscibroda and Victor Bahl. Another collaborator, Rohan Murty of Harvard University, drove 1500 miles around Washington state to gather data that tested SenseLess’s predictions against the real world. “We had zero false positives,” says Chandra. “Never did we say that a channel was free but was actually occupied.”
Trials of SenseLess have also shown that devices do not need to have a very accurate location fix, says Chandra—good news, because many white spaces devices will need to work indoors, where GPS is less accurate. “With the right models, a device can be only accurate to within 0.6 miles and lose access to less than 2 percent of spectrum,” says Chandra. A description of the SenseLess system was presented at the the IEEE Dynamic Spectrum Access Networks conference in Aachen, Germany, earlier this month. It was one of three papers chosen to be fast-tracked for publication.
Chandra and colleagues are working on ways to cram more connections into the white spaces than the FCC has decreed it will allow, in the hopes of influencing it and other regulators to make more space available. One strand of research has shown that an HD movie stream can be transmitted over the same channel being used by a wireless microphone (which use frequencies close to TV broadcasts) without causing any noticeable degradation to the sound recorded. That shows that the current rules may be too conservative, says Chandra. Currently white spaces devices must avoid any channel in use by a wireless microphone as well as the channels on either side of a TV broadcast. “We’ve found that you really don’t need to vacate the entire channel,” says Chandra.
“We intend to leverage all of the work that we’ve done, even if there are some we can’t implement under the FCC rules so far,” says Paul Garnett, Microsoft’s director of interoperability and standards. “The U.S. rules may change in the future, or different countries may take fuller advantage of the capabilities of white spaces networking.”
The SenseLess system has already been modified for use in other countries, including Finland, Singapore, and the U.K., in preparation for white spaces initiatives expected from regulators there. Microsoft has also made it possible for certain devices, such as wireless microphones, to ask SenseLess to reserve them a channel and ask other devices using the database to make room.
This kind of dynamic functionality hints at the broader potential for using a central, dynamic system to manage wireless spectrum beyond just TV white spaces. Kevin Werbach, who researches Internet and communications policy at the Wharton Business School, University of Pennsylvania, says a database that managed more than just white spaces could enable more efficient use of the limited radio spectrum.
“A database system like this could be the basis of a new system of access control, a universal intermediary between devices that want wireless connections and those with the capability,” says Werbach. Like the DNS system that directs Internet traffic to the correct destinations, such a service could direct requests for wireless access to vacant pieces of spectrum, he says, a method that could use available bands more efficiently than the current system that allocates fixed blocks to certain companies and uses.
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