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.