Microsoft details a fix for 'white space' interference

Microsoft researchers may have taken a step closer to finally turning unused analog TV spectrum, known as "white spaces," into unlicensed spectrum that can be used to deliver new wireless broadband services.

Researchers from the software giant, along with academics from Harvard University, have developed a protocol that the company claims could be the foundation for products that meet Federal Communications Commission requirements for avoiding interference when using unlicensed "white space" spectrum. The researchers presented their ideas this week at the ACM SIGCOMM 2009, a communications conference held in Barcelona, Spain, according to an article published on MIT's Technology Review Web site.

"White space" spectrum is unused wireless spectrum that sits between analog TV channels. Because much of this spectrum operates at lower frequencies, it can travel longer distances, much longer distances than the unlicensed frequencies used for Wi-Fi devices. Most Wi-Fi devices operate at 2.4 GHz, whereas, white space spectrum between analog TV channels 21 and 51 operate in the 512 megahertz and 698 megahertz range. (Signals transmitted over lower frequencies tend to travel longer distances at slower speeds, while signals transmitted at higher frequencies tend to be faster over shorter distances.)

Technology companies, such as Google, Motorola, Microsoft, and Dell, have been lobbying the FCC for years to open this spectrum for unlicensed use. The hope is that the spectrum could be used to augment existing wireless services or eventually be used to create new wireless broadband services.

But TV broadcasters and wireless microphone companies have long opposed the use of this spectrum, saying it will interfere with their services.

After a series of prototype tests, the FCC in November finally agreed to open up unused broadcast TV spectrum for unlicensed use. But the commission put together a set of strict guidelines designed to ensure that devices using the unlicensed spectrum would not interfere with existing TV broadcasters or other devices using the same unlicensed spectrum, such as wireless microphones.

Microsoft has designed a set of protocols it calls "White-Fi," which it claims will be able to avoid interference to make the best use of the spectrum. One of the biggest challenges with avoiding interference is that new devices using the same frequency can be introduced to the network at any time. For example, a wireless microphone can be turned on and off. And the tiniest bit of interference can cause interference problems.

To avoid this problem, Microsoft's White-Fi protocol is designed so that each device measures the spectrum conditions around it, finds available frequencies, and is able keep searching for interference. And if interference is detected, it can move to a different sliver of spectrum at anytime, the article said.

The way it works is that the device connects to the available "white space" frequency. But it will also maintain a backup radio frequency channel, so that if another device that uses the same frequency comes into range, it could switch to the backup frequency.

Ranveer Chandra, a Microsoft researcher on the project, told Technology Review that Microsoft has recently received an experimental license from the FCC to build a prototype White-Fi system on the Microsoft Research Campus in Redmond, Wash. The company plans to send its findings from these tests to the FCC.

Companies that support the use of white space spectrum say the unlicensed spectrum could be used to extend Wi-Fi networks in urban areas for low-cost broadband access or provide broadband services to rural communities where traditional wireless providers have been reluctant to offer services because it's too sparsely populated.

Servicing underserved populations, such as low-income urban families and people who live in rural communities has become a focus of the Obama administration. And a portion of the $7.2 billion economic stimulus package passed by Congress earlier this year has been set aside for companies looking to provide broadband service in these areas.

While many firms seeking these funds hope to use wireless technology to provide broadband, none is able to take advantage of white space technology yet. But with advancements such as the White-Fi technology from Microsoft, some service providers may be able to use unlicensed white space spectrum in the future.

Still, it's unlikely that a nationwide White-Fi service would emerge to compete against the big wireless operators, such as AT&T, Sprint Nextel, T-Mobile USA, or Verizon Wireless. White space spectrum differs depending on where you are, and it's not likely that a service provider would be able to patch together a nationwide service. By contrast, the big carriers use licensed spectrum, which guarantees coverage in certain areas.

That said, the White-Fi technology or similar technology used to access white space spectrum could be used to augment existing wireless services that use licensed spectrum.

The big wireless operators have already begun to mix their cellular wireless services, which use licensed spectrum, with unlicensed Wi-Fi. For example, T-Mobile USA has its @Home service that allows its cell phone subscribers to use Wi-Fi-enabled handsets to switch between its cellular network and Wi-Fi hotspots at home.

Verizon Wireless and Sprint Nextel are each selling Mi-Fi routers, which allow its wireless broadband subscribers to attach a device to convert the cellular broadband signal into a Wi-Fi signal that can be shared and used with Wi-Fi-enabled devices, like laptops, cameras and music players.

AT&T and Verizon are also using access to public Wi-Fi hotspots to extend broadband service for their high-speed Internet customers.

"Hopefully, we will continue to see technology innovation in white spaces," said Chris Guttman-McCabe, vice president of regulatory affairs for CTIA, the trade association that represents the major wireless carriers. "We hope that the spectrum gets put to good use. And as we've seen with Wi-Fi, I'm sure that as the technology and services evolve, carriers will contemplate incorporating white space spectrum into their own services."