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Microsoft details Wi-Fi use for 'white spaces'

Marguerite Reardon CNET News

Published: 20 Aug 2009 09:39 BST

Microsoft researchers may have taken a step closer to turning unused analogue 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 the US 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, according to an article published on MIT's Technology Review website.

White space spectrum is unused wireless spectrum that sits between analogue 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 2.4GHz, whereas white space spectrum between analogue TV channels 21 and 51 operate in the 512MHz and 698MHz 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.

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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 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 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 licence from the FCC to build a prototype White-Fi system on the Microsoft Research Campus in Redmond, Washington. 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...