Wi-Fi via White Spaces

A network design that uses old TV spectrum could produce better long-range wireless connectivity.

Long-range, low-cost wireless Internet could soon be delivered using radio spectrum once reserved for use by TV stations. The blueprints for a computer network that uses "white spaces," which are empty fragments of the spectrum scattered between used frequencies, will be presented today at ACM SIGCOMM 2009, a communications conference held in Barcelona, Spain.

White spaces: Accessing the Internet over unused portions of TV spectrum could provide good long-range connectivity in rural areas, and help fill in gaps in city networks. Microsoft researchers tested a new protocol, called White Fi, using the device shown here. Credit: Microsoft Research

TV stations have traditionally broadcast over lower frequencies that carry information longer distances. However, with the ongoing transition from analog to digital broadcasts, more unused frequencies are opening up than ever.

By tapping into these lower frequencies, it should be easier to provide broadband Internet access in rural areas and fill in gaps in city Wi-Fi networks. For example, the spectrum between 512 megahertz and 698 megahertz, which was originally allotted to analog TV channels from 21 to 51, offers a longer range than conventional Wi-Fi, which operates at 2.4 gigahertz. "Imagine the potential if you could connect to your home [Internet] router from up to a mile," says Ranveer Chandra, a member of the Networking Research Group at Microsoft Research behind the project.

The FCC ruled last November that companies could build devices that transmit over white spaces but also gave strict requirements that this should not interfere with existing broadcasts, both from TV stations and from other wireless devices that operate within the same spectrum. Chandra and his colleagues designed a set of protocols, which they call "White Fi," to successfully navigate the tricky regulatory and technical obstacles involved with using white spaces.

"It's a totally different paradigm for wireless networking," says Chandra. "Until now, in wireless networks, you were given a spectrum, and you would share it with everyone else. Everyone was an equal stakeholder. Now, you have this spectrum where there are certain people who are primary users."

One of the main obstacles for Chandra's group was dealing with a network of different devices; in the past, work focused on sending and receiving signals between individual devices over white spaces.

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Setting up a group of devices to communicate over white-space frequencies is a more complicated proposal, because white-space devices have to find available spectrum, which can change depending on where and when the device is operating. The researchers designed a system consisting of a wireless access point, like the router used in Wi-Fi networks, and the mobile devices communicating with it.

White Fi is designed so that each device measures the spectrum conditions around it and works with the others to find available frequencies. Because interference can happen at any time, the system can move to a different slice of spectrum if need be.