Wi-Fi via White Spaces
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.
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.
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.
Some of the challenges that faced the group came about because of the undefined nature of white-space frequencies. . The researchers designed their algorithms to determine the ideal amount of frequency bandwidth to use for a broadcast, balancing the desire for strong signal against the possibility of interference with neighboring frequencies. They also had to design a way for mobile devices to find a signal from an access point.
One of the most important parts of the White Fi system is a protocol for dealing with collisions among different signals (particularly those from wireless microphones, which can turn on at any time). Even a single packet of interference is enough to produce audible disruptions for a microphone. Even if interference affects only one device on the network, strict regulations forbid all devices on the network from using that channel. The researchers got around this by designing the access point so that it maintains a backup channel. If another user is detected, the white-space device or access point immediately switches to the backup channel, which reassigns bandwidth use as needed.
Peter Steenkiste, a professor of computer science at Carnegie Mellon University who specializes in networking, says that previous work on white spaces has focused on addressing one problem at a time. “The thing that I think is very interesting about this paper is that it really has looked at how you put a complete system together,” he says.
Steenkiste adds that “there are a lot of practical issues that they’ve worried about.” In particular, he says, the researchers did not assume an ideal, controlled environment for their system. Rather, they took into account such problems as measurement “noise” and the unpredictable behavior of wireless microphones. “[The research] has an answer for every question,” Steenkiste says.
Chandra says that his group recently received an experimental license from the FCC that allows them to build a prototype White Fi system on the Microsoft Research Campus in Redmond, WA. They plan to send their findings to the FCC in the hope that the data will help determine future white-space regulations. Chandra notes that since the transition from analog to digital television is happening worldwide, there is a high level of international interest in US white-space experiments. Researchers and companies all over the world are looking for technologies to take advantage of the fragments of spectrum that will open up in the coming years, he says.
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