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Low-cost wireless technology proposed
2009-08-24 Article Rating:   / 1
Microsoft researchers have devised a way to offer low-cost, long-range wireless Internet via radio spectrum previously reserved for TV stations. The "WhiteFi" system could adaptively switch frequencies using a technique called SIFT (Signal Interpretation before Fourier Transform), the researchers say.
In a paper that was presented this week at the ACM SIGCOMM in Barcelona, a team of three Microsoft researchers (plus two colleagues from Harvard) pointed out that there are many unused portions of the UHF spectrum. These include, but are not limited to, channel 21 (512MHz) to 51 (698MHz), with the exception of channel 37, says the paper. At the end of last year, the FCC issued a ruling permitting the use of unlicensed devices in these "white spaces." The catch is, any new users are not allowed to interfere with any licensed incumbents, which include not only TV stations but also wireless microphones. According to the paper, the latter present the most difficult problem, since they can be turned on and off at any time. The authors write, "Our experiments show that even a single packet transmission causes audible interference during wireless microphone transmissions." They add that for wireless networking equipment to share UHF spectrum, "both the AP [access point] and its clients must disconnect and then rapidly reconnect using a different available channel." The proposed "WhiteFi" wireless network would include APs that can use any available channel width and operate in any portion of the 180MHz white spaces. The novel SIFT technique would analyze incoming signals to detect transmissions over different channel widths, without having to change the actual channel width of a wireless receiver, claims the paper. As a result, clients could discover when APs have moved to a different operating frequency, the researchers say.  The KNOWS prototype (Click to enlarge) According to one of the researchers, Ranveer Chandra, Microsoft recently received an experimental license from the FCC that will allow it to build a prototype, "campus wide" WhiteFi system on the company's Redmond campus. According to their paper, the team has already built a hardware device (above) nicknamed KNOWS, which includes a PC, a scanner, a standard WiFi card, and a UHF translator that converts incoming and outgoing 2.4GHz signals to the 512 to 698MHz band. It's said the scanner samples the UHF spectrum to detect the presence of TV broadcasts and wireless microphone signals. Meanwhile, the SIFT technique is capable of detecting data transmissions, even when they are intermittent and received over multiple channel widths. Finally, WhiteFi provides a "chirping" protocol that lets clients signal when they are disconnected due to a channel conflict, without interfering with a wireless microphone or other primary user. Availability The ACM SIGCOMM paper by Paramvir Bahl, Ranveer Chandra, Thomas Moscibroda, Rohan Murty, and Matt Welsh is theoretical and does not address issues such as how fast WhiteFi networks would operate and what, if anything, they would cost to operate. Because WhiteFi uses lower frequencies than WiFi, however, signals could travel further, but couldn't offer as much bandwidth. However, it's hoped that WhiteFi networks could provide low-cost access in rural areas where traditional carriers, using licensed spectrum, have been reluctant to offer services. For more information about WhiteFi, download the paper in PDF format from the SIGCOMM website, here, or see an article on CNET covering the topic, here. Related Stories:
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