PROBLEM: Wi-Fi uses frequencies that can’t carry a signal more than a few tens of meters. TV stations, on the other hand, use a portion of the radio spectrum that lets signals travel long distances, and the end of analog television has opened up unused slices of the spectrum between stations. They could be used for wireless Internet service, but it has been difficult to take advantage of these so-called white spaces without causing interference, because the exact frequencies used by TV stations vary geographically.
SOLUTION: Ranveer Chandra made the Microsoft campus in Redmond, WA, his laboratory for the first large-scale network to demonstrate the potential of using white spaces to deliver broadband wireless. Links in the prototype network can span up to two kilometers. To avoid treading on the toes of TV broadcasters, his system uses GPS to determine its location; then it checks the Web to find out what stations are active in the area. Chandra’s devices can also listen for nearby transmissions from wireless microphones, which use the same bands. When a conflict is detected, they switch to a backup slice of unused spectrum on the fly.
If such a system gains currency, “all of us should be connected and better connected, and not just here in the U.S.,” says Chandra. Spectrum regulators from Singapore, India, Brazil, and China have all come to visit his prototype network to explore the potential for white-space signals to connect large rural areas with minimal infrastructure. –Tom Simonite
Self-aware: To avoid interference, a base station checks the Web and finds out which frequency bands are in use by local TV stations (A). It also listens for any wireless microphones in range (B) and picks a free band (C).
Credit: Emily Cooper