A Cell-Phone Network without a License
A trial system offers calling, texting, and data by weaving signals around the chatter of baby monitors and cordless phones.
A trial cell-phone network in Fort Lauderdale, Florida, gets by without something every other wireless carrier needs: its own chunk of the airwaves. Instead, xG Technology, which made the network, uses base stations and handsets of its own design that steer signals through the unrestricted 900-megahertz band used by cordless phones and other short-range devices.
It’s a technique called “cognitive” radio, and it has the potential to make efficient use of an increasingly limited resource: the wireless spectrum. By demonstrating the first cellular network that uses the technique, xG hopes to show that it could help wireless carriers facing growing demand but a relatively fixed supply of spectrum.
Its cognitive radios are built into both the base stations of the trial network, dubbed xMax, and handsets made for it. Every radio scans for clear spectrum 33 times a second. If another signal is detected, the handset and base station retune to avoid the other signal, keeping the connection alive. Each of the six base stations in xG’s network can serve devices in a 2.5-mile radius, comparable to an average cell-phone tower.
“In Fort Lauderdale, our network covers an urban area with around 110,000 people, and so we’re seeing wireless security cameras, baby monitors, and cordless phones all using that band,” says Rick Rotondo, a vice president with xG, which is headquartered in Sarasota, Florida. “Because our radios are so agile, though, we can deliver the experience of a licensed cellular network in that unlicensed band.”
While most radios can only use frequencies that are completely clear, xG’s radios can unlock more free space by analyzing channels whose use varies over time, Rotondo says. Signals can then be inserted in between bursts of activity from a device using that channel.
“Where a more conventional radio would see a wall of signals, we are able to put our packets in between them and move around between those gaps,” he explains. “Using that method, we find that even in an urban area, the 900-megahertz band is really only around 15 percent occupied at any time.”
The company recently won a contract to install an xMax network to cover a large chunk of the U.S. Army’s Fort Bliss training base in New Mexico. “They’re interested in the possibility of one day being able to create cellular networks for use on their bases for everything we use cell networks for: voice, texting, e-mail, and data access,” Rotondo says, “or rapidly deploying a version on the battlefield.”
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