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.”
Craig Mathias, an analyst with the Farpoint Group, which specializes in the wireless industry, has inspected the Fort Lauderdale network. “It really is just like using a regular cellular system, even though the technology is so different,” he says.
The potential for cognitive radio to make better use of spectrum has motivated many companies and academic labs to work on the technology in recent years, says Mathias. “The real advance of xG’s system is that it can be deployed in exactly the same way as a conventional cell-phone network,” he says. But exactly how xG will bring the technology to market is unclear. “One option may be for a carrier to use this in an area or market where they don’t have spectrum, or to serve rural areas without coverage.”
Rotondo says that xG wants to offer its approach as a complement to existing networks. “We are interested in having devices able to dynamically access different areas of spectrum–both licensed and unlicensed,” he says. Wireless carriers like AT&T are turning to Wi-Fi hot spots to offload some of the load on their licensed spectrum, he points out. Being able to have devices switch to the 900-megahertz band at times of high load could be an attractive option, because it can perform much more like a cell network. The radios developed by xG could be built into commercial phone handsets, says Rotondo.
Alternatively, the system could augment emerging networks that operate in the unlicensed “white spaces” recently freed up by the end of analog TV broadcasts, Rotondo says. A recent study by University of California-Berkeley academics revealed how the density of TV stations in metropolitan areas could reduce the availability of white spaces in such areas.
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