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A Personal Cell-Phone Tower

Small, low-power access points boost wireless speeds indoors and in busy areas.

To accommodate explosive growth in demand for wireless data, many mobile carriers have begun touting next-generation “4G” networks. But to consistently reach good speeds, especially indoors in densely populated areas, so some carriers are starting to offer small, low-power indoor cellular access points called “femtocells.”

In the air: Airvana’s HubBub is a femtocell that boosts a wireless signal indoors. Femtocells can improve the quality of voice calls and relieve some of the strain on traditional cell phone towers.

The move from analog to digital to 3G networks was largely a matter of upgrading the technology in cell phone towers. But the explosive growth of smart phone use means that carriers need new ways of boosting bandwidth. One way to ensure high speeds, especially inside buildings where interference is common, is to use small cellular base stations (about the size of a Wi-Fi router) that route traffic over the internet.

AT&T, Verizon, and Sprint have started selling femtocells that can be used in the home to boost spotty wireless reception. Some carriers are also exploring the use of femtocells as a way to improve the performance of the their networks, since the transmitters that are small enough to be installed almost anywhere, from overhead power lines to street lamps and pay phones.

Woo June Kim, vice president of technology for femtocell manufacturer Airvana, says it may be possible to achieve speeds that are 10 times faster than 3G using existing network infrastructure, but he adds, “if you want to increase significantly beyond that, you really need what’s called ‘spatial reuse.’ ” Spatial reuse exploits the proximity of users to a base station–fewer users share each access point, but the capacity of individual access points does not change.

In the past, femtocells have been too expensive for widespread rollout, but femtocell maker Ubiquisys recently developed a $100 unit. AT&T also recently announced that it would offer a $150 femtocell.

Rich Kerr, CEO of Public Wireless, which installs femtocell infrastructure and leases access to it, says that splitting an area of wireless coverage into two smaller units using femtocells increases the wireless capacity by about 85 percent.

Femtocells also have a number of advantages over Wi-Fi, according to Kim. Users do not need to install software, such as Skype, in order to use them. The devices also offer more range (up to two kilometers when used outdoors), and they are designed to avoid interfering with other cellular transmitters by communicating with a carrier’s central computer and adjusting signal strength accordingly. Femtocells can hand off voice and data transmissions to adjacent cells as a user walks or drives through a coverage area.

Public Wireless has been able to deploy cell service to areas where it would be impossible to get permission to put up large towers, says Kerr.

For example, T-Mobile asked Public Wireless to extend its coverage to Diamondhead, a large crater near Waikiki beach in Honolulu, Hawaii. “Everyone’s spent millions trying to get a cell tower on that venue,” says Kerr. “We installed production-level coverage in less than one day, and that unit has been in operation for nine months.”

Femtocells could also solve a problem unique to the switchover from 2G to 3G networks. Currently, when towers make the switch, the radius of their effective coverage shrinks by 20 percent, leaving holes in areas that would have previously seen overlap from multiple towers. “That hole is almost impossible to address with any other technology,” says Kerr.

Kim believes that it’s only a matter of time before femtocells will be ubiquitous both in the home and in densely populated urban areas. “Once operators get comfortable, they’ll do massive rollouts,” he says. “We can expect AT&T to jack up the press on femtocells in the future.”

Market research iSuppli estimates that shipments of femtocells could grow from less than 600,000 this year to 40 million by 2013.

Femtocells do have a handful of disadvantages compared to the existing infrastructure. First of all, home femtocells are dependent on a user’s existing broadband Internet connection. Femtocells are also less suitable for users traveling in a car or train, because calls would need to be passed between cells too often. And in rural areas, femtocells still can’t compete with old-fashioned 120-foot towers with five miles of coverage in every direction.

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