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Dual Antennas Would Boost Cell-Phone Signals

New design would mean fewer dropped calls, longer battery life.
August 12, 2010

The recent furor over the iPhone 4’s antenna has made consumers aware of the constraints designers face when trying to build sleek, compact gadgets that also get a good connection. Researchers at Rice University have come up with a design that could make signal worries a thing of the past, and extend battery life as well. The design uses two antennas that focus their power in different directions.

Radio circuit: This wireless transmitter has been connected to three directional antennas, a setup that shows how cell phones could save power by directing radio signals toward cell towers.

The antenna in your current cell phone beams a signal out in every direction. “Only a tiny fraction of that energy actually reaches the base station,” says Lin Zhong, whose research group investigates ways to make computing devices more efficient. Not only is this a waste of a cell phone’s battery life, he says, but it creates unnecessary interference for other users.

Antennas that beam out energy in a narrower band can be more efficient. Cell towers use this tactic, with several antennas pointing in different directions to better serve users. Zhong’s group, in collaboration with colleague Ashutosh Sabharwal, has developed a prototype antenna system that could give that ability to cell phones themselves, saving energy and extending battery life.

The researchers connected a small Wi-Fi transmitter to three cheap directional “patch” antennas facing in different directions, as well as a standard omni-directional antenna. Only one of the four was active at any one time, with the directional antenna being used to send data to the base station, and the omni-directional one used to receive it.

The system was mounted on a motorized spinning platform to test its most crucial quality: the ability to stay connected when the orientation of the device changed as it rotated (by switching to whichever antenna was generally oriented toward the base station). The directional antennas monitored the quality of packets received from the base station to decide which among them should send the next packet.

Even when spun at one revolution per second, the prototype could stream video with almost zero interference. That suggests the approach would work well in real situations, says Zhong. He collected a week of data from the accelerometers and compasses of 11 smart phones actually in use by real people, and found that these phones were rarely rotated at more than a third of a revolution per second while connected.

The current prototype used Wi-Fi frequencies because off-the-shelf directional Wi-Fi antennas are easier to acquire. But the results apply to any frequency, says Zhong. His group is working on a version that connects multiple antennas to Nexus One cell phones running the Android operating system, as well as computer simulations of the approach. Results so far back up the Wi-Fi tests, he says.

All the results suggest that using just two antennas in a phone would offer significant savings, says Zhong. “With one antenna on the back and one on the front you could double the signal strength at the edge of the network compared to a regular antenna, and halve the energy needed when at the center of the network.”

Spencer Webb, president of AntennaSys, a New Hampshire company that designs antennas for everything from cell phones to medical devices, agrees that two separate antennas front and back could help a cell phone in a variety of situations. “One example is that when you place a device down on a surface, it could switch to the uppermost antenna to avoid interference,” says Webb.

Multiple antennas might also make it possible to sidestep the problem of a user’s grip hindering the connection, an issue many are now familiar with thanks to the publicity surrounding Apple’s iPhone 4 launch, Webb adds. “If you had, say, dual antennas and used them wisely, you could perhaps prevent some of those issues by switching antenna when a person changes their hand position,” he says.

However, one reason for the controversy over Apple’s iPhone–dubbed “antennagate”–is the tight constraints on space and design in smart phones, Web adds. Devices like the Droid X and HTC Evo have suggested that larger smart phones are becoming popular with designers, says Webb. “But did they put more antenna space in there? No.”

Antenna engineers typically lose out in the battle for space to the battery or other components, Webb says. Compact components that offer large space savings will be needed for manufacturers to make room for an extra antenna. That could become possible as the necessary chips drop in cost and size, and as cell-phone carriers move toward next generation mobile networks, such as LTE and 4G, that use smaller antennas, says Zhong, although the strategy may gain more immediate traction in other types of device. “In a laptop, for example, more space like the back of the screen is available,” says Webb.

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