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.
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.