PhotoOne of the many considerations consumers make when selecting a smartphone is its battery life. But a new invention just might level the playing field, making that less of a consideration.

University of Michigan computer science and engineering professor Kang Shin and doctoral student Xinyu Zhang have invented what they call a "subconscious mode" for smartphones and other WiFi-enabled mobile devices could extend battery life by as much as 54 percent for users on the busiest networks.

Still being tested

The researchers will present their invention at the ACM International Conference on Mobile Computing and Networking next week in Las Vegas. The approach is still in the proof-of-concept stage and is not yet commercially available.

Even when smartphones are in power-saving modes and not actively sending or receiving messages, they are still on alert for incoming information and they're searching for a clear communication channel. The researchers have found that this kind of energy-taxing "idle listening" is occurring during a large portion of the time phones spend in power-saving mode -- as much as 80 percent on busy networks.

More efficient listening

The researchers say they have simply tried to make smartphones perform this idle listening more efficiently. It's called E-MiLi, which stands for Energy-Minimizing Idle Listening.

To find out how much time phones spend keeping one ear open, Shin and Zhang conducted an extensive trace-based analysis of real WiFi networks. They discovered that, depending on the amount of traffic in the network, devices in power-saving modes spend 60 to 80 percent of their time in idle listening. In previous work, they demonstrated that phones in idle listening mode expend roughly the same amount of power as they do when they're fully awake.

"My phone isn't sending or receiving anything right now," Shin said, lifting his power-skinned iPhone, "but it's listening to see if data is coming in so I can receive it right away. This idle listening often consumes as much power as actively sending and receiving messages all day."

How it works

Here's how E-MiLi works: It slows down the WiFi card's clock by up to 1/16 its normal frequency, but jolts it back to full speed when the phone notices information coming in. While it's fairly easy to slow a device's clock to save energy, the hard part, Shin said, is getting the phone to recognize an incoming message while it is in this slower mode.

"We came up with a clever idea," Shin said. "Usually, messages come with a header, and we thought the phone could be enabled to detect this, as you can recognize that someone is calling your name even if you're 90 percent asleep."

44 percent power reduction

When used with power-saving mode, the researchers found that E-MiLi is capable of reducing energy consumption by around 44 percent for 92 percent of mobile devices in real-world wireless networks.

Widespread changes in the industry would be required to make all of this a reality. In addition to new processor-slowing software on smartphones, E-MiLi requires new firmware for phones and computers that would be sending messages.

They need the ability to encode the message header -- the recipient's address -- in a new and more detectable way. The researchers say they have created such firmware, but in order for E-MiLi use to become widespread, WiFi chipset manufacturers would have to adopt these firmware modifications and then companies that make smartphones and computers would have to incorporate the new chips into their products.

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