A new “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. 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. It’s well known that you can slow a device’s clock to save energy. The hard part, Shin said, was getting the phone to recognize an incoming message while it was 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.”
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.
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 detectable way. The researchers 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.
Shin points out that E-MiLi is compatible with today’s models, so messages sent with future devices that use E-MiLi’s encoding would still be received as usual on smartphones without E-MiLi. E-MiLi can also be used with other wireless communication protocols that require idle listening, such as ZigBee.
WiFi interface is known to be a primary energy consumer in mobile devices, and idle listening (IL) is the dominant source of energy consumption in WiFi. Most existing protocols, such as the 802.11 power-saving mode (PSM), attempt to reduce the time spent in IL by sleep scheduling. However, through an extensive analysis of real-world traffic, we found more than 60% of energy is consumed in IL, even with PSM enabled. To remedy this problem, we propose E-MiLi (Energy-Minimizing idle Listening) that reduces the power consumption in IL, given that the time spent in IL has already been optimized by sleep scheduling. Observing that radio power consumption decreases proportionally to its clock-rate, E-MiLi adaptively downclocks the radio during IL, and reverts to full clock-rate when an incoming packet is detected or a packet has to be transmitted. E-MiLi incorporates sampling rate invariant detection, ensuring accurate packet detection and address filtering even when the receiver’s sampling clock-rate is much lower than the signal bandwidth. Further, it employs an opportunistic downclocking mechanism to optimize the efficiency of switching clockrate, based on a simple interface to existing MAC-layer scheduling protocols. We have implemented E-MiLi on the USRP software radio platform. Our experimental evaluation shows that E-MiLi can detect packets with close to 100% accuracy even with downclocking by a factor of 16. When integrated with 802.11, E-MiLi can reduce energy consumption by around 44% for 92% of users in real-world wireless networks.