Your Smartphone Battery's Worst Enemy? The Wi-Fi Radio
Battery life is the chief obsession of smartphone makers these days, with companies making news either for improving it (Apple claims the iPhone 4 battery lasts 30 percent longer than previous models) or doing an especially bad job of it (according to criticism leveled at the EVO 4G).
Vendors have targeted a variety of phone components as being the chief culprit for energy drain, but now a study by university researchers in partnership with Microsoft Research India have found the real guilty party may be a phone's Wi-Fi radio. To be more precise, the actual culprit manifests itself in how the Wi-Fi radio interacts with an access point.
According to Eric Rozner, a PhD student at the University of Texas at Austin who worked with the University of Wisconsin-Madison and Microsoft Research India, the problem is in a protocol that's supposed to save power.
"Wi-Fi is more energy-efficient than 3G when it comes to transferring data, so that is why 3G sucks up so much battery in that regard," he said in an e-mail to InternetNews.com. "Alternatively, when you look at how much energy the device consumes while idling, Wi-Fi is much more inefficient."
He said some studies have shown that Wi-Fi communication can account for up to 60 percent of a phone's energy usage on average, and some devices in particular fared far worse. He found that the HTC Tilt's total power consumption increases threefold when using Wi-Fi.
"This is why chipmakers have developed Power-Saving Mode (PSM). However, we show that PSM starts to break down under competing network traffic (for instance, in a dense hotspot)," Rozner said.
Power-Saving Mode is supposed to keep the Wi-Fi radio from draining the battery too quickly. But Rozner and other researchers found that the setup wasted power and misprioritized power management of other devices.
For example, Power-Saving Mode might come on to ping the Wi-Fi access point, then goes back to sleep if it gets no response. But an access point may serve many devices and it will put the phone at the back of the queue while it serves laptops, iPads, additional phones and other devices.
That means each user's phone has to wait for a response -- sometimes several seconds. Some phones, like the iPhone, don't wait for more than a few tens of milliseconds and go back to sleep if it doesn't receive any data. Others, however, sit and wait. Some access points push a device in Power-Saving Mode to the head of the queue, but then everyone else suffers for it.
Rozner recently presented a research paper (here in PDF format) at the International Conference on Mobile Systems, Applications and Services (MobiSys), held last week in San Francisco. He disclosed a program called NAPman, or Network-Assisted Power Management, which minimizes Wi-Fi energy consumption for mobile devices.
NAPman is described as implementing "a new energy-aware fair scheduling algorithm at the AP [access point] that minimizes Wi-Fi radio wakeup time and eliminates unnecessary retransmissions in the presence of competing traffic."
Rozner said NAPman is designed to be easily deployable in today's Access Points. "The only change required to the access point is a simple software upgrade and thus no changes are required to clients. We hope that the design choices in NAPman will make their way into today's deployments," he said.
The performance of the iPhone and other devices when using Wi-Fi doesn't surprise Avi Greengart, research manager for consumer and mobile devices at Current Analysis.
"Apple had woeful battery life when the iPhone first came out, but more recently has gotten much, much better. RIM has also done a terrific job. They have their own radio stacks," he told InternetNews.com, adding that they also "write their own cellular wireless stacks specifically so they can manage everything."
Meanwhile, Greengart said he's having the most trouble with battery life on Android phones. "I don't know if that's an issue with the software, firmware or hardware, but it does mean the guys who build vertically integrated devices like Apple and RIM have a better handle on this," he said.