The Beacon

Extended battery life with 802.11ac

Matthew Gast

This post originally ran in February 2013 on the Aerohive blog where Matthew Gast has his own author blog.

Getting out and talking with customers is one of the activities that keeps me fresh. Some of my best ideas have come directly out of customer conversations, which is one of the reasons you’ll always find me willing to speak at one of our Hive User Group events.

On my last trip out to see customers, I gave my current talk about 802.11ac. At the very last minute at the Philadelphia-area event, I noticed that the restaurant was playing Tony Bennett singing “The Best is Yet to Come,” and I couldn’t resist re-titling my talk and throwing in a picture of Tony to express the same sentiment about Wi-Fi.

While there are many reasons that the best days of Wi-Fi with 802.11ac are yet to come, I’ll focus on one in this post that I haven’t really written about before: improved battery life.

Yes, battery life. 802.11ac will help extend battery life in mobile devices. You read that right. Heck, earlier this week, HTC announced that the HTC One (picture right) is coming with 802.11ac.

The major components of a Wi-Fi interface, in the order that an incoming signal hits them, are:

  1. The antenna system. This one is obvious. You can’t receive radio waves without an antenna.
  2. Analog processing. The signal that comes off the antenna is very weak. A high-quality amplifier is used o boost the signal off the antenna system to feed to the radio chip. Because the signal is so weak, one of the most important attributes of this first amplifier is that it introduce as little noise into the amplified signal as possible. Amplifiers take some power, but not as much as you might think compared to components further along the chain.
  3. The digital signal processing (DSP) stage. There are multiple components here – each antenna’s received signal goes through demodulation and through a Fourier transform, and then multiple spatial streams are pulled apart. The computational power increases dramatically with multiple streams. For a single stream, this step is a straightforward single Fourier transform. With multiple streams, it’s a Fourier transform per stream, possibly with complex applied mathematics to get at each spatial stream.
  4. Decoding the recovered bit stream to correct errors and passing the frame to higher software layers.

What kills battery life is not the amplifier in the analog section. Yes, amplifiers do require power, but the real power hog in multi-stream MIMO systems is the digital processing used to recover parallel spatial streams. If you eliminate the requirement to receive multiple spatial streams, the power required in the digital stage drops dramatically.

If you are making a mobile device, the easiest way to reduce power requirements is to build a single-stream device to minimize the power drain in the DSP stage. Not surprisingly, that’s exactly what HTC did with their new phone. Yes, it’s 802.11ac, but it’s single-stream 802.11ac.

With single-stream 802.11ac, the HTC One can reduce the time needed to send or receive data and increase battery. Although the digital stage for 11ac is slightly more complicated than it would be for 11n, that is more than made up for by the savings that come from running the analog amplifier for shorter periods of time.

(Note that my analysis depends on this being single-stream 11ac and single-stream 11n. If you need a complex multi-stream digital stage, all bets are off because you need much more intensive digital processing to make multiple streams work.)

With Mobile World Congress happening next week in beautiful Barcelona, I wouldn’t be surprised to see a number of additional phones announced with 802.11ac.

When power-conserving devices like battery-powered handhelds are being built with 802.11ac, you can tell that we’re on a roll. As the immortal Tony Bennett put it, the best is yet to come …

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Typically, the biggest power consumption in a Wi-Fi interface will be the signal processing required to "tear apart" multiple data streams at the receiver. I don't expect we'll see multi-stream phones or tablets any time soon, especially with 802.11ac offering up to 433 Mbps to single-stream devices.

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Matthew Gast

Matthew Gast is the Director of Product Management at Aerohive Networks where he leads development of the core software technologies in Aerohive's controller-less Wi-Fi network system.