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Gesture Recognition

June 17, 2013

At the start of my undergraduate education, I briefly attended a college whose emphasis was on engineering. This was a mistake for a physics major, so I left a short time later. I did, however, gain some valuable extracurricular knowledge about radio broadcasting. Since it was a select school, I met quite a few creative and interesting people. The school was so select, in fact, that fully half the students were above average.[1]

The conversations at group lunches were highly technical. We would often invent useful devices, at least on paper - napkin paper, that is. One of these was a gesture control system in which waving an unaided hand would control a device such as a television. Nowadays, there are quite a few technologies to tackle this problem, the most effective being machine vision systems, as used in the Microsoft Kinect device.

In the late 1960s, however, our technology choices were limited, but there were two that would have enabled such a device. The first is radar, since Doppler signals from a waving hand would be easy to detect. The radar generator would have been something like a klystron, although solid state microwave generators, such as the IMPATT diodes, tunnel diodes were just becoming available.

Cutaway of a Varian V-260 reflex klystron

Cutaway of a Varian V-260 reflex klystron. Such devices attached to rectangular waveguides, usually at the X band.

(Cutaway model provided by Walter Z. Seldon and photographed by Erbade, via Wikimedia Commons.)

Another, more easily built device would be based on the Theremin. I mentioned Léon Theremin, the inventor of the Theremin, in a previous article (Cyber Warfare, November 10, 2011). The Theremin uses the electrically conducting human body as a plate in the variable capacitance of a radio frequency oscillator; so, proximity of a body part, such as a hand, will cause a detectable frequency change.

We sit in our offices and living rooms immersed in electromagnetic fields, some of which are intentional, and some are not. The most useful is the light that comes from our lamps and display screens. The most annoying is the power line emission that gives us the buzz on our AM radios and stereo music systems.

Environmental fields can serve as the basis for object detection. One interesting example of this was the use of cosmic rays by Nobel Physics Laureate, Luis Alvarez, in a search for hidden chambers in an Egyptian pyramid in the 1960s. Scientists in Mexico have applied this same technique to the pyramid at Teotihuacan.[2] Recently, more primitive methods have been used at the Nohmul complex in Belize.[3]

The Pyramids at Gizah

The Pyramids at Gizah in Egypt. During my college years in the 1960s, many New Agers believed that the pyramid shape focused "cosmic energy."

(Photograph by Ricardo Liberato, via Wikimedia Commons.)

One type of electromagnetic radiation pervading our home and office environments is Wi-Fi, which is a physical layer in local area networking. Tablet computers, laptop computers, desktop computers, game consoles, computer peripherals and smartphones all have Wi-Fi capability. It sure beats snaking Ethernet cable throughout a house, as I did many years ago.

University of Washington computer scientists have decided to harness this sea of Wi-Fi radio waves as a means for gesture recognition. The Washington research team includes Shwetak Patel and Shyam Gollakota, both assistant professors of Computer Science and Engineering, and doctoral student, Sidhant Gupta. Their technology, which they call "WiSee," will be presented at The 19th Annual International Conference on Mobile Computing and Networking later this year, but details are available online.[4-7]

Figure caption

A display showing the change in wireless signal strength across frequency channels in real time as a user moves his hand. (University of Washington image.)

One advantage of this method is that Wi-Fi signals travel through walls, so gesture detection is not limited by the line-of-sight restriction of camera systems. The method is based on the Doppler effect, which in the gesture case is just a few hertz. The entire Wi-Fi spectrum is scanned, looking for subtle changes in signal strength as a function of frequency.[4]

At this point, the WiSee system can identify nine different whole-body gestures, such as pushing, pulling and punching. In a test environment of five users in a two-bedroom apartment and an office environment, 94% of 900 gestures were accurately detected.[4] As for security, a specific sequence of gestures can be used as a password.[4] My college classmates and I were just a few decades too far ahead of the technology curve on this one.


  1. I'm sure you caught that joke. If you haven't, read the statistics page on Wikipedia.
  2. Claire Marshall, "Physicists probe ancient pyramid," May 13, 2004.
  3. Elizabeth Snodgrass, "Ancient Maya Pyramid Destroyed in Belize," National Geographic, May 15, 2013
  4. Michelle Ma, "Wi-Fi signals enable gesture recognition throughout entire home," University of Washington Press Release, June 4, 2013.
  5. Web Site of The 19th Annual International Conference on Mobile Computing and Networking, Miami, Florida, Sept 30 - Oct 4 2013.
  6. Whole-Home Gesture Recognition Using Wireless Web Site at the University of Washington.
  7. YouTube video about University of Washington gesture capture research.

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