US boffins propose yet another low-low power Wi-Fi for Things

A sober reflection on wireless comms

University of Washington boffins are touting extremely low-power Wi-Fi transmissions – if your application can put up with a maximum 11 Mbps capacity.

The students and boffins, Bryce Kellogg, Vamsi Talla, Shyamnath Gollakota and Joshua Smith of the University of Washington, have in mind the kind of low-power devices that will proliferate as household sensors in the Internet of Things era.

They describe their scheme in this paper, set down for delivery at the Usenix Symposium on Networked Systems Design and Implementation in March.

Gollakota has a long record in fooling around with backscatter. In 2013 with Sidhant Gupta, he demonstrated a scheme to use Wi-Fi for gesture recognition. He improved on that in 2014 (with Kellog and Rajalakshmi Nandakumar), by putting gesture recognition into unmodified laptops.

Last year, Gollakota and Smith – the academics in the author list – set up a company called Jeeva Wireles to pursue their interest in low power communications.

The scheme their students Kellog and Talla worked on has two key power-saving techniques. First, they offload all of Wi-Fi's carrier sensing activities to devices like routers, since these are always-on anyhow. Second, they've created a way to modulate 802.11 signals on the back-scatter from a device.

These approaches mean a device can be entirely passive and still communicate, albeit at a limited data rate – the group demonstrated communications at 1 Mbps (consuming 14.5 microwatts) and 11 Mbps (consuming 59.2 microwatts), which aren't going to excite people signed on for Netflix 4K services.

Their starting point is simple: analogue circuits like ADC/DACs and amplifiers are power hogs, and unlike digital circuits (for example CMOS) they don't benefit from digital scaling. The aim of “passive Wi-Fi” is to get rid of those analogue components.

Offloading carrier sensing to the Wi-Fi hotspot is straightforward: whether that hotspot is a smartphone or a broadband router, it will be sensing the radio channel whether or not the sensor is transmitting.

There's a lot more smarts in how the technology uses “backscatter” – the router's signal reflected back towards it – to create an 802.11 channel from the sensor.

The mathematics is in the linked paper, but the key point of manipulating backscatter is this: characteristics like frequencies in the reflected signal can be manipulated by changing the antenna that's doing the reflecting.

That can be done with a single digital switch “that toggles between two impedance states, resulting in a binary signal”, they write. So instead of the power budget of a radio transmitter, passive Wi-Fi-equipped switches only need to power that binary switch.

Everything in the group's demonstrator was made with off-the-shelf components, they note, but a custom comms stack is required for the plugged-in device to receive the sensor's communications.

The Register would note that there's at least one good reason why more work is needed for “passive Wi-Fi” to be a practical proposition: the University of Washington paper doesn't mention either security or encryption. ®

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