The Internet of Things world could power a lot of things with WiFi signals, if only access points broadcast all the time instead of when they've got something to say.
There's a bucket of research into scavenging ambient signals to power
junk stuff things, and WiFi is a popular energy source, but it's intermittent.
As the University of Washington in Seattle boffins led by Vamsi Talla found in this paper at Arxiv, WiFi's got some useful characteristics for power delivery.
For example, they write, the OFDM protocol produces a signal that's good for power delivery, with a high peak-to-average power ratio, and its ubiquity means there are lots of cheap chipsets available.
It's the intermittency that spoils the party: a sensor placed a few metres – oh, all right, since it's an American paper, ten feet – from a wireless router couldn't get enough power to stay at the minimum 300 mV it needed to operate.
On the other hand, if you the router talks all the time, the users are bound to notice that they're not getting the performance they expect.
Talla et al's PoWiFi – power over WiFi – proposal is to hack the router so it “imitates a continuous transmission while minimising the impact on WiFi performance”.
Those pesky silences spoil WiFi as a power source
So they don't ruin life for users, the PoWiFi proposal is that power signals need also be frequency-agile (just like data transmissions). They use a few channels at a time, and leave the rest of the signal alone.
Where the group is claiming a significant breakthrough is that it's difficult to design a power-receiving antenna that works happily across multiple 2.4 GHz WiFi channels.
As they write: “Designing this is challenging because a fraction of the incident signal is typically reflected back into the environment and remains unusable because of impedance mismatches in the hardware … Our approach is to co-design all the harvesting hardware components—rectifier, matching network and DC–DC converter—to achieve reflection losses less than −10 dB across the desired 72 MHz Wi-Fi band”.
With more efficient harvesting at the sensor end, the relatively straightforward bit of programming at the router is to get it to watch traffic on the air interface: “the router injects power traffic only when the number of packets queued at the WiF interface is below a threshold”.
While all wireless charging schemes are relatively inefficient, The Register notes that the attraction of the University of Washington group's work is that rather than rolling out a new wireless charging infrastructure, they're trying to make efficient use of a signal source that's already there, without turning the WiFi router into a full-time waste of power.
The researchers reckon they've powered temperature and camera sensors at 20 feet and 17 feet respectively. Coin-cell batteries can be charged at greater distances, the boffins write. ®