http://passivewifi.cs.washington.edu/
http://www.pcworld.com/article/3036777/networking/passive-wi-fi-researchers-promise-to-cut-wi-fi-power-by-10000x.html
Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions
Our experimental evaluation shows that passive Wi-Fi transmissions can be decoded on off-the-shelf smartphones and Wi-Fi chipsets over distances of 30 – 100 feet in various line-of-sight and through-the-wall scenarios. Finally, we design a Passive Wi-Fi IC that shows that 1 and 11~Mbps transmissions consume 14.48 and 49.28 µW respectively. This translates to 10000x lower power than existing Wi-Fi chipsets and 1000x lower power than Bluetooth LE and ZigBee.