Now stay connected to a Wi-Fi with up to 99% of low battery consumption. Getting confused? Here is the story of Passive Wi-fi…!
University of Washington students Bryce Kellogg, Vamsi Talla, Shyamnath Gollakota and Joshua R. Smith invented a new hardware for getting connected to the Wi-Fi using antenna signals. The hardware is designed in a way where it will use 10,000 times less power than present Wi-Fi networking equipment. This system is known as Passive Wi-Fi and it works just like a home router, just more efficiently. To give some perspective, the state of the art in low power WI-FI transmissions today consume 100’s of milli-watts of power, whereas the technology the student researchers developed consume only 10-50 micro watts 10,000 times lower power.
Passive Wi-Fi that demonstrates for the first time that one can generate 802.11b transmissions using backscatter communication, while consuming 3 – 4 orders of magnitude lower power than existing Wi-Fi chipsets. Passive Wi-Fi transmissions can be decoded on any Wi-Fi device including routers, mobile phones and tablets. Building on this, we also present a network stack design that enables Passive Wi-Fi transmitters to coexist with other devices in the ISM band, without incurring the power consumption of carrier sense and medium access control operations. We build prototype hardware and implement all four 802.11b bit rates on an FPGA platform. 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 14.5 and 59.2 µW respectively. This translates to 10000x lower power than existing Wi-Fi chipsets and 1000x lower power than Bluetooth LE and ZigBee.
Our design has two main actors: a plugged-in device and passive Wi-Fi devices. The former contains power consuming RF components including frequency synthesizer and power amplifier and emits a single tone RF carrier. It also performs carrier sense on behalf of the passive WiFi device and helps coordinate medium access control across multiple passive Wi-Fi devices. The passive WiFi device back scatters the tone emitted by the plugged-in device to synthesize 802.11b transmissions that can be decoded on any device that has a Wi-Fi chipset. In the rest, we first provide a quick primer for 802.11b physical layer and back scatter communication.