We demonstrate for the first time that one can generate 802.11b transmissions using backscatter communication, while consuming 4-5 orders of magnitude lower power than existing Wi-Fi chipsets. Wi-Fi has traditionally been considered a power-consuming system. Thus, it has not been widely adopting in the sensor network and IoT space where low-power devices primarily transmit data. We believe that, with its orders of magnitude lower power consumption, passive Wi-Fi has the potential to transform the Wi-Fi industry. pdf
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 backscatters 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.
Unlike traditional active radio communication that requires generating RF signals, devices using backscatter communication modulate the radar cross-section of their antenna to change the reflected signal. To understand how backscatter works, consider a device that can switch the impedance of its antenna between two states. The effect of changing the antenna impedance is that the radar cross-section, i.e., the signal reflected by the antenna, also changes between the two different states.
Wi-Fi uses carrier sense to share the ISM band. This however requires a Wi-Fi receiver that is ON before every transmission. Since Wi-Fi receivers require powerconsuming RF components like LNA, frequency synthesizers, mixers and ADCs, this would eliminate the power savings from our design. Instead, we delegate the task of carrier sense to the plugged-in device, which also arbitrates access between multiple passive Wi-Fi devices.
The winner here is battery life for both low and high bandwidth sensors.
See Beta Battery which could easily produce 2x the 0.059 milliwatt signaling power required to reflect at 11 Mbps.