Universal Content-Agnostic Backscatter for OFDM WiFi

Ambient backscatter is one of the most promising solutions for the widespread deployment of low-power IoT. We present CAB, a content-agnostic backscatter system that can demodulate both tag and ambient data from ambient backscattered WiFi alone. In contrast to prior ambient backscatter systems that use ambient data (content) to demodulate tag data, we focus on zero-subcarriers, which are invariant and independent for any ambient OFDM WiFi. The idea of using zero-subcarriers to convey tag data is simple and elegant. Not only does it for the first time remove the dependency of tag-data demodulation on ambient data, but it also significantly improves the practicality of ambient backscatter. CAB is also universal for various OFDM-WiFi signals because the zero-subcarrier we use is virtual and is essentially the phase estimation of the pilot, which is not affected by different pilot patterns. In order to verify the universality, we prototype CAB using off-the-shelf FPGAs and SDRs. Extensive experiments show CAB is universal as it can work with multi-band, multi-stream, and multi-user ambient traffic, including WiFi 3/4/5/6. In addition, CAB can work with excitations at different data rates over various commercial NICs. As the first content-agnostic backscatter system with 340.9 Mbps aggregate throughput, we believe CAB takes a crucial step forward on ubiquitous battery-free IoTs.