PilotScatter: High-Throughput OFDM Backscatter via Pilot Tones

Backscatter is an emerging ultra-low-power wireless communication technology for Internet-of-Things. However, as the widely used modulation scheme in OFDM systems, the combination of QAM and WiFi backscatter is not very satisfactory. To deploy QAM in the WiFi backscatter system, we propose PilotScatter, the first OFDM backscatter system supporting both 16-QAM modulation and non-redundant coding. These changes significantly improve the throughput of PilotScatter over previous backscatter systems. The key insight of PilotScatter is the use of pilot tones and differential demodulation. By modifying the phase and amplitude of pilot tones of the carrier signal, PilotScatter can modulate tag data on ambient WiFi with a 16-QAM scheme. At the receiver, PilotScatter uses a differential algorithm to demodulate tag data. It makes PilotScatter achieve 16-QAM demodulation without relying on ambient WiFi data and past symbols. We prototype PilotScatter using FPGAs, commodity radios, and USRPs. Comprehensive evaluations demonstrate that PilotScatter achieves up to 932.22 kbps throughput for WiFi 802.11g, and the backscatter communication range (Tag-to-Rx) is up to 19 m in Line-of-Sight (LoS) and 16 m in Non-Line-of-Sight (NLoS). Compared with the symbol-level backscatter research, PilotScatter has 7.49x and 3.78x goodput gains over MOXcatter and RapidRider, respectively.