Spectro-Temporal Combining in Bistate WiFi Backscatter Communication With Frequency Shift

In WiFi backscatter communication, the frequency shift technique allows a backscattered signal to appear not in the frequency channel of the carrier signal but in adjacent ones, thus avoiding noisy OFDM-based carrier signals and increasing the communication range. Through testbed experiments, we observe that frequency shift is effective in mitigating the impact of the inherent fluctuation of WiFi signals, particularly in bistate backscatter communication; however, due to the weak strength of the backscattered signal, other signals from incumbent transmitters may appear in the shifted frequency channels, significantly interfering with the backscattered signal. To combat this challenge in a way that is nondisruptive to incumbent transmitters, we propose a receiver-side spectro-temporal combining scheme in which spectrum combining is performed to suppress interference appearing in one of the shifted channels, while temporal combining is performed with transmission repetitions to suppress bit errors resulting from residual interference. The scheme's on-the-fly spectrum combining and bit-sequence temporal combining require minimal buffer memory. Through system prototyping and testbed experiments, we demonstrate that the proposed scheme outperforms the conventional and temporal-combining-only cases in terms of the bit error rate and throughput under various conditions.