LoMu: Enable Long-Range Multi-Target Backscatter Sensing for Low-Cost Tags

Backscatter sensing has shown great potential in the Internet of Things (IoT) and has attracted substantial research interest. We present LoMu, the first long-range multi-target backscatter sensing system for low-cost tags under ambient LoRa. LoMu analyzes the received low-SNR backscatter signals from different tags and calculates their phases to derive the motion information. The design of LoMu faces practical challenges including near-far interference between multiple tags, phase offsets induced by unsynchronized transceivers, and phase errors due to frequency drift in low-cost tags. We propose a conjugate-based energy concentration method to extract high-quality signals and a Hamming-window-based method to alleviate the near-far problem. We then leverage the relationship between the excitation signal and backscatter signals to synchronize TX and RX. Finally, we combine the double sidebands of backscatter signals to cancel the tag frequency drift. We implement LoMu and conduct extensive experiments to evaluate its performance. The results demonstrate that LoMu can accurately sense 35 tags at the same time. The average frequency sensing error is 0.7% at 400m, which is 4× distance of the state-of-the-art.