$\textsf{Lotus}$ : Rethinking Polarization Mismatch for Carrier Cancellation in Backscatter Systems

Carrier interference is a fundamental research challenge in backscatter systems. Existing solutions leverage frequency shifting or full duplex designs to mitigate carrier interference in the analog or digital domain. However, these solutions introduce extra spectrum usage, protocol overhead, and power consumption, all of which are undesirable in backscatter systems. In this paper, we revisit polarization mismatch and propose Lotus, a low-cost analog design to combat carrier interference for backscatter systems. Lotus comprises a novel antenna design and a backscatter tag design. At runtime, Lotus antenna replaces the receiver default antenna to cancel out the carrier interference, without any protocol or hardware overhead. Lotus tag mitigates the power loss caused by polarization mismatch and remains compatible with all existing backscatter radios. Experimental results show that Lotus achieves comparable cancellation gain (i.e., 42 dB) with the state-of-the-art frequency-shifting baseline. Meanwhile, Lotus outperforms the baseline by 2× and 6.5× in spectrum and power efficiency, respectively. Additionally, Lotus achieves comparable performance to the frequency shifting baseline regarding backscatter range and throughput across three backscatter technologies, including Wi-Fi, Bluetooth, and LoRa.