Toward Software-Defined Backscatter Modulation via Signal Emulation

The vision of backscatter communication always incorporates compatibility with active radios to enable low-cost and easy deployment. However, recent innovations lack the flexibility to communicate with heterogeneous wireless devices directly. In this paper, we design and implement a flexible backscatter system, i.e., Flexcatter, which can support various modulation schemes in a software-defined way, to be compatible with different kinds of active radios. The key technique is signal emulation, where the tag can vary the reflection coefficient in the time domain to emulate desired baseband signals. We first carefully design a cost-effective impedance network, which employs two radio frequency (RF) switches to provide up to 16 reflection coefficients. Next, we establish and model the emulation mapping between desired baseband signals and available reflection coefficients. Besides, the emulation frameworks for different modulation schemes are presented. After that, to face the emulation distortion for orthogonal frequency division multiplexing (OFDM), we introduce the oversampling method in the baseband modulation process. We further build the prototype hardware, and experiment results show that Flexcatter can flexibly generate various kinds of backscatter signals, including Wi-Fi, BLE, and LoRa. Especially the OFDM transmission generated by Flexcatter can achieve a throughput of 25.1 Mbps.