Towards robust general-purpose backscatter with commodity radios

Over the past decade, backscatter nodes have received booming interest for many emerging mobile applications, such as sports analytics and interactive gaming. However, problems remain in making backscatter communication into general-purpose battery-free data transfer for IoTs. First, backscatter networks are not ready to provide a high-throughput and stable communication platform for billions of such mobile nodes. The common mapping paradigm that chooses the optimal rate based on RSSIs is hardly adaptable to hardware diversity, and the current probing processes are not optimized for mobile scenarios due to inefficient probing trigger. Second, although backscatter communication can already be achieved with ambient signals, the current system suffers from several key issues, including redundant modulation, productive-data dependency, and lack of interference countermeasures. To address those issues, we propose a mobility-aware rate adaptation link-layer that fully exploits the mobility hints from PHY information to deliver a high-throughput link-layer for mobile backscatter networks. The key insight is that mobility-hints can greatly benefit linklayer design, including rate selection and channel probing. Moreover, we introduce robust modulation designs with ambient signals, e.g., BLE, WiFi. Specifically, we propose direct frequency shift modulation with the single tone generated by an excitation BLE device, making robust single-bit modulation possible. Besides, we present a novel backscatter modulation design that can take uncontrolled OFDM WiFi signals as excitations and efficiently embeds tag data at the single-symbol rate. The prototype is implemented using COTS RFID readers, commodity radios, commercial RFID tags, and customized tags with FPGAs. Our extensive experiments demonstrate that the mobility-aware link-layer achieves up to 3.8x throughput gain over the state-of-the-art methods across a wide range of mobility, channel conditions, and tag types. Besides this, the BLE modulation design achieves more than 17x uplink goodput gains over FreeRider under indoor and outdoor environments. And the maximum throughput of our WiFi modulation design is 3.92x and 1.97x better than FreeRider and MOXcatter.