23.1 A 44μW IoT Tag Enabling 1μs Synchronization Accuracy and OFDMA Concurrent Communication with Software-Defined Modulation.

Backscatter tags have proven advantageous in reducing the power consumption of ultra-low upload data-rate IoT devices from milliwatts to tens of microwatts [1, 2]. Previous backscatter ICs generally employ codeword translation and rely on the incident signal being a modulated wave compliant with 802.11b [1] or utilizing backscatter modulation of the incident single tone to generate a signal that adheres to established standards [2]. However, packet collisions occur in conventional backscatter IC systems with multiple tags in the following scenarios: 1) all tags synchronize with the incoming packet and concurrently backscatter their data in response to a modulated Wi-Fi or BLE signal; 2) the incident signal consists of an unmodulated single tone, and each tag individually performs standard-compliant modulation. In [3], a backscatter system design is presented that can realize Orthogonal Frequency Division Multiple Access (OFDMA) in the 802.11g framework. Nevertheless, its TX-to-tag distance is limited to 2m. The multi-tag issue in backscatter networks limits the widespread deployment of IoT devices. To date, none of the existing works could support collision-free multi-tag transmissions while maintaining microwatt-level power consumption.