Transfer Beamforming via Beamforming for Transfer.

Although billions of battery-free backscatter devices (e.g., RFID tags) are intensively deployed nowadays, they are still unsatisfying in the two major performance limitations (i.e., short reading range and high miss reading rate) resulting from the current harvesting inefficiency. The classic beamforming technique is regarded as the most promising solution to address the issue. However, applying it to backscatter systems meets the deadlock start problem, i.e., without enough power, the backscatter cannot wake up to provide channel parameters; but, without channel parameters, the system cannot form beams to provide power. In this work, we propose a new paradigm called transfer beamforming ( ${\sf TBF}$ ), namely, the beamforming strategies can be transferred from reference tags with known positions to power up other unknown neighbor tags of interest. In short, transfer beamforming (is accomplished) via (launching) beamforming (to reference tags first) for (the purpose of) transfer. To do so, we adopt the semi-active tags as the reference tags, which can be powered up with a normal reader in a wide range. Then the beamforming is initiated and transferred to power up the low-sensitive but cost-effective passive tags surrounded by reference tags. A prototype evaluation of ${\sf TBF}$ with 8 transmitting antennas presents a 99.9% inventory coverage rate in a crowded warehouse with 2,160 RFID tags. Our comprehensive evaluation reveals that ${\sf TBF}$ can improve the power transmission by 6.9 dB and boost the inventory speed by 2× compared with state-of-art methods.