Redefining Passive in Backscattering with Commodity Devices

Publication Date: 9/21/2020

Event: The 26th Annual International Conference on Mobile Computing and Networking (MobiCom 2020)

Reference: pp. 1-13, 2020

Authors: Mohammad Rostami, NEC Laboratories America, Inc., University of Massachusetts Amherst; Karthikeyan Sundaresan, NEC Laboratories America, Inc.; Eugene Chai, NEC Laboratories America, Inc.; Sampath Rangarajan, NEC Laboratories America, Inc.; Deepak Ganesan, University of Massachusetts Amherst

Abstract: The recent innovation of frequency-shifted (FS) backscatter allows for backscattering with commodity devices, which are inherently half-duplex. However, their reliance on oscillators for generating the frequency-shifting signal on the tag, forces them to incur the transient phase of the oscillator before steady-state operation. We show how the oscillator’s transient phase can pose a fundamental limitation for battery-less tags, resulting in significantly low bandwidth efficiencies, thereby limiting their practical usage.To this end, we propose a novel approach to FS-backscatter called xSHIFT that shifts the core functionality of FS away from the tag and onto the commodity device, thereby eliminating the need for on-tag oscillators altogether. The key innovation in xSHIFT lies in addressing the formidable challenges that arise in making this vision a reality. Specifically, xSHIFT’s design is built on the construct of beating twin carrier tones through a non-linear device to generate the desired FS signal – while the twin RF carriers are generated externally through a careful embedding into the resource units of commodity WiFi transmissions, the beating is achieved through a carefully-designed passive tag circuitry. We prototype xSHIFT’s tag, which is the same form factor as RFID Gen 2 tags, and characterize its promising real-world performance. We believe xSHIFT demonstrates one of the first, truly passive tag designs that has the potential to bring commodity backscatter to consumer spaces.

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