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Radio ranging with ultrahigh resolution using a harmonic radio-frequency identification system


The accurate sensing of the location of specific objects in an indoor setting is critical for applications including robotic feedback control and non-intrusive structural integrity monitoring. Current optical and ultrasound approaches often suffer from insufficient accuracy, obstruction by other objects, and ambiguous identification. Alternatively, conventional radar-like radio-frequency (RF) methods can suffer from problems such as multipath ambiguity, small time of flight, and limited item recognition. Attachment of a passive RF identification (RFID) tag can provide a unique marker by modulating the backscattering signal, but current systems struggle with high interference and noise, and thus have poor ranging accuracy. Here we show that a 1 GHz harmonic RFID system can provide a ranging resolution of less than 50 micrometres with a sampling rate of greater than 1 kHz. The fundamental limits on ranging precision in our system are traced to the phase noise of the RF source and the aperture jitter of the data converter. The small passive tag required for the approach can be embedded in indoor or underwater objects, as well as within building structures.

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Fig. 1: The experimental set-up of the harmonic RFID ranging system.
Fig. 2: The experimental results of ranging variations with respect to the frequency strategy.
Fig. 3: The experimental results of the quasi-static ranging.
Fig. 4: The experimental results of the tag movement and permittivity based on the two-tag structure.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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This project is supported by Department of Energy (DoE) of the United States under the Advanced Research Projects Agency – Energy (ARPA-E) project numbers: DE-AR0000528 and DE-AR0000946. The authors thank G. C. McLaskey and A. Lal for discussions.

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Authors and Affiliations



X.H. and E.C.K. perceived the fundamental concepts and brainstormed the design of experiments together. X.H. conducted all the experiments, data processing and manuscript preparation. E.C.K. supervised the project direction and helped with revisions of design and writing.

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Correspondence to Xiaonan Hui.

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The authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary Figures 1–10

Supplementary Video 1

Real-time experiment with 25 μm step size in water.

Supplementary Video 2

Real-time experiment with 5 cm round trip in water.

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Hui, X., Kan, E.C. Radio ranging with ultrahigh resolution using a harmonic radio-frequency identification system. Nat Electron 2, 125–131 (2019).

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