Project CRADLE: Combined RF/Acoustic Detection and Localization of Passive Tags

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As the number of wirelessly connected devices continues to grow, new opportunities emerge to collect data that can deepen our understanding of human activity and surrounding environments. However, managing these devices becomes increasingly challenging as form factors shrink to the centimeter scale and power budgets approach zero. Passive (battery-free) tags are particularly attractive for large-scale deployment, but their operation poses major challenges in remote powering, communication, and localization. Existing approaches—such as duty cycling and far-field wireless powering—depend critically on accurate tag localization. Yet, localizing centimeter-scale passive devices remains difficult because their radar cross-sections are extremely small, rendering them nearly indistinguishable from environmental clutter in conventional imaging or radar systems [1].

To overcome these limitations, we developed CRADLE (Combined RF/Acoustic Detection and Localization of passive tags), a hybrid RF–acoustic system that detects and localizes passive tags by jointly leveraging electromagnetic and acoustic domains. Each tag consists of a passive electrical network connecting an RF antenna to an ultrasonic transducer. The reader simultaneously transmits continuous-wave (CW) RF and pulsed ultrasound to interrogate the environment. When the ultrasonic pulse reaches a tag, it modulates the transducer’s capacitance, which in turn produces a time-varying load on the antenna. This modulation parametrically alters the reradiated RF signal, embedding the acoustic time-of-flight information into the received RF waveform. By demodulating this backscattered RF signal, the reader can precisely recover the tag’s range and position [1].

We have implemented a proof-of-concept CRADLE platform with custom-designed tags and reader hardware to experimentally validate the concept. Measurements demonstrate sub-decimeter localization accuracy at distances up to 6 m, significantly outperforming conventional RF-only localization methods. With continued refinement of tag architecture and reader signal processing, we anticipate extending this capability to centimeter-scale passive tags with sub-centimeter accuracy over multi-meter ranges [1].

Publications

[1] A. S. Rekhi, E. So, A. Gural, and A. Arbabian, “CRADLE: Combined RF/Acoustic Detection and Localization of Passive Tags,” IEEE Transactions on Circuits and Systems I: Regular Papers, vol. 68, no. 6, pp. 2555–2568, June 2021, doi: 10.1109/TCSI.2021.3064990.