[Paper Review] Rebooting Ultrasonic Positioning Systems for Ultrasound-incapable Smart Devices

Qiongzheng Lin, Zhenlin An+1 more|arXiv (Cornell University)|Dec 6, 2018

Indoor and Outdoor Localization Technologies46 references4 citations

TL;DR

This paper proposes UPS+, a novel ultrasonic positioning system that enables ultrasound-incapable smart devices to achieve centimeter-level indoor localization by exploiting the nonlinear distortion in device microphones to downconvert high-frequency ultrasonic signals into audible frequencies. The system uses two types of beacons—uBeacons for trilateration and a cBeacon for frequency mixing—achieving sub-centimeter accuracy without requiring hardware modifications to existing devices.

ABSTRACT

An ultrasonic Positioning System (UPS) has outperformed RF-based systems in terms of its accuracy for years. However, few of the developed solutions have been deployed in practice to satisfy the localization demand of today's smart devices, which lack ultrasonic sensors and were considered as being `deaf' to ultrasound. A recent finding demonstrates that ultrasound may be audible to the smart devices under certain conditions due to their microphone's nonlinearity. Inspired by this insight, this work revisits the ultrasonic positioning technique and builds a practical UPS, called UPS+ for ultrasound-incapable smart devices. The core concept is to deploy two types of indoor beacon devices, which will advertise ultrasonic beacons at two different ultrasonic frequencies respectively. Their superimposed beacons are shifted to a low-frequency by virtue of the nonlinearity effect at the receiver's microphone. This underlying property functions as an implicit ultrasonic downconverter without throwing harm to the hearing system of humans. We demonstrate UPS+, a fully functional UPS prototype, with centimeter-level localization accuracy using custom-made beacon hardware and well-designed algorithms.

Motivation & Objective

• Address the limitation of existing ultrasonic positioning systems (UPS) that require specialized ultrasonic sensors, which are absent in most modern smart devices.
• Overcome the challenge of enabling ultrasound-based localization on devices without ultrasonic hardware, which are considered 'deaf' to ultrasound.
• Leverage the nonlinear response of smartphone microphones to convert high-frequency ultrasonic signals into audible frequencies that can be processed by standard devices.
• Design a practical, energy-efficient, and deployable indoor positioning system that achieves sub-centimeter accuracy using off-the-shelf components and minimal device modifications.
• Demonstrate real-world applicability through use cases such as locating AirPods, pairing iPads, and tracking hand movements with an Apple Watch.

Proposed method

• Deploy two types of custom beacon devices: uBeacons (battery-powered, for trilateration) and a cBeacon (cable-powered, for downconversion).
• Transmit ultrasonic beacons at two distinct high frequencies (e.g., f₁ and f₂) from uBeacons, while the cBeacon transmits a second ultrasonic tone at a different frequency.
• Exploit the nonlinear mixing effect in smartphone microphones to generate a difference frequency component at |f₁ - f₂|, effectively downconverting the ultrasonic signal into the audible range.
• Ensure the downconverted signal falls within the 2–15 kHz range to be compatible with the sampling and filtering characteristics of common smartphone microphones and audio formats.
• Use precise time synchronization via PTP (Precision Time Protocol) to enable accurate round-trip time-of-flight measurements for ranging.
• Implement a low-duty cycle operation to minimize energy consumption, extending battery life of uBeacons to up to 8 months under optimal settings.

Experimental results

Research questions

• RQ1 Can the nonlinear distortion in smartphone microphones be harnessed to enable reception of ultrasonic signals without hardware modifications?
• RQ2 To what extent can a heterogeneous beacon architecture (uBeacon and cBeacon) enable centimeter-level localization accuracy on ultrasound-incapable devices?
• RQ3 How can the system maintain high accuracy while operating entirely within the ultrasonic spectrum to avoid human and pet hearing interference?
• RQ4 What is the practical energy efficiency of the system, and how can duty cycling extend the operational lifetime of battery-powered uBeacons?
• RQ5 Can the system be effectively deployed in real-world applications such as locating small devices (e.g., AirPods), pairing devices, and tracking human limbs?

Key findings

• UPS+ achieves centimeter-level localization accuracy using only off-the-shelf ultrasonic transducers and standard smartphone microphones, without requiring hardware modifications.
• The system successfully leverages microphone nonlinearity to downconvert ultrasonic signals (e.g., 20–22 kHz) to the audible band (2–15 kHz), enabling reception on standard devices.
• The cBeacon and uBeacon architecture enables robust trilateration with sub-centimeter accuracy in controlled and real-world environments.
• Energy consumption is significantly reduced by adopting a low-duty cycle: increasing the PTP synchronization interval from 1 second to 64 seconds extends uBeacon battery life from 5 to 8 months.
• The system is validated in real-world applications, including locating AirPods, pairing iPads within 10 cm, and tracking hand-arm motion with an Apple Watch, demonstrating practical utility.
• The downconverted signal spectrum is carefully tuned to 5–15 kHz to ensure compatibility with common audio formats (e.g., M4A) that filter out frequencies above 16 kHz.