Caraoke: An E-Toll Transponder Network for Smart Cars


The inventors have developed Caraoke, a networked system for delivering smart services using existing e-toll transponders. Caraoke presents a new reader design that can count, localize, and estimate the speed of the cars on the road using collision signals from their e-toll transponders.

Problem Addressed

Electric toll collection transponders (e.g. E-ZPass) are simple devices consisting of a battery-powered RFID. They are perhaps among the most-widely used wireless communication technologies. There is a big opportunity for using e-toll transponders to enable smart cities, for example by deploying readers on traffic lights that query the transponders and track the number of cars at every intersection. Unfortunately, e-toll transponders are designed under the assumption that only one transponder transmits at any point in time, and hence have no MAC protocol to prevent collisions. Replacing the large infrastructure of deployed transponders to support a MAC protocol would be time and cost prohibitive. The Inventors have developed Caraoke, a system that exploits the structure of the existing e-toll transponder signal and its properties in the frequency domain to deliver smart services.


Caraoke presents a new reader design that can count, localize and estimate the speed of cars on the road using collusion signals from their e-toll transponders. The key feature that enables Caraoke to work in the presence of collision is its ability to exploit the transponders’ carrier frequency offset (CFO) as RFID devices. The Inventors consider the collision in the frequency domain as opposed to the time domain, and show that each collision exhibits spikes that correspond to the CFO of the colliding transponders. Further, e-toll transponders have particularly large CFOs that span 1.2HMz, creating a significant separation between the spikes. Thus, they can estimate the number of transponders by counting these spikes.

They also use the differences in CFO to measure the wireless channels to the individual transponders, and hence apply RF-based localization to track cars and measure their speeds. Caraoke can also decode the IDs of the colliding transponders, say to charge a car for parking or speeding. To do this, Caraoke leverages the channels and CFO measurements to combine multiple collisions in a manner that the signals from the target transponder add up coherently, whereas the signals of other colliding transponders combine incoherently. This allows Caraoke to boost the SNR of the target transponders above others, and enable it to decode the ID of the target transponder. 


  • Small and low-cost device amenable to large-scale deployment
  • Caraoke is built into a custom designed printed circuit board (PCB) and connects to the Internet via an LTF modem; energy harvested from solar power, making it easy to deploy on street-lamps.