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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.