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.