Caraoke: An E-Toll Transponder Network for Smart Cities – Public Review

E-Toll transponders are commonplace in the US: they allow vehicles to pay highway tolls electronically; and in many highways, they are actually required. The core of an E-Toll transponder is a battery-powered RFID. When queried by a radio signal from a reader, it responds by transmitting a radio signal at a fixed carrier frequency with its identity encoded. By decoding this radio signal, the reader can identify and bill the vehicle properly. However, existing transponders and readers are designed with the assumption that only one transponder responds per query. When multiple transponders responds to the same query, their responses collide because they are using the same carrier frequency and subsequently the reader will fail to decode them. Therefore today’s E-Toll readers have to use directional antennas and restrict the movement of vehicles so that only one transponder responds to a query.