Paper published by the University of Washington: Ambient backscatter: wireless communication out of thin air.

Definition of a system to allow very near ultra-low-power devices to communicate by backscattering ambient radio signals like TV, also leveraging these ambient signals as power supply. Thus contrary to RFID cards no device involved in the communication requires an external power source. Includes the specification of a communication protocol to allow devices to wake up only when an actual transmission is ongoing.