Patent ID: 8633805

Claim:
A method, comprising: transmitting at least one first radio frequency signal to a plurality of devices positioned in a field, each respective device in the plurality of devices configured to store an identifier to uniquely identify the respective device among the plurality of devices, and selectively respond to radio frequency signals broadcast to the field based on the identifier stored in the respective device and commands modulated on the radio frequency signals, wherein the at least one first radio frequency signal is configured to request a first device having a first identifier in the plurality of devices to response to a subsequent radio frequency signal and to request second devices having second identifiers different from the first identifier in the plurality of devices to generate no response to the subsequent radio frequency signal; transmitting the subsequent radio frequency signal to the field; receiving at least one second radio frequency signal, responsive to the subsequent radio frequency signal, from the first device having the first identifier; removing data modulation from the at least one second radio frequency signal to determine a Doppler frequency shift in the at least one second radio frequency signal; and identifying a motion of the first device, based on the Doppler frequency shift.