Patent ID: 7286042

Claim:
A passive communication device for use in a communication system also having a base station, the communication device comprising an internal power source, receiver means for receiving signals transmitted from the base station, and processing means selectively responsive to an interrogation signal received from the base station by said receiver means to generate a data signal for transmission to the base station, wherein said receiver means comprises a first receiver element and a second receiver element, said first receiver element having a lower power consumption when energised than said second receiver element does when energised, said communication device having a standby state in which said internal power source energises said first receiver element so as to detect the presence of a received signal, and in which said second receiver element and said processing means are de-energised, said first receiver element being responsive to detection of a received signal to trigger a state of operation of said communication device in which said internal power source energises said second receiver element, said second receiver element, when energised, being selectively responsive to a characteristic of the signal received to enable said internal power source to energise said processing means and to trigger an active state of operation in which said processing means is actuated, and said processing means comprising demodulator means responsive to a modulation of the received signal when said demodulator means is actuated.