Abstract: An RFID tag (200) and similar control/sensing unit (300) for a communications system are described. The control/sensing unit has a control element (301) having one or more states, and include an antenna (207) as well as an electrical circuit (201-206) generating a pulse control signal for controlling transmission from the antenna and varying a parameter of the pulse control signal in dependence on the state of the control element (301). Also disclosed is a receiving communication unit in the form of a base unit or system controller for such a communications system, the unit including a receiver (100) for receiving a pulsed RF signal and an electrical circuit (105-106) for detecting a parameter of the pulsed RF signal. An output circuit (107) then produces an output control signal dependent on the detected parameter of the pulsed RF signal.

Abstract: An RFID transceiver tag is proposed for use in a monitoring system for detecting a change in position of the tag relative to an associated RFID tag or to the environment surrounding or between the tags. The transceiver tag has a detector for detecting a characteristic of a signal transmitted between the transceiver tag and the associated RFID tag and for creating a trigger signal if the detector detects that the characteristic has changed beyond a predetermined extent; and a transmitter for transmitting a signal to a remote controller on receipt of the trigger signal from the detector.

Abstract: A system 100 is disclosed for monitoring the position of one or more RFID tags 201. The system has a detector 301 incorporating circuitry 304 for detecting changes in the range of an RFID tag 201 from the detector and for triggering an alarm 401 if a detected change in range of an RFID tag 201 exceeds a predetermined threshold or if the RFID radio tag cannot be detected by the detector 301. Range may be detected, for example, by measuring the time of a returned radio signal from a tag 201, by measuring the strength of a returned radio signal from a tag, or by detecting changes in a periodic interval at which energy is transmitted by a tag.