An RFID monitoring arrangement comprises a first RFID device and a second RFID device for monitoring the first RFID device. The first RFID device transmits at least two messages each including unique identifying information and a power level code, wherein the transmitter power level corresponds to the...http://www.google.com/patents/US7319397?utm_source=gb-gplus-sharePatent US7319397 - RFID device for object monitoring, locating, and tracking
