An RFID tag has a frequency agile RF transceiver that transmits data from the RFID tag to a reader and a direct sequence spread spectrum RF receiver that receives communications from the reader. Messages between the reader and the RFID tag are divided into frames, and each frame contains a frame header...http://www.google.co.uk/patents/US7044387?utm_source=gb-gplus-sharePatent US7044387 - RFID tag and communication protocol for long range tag communications and power efficiency
