Realization of a CDMA-based RFID System Using a Semi-active UHF Transponder

RFID systems may increase the efficiency of logistic systems, e.g., inventory of stocks. Unfortunately, present existing systems come fast to their limits if a great amount of RFID transponder must be inventoried in very short time, as the channel access method of current RFID systems is Time Division Multiple Access. To shorten the inventory process, this work shows a first realization of an RFID-based semi-active UHF transponder using a Code Division Multiple Access method. The work concentrates on the uplink channel (tag to reader) that covers the transponder´s backscattered signals. This channel access method enables simultaneous transfer of data, meaning that all transponder in the field may respond at the same time within the same frequency band. The data transfer realized for the uplink channel, uses a certain set of orthogonal spreading sequences (modified Gold codes) being different for every transponder. The RFID reader used in this work despreads the backscattered signals and decodes the data of the different transponder. Briefly, this work shows, in principle, the opportunity for a simultaneous data transfer on the uplink channel in RFID systems, which in turn may shorten very much the time for a complete inventory round.