The application of multiport theory for MIMO RFID backscatter channel measurements

This contribution deals with channel measurements in multiple-input multiple-output radio-frequency identification systems (MIMO RFID). Herein, multiple antennas are applied at the reader and at the RFID tag as well. A method for the determination of the channel coefficients between all antennas is presented. The channel information is needed to apply MIMO techniques to enhance the data rate or the reliability or range of the backscatter link. Since no measurement can be conducted at the tag antenna ports the method is based on the theory of determining the scattering parameters of a multiport (n-port) with a reduced number of measurement ports. Herein, the load impedances within the RFID tag being normally used to transmit information via backscatter modulation are used as known reflection standards. An internal thru standard between the tag antenna ports is introduced to reduce phase ambiguities. The method could be applied in future generations of multi-antenna RFID tags or could be used to characterize the channel for MIMO RFID prototype systems to evaluate their performance limits e.g. by calculating the MIMO capacity. Measurement results for the proposed method for a system with two reader transmit and receive and two tag antennas (2×2×2) are presented (5 to 6 GHz).