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).

I. Introduction

Within the last years the proliferation of RFID systems and technologies has increased enormously [1]. Besides the purpose of identifying goods, e.g. in supply chain management, RFID technologies become more and more important in other scenarios of data exchange, e.g. as an alternative to Bluetooth or ZigBee. For passive and semi-passive, i.e. battery-assisted RFID tags, the transmission of data between tag and reader relies on the principle of backscatter modulation by varying the mismatch between the tag antenna and its load. Thus, due to its dyadic channel structure, a backscatter system suffers from heavy path loss, which increases with the fourth power of the distance like in a radar system [2]. In multipath environments the system performance is additionally limited by fading effects, which are more severe than in classical “one way” communication systems [3]. Figure 1. MIMO RFID system