SAW-tag system with an increased reading range

Abstract

Many applications of passive RFID labels (tags) require a reading range larger than 10m compounded with the limited to lOmW power radiated by the interrogation system. In this paper it is shown that SAW-tags operating in 2.45 GHz ISM band can meet such criteria. The losses in a "D7" SAW-tag with 24 bit of code ( ~1.7 *10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ) were minimized to a record level of about 37dB for code responses by using a uni-directional transducer (SPUDT, group type) and by decreasing the total number of reflectors. The reflectors were carefully weighted using FEM/BEM simulator FEMSAW and the code responses amplitudes had uniformity ±1.5dB. We use time-positioning coding with one reflector per 16 possible positions and, thus, sacrificed some code capacity, but visibly reduced the losses. The calibration reflectors, the check sum reflectors and increased guard time delays between possible time positions of responses were used to generate a robust reading algorithm. The tag and the reader antennas had directivities of 8dBi and 16.5 dBi respectively. The reading distance of the D7 tag was up to 11m for indoor measurements and 13m in open space. Such a distance is achieved with truly passive tags requiring no battery or other power supply. We show that the same loss level can be expected for D9 SAW-tag with 1 billion of possible codes. The read error rate as a function of signal-to-noise ratio is estimated.