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 10mW 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 7 ) 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.

SAW sensor read range limitations and perspectives

“…The receiver antenna converts the RF power to DC as long as 100 mV of voltage is induced. The relation between the induced voltage and communication range is explained by [21] as in (14).…”

“…There are still a need and future perspectives to increase the readout distance. Authors in [14] proposed the operation of a SAW device at 2.45 GHz ISM band and demonstrated a reading distance of up to 11 m for indoor measurements and 13 m in outdoor operation. Time-position coding was used with one reflector per 16 possible positions to visibly reduce the losses and increase the read range.…”

Ultra-wide-band SAW sensors and tags

“…Due to techological constraints, initial sensor developments were performed in order to operate in the lower frequency range 200 MHz-400 MHz [6] and are discussed here to demonstrate basic principles of UWB readout. A single crystal (YXl)/128 • LiNbO 3 substrate is patterned with Interdigitated Transducer (IDT) AlCu electrodes along a geometry allowing for Linear Frequency Modulated (LFM) chirp generation (Fig.…”

“…Because of the rather high acoustic propagation loss in this frequency range, about 6 dB/μs, the possible code signal duration is limited to T < 3μs. Although the theoretical informative capacity, proportional to B · T , is 200 bits, recent practical implementations use 30 bit [6]and the measured devices with 64 Data Bits, and 16 Bit Error Detection were demonstrated [7]. The collision problem is not addressed in these considerations and only allow for a fraction of this information capacity to be used by any single sensor at a given time.…”

Model-Based Identification Method for Pulse Position Coded Surface Acoustic Wave Identification Tags

“…Up to now, grid estimation has commonly been performed using so-called calibration or framing reflectors on fixed positions. These framing reflectors reduce the space left for data and limit the capability of simultaneously identifying multiple tags [11], [12].…”