Achievable performance and effective interrogator design for SAW RFID sensor tags

Remote sensing is a critical application that supports activities such as environmental monitoring, planetary science, structural shape and health monitoring, non-destructive evaluation, etc. that are critical to many NASA missions. The utility of the remote sensing devices themselves is greatly increased if they are "passive" - that is, they do not require any on-board power supply such as batteries - and if they can be identified uniquely during the sensor interrogation process. In this paper, we consider one very promising passive sensor technology, called surface acoustic wave (SAW) radio-frequency identification (RFID), that satisfies these criteria. Although SAW RFID tags have great potential for use in numerous space-based remote sensing applications, the limited collision resolution capability of current generation tags limits the performance in a cluttered sensing environment. That is, as more SAW-based sensors are added to the environment, numerous tag responses are superimposed at the receiver and decoding all or even a subset of the telemetry becomes increasingly difficult. Background clutter generated by reflectors other than the sensors themselves is also a problem, as is multipath interference and signal distortion, but the limiting factor in many remote sensing applications can be expected to be tag mutual interference. In this paper, we present the results of a research effort aimed at providing answers to the following questions: 1) What are the fundamental relationships between tag parameters such as bit-rate, time-bandwidth-product, SNR, and achievable collision resolution? 2) What are the differences in optimal or near-optimal interrogator designs between noise-limited environments and interference-limited environments? 3) What are the performance characteristics of different interrogator designs in term of parameters such as transmitter power level, range, and number of interfering tags?