SAW RFID spread spectrum OFC and TDM technology

SAW based RFID sensors can offer wireless, passive operation over harsh environments, and various device embodiments are used for retrieval of the sensed data. SAW has many unique advantages over possible competing technologies, including the following characteristics: passive, radiation hard, operable over wide temperature ranges, small, rugged, inexpensive, and identifiable. In a multi-sensory environment, it is necessary both to identify the sensor and to retrieve the sensed information. The OFC SAW technology approach has been funded by NASA for possible inclusion in ground, space flight, and space exploration sensor applications. The purpose of this paper is to present the concept of encoding SAW-based sensors for use in a multiple-sensor environment. The emphasis will be on orthogonal frequency coded (OFC) devices overlaid with a form of time division multiplexing (TDM). It will be shown that in addition to the benefits of OFC such as enhanced processing gain and lower interrogation power spectral density (PSD), the TDM allows for a decrease in overlapping energy and therefore a decrease in intersensory collisions which are shown to cause peak ambiguity. This approach should be applicable to many different SAW-based sensors (such as temperature, pressure, liquid, gas, etc.), as well as allow for proper IDing. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance. This paper will discuss the use of SAW OFC in a temperature sensor application. Devices are then used in computer-simulated transceiver design, and the results of a prototype sensor system are discussed.