SAW Spread Spectrum RFID Tags and Sensors

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SAW Spread Spectrum RFID Tags and Sensors D.C. Malocha, B. Fisher, D. Gallagher, N. Kozlovski, J.M. Pavlina, M. Roller, N. Saldanha, and B. Santos School of Electrical Engineering and Computer Science University of Central Florida, Orlando, FL 32826 [email protected]

Abstract— The purpose of this paper is to discuss recent research results of SAW orthogonal frequency coded (OFC) RFID tags and sensors. Efforts have successfully demonstrated many different SAW OFC embodiments that have overcome a number of technological challenges uncovered over the course of the research. Current embodiments have included coding approaches using time, frequency, and phase diversity to reduce code collisions, increase range, lower loss and increase the number of possible codes available. Devices have been successfully demonstrated at 250 MHz, 456 MHz and 912 MHz. Several device embodiments, including the original OFC demonstrated devices at 250 MHz with a 69 MHz bandwidth, operate as ultra wide band (UWB) communication links. Harmonically operated devices have been demonstrated and have shown good performance. Interrogation of devices at 250 MHz has been successfully demonstrated and ranges of several meters achieved. The paper will present a review of the OFC device concept and several new device embodiments at 250, 456 and 912 MHz. System considerations and performance calculations related to sensor interrogation will be shown. A discussion of coded OFC transducers and unidirectional embodiments, which can lead to very low loss and highly diverse devices, will conclude the paper.

I. INTRODUCTION The use of spread spectrum techniques in SAW device technology dates back to the inception of the technology in the early 1970’s with the introduction of chirp and coded transducers. The application of spread spectrum techniques has more recently been applied to device embodiments for encoding RFID tags and RFID sensors, which has the advantageous of code diversity, security, processing gain, and ultra wide band operation.[1-6] The use of spread spectrum in commercial SAW CDMA [7-9] and prototype OFC RFID tags has been successfully demonstrated.[10-13] However, the two types of devices will most probably serve different applications; based on cost versus performance trade-offs. The commercial applications require small footprints, including antenna, huge numbers of codes, and extremely low cost. The chief competition is the silicon (Si) RFID tags which have similar constraints. The primary difference is that the SAW CDMA tag is wireless and passive, while the Si tag is an active tag that requires either a battery, external transmitted or “scavenged” power. The SAW OFC tag, while still low cost and has similar advantages to the CDMA approach, will probably not meet the commercial cost goals of a few cents each. The OFC SAW RFID tag and sensor

research to date has focused on applications that do not require extremely low cost, and/or require an integrated or external sensor capability. One of the primary applications examined is space ground and flight operations and sensing needs. SAW devices provide unique and wide ranging capabilities that include radiation hardness, wireless, passive, low loss, multisensor, secure, small, and rugged. Our group first published the use of OFC within SAW RFID tags and sensors embodiments in 2004.[10] These first devices operated at 250 MHz and had fractional bandwidths of over 25%, exceeding the FCC definition of UWB, which requires greater than 20%. The frequency was chosen for convenience; low enough for ease of device fabrication, yet high enough to produce a reasonably sized prototype system. OFC reflectors and transducers can use a variety of coding techniques within the RFID sensor or tag design, which include multiple carrier frequencies (FDM), delay (TDM) and phases (PN). This multi-layer coding approach of FDM, TDM, and PN coding results in a large number of possible codes, and decreases the effects of code collisions in a multisensor or tag system. Because of the diversity of coding, the devices provide processing gain and secure, spread spectrum signal operation. The OFC approach provides low loss reflector designs, which translates to greater device range capabilities. A typical single frequency CDMA SAW RFID tag has 30+ dB loss, while an OFC SAW tag may have 6-15 dB loss [7,13]. If the device is used with a unidirectional transducer, theoretically zero loss is achievable. The coding approaches for the reflectors are applicable to transducers, providing even more coding diversity. An OFC type transducer can be used in conjunction with OFC reflectors to provide lower loss, larger bandwidths, greater processing gain, and greater code diversity. OFC transducers have been analyzed, designed and tested and have shown expected correlation properties and operation for UWB communication systems [14]. The following sections will discuss several SAW OFC design considerations for tag and sensor applications. II. BACKGROUND The basic multi-sensor system is shown schematically

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Interrogator Clock Post Processor Sensor 3

The work was funded by NASA Graduate Student Research Program Fellowships, and NASA contracts and industrial collaboration with Applied Sensor Research and Development Corporation, contracts NNK07EA38C, and NNK07EA39C, and Mnemonics Corporation, contract NNX08CD43P.

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Figure 1 Passive, wireless, multisensory system block diagram.

2 some OFC sensor implementations will be discussed. III. SAW ACOUSTIC COMPONENT DISCUSSION f1

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A. OFC Chip and Reflector Insertion Loss The OFC reflectors, when properly designed, look nearly transparent to the other chip frequencies, which allows high chip reflectivity, and each reflector chip can be examined independently of all others, at least to first order. The reflectivity of a periodic structure at synchronous frequency can be obtained from transmission line or coupling of mode theory (COM) and is given as

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Figure 2 Schematic diagram of a 7 chip OFC RFID tag, and OFC measured and COM predicted time response.

in Figure 1. The system has multiple tags located at arbitrary locations and each sensor is wireless and passive. A schematic of an OFC SAW embodiment and the chip time response is shown in Figure 2. For this illustration, it is depicted with a simple interdigital transducer (IDT) connected to an antenna being interrogated with a chirp input signal, and the input signal is converted to a SAW and is reflected off the OFC reflector chips, encoded and returned to the antenna for re-transmission to the receiver.[6,7] The OFC device is represented as a one-port network and the full device characteristics are obtained from the S11 transducer response, an example shown in Figure 3. The combination of device and antenna can be considered analogous to a target radar cross section. It is desired to optimize the target reflectivity (cross-section) and return as much of the coded interrogation signal as possible. The SAW has the ability to encode the retransmitted signal by changing amplitude, phase, and or delay of the interrogator. This is a rather unique capability for a wireless, passive device, which makes the SAW very attractive for many applications. The antenna is a key component of the passive sensor, and the transceiver is a key system component, but these elements are outside the scope of this paper. Although system standards are important, some applications, such as space exploration, allow the study of wider possibilities of frequency, bandwidth, etc., and no limits are placed on possible device parameters. The current research is to find optimum device and system operational parameters. This paper will focus on the SAW OFC RFID tag design considerations and a few representative examples of

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where r = reflectivity per electrode and Ng = number of chip reflectors. For |Ng*r| greater than approximately 3, the chip reflection is nearly unity, however, the chip length is long, the bandwidth narrow, and there are many chip intra-reflections. To minimize reflector loss while minimizing second order effects, a good design rule is .6