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PASSIVE RFID TAGS Ali Sadeq Abdulhadi Jalal Assistant Professor, SMIEE College of Information Engineering, Al-Nahrain University 10070, Al-Jadriya Complex, Baghdad, Iraq E-mail: [email protected]
, [email protected]
(Correspondent) Hand Phone: +964-790-1750151 RFID stands for Radio Frequency IDentification. The ability to access information through a non-line-of-sight storage in a tag can be utilized for the identification of goods, locations, animals, and people. RFID tagging overcomes the limitations of optical barcodes, which are line-of-sight and weather dependent and need manual operation. The three basic components of a typical RFID system are; an antenna or coil, a transceiver (reader with decoder), and a transponder (RFID tag) with electronically programmed information. Most RFID tags are comprised of an antenna and integrated circuit (IC). Passive tags do not have any on-board power supply. RFID tags, which use on-board power supply (such as batteries) are called active RFID tags. Passive RFID tags offer lower prices at the cost of shorter reading ranges (up to 3 m) when compared to the more expensive long-range active RFID tags (read up to 100 m). Therefore, efforts have been put in developing chipless RFID tags with no ICs to reduce the cost of the tag. So far, the only promising chipless RFID tag is the surface acoustic wave (SAW) tag. A comparison made between SAW tags and IC-based semiconductor device. A novel classification of passive RFID transponders based on a comprehensive literature review is also presented. Keywords: passive; transponder/tag; Radio Frequency Identification (RFID); Surface Acoustic Wave (SAW). INTRODUCTION The first RFID systems appeared during World War II for identification of airplanes. Two key issues must be considered for the design of an RFID system. First, the number of possible codes that can be stored in a tag; second, the capability of manipulating and communicating information. Nowadays, tags are produced at low cost because of the continuous progress of semiconductor technology. Tags with a chip size on the order of 1 mm 2 and smaller are being fabricated using lithographic technologies. They operate in the GHz range, where sufficiently wide frequency bands are available. The use of the unlicensed Industrial, Scientific and Medical (ISM) frequency bands with limited radiated power, allowed for a practically unlimited number of codes to be written and read at microsecond time intervals. Potential applications of RFID systems include but are not limited to: • Traffic control of vehicles, wagons, ships, etc., • Identification of containers, pallets, bags in airports, etc., • Individual goods control and inventory in stocks, shops, etc., • Tracing of animals and products of animal origin, • Tracking of wild animals, marking of trees in forests, etc., • Access to buildings, parking, restricted areas, computers, etc., • Ambient assisted living for the disabled and the elderly, • Identification of parts, equipment, machines, and cars assembled on conveyer lines, • Tracing of dangerous and explosive substances, and • Security and guard services. Digital Communication
Figure 1. RFID system block diagram.
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A typical RFID system, as shown in Figure 1, consists of an RFID tag which carries the ID data, an RFID interrogator/reader, which interrogates the tag and extracts the data from it, and application software acting as an interface between the user and the RFID system. In brief, RFID technology is based on radio waves in order to transmit data by the reader to the tag, and in return, it receives modulated returned echoes from the tag via the reader. The tag modulates the EM wave and transmits the data back to the reader, where it is processed for real-time ID, asset tracking, security surveillance and many other authentication and management purposes. Silicon-based RFID tags are generally classified into three major types based on their power supplies; active tags, semiactive tags and passive tags. Active RFID tags have an on-board power supply in the form of a battery. It uses the battery power to amplify the interrogating signal. Therefore, active tags do not need to use the RF interrogating signal to energize the tag for data processing and hence possess a longer reading range. Active tags can generally be differentiated by their digital section. The digital section provides the ID code as well as embedded security protocols and encryption techniques. The data processing and protocol execution are controlled by the processor, which, in some cases, has additional coprocessors to perform the encryption and data processing instructions (Kossel et al., 1999). Although the active tag has an on-board power supply, additional techniques for extending the battery life with low-power consumption have been implemented in the form of sleep modes (Kossel et al., 1999). Active tags that do not detect the interrogation zone of a reader go into a sleep mode, and thus they do not waste power (Kossel et al., 1999). The most significant advantage of active RFID tags is that they are reprogrammable, and therefore, can be used on a variety of items repetitively until the battery power is exhausted. Semiactive RFID tags have the provision of an on-board power supply for minor signal processing tasks but this power is not utilized for amplification of the received and transmitted signals. Thus a semiactive tag consumes much less power from the onboard battery and has a longer life compared to an active tag. Semiactive tags have less reading ranges compared to an active tag due to this budgeted power allocation that is dedicated only for the signal processing unit. Therefore, the semiactive tag is an intermediate approach compared to a fully active tag and a battery-less fully passive tag. Passive RFID tags do not have an on-board power supply and therefore rely only on the RF interrogating power emitted from the reader for both data processing and transmission. Accordingly, some passive tags perform data processing, but others do not. Passive tags are usually in the form of Electronic Article Surveillance (EAS) tags commonly found in retail shops for security purposes or might be chipless as surface acoustic wave (SAW) tags (Kossel et al., 1999).
RF Analog Front End
Figure 2. Passive UHF RFID tag block diagram. Vdd
Demodulator Rx Enable
Figure 3. RFID tag block diagram containing the RF-analog front end (Ashry et al., 2007).
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Most passive tags have low power consumption and low cost due to their simple design. Passive tags must have an RF front end, an analog circuit, and a digital circuit tailored to their data processing techniques, since they rely completely on the reader’s interrogating RF signal to gain its operating energy. The block diagram of a passive RFID tag using backscatter modulation is shown in Figure 2. The tag consists of an antenna and tag’s chip. The chip contains a RF-analog front end, a digital control block, and a non-volatile memory (Cristina, 2009). The RF-analog front end includes a voltage rectifier, a demodulator, a clock generator, and a modulator (Ashry et al., 2007), as shown in Figure 3. The rectifier is the most important block in the passive RFID tag. It should be capable of supplying the needed DC voltage with maximum possible efficiency. The demodulator is simply a peak detector which detects the gaps in the input RF signal. The modulator is an Amplitude Shift Keying (ASK) consisting of a simple transistor switch that short circuits the input of the RFID tag chip. The clock generator circuit is based on RC relaxation oscillator. The control logic has to switch the tag between two main operating modes. The first is the reading mode which is responsible for monitoring the received data when the received pattern is recognized. The second is to switch the system to back scattering mode which is responsible for controlling the reply process (Ashry et al., 2007). The RF front end consists of the antenna and associated impedance matching circuit in order to minimize signal reflection between the antenna and tag circuitry. The analog part of the passive tag may comprise an LC tuning circuit and a rectifier (Ghovanloo et al., 2004). An efficient RF-DC converter rectifies and multiplies the received signal generating a practical DC voltage, far higher than the incident RF signal amplitude, increasing the range between the reader and the tag (De Vita et al. 2005). The digital part of the passive RFID tag may have an IC, Application Specific Integrated Circuit (ASIC), or just a memory block of a few kilobits according to the required application (Kraiser et al., 1995). Passive RFID tags can be made using printing techniques (Redinger et al., 2003a). There have been tremendous efforts and interests in direct printing of RFID tags on plastic, fiber, and other low-cost laminates to compete with the ultra-low-cost optical barcodes. Also, existing ink-jet-deposition processes capable of creating high quality passive devices for RFID applications have been investigated and are under development (Redinger et al., 2003a; Redinger et al., 2004b; Subramanian et al., 2005). Due to the absence of on-board power supplies, passive RFID tags have a much shorter reading range (up to 2m). They are more vulnerable to environmental effects and have poor or no data processing abilities and hence cannot be easily reprogrammed. The advantages of passive RFID systems are low cost and low maintenance. Due to these prominent features, passive tags are used in a wide range of applications such as medical, supply chain management, and wireless sensing [(Philipose et al., 2005; Dehaene et al., 2009). In this paper, the current status of the development of passive RFID tags is discussed based on a novel classification as shown in Figure 4. We mainly focus on the tag devices, omitting issues related to the reader design and the corresponding signal processing. In section 2, silicon based RFID tags are studied with emphasis on their digital part. In section 3 SAW-based chipless RFID tags are shown. Section 4 covers spectral-signature based passive tags. In section 5 passive tags are reviewed based on back scatter modulation. Finally, Section 6 concludes the article and provides some perspectives. SILICON-BASED PASSIVE RFID TAG ARCHITECTURE RFID transponders that contain ICs usually comprise digital circuits operating as memory blocks and microprocessors or microprocessor systems, or ASICs (Curty et al., 2005; Khaw et al., 2004; Teh et al., 2004).Some of these tags are passive, others are active. Manufacturers of RFID chips are tending to produce low-cost, low power RFID tags that are direct competitors to bar codes. A 900 MHz passive tag consists of an antenna along with a matching circuit, an analog section, a digital section, and a memory block, is shown in Figure 5 (Liu et al., 2003). The RF front end of the tag has the function of receiving and transmitting the RF signal from and back to the reader. It is represented by the RF antenna block which includes the matching circuit and may have various functions depending on the operation principle of the tag. Mainly, it has to rectify the induced voltage from the reader’s received interrogating signal in order to support the operation of the digital section and memory block. The digital section usually consists of sequential circuit and acts like a finite state machine.
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Passive RFID Tags
Backscatter Modulation Based
Capacitive Tuned Dipoles
LH Delay Lines
Space Filling Curves
Remote Complex Impedance
Stub-Loaded Patch Antenna
The Global Tag
Programmable IDT Tag
Chirp IDT Tag
Dual Band Resonators
Figure 4. Classification of passive RFID tags.
Figure 5. System construction of the RFID tag (Liu et al., 2003). EEPROM-based ــــThe memory block can be an Electrically Erasable Programmable Read Only Memory (EEPROM), a Static Random Access Memory (SRAM) or a Ferroelectric Random Access Memory (FRAM). EEPROM has high power consumption during writing operations and a limited write cycle. It is used in a wide
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range of applications due to its low manufacturing cost and high number of possible reprogramming cycles. The FRAM chips are preferred due to their low read power consumption in comparison to the EEPROM as well as their significantly lower write time, but they are rarely used because of manufacturing difficulties. Microprocessor-based ــــDedicated Central Processing Units (CPUs) are used to perform complex functions, such as anticollision, clocking and authentication, in spite of the fact that their addition increases power consumption. They can achieve high data rates in the high magnetic field emission from interrogators in the 13.56 MHz ISM band (Masui et al., 1999). Tags with microprocessors will become increasingly common in applications using contactless smart cards in the near future due to their excellent processing capabilities. Efficient and high data rate transfers are achieved by introducing tags with embedded digital signal processors (Engel, 2002). Since microprocessors tend to have high power consumption, great efforts have been dedicated to produce low power prototypes so that they can be used in RFID tags for complex signal and data processing applications (Page, 1993). RFID tags containing microprocessors usually use interrogator driven procedures for communication due to their ability to be interrogated and answer upon a request or command. The operation sequence of this type of RFID tags is comprised of three steps: first receiving and decoding the request signal from the reader, then perform data processing, and finally, data encryption and transmission back to the reader. Figure 6 depicts one of the tag architectures that can perform these steps efficiently.
CPU RF Front End Antenna
text Crypto (CODEC)
ROM (Operating System)
EEPROM (Application Data)
Figure 6. Block diagram of a RFID transponder with a microprocessor.
The transponder module comprises the antenna, RF front end, microprocessor (CPU) with internal random access memory (RAM) and encryption coprocessor, and RO memory (ROM) for data storage. The antenna and RF front end enable effective data transmission between the reader and the tag and thus establish the link between the RF and digital circuitry in the tag architecture. The RF front end consists mainly of the feeding and impedance matching network, modulation/demodulation circuit and AC/DC conversion circuit for supplying power to the digital section. The CPU consists of the microprocessor, internal registers, the encryption coprocessor, and the microprocessor’s internal RAM. Complex systems like these require an efficient operating system. The transponder’s operating system is implemented in the external ROM and thus cannot be deleted after the loss of the power upon leaving the interrogation zone of the reader. The operating system consists of software drivers and applications that manipulate hardware for anti-collision procedures, data processing, and authentication. Encryption and authentication procedures and protocols are needed since these types of transponders can process and retrieve valuable data (bank account numbers, personal ID numbers (PINs), etc). The application data is stored in an EEPROM and can be changed according to the provided service. These types of tags are usually found in the form of smart cards (e.g. for banking cards, cellular telephones, health insurance cards, etc.) or in pricing labels, baggage identification tags, animal identification tags (Da Costa, 2000), and are at a higher cost than chipless transponders or those with simple memory functions. ASIC-based ــــThe use of ASIC in RFID tag design in recent years has developed rapidly. Research and development companies around the world have been working on ways to minimize the size of RFID transponder ICs, to lower power consumption, and finally, to produce a low-cost, efficient RFID tag IC (Masui et al., 1999). Using high operating frequencies for RFID has minimized the physical dimensions of passive components.
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Furthermore, ASICs that can perform complex data processing are developed. Nowadays, almost all RFID ASICs are made from Complimentary Metal-Oxide Semiconductor (CMOS) technology (Engel, 2002; Page, 1993; Da Costa, 2000), which consumes very little power. Figure 7 shows a typical RFID transponder IC (ASIC). It is composed of an RF front end (tuning capacitor, voltage rectifier, voltage limiter, modulator, demodulator and clock recovery), an analog front-end (voltage reference, regulator, Power-On-Reset) and a digital part (Villard et al., 2002).
RF Front End Voltage Rectifier & Limiter Modulator text
Analog Front End Voltage Reference & Regulator
Clock Recovery & Divider Transponder IC
Figure 7. Transponder ASIC block diagram (Villard et al., 2002).
Most RFID tags operate in two phases: charge up phase and data transmission phase. During the charge up phase, the storage circuit collects energy from the reader’s EM waves. The collected DC power is used to support the operation of the digital section of the ASIC. In the data transmission phase, the tag transmits data back to the reader. In this phase, the modulator block is used to power the modulation of the carrier signal received from the reader in order to send valid information. The demodulator block is used to decode any commands from the reader toward the tag. The reader usually transmits ASK modulated signals that can be simply demodulated in the tag with a single diode as an envelope detector. Backscattering modulation is performed by varying the RF input impedance of the tag and influencing the antenna radar cross section (RCS). The EEPROM contains stored data and allows the incoming data to be stored and preserved after the power supply is cut off. It also allows greater flexibility and extends the RFID system’s applications. Researchers have also been working on printing RFID transponders onto organic substrates, such as paper, for UHF RFID designs for anti-counterfeiting and security (Rida et al., 2007a). Yang, and Tentzeris (Yang et al., 2007) presented the design and characterization of novel paper based ink-jet printed UHF antennas and transponders. They reported the use of two methods for printing RFID tags onto paper. The first method was an ink-jet printing where a low-cost Di-matrix printer system was used with a special conductive silver ink. The second method was based on conventional lamination and copper etching chemistries. The integration of sensors with RFID transponders is also reported in order to allow large-scale production of wireless sensing systems using ink-jet printing systems. The outcome in the near future would be the design of a flexible three-dimensional package with embedded actives and passives and thin film battery in paper substrates that is expected to be among the cheapest solution for wireless sensing with RFID transponders for large volume applications (Rida et al., 2007b). Transponder ASICs have gone a step further by embedding an antenna on an RFID chip which is an ideal method for reducing the area of RFID tags and their cost. The shape of the RFID embedded antenna is similar to a coil pattern because the chip-received electromagnetic energy of induction is larger than that of radiation in the near region of a reader antenna. Hitachi has disclosed this invention by presenting their embedded RFID microchip (Usami, 2008). The embedded antenna structure of the ultra small RFID microchip is shown in Figure 8. Further size reduction is achieved with a 3.3mm x 3.3mm IC that is integrated into an RFID transponder along with a MEMS pressure sensor and FRAM memory. The sensor and memory ICs are independently addressable via the serial peripheral interface (SPI) standard. The IC was designed to implement a RF front-end, SPI Master and digital Finite State Machine (FSM). The wireless front-end operates via a 13.56 MHz inductively coupled system for power transfer and bi-directional data transmission to a reader and has on-chip voltage regulation, data recovery, and load modulation circuits. The digital logic implements a custom developed wireless SPI protocol, allowing for direct
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communication with SPI peripherals via the RF front-end. Additionally, a digital FSM is responsible for data logging operation, in which pressure data is autonomously measured and stored in memory over constant intervals (Tumer et al., 2010). 0.4 mm Embedded RF Antenna
Ultra-Small RFID Chip
Figure 8. Embedded antenna structure of ultra-small RFID microchip (Usami, 2008). Another design and implementation of flexible, fully integrated sensor motes (low power wireless sensor network devices) for ubiquitous sensor networks based on an established, long-range RFID technology is developed (Gay et al., 2010). The batteryless mote is made up of a single-chip solution incorporating a microwave RFID-front-end, an 8-bit microcontroller, a low-power temperature sensor, a reconfigurable sensor interface, a 10-bit SAR-ADC, a fully developed I2C digital interface and nonvolatile memory for program and sensor data. SAW-BASED RFID TAGS The RFID market is currently dominated by integrated circuit (IC) based RFID. In recent years, with the emergence of commercialized systems, SAW RFID provides identification solutions in harsh environments or when long read ranges are required, where common silicon IC based RFID technologies have been unsuccessfully deployed. It turns out to be fairly complementary to IC RFID in different applications. As a matter of fact, SAW devices have been mainly used as the radio frequency filters in signal processing devices in military and radio frequency filters in everyday applications, such as mobile phones and televisions. SAW tags are similar to the RF SAW filters that are widely used in mobile phones. Both of them use basically the same technology. SAW tags, are linear, time-invariant systems which simply reflect the interrogation signal in a coded form that carries the tag information. SAW tags achieve the necessary separation between the request and the response signal by using a time division employing a SAW delay line. SAW tags can achieve reading distances of several meters due to the minimum signal-to-noise ratio for decoding the information of the reflected signal in the reader and the limited licensed radiated power from the reader. SAW tags feature low losses, large delay times, and small dimensions. In addition, they have a simple and robust structure. Advantages of SAW tags versus IC-based tags: 1- SAW RFID tags are truly passive (needing no power supply). By contrast, silicon RFID tags need an RF interrogating powering signal which is rectified into useable DC power to operate the IC chip. 2- Although the smallest signal for operating the IC-based RFID tag can be lower than 0.1 volt, the reading signal of SAW tags is about 100 times smaller than that of IC-based tags. 3- SAW tag has a stronger capability for a longer reading range, better signal penetration, tagging on metallic containing cases or liquid at low reading power consumption (Hartmann et al., 2007a). 4- SAW tags operate in the free 2.45-GHz ISM band, while the use of IC tags requires specific certification in the 860-960 MHz range. 5- SAW tags can work over very wide temperature ranges from cryogenic up to several hundred degrees and can withstand high energy x-rays, or gamma ray for which semiconductor devices are useless. 6- Because of low radiated power, SAW tag readers have a substantially higher interference resistance than IC tag readers to systems radiating few watts in the same frequency like Bluetooth, WLAN, etc.
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7- SAW RFID can be a multifunctional device, such as simultaneous identification, sensing and/or real-time location. Currently, there are some commercialized wireless and passive SAW sensors, such as temperature sensors, pressure sensors, torque sensors, humidity sensors, vibration sensors and so forth (Reindl et al., 1998). 8- SAW tags can identify objects with high moving speeds. Since the readout procedure requires only few microseconds, 1×105 interrogations can be performed per second, thus permitting reliable identification of particularly fast moving objects. However, standard regulations limit the IC tag reader to low rate. The SOFIS SAW RFIDs made by Siemens were used in Munich’s subway system in the early 1990s. The maximum velocity of a train passing by can be up to 350km/h (Scholl, 2003). Disadvantages of SAW tags versus IC-based tags: 1- SAW RFID is read-only. The encoding of SAW RFID is dependent on the reflectors, whose position and number is predetermined in the photolithography processing. There is no simply way for fabricating large volumes of different coding tags. 2- There has been no complete anti-collision solution yet. Since SAW tags are truly passive devices, they cannot actively stop transmitting their echoes according to the reader’s instruction. For example, although the time separation based anti-collision helps relieve the problem, it is likely to have restricted use in practical applications because the number of non-overlapping time segments is rather limited and such tags require SAW devices that have longer delay times. This would lead to devices with extra cost and tag losses. 3- Other than IC based RFID, the code capacity of SAW RFID is directly related to the modulation scheme and the number of reflectors. Hence, the increase of code capacity is obtained at the expense of substrate dimension, and finally the number of reflectors is limited by the multi-reflection. Most of commercialized SAW RFID systems have 32-bit of code capacity Scholl, 2003; Fachberger et al., 2004). In order to be compatible with EPC-96 and EPC-128 RFID specifications, the use of phase information in code identification is highly regarded in SAW RFID research and development, since it can enlarge the code capacity greatly under the same number of reflectors. Principle of Operation: The operation of SAW devices is based on piezoelectricity, a coupling between electrical and mechanical properties of a material. In certain dielectric crystals, an electric potential difference is produced at the application of a mechanical stress and, conversely, the crystal produces a mechanical vibration when an electric field is applied. In a SAW device, an interdigital transducer (IDT) is used to achieve the transduction between an acoustic signal and an electrical signal and vice versa. The IDT consists of two interlaced combs-like metal structures deposited on the surface of a piezoelectric substrate. The schematic diagram of the principle of operation of a reflector-based SAW tag is shown in Figure 9 (RFSAW, 2004).
Tag Antenna Radio Waves
IDT SAW RFID Chip
Reader SAW Pulses
Figure 9. Operation of a SAW tag system (RFSAW, 2004).
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A reader transmits a request pulse, which is received by the tag antenna that is directly connected to the IDT. The IDT transforms the electrical signal into a nano-scale surface acoustic wave. The generated SAW wave then propagates along the surface of the piezoelectric substrate made of lithium niobate (LiNbO3) (RFSAW, 2004). The SAW wave is partially reflected and partially transmitted by each of the code reflectors that are placed at predetermined positions on the chip. These reflectors usually consist of one or a few narrow aluminum strips. The reflected SAW pulse returning to the IDT carries a code based on the positions of the reflectors. When the train of reflected SAW pulses returns to the IDT, the acoustic signal is then converted back into an electrical form and transmitted by the tag antenna. The response signal is then detected and decoded by the reader. A review on available SAW tags is presented next based on reported literature. IDT-based SAW tags ــــTransducer-based SAW tags consist of one large transducer, the input transducer, and several smaller coding transducers, called output IDTs as shown in Figure 10 (Cole et al., 1972; Nysen et al., 1983a; Nysen et al., 1999b). Both the input and all output IDTs are connected in parallel. When an electrical signal is applied to this common electrical port, SAWs will be generated by all transducers. The insertion attenuation of a signal generated by the input IDT and picked up by one output IDT is the same as the signal generated by this output IDT and picked up by the input IDT since the forward and reverse transfer functions are equal in SAW devices. When neglecting propagation losses, it can be easily shown that best amplitude uniformity of the code signal is achieved by an equal distribution, and thus equal matching, of the input signal to all code IDTs. Similarly it can be shown that an equal distribution of the input signal to the input IDT and to the summing network of the output IDTs leads to minimum insertion attenuation of transducer-based SAW tags. A multi-IDT tag might show lower insertion loss when compared with reflector tags. In the single-track design [Figure 10(a)] each coding structure is passed only once and thus the associated losses show up only once. The conversion efficiency of an IDT can be adjusted very finely, resulting in a good uniformity in amplitude of the delayed impulses. Electrically loaded IDTs show a reflection of the acoustic wave, which might lead to further spurious signals when picked up by other coding IDTs. When all code signals have the same or similar time distant, all reflected acoustic waves are picked up by the output IDT which is situated one before. The time position of this spurious signal, which is growing with the number of output IDTs lined up in a track, is identical with the next code position and can cause the code confusion. In multi-track case [Figure 10(b)] these multiple reflections are partly reduced, but the width of device increased. If both, input and output IDTs are built up as unidirectional transducer the losses are reduced to propagation losses and track losses (Reindl et al., 1993b).
(a) Input IDT
Figure 10. Transducer-based SAW tags (Cole et al., 1972).
Reflector-based tags ــــX-Cyte tags have 16 reflectors distributed in 4 acoustic tracks (Nysen et al., 1983a; Nysen et al., 1999b). The multi-track design included separate transducers in each acoustic channel and 2 reflectors on
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both sides, each with up to 3 preceding phase shifting elements (see Figure 11). The SAW thus has to pass the bus bars twice.
Figure 11. Reflector-Based Tag from X-Cyte Inc. (Nysen et al., 1999b). Reflector-based tags with folded propagation path of SAWs allow a twice reduction in size, whereas multi-track geometries cause the chip area to increase. A multi-track design with only 2 reflectors per track, as shown in Figure 11, leads to increased losses which cannot be compensated. The X-Cyte tags suffered two main drawbacks which prevented them from commercialization. First, they have no simple method for testing the ready fabricated chips on the wafer before coding and packaging. The amplitudes of the reflected signals could be measured using wafer probers and network analyzers. To extract the critical phase differences between the signals, the carrier frequency must be demodulated in the complex time domain data, which cannot be done by a network analyzer. Thus, all chips were coded and assembled completely with the packaging to tags and then tested. Second, because the reflectors are distributed all over the chip, any variation of the SAW velocity over the chip is very critical for the phase coding. Siemens SAW tag ــــSiemens developed SAW-tag RFID systems and tags with 20- and 31-bit operation in the international ISM band at 2.45 GHz in the early 1990s (Reindl et al., 1998c; Reindl et al., 1994d). The first generation used an ASK modulation scheme with reflective and non-reflective structures. The 33 reflectors were distributed in 4 tracks on both sides of the input transducer, always 8 reflectors in a line, according to the design rules given in (Reindl et al., 1993b). To compensate the attenuation difference between the 4 groups caused by the difference in initial delay, the groups were weighted with different apertures. For equal distribution, the 1st and the 4th group were placed on one side of the input transducer and the 2nd and 3rd on the other side as shown in Figure 12.
Figure 12. Layout of a mounted SAW RFID tag using an ASK coding in the 2.45-GHz band with 33 reflectors in 4 tracks. The outside dimensions are 16 × 9 mm (Reindl et al., 1998c).
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The global SAW tag ــــHartmann’s global tag has made a significant step toward practically infinite numbers of codes (RFSAW Inc, 2004; Hartmann, 2002b; Hartmann et al., 2004c). Time position encoding is used; however, time slots for the position of the center of a pulse were radically reduced, due to prescribing to each slot some phase which is systematically growing along the array of slots inside a given group of slots, Figure 13. Although the pulse width remains much wider than one time slot, the pulse position can be localized due to phase information. Code capacity up to 256 bits is predicted and devices with 128 bits were demonstrated (Hartmann et al., 2004c). Opencircuit reflectors with a diffraction compensating shape were used. The anti-collision problem of the simultaneous presence of few tags was discussed in (Hartmann et al., 2004d; Brandl et al., 2008a; Brandl et al., 2009b) and some solutions are proposed. Group 1
Figure 13. Combined time position and phase coding (Hartmann et al., 2004d). Programmable IDT-based SAW tags ــــThese passive tags are also called impedance-loaded SAW sensors (Reindl et al., 1993b). The SAW delay line consists of three IDTs. The first one connected to the antenna is the transceiver IDT. The second is used as a reference one. The third is connected to an external sensor to be used as a programmable reflector. The operating principle is briefly described in Figure 14 (Fu et al., 2005). The transmitted pulse from the reader is received by the antenna and applied to the transceiver IDT. Then a SAW is excited on the piezoelectric substrate and propagates towards the reference and the programmable reflectors where it is partially reflected. The transceiver IDT converts the reflected SAW into an electrical signal and sends it back to the reader through the antenna. Since the amplitude and/or phase of the reflected signal vary with the impedance variation of the external sensor, the sensor information is included in the echo of the programmable reflector. Finally, the echoes are transferred to a PC or other devices for post processing. Two advantages of this approach compared with the conventional double-electrode-type one discussed earlier were reported (Fu et al., 2005). Firstly reducing the manufacturing cost, and secondly increasing the operating frequency. It has been shown that this programmable reflector can be used as wireless passive SAW capacitive and resistive sensors. Antenna Piezoelectric Substrate
Transmitting/ Receiving IDT Transmitted Pulse
Classical Sensor + Matching Circuit
Figure 14. Programmable reflector SAW tag. Response 1 is the echo of the reference. Response 2 is the echo of the programmable reflector (Fu et al., 2005).
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Chirp transducer SAW tag ــــRecent developments of UWB technology offer many attractive possibilities for the design of SAW RFID tags. According to the Federal Communications Commission (FCC) regulations (Breed, 2005), an UWB device is a device emitting signals with a fractional bandwidth greater than 20% or a bandwidth of at least 500 MHz SAW tags operating at 2.5 GHz with a band of 500 MHz would satisfy this criterion. A schematic drawing of a chirp transducer UWB SAW tag is shown in Figure 15. It is having signal processing partly performed within the tag (Harma et al., 2009). This will allow for a single matched tag response, which, after being modified within the tag, will be different from the environmental echoes received by the reader. One advantage of this is to make the system more resistant to environmental interference, because the reader is now able to distinguish between the signal reflected by the SAW tag and those reflected by objects outside the tag. The second is that the UWB SAW tag system transmits very low power levels, since the principle of the UWB technology is to reuse an already occupied frequency spectrum, but with very low power. Chirp Transducer
l1 l2 l3
Figure 15. Chirp transducer SAW tag with an array of wideband code reflectors (Harma et al., 2009). OFC SAW Tag ــــOrthogonal Frequency Coding (OFC) techniques have also been developed in SAW tags (Malocha et al., 2008; Wilson et al., 2009). The asynchronous nature of SAW passive tags results in little or no advantage when implementing orthogonal code sets with respect to code collisions. To minimize OFC device code collisions, multilayer coding techniques have been implemented, applying diversity in frequency (OFC) coding, phase (PN), and time and frequency division multiplexing (TDM and FDM). These approaches also provide large enough code sets for sensors. The principle disadvantages are increased device length and characterization of free versus metalized propagation delay effects. However, the advantage is a decreased effect of code collisions, which yields to more useful tags in a given range, and an even larger number of code sets. This is schematically shown in Figure 16. Sensor 1
Figure 16. Schematic of two OFC TDM devices, demonstrating FDM system embodiment; each sensor occupies a different frequency sub cell (Wilson et al., 2009). SPECTRAL SIGNATURE-BASED RFID TAGS Spectral signature-based chipless tags encode data into the spectrum using resonant structures. Each data bit is usually associated with the presence or absence of a resonant peak at a predetermined frequency in the spectrum. The advantages of these tags are that they are fully printable, robust, and are low cost. The disadvantages of these tags are large spectrum requirements for data encoding, tag orientation requirements, size, and wideband dedicated RFID reader RF components. Planar circuit chipless RFID tags are designed using standard planar microstrip/coplanar waveguide/ stripline resonant structures, such as antennas, filters, and fractals. They are printed on thick, thin, and flexible laminates and polymer substrates (Jalaly et al., 2005).
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Capacitive tuned dipoles tags ــــThese chipless tags consist of a number of dipole antennas, which resonate at different frequencies (Jalaly et al., 2005). The capacitively tuned dipole tag is shown in Figure 17. When the tag is interrogated by a frequency sweep signal, the reader looks for magnitude dips in the spectrum as a result of the dipoles. Each dipole has a 1:1 correspondence to a data bit. Issues regarding this technology include tag size (lower frequency longer dipole—half wavelength) and mutual coupling effects between dipole elements.
Figure 17. Capacitive tuned dipoles arranged as a 11-bit chipless RFID tag (Jalaly et al., 2005). Space-filling curves tags ــــThese are spectral signature encoding RFID tags which were first reported in (McVay et al., 2006).The tags are designed as Piano and Hilbert curves with resonances centered around 900 MHz. The tags represent a frequency selective surface, which is manipulated with the use of space-filling curves (such as the Hilbert and Piano curves). The space-filling curve exhibits an interesting property of resonating at a frequency, which has a wavelength much greater than its footprint. This is an advantage since it allows the development of small footprint tags at UHF ranges. Figure 18 shows a 5-bit space-filling curve chipless tag, comprising an array of five second-order Piano curves, which creates five peaks in the radar RCS of the tag. The chipless tag was successfully interrogated in an anechoic chamber. Only 5 bits of data have been reported to date. The advantage of the tag is its compact size due to the properties of the space-filling curves. However the disadvantage of the tag is that it requires significant layout modifications in order to encode data. DUROID 5870
Figure 18. An array of the 2nd order Peano-curve elements. The total length of the array is about 163 mm (McVay et al., 2006). LC resonant tags ــــThese tags comprise of a simple coil, which is resonant at a particular frequency as shown in Figure 19. They are considered as 1-bit RFID tags. The operating principle is based on magnetic coupling between the reader antenna and the LC resonant tag. The reader constantly performs a frequency sweep searching for tags. Whenever the swept frequency corresponds to the tag’s resonant frequency, the tag l starts to oscillate, producing a voltage dip across the reader’s antenna ports. The advantage of these tags is their price and simple structure (single resonant coil), but they are very restricted in operating range, information storage (1 bit), operating bandwidth, and multiple-tag collision. These tags are mainly used for electronic article surveillance (EAS) in supermarkets and retail stores (Tagsense Inc, 2006).
Figure 19. An LC Resonant tag (Tagsense Inc, 2006).
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Multiresonator-based tags ــــThese tags consist of a vertically polarized UWB disc-loaded monopole Receive (Rx) tag antenna, a multiresonating circuit, and a horizontally polarized UWB disc-loaded monopole Transmit (Tx) tag antenna (Preradovic et al., 2008a; Preradovic et al., 2009b; Preradovic et al., 2009c; Preradovic et al., 2008d; Preradovic et al., 2008e; Preradovic et al., 2010f). The tag is interrogated by the reader by sending a frequency swept continuous wave signal. When the interrogation signal reaches the tag, it is received using the Rx monopole antenna and propagates towards the multiresonating circuit. The multiresonating circuit encodes data bits using cascaded spiral resonators, which introduce attenuations and phase jumps at particular frequencies of the spectrum. After passing through the multiresonating circuit, the signal contains the unique spectral signature of the tag and is transmitted back to the reader using the Tx monopole tag antenna. The interference between the interrogation signal and the retransmitted encoded signal containing the spectral signature is minimized by the cross polarization between the Rx and Tx tag antennas. Figure 20 shows a 35-bit tag designed on Taconic TLX-0 (Ԑr = 2.45, h = 0.787 mm, tan δ = 0.0019) (Preradovic et al., 2009b). The tag encodes data in both amplitude and phase and operates in the UWB region, the tag supports simple spiral shorting data encoding and the tag responses are not based on RCS backscattering but on retransmission of the cross-polarized interrogation signal with the encoded unique spectral ID. The chipless tag is designed for printing on the Australian polymer banknote as an anticounterfeiting security feature (Preradovic et al., 2009g).
Rx Antenna Tx Antenna
Multiresonator with 35 Spirals
Taconic TLX-0 Substrate
Figure 20. Photograph of 35-bit chipless RFID tag (length = 88 mm, width = 65 mm) (Preradovic et al., 2009b). Multiresonant dipole-based tags ــــThis tag is based on a concept similar to the multiresonator-based chipless tag. However, the tag’s designers seek to build on the concept of the multiresonator tag by replacing the stop-band spiral resonators and the second tag antenna with a novel multiresonant dipole antenna (Balbin et al., 2009). The multiresonant dipole antenna comprises a set of parallel loop antennas, which resonate at different frequencies. Each loop antenna corresponds to a single bit of data. The multiresonant dipole-based chipless RFID tag is shown in Figure 21. The multiresonant dipole antenna comprises a series of folded half-wave dipole antennas. The half wavelength dipole antennas produce peaks in the return loss at their resonant frequencies. By removing any of the half wavelength dipoles, the corresponding resonant peak disappears without influencing the resonances of the other dipoles. The main benefit of using the multiresonant dipole antenna is that the size of the entire tag can be reduced and spatial efficiency is enhanced.
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UWB Monopole Antenna (Rx) R
Feed Extemsion Multiresonant Ground Dipole Plane Antennas (Tx)
Figure 21. Multiresonant-dipole-based chipless RFID tag (Balbin et al., 2009). (red—top layer, gold—bottom layer) Hybrid Coding Tags ــــThe specific design presented in this section is based on the association of multiple uncoupled resonating elements or Elementary Coding Particle (ECP) having a “C”-like structure (Vena et al., 2011). Each resonating element plays the role of an antenna and a resonator as shown in Figure 22. The number of resonators for each tag configuration is five, and the size of this structure is nearly 2 cm × 4 cm. The tag combines two nearly independent parameters for data encoding, phase deviation and frequency position to get a hybrid coding technique. Two independent codes can be allocated for each resonator, hence a real improvement is made in terms of coding efficiency. It has been shown that instead of having only 10 bits in  where only the phase deviation was used, the coding capacity is 22.9 bits within this reduced size structure. The hybrid tag makes it possible to reach a low unit cost with printing techniques, since only one conductive layer is used for tag fabrication without a ground plane. FR4
Figure 22. Hybrid Coding tag with different slot lengths and gaps (Vena et al., 2011).
Dual-Band Resonators Tags ــــThe integration of dual-band resonators in frequency-coded chipless RFID tags is proposed in (Girbau et al. 2012). These tags increase the number of bits coded in each resonator and also increase the operation bandwidth by using dual-band resonators. Dual-band resonators permit control of the second resonance and use it to code information. Moreover, two processing techniques were proposed to extend the read range in frequency-coded chipless RFID: background subtraction and time gating. When using normal (halfwavelength) resonators (section 4.4), each resonator codes one bit in its first resonance, leading to two possible combinations (0 and 1). When using stub-loaded resonators (Figure 23), each resonator can code information in the first and second resonances, and then three combinations can be obtained (00, 11, and 10). In consequence, while a code of 2N words can be obtained with N normal resonators, a code of 3N words can be obtained with N stub-loaded resonators.
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AMPLITUDE-PHASE-BACKSCATTER-MODULATION-BASED CHIPLESS TAGS These tags require less bandwidth for operation than time-delay-based and spectral signature-based chipless tags. Data encoding is performed by varying the amplitude or phase of the backscattered signal based on the loading of the tag’s antenna. The variation of the loading is controlled by reactive loading of the tag’s antenna rather than by an on/off switch between two impedances. The antenna loading influences the RCS of the antenna in amplitude or phase, which can be detected by a dedicated RFID reader (Rao et al., 2005) .The reactance of the load may vary due to the fact that the antenna load is an analog sensor or left-handed (LH) delay line, or that the antenna is terminated by a microstrip based stub reflector. The advantages of these types of chipless tags are that they operate over narrow bandwidths, and they have simple architecture. The disadvantages are the number of bits that can be detected, and that data encoding is performed by lumped/chipped components which increase their cost. Following are four types of chipless RFID tags based on the data encoding by antenna loading element.
Figure 23. Stub-loaded dual-band resonator tag (Girbau et al. 2012).
LH delay line ــــAntenna loading is performed by utilizing analog circuits for phase modulation and increasing the response time of the tag using the slow-wave effect of LH delay lines (Schuler et al., 2009), which also minimizes the size of the tag. The operating principle of the tag is presented in Figure 24. The tag is interrogated by a bandlimited pulse transmitted from the RFID reader. The interrogating pulse is received by the tag antenna and propagates through a series of cascaded LH delay lines, which represent periodical discontinuities. The received interrogating pulse is reflected upon reaching each discontinuity and the information is coded by the phase of the reflected signal with respect to a reference phase. The envelope of the reflected signals with encoded data maintain similar magnitudes (envelopes) while the phase variation differs due to different Γ1, Γ2, and Γ3 with phase values φ1, φ2 and φ3, respectively. The LH delay line-based tag encodes data using a higher order modulation scheme, such as Quadrature Phase Shift Keying (QPSK), which enables greater throughput but requires a higher signal-tonoise ratio for successful tag detection (Mandel et al., 2009). The QPSK modulator used within the tag is based on a variable reactive element, which minimizes the variation of the amplitude and maximizes the phase variation.
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Figure 24. Operating principle of left-hand-delay-line-based chipless RFID tag (Schuler et al., 2009). (Reprinted with permission).
Remote complex impedance-based tags ــــThese tags comprise printable antennas, which are terminated by lossless reactance. The tag antenna is chosen to be a scattering antenna (such as a patch antenna) instead of a typically used minimum scattering dipole antenna. A scattering antenna when terminated with an open or short, should scatter back the same power, irrespective of the type of lossless termination, while the minimum scattering antenna will scatter almost no power back in open circuit conditions (Nikitin et al., 2006). This property of scattering antennas is envisaged in (Mukherje, 2008a) to encode data by means of loading a scattering antenna with microstrip stubs, which represent different inductances, and therefore manipulating the phase component of the antennas RCS and backscattered signal. The chipless RFID system based on remote measurement of complex impedance can be modeled as a two-port network where the reader is considered to be the source while the reactive impedance is considered to be the load as shown in Figure 25. The transmitted interrogating signal is defined by the S21 parameter while the S12 parameter is the backscattered tag response signal with phase signature. It is possible to create different phase signatures in the backscattered response signal by having tags with different inductive loadings of their antennas (Mukherje, 2007b). The reactive loadings are designed to be microstrip stubs in order to make the tag fully printable and low-cost. Free Space a ·
Figure 25. Scattering Antenna modeled as a 2-port (Mukherje, 2008a). Stub-Loaded-microstrip-patch-antenna (SLMPA)-based tags ــــThese are a newer generation of backscatter phase signature tags similar to the remote complex impedance based tag presented earlier (Balbin et al., 2009b). They are using polarization diversity to isolate the interrogation signal and the backscattered signal to increase the robustness of the system and allow commercial solutions to be developed [54,55]. The operating principle of the SLMPA tag is based on basic principles of vector backscattered signals from multiple planar reflectors. The SLMPA-based tag is shown in Figure 26.
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Meandering O/C Stubs
Inset Width L2
Spacing Element 1
Taconic TLX-0 Substrate
Figure 26. Stub-loaded-patch-antenna-based chipless RFID tag comprising three SLMPAs loaded with meander line stubs (Balbin et al., 2009b). The tag consists of multiple patch antennas, which are selected according to their scattering properties. The planar reflectors are in the form of meander stubs in order to minimize area and cost. The numbers of bits that can be encoded by the tag depends on the number of patches (n) and the available meander line inductances. The chipless tag is interrogated by transmitting n different continuous wave (CW) signals from the reader at n frequencies corresponding to the operating frequencies of each patch antenna. When the tag is read by directive reader antennas, a bit sequence can be detected using the relative phase difference of the backscattered signals. The relative phase refers to the phase difference between the E-plane and H-plane signals at the reader, adding another degree of differentiation. It is important to notice that this type of tag requires interrogation and reading with a directional dual polarized reader antenna and not circularly polarized due to the tag’s operating principles. CONCLUSION A comprehensive study on passive RFID tags available in the open literature and some of those on the market has been presented. As the requirement for cheaper RFID tags for various applications grows, there are a greater number of different passive chipless RFID tags that can be classified in a wide range of different types. Variations of RFID transponders are presented in the form of different types of power supplies, technological and topological structures. Silicon-based and chipless RFID tags have been introduced to the main classification of passive RFID transponders. Different types and forms of chipless passive RFID tags have also been introduced in this article. Passive SAW tags were also presented under the chipless classification. SAW tags have clear advantages over silicon based RFID devices in many aspects: very large number of codes, larger reading distance, smaller size, robustness, etc. For these aspects, the development of smart SAW tag systems requires some necessary technological tools and infrastructure. Passive SAW tags offer an excellent technical solution over IC based ones. Other main classification of chipless tags is based on modulation techniques, which are spectral signature-based and backscatter modulation-based chipless RFID tags. We conclude this paper by Table 1which shows a comparison between different types of spectral signature-based tags. All of them are printable and developed for certain applications. Although the majority of chipless RFID tags are still in prototyping stages it remains to be seen whether they will make it into the mainstream market. However, the progress of chipless RFID technology in recent years enthusiastically suggests that the best of chipless RFID is yet to come.
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Table 1: Comparison of Spectral Signature-Based RFID Tags Tag Type Frequency No. of Size (Range) Bits 11 12×4 cm Capacitive tuned 5.8 GHz dipoles 900 MHz 5 16×3 cm Space-filling curves
Read Range 1m
Require significant modification for data encoding Cheap and simple structure. Mainly used in EAS. Fully printable. Use 2 cross polarized antennas to avoid mutual coupling. Use parallel dipole antennas for frequency signature. Use phase and frequency coding. Very compact. Dual-band stub loaded. Each stub perform 3 different codes (3N).
Multiresonatorbased Multiresonant dipole-based Hybrid Coding
8.8×6.5 cm 4×4 cm
~ 5×5 cm
Easy fabrication (Printable)
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Ali Sadeq Abdulhadi Jalal received a degree in electrical engineering from Al-Rasheed College / University of Technology / Baghdad / Iraq in 1980 and received his M.Sc. in communication engineering from the same college in 1986. He received his PhD in wireless communication engineering from University Putra Malaysia / Malaysia in 2013. He is now a senior member in IEEE, an assistant professor and deputy dean at the College of Information Engineering / Al-Nahrain University / Baghdad / Iraq. His main research interests are design, analysis, fabrication and [email protected]
of RFID tag antennas and microwave filters.