review on saw rfid tag

08853010/$25.00 2010 IEEE

654 IEEE TransacTIons on UlTrasonIcs, FErroElEcTrIcs, and FrEqUEncy conTrol, vol. 57, no. 3, March 2010

AbstractSAW tags were invented more than 30 years ago, but only today are the conditions united for mass application of this technology. The devices in the 2.4-GHz ISM band can be routinely produced with optical lithography, high-resolution radar systems can be built up using highly sophisticated, but low-cost RF-chips, and the Internet is available for global ac-cess to the tag databases. The Internet of Things, or I-o-T, will demand trillions of cheap tags and sensors. The SAW tags can overcome semiconductor-based analogs in many aspects: they can be read at a distance of a few meters with readers radiating power levels 2 to 3 orders lower, they are cheap, and they can operate in robust environments. Passive SAW tags are easily combined with sensors. Even the anti-collision problem (i.e., the simultaneous reading of many nearby tags) has adequate solutions for many practical applications.

In this paper, we discuss the state-of-theart in the devel-opment of SAW tags. The design approaches will be reviewed and optimal tag designs, as well as encoding methods, will be demonstrated. We discuss ways to reduce the size and cost of these devices. A few practical examples of tags using a time-position coding with 106 different codes will be demonstrated. Phase-coded devices can additionally increase the number of codes at the expense of a reduction of reading distance.

We also discuss new and exciting perspectives of using ultra wide band (UWB) technology for SAW-tag systems. The wide frequency band available for this standard provides a great opportunity for SAW tags to be radically reduced in size to about 1 1 mm2 while keeping a practically infinite number of possible different codes. Finally, the reader technology will be discussed, as well as detailed comparison made between SAW tags and IC-based semiconductor device.

I. Introduction

In this paper, we briefly review the current status of the development of radio frequency identification (rFId) tags based on saW technology. We mainly discuss the tag devices, omitting issues related to the reader design and the corresponding signal processing issues.

The first rFId systems appeared already during World War II for identification of airplanes. however, it is only now that the technical conditions are right for widespread use of rFId. The 2 key issues for rFId technology are the number of different codes that can be stored on a tag and the possibility of transferring and communicating infor-mation. Because of the ongoing progress of semiconductor technology, mass production of such devices at a low cost has become possible. Micro- and nanometer lithographic technology enables the fabrication of very small tags with a chip size on the order of 1 mm and smaller, operat-

ing in the Ghz-range, where sufficiently wide frequency bands are available. These industrial, scientific, and medi-cal (IsM) frequency bands can be used without licensing when using a limited radiated power.

The wide frequency bands finally allow for a practically infinite number of different codes to be written and read at microsecond time intervals. The omnipresent internet, intranet, and similar communication networks enable the processing of databases and development of smart systems that use the information automatically read from rFId tags. The dramatic development of mobile phones, which only combine a transmitter with a receiver, both used in radio communications for a century by now, was based on exactly the same 2 reasons: first, the development of tech-nology enabling the use of high and wide frequency bands which support a large number of subscribers, and second, computer databases with high-speed data links enabling fast communication. The type of rFId tag introduced in this paper, the surface acoustic wave (saW) tag, is similar to rF saW filters that are widely used in mobile phones. saW tags and saW filters use basically the same technology.

rFId tags will be omnipresent. Below is a small list of possible applications:

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 originTracking of wild animals, marking of trees in forests, etc.access to buildings, parking, restricted areas, com-puters, etc.ambient assisted living for the disabled and the el-derlyIdentification of parts, equipment, machines, and cars assembled on conveyer linesTracing of dangerous and explosive substancessecurity and guard services

These applications will demand trillions of tags per year, which may result in an industry larger than the saW industry of today. Mass application of tags was predicted for the first time by c. hartmann many years ago [1].

II. active and Passive rFId Tags

Both semiconductor integrated circuit (Ic)-based and saW-based rFId tags use no transceiver stage to gener-

Review on SAW RFID TagsVictor P. Plessky, Senior Member, IEEE, and leonhard M. reindl, Member, IEEE

(Invited Paper)

Manuscript received May 21, 2009; accepted november 23, 2009. V. P. Plessky is with GVr Trade sa, Bevaix, switzerland (e-mail:

[email protected]).l. M. reindl is with the Institute for Microsystems Technology, Uni-

versity Freiburg, Germany.digital object Identifier 10.1109/TUFFc.2010.1462

ate the response signal, which carries their identification number, but send back a modulated version of the request signal when requested by the reader (see Fig. 1). Ic-based tags use an aM backscatter modulation and saW-based tags a time modulation.

Ic-based tags can be subdivided into 2 categories: pas-sive and active labels. Whereas active tags usually have an on-board battery, passive tags power their circuitry by rectifying a part of the request signal energy transmitted by an external reader. Incorporation of a battery increases the cost of a device, limits its life-time, and, furthermore, adds risk for environmental pollution. The minimum volt-age for operating the rectifier stage for extracting the supply power from the request signal, together with the limited licensed radiation power for the read-out signal restricts the reading distance of passive Ic-based rFId tags to a very limited range. The separation between the strong request signal and the small response signal of the tag is achieved by using the subcarrier frequency offset between both signals.

saW tags, on the other hand, are linear, time-invariant systems which simply reflect the request signal in a coded form that carries the identification information. saW tags achieve the necessary separation between the request and the response signal by using a time division employing a saW delay line. The minimum signal-to-noise ratio for decoding the information of the reflected signal in the reader, together with the limited licensed radiation power for the read-out signal allows a reading distance of several meters for passive saW-based rFId tags. saW tags fea-ture low losses, large delay times, and small dimensions. In addition, they have a simple and robust structure.

as compared with the widely used barcode, Ic-based and saW-based rFId tags both have the following advantages:

They can be read automatically, that is, without hu-man presence. This allows for an unambiguous identi-fication of objects, people, and animals.They do not need to be in line-of-sight to the reader, nor is any particular tag orientation required.They can have a reading distance as large as 10 m or even larger, depending on the system used. For bar-codes, reading distance is limited to about 30 cm.

III. saW rFId Tags

The main feature of saW tags is the high quality fac-tor, Q, of piezoelectric single crystals that allows a passive storage of the request signal until all environmental echoes have died out. Their operation is based on micro-acoustics of piezoelectric crystals instead of semiconductor physics.

They do not require any dc power, because they totally passively store, reflect, and re-emit the request signal, be-ing a linear device. Moreover, the request signal can be about 100 times smaller than for integrated circuit (Ic)-based tags.

another attractive feature of saW tags is their simple structure. saW tags are fabricated using single-metal-lay-er photolithographic technology although operation in the microwave region requires submicron lithography (about 0.3-m-wide electrodes), which is a standard tool today in Ic fabrication. This enables the fabrication of devices working in the 2.45-Ghz frequency band reserved globally for IsM applications. saW tags utilize the unique feature of piezoelectric materials which allows for a transforma-tion of electromagnetic waves into 100 000 times slower surface acoustic waves. saW tags can hence operate as delay lines and provide a sufficient delay with a relatively small substrate length for temporally separating the tag response signal from the read-out signal.

A. SAW Tags Versus IC Tags

compared with passive Ic-based tags, saW tags dem-onstrate the following advantages [2]:

saW tags operate with low-level rF pulses of about 10 mW. Ic tags at the same distance require a con-tinuous radiation of a few watts.saW tags operate in the 2.45-Ghz IsM bandcom-pliant with rF emission regulations throughout the world. The use of Ic tags demands specific certifica-tion.saW tags can be attached on metal objects. saW tag systems achieve greater penetration into pallets containing metal or liquid items [3].Because of low radiated power, saW tag readers have a substantially higher interference resistance than Ic tag readers which radiate a few watts in the same frequency as Bluetooth, Wlan, etc.The reading process of saW tags additionally permits a direct and accurate measurement of the tag tem-perature. saW tags thus show an inherent capability for operating as sensors.saW tags are very robust and can be operated in challenging environments (e.g., they can withstand high temperatures). Ic-based tags are more sensitive to such harsh conditions.

Because Ic tags contain a memory and a processor, any information in these tags might be re-written if the proper memory technology is used and the volume of information written in a tag is relatively large. This is considered as principle advantage of the Ic tags, which are small in size and relatively cheap. Ic tags can also reach a reading dis-tance of a few meters. however, the possibility to rewrite a tags code will inevitably make readers easily available and accessible. The tag information can be easily read, modified, or even falsified without authorization.

655plessky and reindl: review on saW rFId tags

Fig. 1. operating principle of rFId tags.

Because manufacturing of saW tags only requires one photolithographic step, they might become cheaper in mass production than Ic-based tags, which also need an expensive antenna for harvesting EM power. To achieve a reading distance comparable to that of saW tag readers, Ic tag readers have to radiate 100 to 1000 times more rF power, up to a few watts. such a concentration of electro-magnetic radiation may lead to health hazards and would create strong interference with other communications sys-tems using the same frequency range. saW tags can also easily incorporate sensor functions.

B. Principle of Operation

The operation of saW devices is based on piezoelectric-ity, a coupling between a materials electrical and mechan-ical properties. In certain dielectric crystals, the applica-tion of mechanical stress produces an electric polarization and, conversely, such a crystal undergoes a mechanical distortion when an electric field is applied. This property is used in saW devices and in many other applications to produce a mechanical output from an electrical input or vice versa. In saW devices, the transduction between an electrical signal and an acoustic wave is achieved by utiliz-ing an interdigital transducer (IdT), consisting of 2 inter-laced comb-like metal structures deposited on the surface of a piezoelectric substrate. The principle of operation of a reflector-based saW tag is shown schematically in Fig. 2. a reader emits a request pulse, which is received by the tag antenna, directly connected to an IdT.

The IdT transforms the electrical signal into a nano-scale surface acoustic wave, which is a mechanical wave of particle displacements. The generated saW pulse then propagates along the surface of the substrate, which is usually made of a material with strong piezoelectricity, such as lithium niobate (linbo3). The saW pulse is par-tially reflected and partially transmitted by each of the code reflectors, placed at precisely determined positions on the chip. These reflectors usually consist of one or a few narrow aluminum strips. The reflected saW pulse return-ing to the IdT carries a code based on the positions of the reflectors. In other words, this encoding method is based on the time delays of reflected pulses. It is known as time position encoding or pulse position modulation (PPM) and is described in further detail in section III-3. When the train of reflected saW pulses returns to the IdT, the acoustic signal is then reconverted into an electrical form

and retransmitted by the tag antenna. The response sig-nal is then detected and decoded by the reader. In saW tags, a surface acoustic wave is used to read a sub-micron barcode of properly arranged reflectors. In real reader systems, the described pulse signals are rarely used and more simple frequency domain reading methods are used, in which S11(f) of the tags is measured and then Fourier-transformed to the time domain, enabling cheaper reader designs.

IV. Milestones of saW Tag history

A. SAW Tags Based on IDTs

Passive reflective tags with similar functionalities like current saW tags, which achieve the necessary delay of about 1 microsecond by using an electromagnetic delay line were described first in 1966 [4]. saW tags were pro-posed first in the 1970s [5][8] and included the multi-IdT and in [5] also reflector structures in one or a few acoustic channels. Fig. 3 schematically demonstrates the idea of multi-IdT tags.

Transducer-based saW tags usually consist of one large and strong transducer, the input transducers in Fig. 3, and several coding transducers, called output IdTs in Fig. 3. Both the input and all output IdTs are electrically wired in parallel. When an electrical signal is applied to this common electrical port, saWs will be generated by all transducers. Because the forward and reverse transfer function is equal in saW devices, the insertion attenua-tion of a signal generated by the input IdT and picked up by one output IdT will be the same as the signal gener-ated by this output IdT and picked up by the input IdT. Thus, in the time domain we first get spurious signals which originate by a transfer from one code transducer to another. If the initial delay of the tag, before the first code


Fig. 2. operation of a saW tag system.

Fig. 3. Transducer-based saW tags.

signal is expected, is chosen equal to or larger than the largest time delay of these spurious signals between the first coding IdT and the last one in a track, these spurious signals do not influence the code signal. When neglect-ing propagation losses, it easily can be 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 1-track design [Fig. 3(a)] each coding structure is passed only once and thus the associated losses show up only once. The conversion ef-ficiency 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 acous-tic 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 spuri-ous 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 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 [9].

special care must be taken on electrical longline effects. all high coupling piezoelectric materials show a relatively high dielectric constant between 50 and 100. Therefore, the electric wavelength el on these materials is reduced by a factor of approximately 10 when compared with the vacuum wavelength. If the length of the connecting bus-bars become larger than 0.1 el, electric waveguide effects become important. To achieve a uniform summation of all signals from the individual output IdTs, a special rF summation network has to be implemented, which resem-bles the summation and output network used for saW convolvers in the 1970s and 1980s, see Fig. 4. Further care

has to be spent on the impedance matching of the cod-ing IdTs and between them and the input IdT. Because omitting an IdT would destroy this sensitive balance, an amplitude shift keying (asK) coding seems not to be pos-sible, but a pulse position coding or a phase coding seems feasible.

B. Reflector-Based Tags

X-cyte tags [8] have 16 reflectors distributed in 4 acoustic tracks. The multi-track design included separate transducers in each acoustic channel and 2 reflectors on both sides, each with up to 3 preceding phase shifting elements (see Fig. 5). The saW thus has to pass the bus bars twice.

reflector-based tags with folded propagation path of saWs allow a 2 times reduction in size, whereas a multi-track geometry causes the chip area to increase. a multi-track design with only 2 reflectors per track, as shown in Fig. 5, leads to increased losses which cannot be compen-sated.

X-cyte tags were designed for the american 900-Mhz IsM band and used a 4 PsK (phase shift keying) coding scheme, where 2 bits are coded by one symbol (reflector), which led to a further reduction of the number of reflec-tors and thus chip size and, in principle, also insertion loss. The first 8 symbols were generated in the left side of the chip shown in Fig. 5 and the last 8 symbols on the right side. For coding, the appropriate number of phase shifting elements per reflector were etched away in a sec-ond step. The phase shifting elements were built up as metalized area, which leads to an additional phase shift of 45 in phase for the passing saW when compared with the free surface. Because the saW passes each phase shifting element twice, the removal of such an element would shift the phase of the corresponding reflected signal by 90.

The X-cyte tags suffered 2 major drawbacks which hindered them from becoming a commercial success. First, they had no strategy for testing the ready fabri-cated 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


Fig. 4. output summing network for 8 code IdTs in a transducer-based saW tag. The number of overlaps is growing from left to right to com-pensate the attenuation of the saW by passing the IdTs before. Fig. 5. X-cyte tag.

critical phase differences between the signals, the carrier frequency must be demodulated in the complex time do-main data, which a network analyzer usually does not do. Thus, all chips were coded and assembled completely with the packaging to tags and then tested. a low yield thus leads to high fabrication cost. second, because the reflec-tors are distributed all over the chip, any variation of the saW velocity over the chip is very critical for the phase coding. The number n of acoustic wavelengths stored in a saW tag can be calculated with the center frequency f0 and the maximum delay time max with

n = f0 max.

Typical saW tags incorporate 5000 to 10 000 acoustic wavelengths, depending on the number of symbols, the relative bandwidth which determines the minimum sepa-ration of the symbols, and the minimum initial delay. If we demand that the phase must be within 10 at the exact phase, we end up to an accuracy of about 1:360 000, or 3 ppm. The saW phase velocity, therefore must be constant to a level of 3 ppm over the whole chip, shown in Fig. 5 to allow a safe extraction of the code.

Both authors tested a phase coding technique in the past. Plessky studied this topology in 1994 [10] for 2.4-Ghz tags (Fig. 6). although the device had very reason-able amplitude performance, the phase coding was impos-sible to achieve at that time with a 2.4 Ghz frequency. The device had no reference reflector.

reindl [11] generated a 4 PsK coding at both frequen-cies, 2.45 Ghz and 433 Mhz, by a small shift of the re-flectors of 1 or 2 times an eighth of an acoustic wave-length around the fixed periodic sampling. Because of the strong temperature dependence of the acoustic velocity on linbo3-yZ of 94 ppm/c, the correct recovery of the phase information needs a careful elimination of the tem-perature shift of the phase. although it was impossible to eliminate these temperature shifts manually, a small computer program could easily recover the correct phase information by using the first and last symbol for refer-ence (see Fig. 7).

C. Micro Design/Epcos Tags

one of the first commercial operations of saW tags rFId systems were installed in the late 1980s and be-ginning of the 1990s by the norwegian company Micro design as (now q-Free asa), a spin-off of the norway Institute of Technology in Trondheim. The tags were pro-duced by saWTEK. The system was installed in the oslo highway toll ring and the airport feeder (see Fig. 8). The cars could pass at a speed of 100 km/h without any stop. The reader replaced the manual tall collecting system, the booth of which can be seen in Fig. 8. In the upper part of the picture the antennas can be seen.

The system used an asK coding scheme (signal at a certain time slot = on; no signal = off) with 32 symbols in a norwegian 856-Mhz IsM band. The readers were fabricated by Micro design. The tags were designed by B. Fleischmann (at that time with Epcos) and fabricated by Epcos. he used a programmable wafer stepper for direct exposure of photoresist on the wafer for manufacturing of the IdT and all reflectors. Each tag got an individual part of the step program. The reflectors were placed in 2 tracks situated on one side of the IdT. For a good uniformity in amplitude of all bits, the increasing loss due to the preced-ing reflectors had to be compensated by choosing stronger and stronger reflectors in the back. Because the asK was implemented by putting or omitting a reflector, a code


Fig. 6. Measured response of phase coded saW-tag device; arbitrary loss reference level.

Fig. 7. radio frequency response of a 4 PsK identification tag in the 434 Mhz band.

Fig. 8. application of saW rFId tags in the norwegian highway toll ring of oslo.

dependence of the attenuation of the saW at each of the back reflector positions showed, which became more and more severe by the last reflectors. B. Fleischmann tried to compensate this code-dependent attenuation by choosing code-dependent reflectors at the rear part. nevertheless, a code dependency of the symbol amplitudes in the rear part remained, which lowered the yield. The code-depen-dent uniformity variation could be solved later by using special structures at off-positions which led to the same attenuation as a reflector, but showed a negligible acoustic reflection. however, this improvement came too late for the norwegian tags.

More than 500 000 tags were shipped and only one single failure occurred during several years of operation, caused by a wire bond break. at the end of the 1990s the saW tag system had to be replaced because of European harmonization, as the 856 Mhz band was no longer free.

D. Siemens SAW Tags

To avoid such problems, siemens 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 [11], [12]. 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, accord-ing to the design rules reported in [9]. 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 (see Figs. 9 and 10). Thus only one group had to be carefully designed to com-pensate internal losses.

In the 32-bit design, the first 1.5 s were used as ini-tial delay and the 33 reflectors were equidistantly distrib-uted over the following 2 s. Fig. 11 shows 2 measurement curves: (left) with a series of 8 on symbols, and followed with 8 off symbols, (right) an alternating series of on and off symbol. The tags were measured with a larger

bandwidth than the IsM band, including frequency bands for which the tags operate only with a reduced efficiency.

To compensate different read-out distances and tem-perature effects, the first and last symbols are used as reference.

The saW rFId tags are used in Munich subway and several local train systems (see Fig. 12). a 20-bit code space was sufficient for most railway applications. The


Fig. 9. 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.

Fig. 10. Photo of the mounted saW rFId tag shown in Fig. 9.

Fig. 11. Two measurement curves of 2 saW rFId tags as shown in Figs. 9 and 10.

Fig. 12. saW Id tag mounted on the side of a railway vehicle.

saW rFId tags are mounted to both sides of the railway vehicles. The readers were located at selected points along the net, near the rails, and linked to a central computer. The tags can be read with a detection range of several meters and up to a train velocity of 350 km/h without any failure. The system works up to an ambient temperature of 400c. The system is still in use and several thousand of the tags have been shipped.

The first reader generations implemented a pulse radar systems, Fig. 13 shows the very first prototype. later gen-erations shifted to highly evolved, but much less expen-sive, frequency modulated continuous wave (FMcW) or frequency stepped continuous wave (FscW) systems.

The system suffered from the relatively high insertion attenuation of the symbols caused by the many reflectors needed by the asK coding scheme, and due to the fact that linbo3-yZ was used as piezoelectric material. lin-bo3-rot128 would have shown the same attenuation of the saW on free surface (about 6 dB/s at 2.45 Ghz). The attenuation of the saW by the transmission of a reflector, however, would be lowered on rot-128 ln, resulting in an overall lower insertion loss. The coding was done using a programmable wafer stepper whereby one reflector after the other was exposed. long processing time and, there-fore, high cost resulted. Most problematic, however, was the high cost for the reader device, which took more than a decade to come down to a reasonable price.

several attempts were made to improve the tag perfor-mance. To totally get rid of near spurious signals originat-ing from multiple reflections within the coding reflectors, designs were studied with only one, but 100% reflector per track (see Fig. 14). Broadband 100% reflector could be achieved using a reflecting multi-strip coupler [13]. The compensation of the different insertion attenuation of the tracks was compensated by different apertures of the re-flectors. Thus, only the insertion losses on free surface and the track losses remained. Because of the wide central in-

put IdT, all reflectors lay within the near field of the IdT and only track losses occur. Because of reciprocity, this holds also for the way back. however, with 33 reflectors, the aperture of the central input IdT became too wide and electrical long line effects destroyed the uniformity of the impulse response.

For the European 433-Mhz IsM band, saW rFId tags were designed which lowered the chip size by applying a multi-strip coupler for track changing [13], [14] (see Fig. 15). The design used a unidirectional sPUdT as IdT (la-beled 2), a track changing element (labeled 4), and coding reflectors (labeled 3). The track of the saW is labeled 5. contact was made by an inductive coupling (labeled 6) to the antenna.

The introduction of pulse position coding by Plessky [10] was very fruitful, and allowed higher-order coding but overcoming the problems associated with phase coding (see Fig. 16).

Most saW rFId tag activities at siemens stopped af-ter all saW-related activities were transferred to Epcos. The train identification and related applications, however, remained within siemens and are still active.

E. BaumerIdent Tags

These tags were developed first in small swiss company TaGIX by r. stierlin, later transferred to BaumerIdent company [15].

The device operated in the 2.4-Ghz IsM band with a split finger IdT, using a 3rd harmonic (see Fig. 17). In one track there are 5 open-circuit /2 wide reflectors situ-ated on both sides of the IdT. Time position coding was used, providing 10 000 different codes. The design included a few innovations:


Fig. 13. Early prototype of pulse radar saW Id tag reader.

Fig. 14. schematic layout of a saW rFId tag with 33 100% reflectors distributed in 33 tracks on both sides of the centered input IdT.

Fig. 15. schematic layout of a saW rFId tag for operation in the small European 433-Mhz IsM band.

a calibration reflector was introduced to simplify compensation for temperature and technology shifts during reading and deciphering of the code (also in-cluded in the Micro design and siemens tags)Time(pulse) position coding was implemented [16], [17] allowing higher-order coding, and thus fewer re-flectors and lower insertion loss, but overcoming the problems associated with phase coding.reflectors were offset by a fraction of wavelength in 2 sub-channels to reduce parasitic multiple reflections [18].

F. Hartmanns Global Tag

a significant step toward practically infinite numbers of codes was made by c. hartmann [3], [19][23]. In his coding scheme, the time position is used, but time slots for the position of the center of a pulse are radically reduced, due to prescribing to each slot some phase which is sys-tematically growing along the array of slots inside a given group of slots, Fig. 18. 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 dem-

onstrated [21]. open-circuit reflectors with a diffraction compensating shape were used. The anti-collision problem of the simultaneous presence of a few tags was discussed [24] and the ways to solve it proposed.

G. Other SAW-Tag Design Ideas

d. Malocha and coworkers recently developed orthogo-nal Frequency coding for saW tags [25]. They use care-fully designed narrowband reflectors arranged in a way such that when one reflector has maximal reflectivity, the reflectivity of the others is close to zero. such a system allows reduction of losses, because the reflectors can be rather strong. This approach can be applied to sensors and for identification of a limited number of sensors, but it can hardly be used for Id tags with large numbers of codes.

Potentially, a bank of saW resonators with slightly different frequencies can be used as an Id tag [26]. one variant of such a tag, although based on FBars, is de-scribed by a. ronnekleive et al. [27]. high q-factor saW resonators would allow use of lower frequency IsM bands (434 Mhz and 868 Mhz), where the available frequency band is too narrow for most reflector delay line tags de-scribed above. This approach requires a bank of indepen-dent high q-factor resonators and its realization would be difficult.

V. design of saW Tags

A. Frequency Bands and Data Capacity

although the idea of saW tags was already proposed decades ago (by davies et al. in 1975 [28]), its final com-mercial breakthrough has not yet occurred. To become a commercially attractive product for mass applications, the data capacity of saW tags must be at least 20 to 32 bits, corresponding to between approximately 1 million and 4 billion codes.

The number of different codes is determined by the BT product (B is the frequency bandwidth and T is the coding time), as suggested by shannons formula [29]. Be-cause a saW tag must be small and cheap, we cannot use more than 2 to 4 s for coding delay. These delays corre-spond to propagation distances of 8 and 16 mm. If a data capacity of 32 bits (or, better, 64 or 128 bits) is desired,


Fig. 16. cut out of a pulse position coded saW rFId tag operating in the 868-Mhz IsM band. on the left side of the chip, a sPUdT input IdT is seen; on the right, 3 reflectors can be seen.

Fig. 17. saW tag from BaumerIdent.

Fig. 18. combined time position and phase coding.

a frequency band of 16 Mhz (or 32 Mhz or 64 Mhz) is needed. such frequency bands are available only at rela-tively high frequencies. Effectively, the only suitable fre-quency range available globally is the IsM band from 2400 to 2483.5 Mhz. This band is now extensively used around the world for local communication systems: Bluetooth, Wlan, wireless keyboards, etc.

achieving a sufficient number of codes in a saW tag therefore requires the use of the 2.45-Ghz range. This requires the use of submicron photolithographic tools be-cause the narrowest linewidths needed at this frequency range are on the order of 0.3 to 0.4 m. It is to be noted that this requirement is rather modest in comparison with the state-of-the-art Ic technology operating with a resolu-tion down to 0.05 m. saW tag technology can therefore reuse equipment from older generations of Ics, which de-creases the fabrication cost.

an identification code can be written on the saW tag in time positions, amplitude, phase, or other suitable sig-nal characteristics of the reflected pulses. The reflected pulses represent the symbols of the tag response signal and can contain codes for one or more bits each. The first commercial saW tags designed according to these principles(from BaumerIdent), are currently used in de-manding industrial environmentsspecifically, for auto-mation of car assembly lines (see Fig. 19). The number of unique codes commercially achievable at present is rather limited, on the order of 10 000. new ideas are currently being developed aiming at a radical increase in the data capacity of saW tags to 64 or even 128 bits (hartmann [3], [19][23]).

B. Best SAW Tag Geometry

The following principles are recommended to design a saW tag with the best geometry:

one track must be used with a unidirectional trans-ducer (sPUdT) to avoid bi-directionality lossopen finger reflectors with variable duty factor yield low loss; diffraction compensation can be usedTime position coding; 4 slots per group is close to op-timal for achieving a maximal number of codes, using given total delay. In Global tag, proposed by c. hart-mann, the number of positions is much higher and a group of slots may include a few pulses.an initial delay of about 1 microsecond remains nec-essaryTwo or 3 calibration reflectors (with fixed position) must be used.Error correction and other auxiliary functions may require the use of several reflectors which cannot be used for coding but occupy space.

C. Time Position Encoding

saW rFId tags can be encoded in several ways. cur-rent saW tag products use time position encoding [10], [16], [17], which represents the most straightforward way of data encoding in saW tags. This is the main method currently used in commercial saW tags [15], [30].

In this encoding scheme, the total time delay is divided into slots of certain duration.

The slot width hereby is not limited by the separa-tion of 2 signals by the radar system given by the time width of the pulses, which is t = 1/B, where B is the frequency band of the overall system (actually determined by the band of signals radiated by the reader). In fact, the slot width is limited by the resolution with which the reader system can measure the time differences between 2 symbols. This resolution is given by the cramer-rao bond, which depends on the bandwidth, number of sam-pling points, and the signal-to-noise ratio [31]. Therefore, 16 time slots seem to be feasible, giving 4 bits per symbol. To avoid overlapping of the symbols, a guard interval is needed between 2 symbols according to the separation of the system.

In Fig. 20, the slots form groups of 5 slots. For such a tag, one of the first 4 slots of each group is occupied by a reflector and the fifth one, the guard slot, is always left


Fig. 19. Test of a saW rFId tag system for automation of car assembly lines.

Fig. 20. schematic of time position encoding in saW tags.

empty (see Fig. 20). Each reflector thus has 4 possible positions (equal to 2 bits of data) and the total number of different realizable codes is 4n for a tag having n reflec-tors. Ten code reflectors will thus yield about 1 million distinct codes. When all the reflectors are placed in-line in one acoustic path, the chip space required by these 10 reflectors is about 2.5 mm.

The advantage of this encoding method is that one al-ways has the same number of reflectors, which makes it easier to design a saW tag with uniform amplitudes of response signals. also, for the reader, the problem then simplifies to searching for a single response from a given group of time slots.

To maximize the number of codes (for a given total coding time) using the time position encoding scheme, about 3 to 4 slots per group must be used. however, in practical devices (BaumerIdent, cTr), as in that shown in Fig. 21, decimal groups are also employed. In such a scheme, a reflector can occupy one of 10 possible positions. a reduced number of reflectors results in lower loss and lower parasitic response level, although the total length of the device slightly increases. commercially available saW tags (BaumerIdent, cTr) have a data capacity of 10 000 different codes, which in the decimal time position system corresponds to 4 code reflectors.

To provide uniform amplitudes of responses (or any oth-er desired variation of amplitudes with the pulse number), the strength of reflectors must be carefully adjusted start-ing with the very last and most strong reflector. some-times it is impossible to keep the magnitudes of reflected pulses uniform up to very first responsesthis would de-mand exceedingly week reflectors at the beginning of the a reflector arrayconsequently higher amplitudes of the first responses may be an option [21].

D. Phase Encoding

For time position encoding using time slots as large as 25 ns, which is far away from the cramer-rao bound which would allow much coarser slot lengths, the exact coordinate of a particular reflector is not so important. It must only be within a 25-ns time slot, which corresponds to about 60 wavelengths at 2.4 Ghz. The calibration re-flectors help to account for inaccuracies in position caused by temperature, technology variations, and other shifts. a code reflector often consists of only one or a few elec-trodes. a single reflector electrode has a width of about 0.4 to 0.6 m, which is significantly narrower than the slot it occupies. The slot width of 25 ns corresponds to about 50 m of physical space.

If the phases of the reflected pulses could be measured accurately, the coding capacity would increase significant-ly. Phase encoding has been discussed for many years but not yet implemented in commercial products. such sys-tems are being developed now by rFsaW [3], [19][23]. The idea of phase coding is simple: by displacing the re-flectors slightly, phase shifts can be realized and phase coding implemented. Fig. 22 illustrates the principle of introducing phase shifts of 90 by shifting reflector posi-tions by multiples of /8 [32]. In such a case, each reflec-tor can have 4 phase positions, which adds 2 additional bits to time position encoding. The previously described saW tag with 10 code reflectors will then have 240 vari-ants of codes, 40 bits, or about 1012 different codes. This is a large number: for every human being on Earth, there will be about 150 tags available with different codes never repeated.

Phase coding can be combined with time position en-coding in a more clever way [3], [19][23]: instead of keep-ing time slots unchanged and introducing phase modula-tion of the reflectors, it is proposed to reduce the size of the time slots and therefore increase the number of slots in a group, keeping the duration of time for the whole group unchanged. Each slot is assigned a definite phase of the re-flector, if the reflector is placed there. In this modulation scheme, the phase is used to determine the time position of the reflected pulse. It is evident that the uncertainty of phase measurement of the reflected responses depends on the signal-to-noise ratio [31], [33], and [34]. The required strength of the signal increases with increasing accuracy of phase values used for encoding. optimal methods for phase encoding and decoding are under intensive investi-gation [34].

E. Encoding Technology

Whichever encoding scheme is employed, each saW tag produced has a unique physical appearance. currently, for the manufacture of only 10 000 different codes, the images of all these tags are placed on a large (but still reasonable)


Fig. 21. Practical example of time position encoding [30].

Fig. 22. schematic of phase encoding.

number of photomasks. This technology will evidently be too expensive for 106 codes and totally unrealizable for 1013 different codes.

one idea is to apply a double-stage photolithography process, wherein the first stage all reflectors in all possible positions are produced (or at least exposed) with high accuracy, and subsequently all redundant reflectors are deleted, say, with some fast and programmable tool or conventional photomasking. This system was tested suc-cessfully; however, it has not yet been implemented on a mass production scale. Programmable reflectors were pro-posed [35] at the expense of loosing tag passivity.

VI. developments in saW tags

The main goals of saW tag design include a reduction of device losses, a reduction of device size, and an enhance-ment of data capacity. a combination of time position en-coding and phase encoding provides a means to increase the information capacity, as described in previous section. This section presents ideas for further solutions and shows that a small saW tag device size can be achieved simulta-neously with a sufficiently large data capacity.

A. Examples of Recent SAW Tag Design

Fig. 23 shows a typical time response of a saW tag. In this case, FEM/BEM software1 has been used to simulate the performance of a tag having 14 code reflectors.

as illustrated by the mask image in Fig. 24, ten of the reflectors are used for encoding, the first and the last re-flector are used for calibration and are typically designed to have stronger responses than the others [36], and the 2 reflectors preceding the very last one are used to create a checksum for error control. The reflector array is de-signed to produce uniform amplitudes for code reflections to achieve a maximal read range.

amplitudes of response signals are adjusted by gradu-ally increasing the reflectivity of code reflectors, by adding electrodes to reflectors, and by increasing their width. This is done to compensate for the losses caused by propaga-tion on the substrate surface, and by reflections and bulk wave conversion from preceding code reflectors. a formula for the proper choice of the amplitude of the responses, and criteria to determine whether more tracks are advan-tageous, are given in [9].

as mentioned previously, a saW tag must provide cer-tain time delay to separate the response signal from the read-out signal. The reflected signals must be received by the reader only after a delay sufficient for the environmen-tal echoes (reflections from walls or other nearby objects) to die away. an adequate initial delay is typically about 1 s and is facilitated by leaving about 2 mm of empty space on the substrate between the IdT and the code reflectors. The free-surface saW velocity on linbo3 is about 4000 m/s.

B. Loss Reduction in SAW Tags

a standard IdT, as depicted in Figs. 2 and 3, consists of electrodes with alternating polarities. as it transforms the electrical signal into an acoustic form, it generates sur-face acoustic wave propagation equally in both directions. When such a bidirectional IdT is used in saW tags, half of the signal energy is already lost in transduction and the same amount in the transduction back.

This problem can be overcome by using a unidirectional IdT that only generates wave propagation in one direc-tion. For a similar reason, saW tags with several parallel acoustic channels will have a higher loss level than a de-vice where all reflectors are situated in the same channel.

however, typical unidirectional transducers (more spe-cifically, single-phase unidirectional transducers, sPUdTs) include electrodes with a width of /8, where is the wavelength of saW on the piezoelectric substrate. at 2.45 Ghz, /8 is about 0.2 m. This means sPUdT-type transducers cant be produced using the photolithography technology currently used in the saW industry.

recently, however, a sPUdT especially designed for saW tag applications was proposed by hartmann & Plessky [23] exploiting the fact that, on 128-linbo3, the reflectivity of short-circuited electrodes can be close to zero at some metal thickness and electrode widths. The proposed transducer uses /4-wide (and wider) electrodes and can be manufactured using current commercially available optical photolithography.


Fig. 23. simulated saW tag response.

Fig. 24. Mask image of a reflector-based saW rFId tag.

1http://www.gvrtrade.com/img/FEMsaW%20simulation%20Tool.pdf

Tags which are used today apply a bidirectional IdT and result in a loss level on the order of 55 dB for 10 000 codes. This will be reduced to about 40 dB for saW tags with a unidirectional IdT and 106 codes [37]. The unidi-rectional transducer may include 0.3-m-wide electrodes. also the reflectors must be rather narrow: for some cases, reflector electrodes must have a width of 0.3 to 0.4 m. Therefore, reliable photolithography capable of produc-ing the line widths of 0.3 m is needed. In addition to reduced losses, the use of a sPUdT in saW tags has the advantage of a lower level of parasitic reflections, includ-ing reflections from the transducer itself.

C. Size Reduction of SAW Tags

replacing the bidirectional IdT with a unidirectional IdT also serves to reduce the chip size. saW tags using a bidirectional transducer are normally designed to have their reflectors on both sides of the transducer. In this case, space for the initial delay must also exist on both sides, which results in an inefficient use of the substrate area. When a unidirectional transducer is employed, all re-flectors must be placed on the same side of the transducer and only one initial delay is needed.

a further reduction of chip size can be achieved by fold-ing the channel used for saW propagation. a Z-path saW tag has been designed, fabricated and tested [37] that uses 2 inclined, strongly reflecting mirrors (each consisting of an array of open-circuit metal strips), as shown in Fig. 25. although such folding demands 2 additional reflectors (and 4 reflections of the signal), which inevitably results in additional losses on the order of 5 to 10 dB, the read-ing distance is reduced not so strongly (less than 50%). This can be an acceptable price to pay for a significant reduction of size and cost of a saW tag. The size of the chip in saW propagation direction 0X (horizontal in Fig. 25) was reduced from 6 mm to about 3 mm. another pos-sibility of folding the acoustic track might be the use of track changers [13].

D. Ultra-Wideband SAW Tags

The currently emerging ultra-wideband (UWB) tech-nology offers many attractive possibilities for the devel-opment of saW rFId tags. according to the regulation of the United states Federal communications commis-sion (Fcc) [38], a UWB device is a device emitting sig-nals with a fractional bandwidth greater than 20% or a bandwidth of at least 500 Mhz. a saW tag operating at

2.5 Ghz with a band of 500 Mhz would satisfy this criteri-on. The UWB band being much wider than the 2.45-Ghz IsM band, a certain value of BT product, determining the data capacity of a tag, can now be achieved with a signifi-cantly shorter coding delay, which enables a considerable reduction of tag size. For example, with B = 500 Mhz, a BT of 200 only requires a coding time of 400 ns instead of the 2 s typical for 2.45-Ghz saW tags. The total chip size can then be smaller than 0.5 1.0 mm. a shorter coding time also implies lower losses. a propagation time of 400 ns corresponds to only about 3 dB propagation loss.

another interesting possibility is to have signal pro-cessing partly performed within a saW tag using, for ex-ample, a chirp transducer [39][41] as illustrated in Fig. 26. This will allow for a matched-to-signal processing of the tag response, which, after being modified within the tag, will be different from the environmental echoes of the request signal also received by the reader. This makes the system more resistant to environmental interference, because the reader is now able to distinguish between the signal reflected by the saW tag and that reflected by ob-jects outside the tag. Because the principle of the ultra-wideband technology is to reuse an already occupied fre-quency spectrum, but with very low power, an UWB saW tag system will also have the additional advantage of very low transmitted power levels.


Fig. 25. Z-path saW tag geometry with 2 inclined reflectors.

Fig. 26. read out process for an ultra-wideband saW tag. (a) an up-chirp linear frequency-modulated signal is used for the request. (b) The signal is compressed by the chirp transducer, reflected by code reflectors, and expanded by the transducer. The output signal has a dispersion op-posite to the signal. (c) reflections from surrounding objects have the same dispersion as the request signal.

VII. discussion

The market demands small size, low cost, and environ-mentally compatible rFId tags. That excludes devices consuming power from batteries. Both semiconductor-based tags and saW tags can be read remotely, and both are small in size. They do not require maintenance and their life-time is limited only by the usual product time of the circuitry. however, saW rFId tags and passive semi-conductor rFId tags are based on fundamentally different physical principles. In this section, we compare in detail these 2 approaches.

A. Power Issues in SAW Tags and in IC Tags

The main feature of saW rFId tags is that they do not use any autonomous power supply such as batter-ies. Moreover, they do not include any such circuitry that would need to be powered. saW tags are passive devices that merely reflect the request signal. This results in a linear operation at any signal level, even at a very low one. The signal energy of the saW tag response must of course be sufficiently high for the reader to be able to receive it, which is determined by the signal-to- noise level. however, using multiple readings and matched-to-signal detection, tag signals with power below the noise level can be read. The total power radiated by the reader is typically on the order of 10 mW. For high-speed long-read-range ap-plications, only a fraction of a microwatt is needed at the tag position [3]. This is the typical power level to which human beings will be exposed when in proximity of saW tag systems. It is about a hundred times lower than the radiation exposure generated by mobile phones.

rFId systems based on semiconductor chips use an Ic to receive and detect the signal sent by the reader, as well as to subsequently decode the signal and generate the response. The functional blocks of a typical Ic tag include power accumulation, computation, and communi-cation. The main feature of Ic semiconductor tags is that they must include a proper dc power source for correct operation. The so-called passive Ic rFId tags that do not carry a battery are obliged to take this power from the rF request signal. The main part of the signal sent by the reader is used to power the Ic and only a small modulation of this signal is used for transmission of data. rectifier circuitry is used to extract sufficient power from the radio signal. The rectifier converts the signal into dc for storage in a capacitor and, ultimately, for powering the chip. The reading of the tag is performed using a predetermined protocol and is only possible if the nec-essary dc power level is maintained throughout the en-tire request cycle. Therefore, a minimum critical power of about 100 W must be received continuously by the tag antenna during the entire decoding period of the tags signal [2]. Below this signal threshold, rectification is not possible. This power restriction is imposed by the phys-ics of semiconductors and thus is fundamental. For saW

tags, on the other hand, no threshold exists because they are linear passive devices. They generate a response at all power levels, usually orders of magnitude lower than what is required for Ic tags.

VIII. conclusions

In this short review, we argued that saW tags have clear advantages over Ic semiconductor rFId devices in many aspects:

saW tags practically have an infinite number of codes sufficient for all reasonable applications.saW tags have an incomparably larger reading dis-tance with the same power radiated by the reader, when compared with passive Ic-chip-based tags.saW tags are small, robust, and can operate in harsh environments where Ic-based tags fail.saW tag readers using correlation techniques for sig-nal processing can read several saW tags simultane-ously [20].saW tags can easily be used as temperature sensors with Id function [42]

To sum up the above arguments, it is evident that the necessary technological tools as well as the necessary in-frastructure and prerequisites are available for the devel-opment of smart saW-tag-based systems. additionally, the development of the internet offers conditions for effi-cient transfer of information to and from databases, which in turn is another pre-condition for the efficient use of rFId tags.

saW tags offer an excellent technical solution. how-ever, to convert this brilliant idea into a multi-billion dol-lar business, several scientific and technological challenges must be solved, and the fabrication cost of reader devices and tags must be decreased drastically.

acknowledgment

V.P. thanks s. hrm from hUT, helsinki for fruitful collaboration during last 5 years. Many of the results au-thored by sanna are included in this review as well as in her Ph.d. thesis [43]. Many thanks to also c. hartmannhis brilliant ideas inspire development in this area. We also thank W. Walker for reading and correcting the text.

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Victor Plessky was born in Gomel, Belarus, in 1952. he has worked for many years in the area of surface acoustic wave (saW) physics and de-vices. he theoretically predicted the surface transverse waves (sTW), a new type of waves now widely used for design of high-q resonators. his theory of leaky wave propagation in periodic grating (the so-called Plessky equation) is now used for the design of low-loss filters. he has pub-lished more than 200 papers and authored about 30 patents. as a Visiting Professor, he collabo-

rated for more than 12 years with the helsinki University of Technol-ogy, Finland, where one of the best groups in saW area was created with his leadership. he also was lecturing at Freiburg University, at the Ecole Polytechnique Fdrale de lausanne, lausanne, switzerland, and in the angstrom lab, Uppsala University, Uppsala, sweden. he was supervisor and consultant of 12 Ph.d. theses. dr. Plessky holds the title of Full Professor, granted to him by the russian Government in 1995. he was a winner of a lenin Komsomol award for young scientists in the Ussr in 1978, and received the outstanding paper award from the IEEE UFFc society in 2001. currently he works as consultant in micro/nano acoustics physics and devices at GVr Trade sa in Bevaix, switzerland.


Leonhard Reindl (M93) received the dipl. Phys. degree from the Technical University of Mu-nich, Munich, Germany, in 1985 and the dr. sc. Techn. degree from the University of Technology Vienna, austria, in 1997. From 1985 to 1999 he was a member of the micro acoustics group of the siemens corporate Technology department, Mu-nich, Germany, where he was engaged in research and development on surface acoustic wave (saW) convolvers, dispersive and tapped delay lines, Id-tags, and wireless passive saW sensors. In winter

19981999 and in summer 2000 he was guest professor for spread spec-trum technologies and sensor techniques at the University of linz, linz, austria. From 19992003 he was university lecturer for communication and microwave techniques at the Institute of Electrical Information

Technology, clausthal University of Technology, clausthal. In May 2003, he accepted a full professor position at the laboratory for electrical in-strumentation at the Institute for Micro system Technology (IMTEK), albert-ludwigs-University of Freiburg.

his research interests include wireless sensor and identification sys-tems, saW devices and materials as well as microwave communication systems based on saW devices. he holds 35 patents on saW devices and wireless passive sensor systems. he has authored/coauthored ap-proximately 130 papers in this field. he is an adcom member of the IEEE Ultrasonics, Ferroelectrics, and Frequency control society and a member of the Microwave Theory and Techniques society. since 2000 he has been a member of the Technical Program committee of the IEEE Frequency control symposium. he also is engaged in technical commit-tees of the German Electrical, Electronic, and Information Technologies and Information Technology society (VdE/ITG).


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