A remote identification system based on passive identifiers

Signal Processing 26 (1992) 369 379 Elsevier

369

A remote identification system based on passive identifiers H.J. Butterweck, A.C.P. van Meer and J.H.F. Ritzerfeld Group Circuits and Systems Jor Signal Processing, Department of Electrical Engineering, Eindhoven University ~/" Technology, Eindhoven, The Netherlands Received 6 May 1991 Revised 30 October 1991

Abstract. A system is presented that enables automatic remote identification of persons or goods using tagging with passive devices. A well-defined system area is interrogated by a magnetic field that is periodically switched on and off. Presence of a tagged object is detected and its identity established by measuring the natural response of the tag during the transmission silence. The tag or identifier consists of a main resonant circuit, the coil of which provides magnetic coupling, and several quartz crystals that determine its identity. High reliability and insensitivity to interference are ensured due to the high quality factors involved. The use of passive identifiers provides a robust and low-cost system that can serve in a wide range of applications.

Zusammenfassung. Es wird ein System vorgestellt, das die automatische Identifikation auf Abstand (remote identification) von Personen oder Giitern erm6glicht, wobei passive Etikettierung benutzt wird. Ein gutdefiniertes Systemgebiet wird durch ein Magnetfeld abgefragt, das periodisch an- und abgeschaltet wird. Durch die Messung der natiirlichen Antwort der Etikette in der LIbertragungspause wird die Anwesenheit des etikettierten Objektes detektiert und seine ldentit/it festgestellt. Die Etikette oder der Identifikator besteht aus einem Hauptresonanzkreis, dessen Spule die magnetische Kopplung herstellt und aus einigen Quarzkristallen, die die ldentit/it festlegen. Hohe Zuverl/issigkeit und Unempfindlichkeit gegenfiber St6rungen werden durch die hohen eingesetzten Qualit/itsfaktoren erreicht. Der Gebrauch passiver ldentifikatoren ergibt ein robustes und billiges System, das in einem weiten Anwendungsbereich eingesetzt werden kann. R~sum& Nous pr6sentons un syst6me permettant l'identification automatique ~ distance de personnes ou de biens fi I'aide d'un marquage par des m+canismes passifs. Une zone bien d6finie du syst6me est interrog6e par un champ magn6tique qui est p6riodiquement enclench6 et d6clenchb. La pr6sence d'un objet marqu6 est d+tect~e et son identit6 6tablie par une mesure de la r6ponse naturelle du marqueur durant une p6riode de non-transmission. Le marqueur ou identificateur consiste en un circuit r6sonnant principal dont le sol6no'/de assure le couplage magn6tique, et de plusieurs cristaux de quartz qui d6terminent l'identit6. Une haute fiabilit+ et une bonne insensibilit6 aux interf6rences sont assur~es du fair des facteurs de haute qualit6 impliqu6s. L'utilisation d'identificateurs passifs produit un syst6me robuste et peu cofiteux pouvant ~tre utilis~ dans une large gamme d'applications.

Keywords. Identifcation, remote sensing, passive tagging, quartz crystals, coded labels.

1. Introduction Presently, a number of electronic systems are available for the detection or identification of persons and goods [2-8]. The use of such systems is becoming increasingly widespread. Applications are in access control, electronic security, vehicle identification, monitoring of livestock, storage

control and logistics. Design of these systems is governed by a number of basic principles that can best be elucidated by considering some typical representatives. In article tagging as applied in anti-shoplifting systems the presence of an (uncoded) electrical label (tag) in some well-defined region of space is detected with the aid of a high-frequency (0.1 to

0165-1684/92/$05.00 ~,(, 1992 Elsevier Science Publishers B.V. All rights reserved

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H.J. Butterweck et al. / A passive remote identification system

50 MHz) magnetic field. Such a label slightly distorts the field generated by some properly designed transmitter coil. The secondary magnetic field resulting from this perturbation is then detected by a receiver coil. In most current systems the two coils are combined in the same structure, which is called an interrogator. The label can be constructed in rather different ways. One option that has found widespread use in public libraries consists of a ferromagnetic strip or wire whose nonlinear magnetic properties are exploited. Activation and deactivation is possible through application of suitable low-frequency fields. Another device that is frequently used in anti-pilferage systems consists of a capacitor connected to a small-sized coil providing the magnetic coupling with the external field. This resonant combination is detected with the aid of a frequencymodulated generator sweeping around the resonance frequency. The receiver is then constructed such that it can discriminate between the resonance curve from the label and more flat responses from other objects in the region of detection. Contrary to such a system with a range of maximally a few meters a passive identification card requires a rather close contact with the interrogator j. On the other hand such a card can store a more or less complicated code. This way an identification of the cardholder is enabled, whereas article tagging is meant for detection only. The code can be contained in a ferromagnetic strip or in an electronic circuit (as in a smart card). If such labels have to function at a considerable distance from the transmitter/receiver coil they require some form of power supply by means of a battery or a high-power microwave activation by a remote source. Of course, such active systems require some amount of maintenance and are, moreover, more expensive than their passive counterparts. Finally we mention certain traffic monitors of the radar type, where an electromagnetic wave at G H z frequencies provides the coupling between the fixed Modern systems can cope with distances up to a few centimeters. In this paper such distances are indicated as 'closerange'. signal Processing

interrogator and the mobile object of detection or identification. The great variety of features exhibited by the above examples asks for some ordering of the systems under consideration. Besides the basic partition into systems for detection and for identification we can classify the various systems according to the following mutually exclusive attributes: passive versus active, remote versus closerange, magnetically coupled versus radiationcoupled 2. Another classification scheme refers to the underlying physical mechanism, e.g. resonance or nonlinear behaviour. Finally we have some measures with a more gradual character: reliability and the degree of sophistication and, in close relation to these figures of merit, the eventual price of the system. The above classification schemes are also required for an unambiguous terminology. The label attached to the mobile object is called a tag in the case of pure detection and an identifier for identification purposes. If remote sensing is enabled, an identifier becomes a transponder. The fixed coils for transmitting and receiving are called antennas, even if the physical dimensions of the system are small compared to the wavelength. The region in space, that is interrogated by the antennas, is called a gate.

2. Anew system In the present state of the art not every combination in the above classification scheme is realizable. For an identification system we can only choose between the pairs passive/close-range and active/ remote, the latter at a relatively high price. Thus a cheap passive remote identification system that operates reliably, appears to be highly desirable. : We speak about a (pure) magnetic coupling if the physical dimensions of the system are small compared to the wavelength. If this condition is not met, radiation coupling applies. Clearly, the type of coupling is determined by the operating frequency of the system.

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Below we show how such a system can be developed through a proper modification of the classical resonant (LC-)tag. For such a tag with a typical linear dimension of 5 cm the quality factor Q can be shown to attain a theoretical maximum of about 1000 in the 200 MHz region [6]. These figures vary with the dimensions such that a linear magnification by a factor T leads to an approximate Q increase according to T '~ occurring at a frequency reduced by the factor T ~. The background for these laws is provided by the two basic loss mechanisms in the tag coil, viz. the ohmic losses in the copper wire (proportional to ~o - 1 2 ) and the radiation losses (proportional to co3). Of course, the dielectric losses in the capacitor and the tag casing still have to be added. Together with the necessity that the operating frequency has to be considerably lower than the above optimum to avoid wave effects in the system, Q values in excess of 200 will seldom be achieved. It is easy to recognize that with such tags an identification system with a sufficient number of codewords cannot be realized. Such a system would require a set of, say, M tags, whose resonance frequencies have a minimum relative spacing of Q- 1. Only then a noticeable overlap of the resonance curves and an associated reduction of the reliability can be avoided. This results in a minimum bandwidth M Q - ~ of the complete system which even for moderate M values exceeds the limits prescribed by international legal requirements. A second factor prohibiting the development of an identification system on the basis of ordinary LC-tags is the impossibility to produce such tags with sufficient reproducibility and constancy of their resonance frequencies. A third argument against such a system is provided by the requirement that other than tagged objects must not be detected. As an example, an anti-shoplifting system should not raise the alarm, if a pram or another large metallic object passes the gate. Although such an object generates a large signal at the receiver, this has to be rejected in favour of the much smaller tag signal. The required

discrimination can only be based on a difference in waveform, which is insufficient if the tag resonances are rather mild. These considerations suggest the line along which the modification of the above system has to proceed. Obviously the Q-factor has to be raised, coupled with an adequate stability and reproducibility of the resonance frequency. This can be accomplished by connecting a quartz crystal to the LC-circuit of the tag. With its well-defined, stable and reproducible resonance frequency the crystal transforms the tag into an identifier. The combination of the low-Q LC-circuit and the high-Q crystal (Q~50,000) leads to a peculiar interference of the individual resonance curves, as shown in Fig. 1 for the case of coinciding resonances in the partial systems. Observe that the inverted crystal resonance curve is superimposed on the main resonance. Apparently the crystal resonator extracts part of the energy from the LC-circuit to cover its own losses. This is a striking illustration of a general principle pertinent to any low-loss dynamic system excited at one of its natural frequencies: Weak coupling to another passive system can only lower the amplitude of the forced oscillation, either because of detuning due to a reactive load or because of absorption due to a dissipative load. Here the LCcircuit is the main system which experiences additional damping by the crystal. Electrically, the crystal identifier consists of the parallel connection of the LC-circuit and the crystal

~0

~d

Fig. 1. Resonance curve of a crystal-loaded identifier. Vol. 26. No. 3, March 1992

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H.Z Butterweck et al. / A passive remote identification system

im q~

qa

X

X

0

r

J~o

IfR1T T main c o i l

crystal

Fig. 2. Equivalent circuit of the crystal-loaded identifier. -~ one-port, that can be represented as a series LCcombination. Taking into account the losses of both circuits in the form of resistors, one arrives at the equivalent circuit of Fig. 2, which has been discussed and analyzed in [6]. For our purposes we are primarily interested in the overall transmission between the two gate antennas. If the constant transmission in the absence of an identifier is left out of consideration, we obtain a two pole-pair transmission, which in the vicinity s~jco0 (where the contributions from the conjugate poles may be neglected) can approximately be written as A B H(s) ~ s + a - j C O 0 t s + / 3 - j c o 0 "

(1)

Thus we have poles at q l = - a + j C O o and q2 = -/3+jCOo with residues A and B. The damping coefficients a,/3 refer to the main and to the crystal circuit such that Ql=COo/2a and Q2=COo/2/3 become their quality factors. Note that Q2 >>Q~ and therefore a >>/3. In (1) we have tacitly assumed that both circuits have the same resonance frequency COo. By this we mean that the poles ql and q2 which are also the system's eigenfrequencies (natural frequencies) have the c o m m o n imaginary part COo. Since H(s) is proportional to the impedance of the identifier [6], q~ and q2 represent its open-circuit eigenfrequencies. On the other hand the short-circuit eigenfrequency is a zero of H(s), to be denoted by r: H(r) = 0. Elementary analysis of the identifier circuit shows that (in a high-Q approximation) the imaginary parts of q~ and q2 equal COo, if both the Signal Processing

re

-0-~ 0

Fig. 3. Poles (x) and zero (O) of the identifier impedance for identical resonance frequencies.

parallel- and the series-circuit are tuned to coo. But then also the imaginary part of r equals coo, and the symmetrical pole-zero pattern of Fig. 3 is obtained 3. With r=jcoo

flA + a B

,

(2)

A+B

following from (1) we can then conclude that the second term in (2) and thus the ratio A / B of the two residues has to be real. This leads to 7 = -Re(r)

flA + a B - - - / 3 A+B

a - fl 4 - - . I+A/B

(3)

Further the inverse character of the resonance curve of Fig. 1 requires that the zero is nearer to the imaginary axis than the pole q2: 0<7
-vo
(4)

Associated with the zero (the short-circuit eigenfrequency) is the unloaded quality factor Q0 = COo/ 27, while QL = Q2 = O9o/2/3 is called the loaded quality factor. The first is concerned with the intrinsic damping of the crystal, as represented by R2 in Fig. 2, whereas the latter stands for the total damping caused by the series connection of R1 and R2. If it were possible that only R~ damped the 3 Obviously the H(s) of the identifier has to satisfy some condition guaranteeing that pole symmetry implies zero symmetry. Thus not any function H(s) satisfying ( 1) can be realized by the circuit of Fig. 2.

H.J. Butterweck et al. / A passive remote identification system

crystal we could speak about a fictive external quality factor Qext. Clearly, the inverse Q-factors add up such that Q{ ~= Qo' + Qe~. Accordingly, we have Qext= (o0/2(fl - 7). Further we conclude from the equivalent circuit of Fig. 2 that

373

3. Transmitting and receiving modes

(7)

Like any other resonant label, a crystal-loaded LC-tag can be detected in a variety of ways. The classical method uses frequency sweeping which detects a tag through searching a certain frequency band for an (anti-)resonance peak in the gate transmission. Under certain conditions (availability of the transmitter signal at the receiver, negligibility of wave effects) the reliability of the detection can be improved by synchronous detection, i.e. by incorporating the transmission phase into the decision strategy. In our system we prefer a pulsed operation in which a transmitting and a receiving mode alternate. In the first mode a constant sine wave is transmitted which brings the crystal inlo resonance. While the receiver is non-active in this mode, the transmitter is switched off in the second mode. Then the receiver observes the crystal's natural oscillation, whose exponential decay is characterized by the time constant r = lift = 2Qc/co0 (for Qc = 14,500 and co0/2rc =27 MHz this assumes the typical value of 0.17 ms). An appropriate choice for the lengths of the transmitting and the receiving phase is 3r for each mode, which amounts to a cycle duration equal to 6r (1 ms in the above example). With this choice, the steady state is almost completely reached in the transmitting phase, while the natural oscillation can be almost completely observed in the receiving phase. It can be shown [6] that shorter but also longer cycle times 4 ultimately lead to a reduction of the decision reliability. One of the main advantages of the pulsed operation is provided by the easy discrimination between the high-Q label and a pram which stands symbolically for any other low-Q object of possibly large physical dimensions. Even if the initial amplitude of the pram's natural oscillation exceeds that of the quartz label by several orders of magnitude, the latter becomes dominating after a sufficient

following from (3). Notice the opposite signs of A and B, reflecting the antiresonance in Fig. 1.

4 TOO long cycle times reduce the possibility of repetition of m e a s u r e m e n t s , as discussed further down.

Qcxt_ R2 Q0 R I

(5)

This ratio that can serve as a measure for the coupling of the crystal to the surroundings can be adjusted with several electrical means. One is to connect the commercially available crystal oneport to only part of the windings of the main coil, another is found by connecting a very small capacitor in series with the crystal one-port. After a transformation of the equivalent circuit again a series RLC-circuit is obtained with a higher R2 and, due to (5), a higher value of Qex, (a lower external damping). Of course, the degree of coupling does not influence the value of Q0 and, as a consequence, that of y and of the zero r. Obviously the coupling only affects fl and the pole q2 (cf. Fig. 3). This implies that the negative peak in the resonance curve of Fig. 1 becomes deeper and wider with increasing coupling. Quantitatively, its relative width is determined by the loaded quality factor as Q[~. The special case R, =R2 resulting in Qo = Qe×t, Qc = Qo/2 is known as critical coupling. The resonance curve then exhibits a 50% dip. In the more general case we have the depression factor H(jco0)

7

Qc_

R2

H0(j(oo)

fl

Q0

R,+R2'

(6)

where Ho denotes the transmission of the crystalfree LC-circuit. Like the ratio H/Ho also the ratio of the residues A, B can be expressed in terms of the quality factors. For future reference we note that A

a

8-p-),

_ Qe,,t

Q, >>1,

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number of periods of the oscillation. Hence we can easily discard the pram's response through delaying the start of the measurement in the receiving phase by a small fraction of r. This way only the crystal's oscillation is observed, because together with the pram's oscillation also that of the main LC-circuit is suppressed. Another advantage of the separation in time between transmission and reception is that it permits the relatively easy use of a combined transmitter/receiver coil. The only requirement for such a solution is a perfect isolation between transmitter and receiver such that neither in the transmitting nor in the receiving phase any signal is directly coupled from the transmitter to the receiver. This problem is well known from radar technology which offers a number of pertinent solutions. Notice that the use of a single coil is rather difficult in sweeping systems, where the inherent direct coupling has to be balanced by means of a broadband compensation circuit. Like the sweep mode, also the pulsed mode permits synchronous detection. Nevertheless, in this paper we confine ourselves to the asynchronous version, because a number of weighty arguments can be found against synchronous detection. First, a connection (of the multi-wire type for an identification system) between transmitter and receiver with a calibrated phase delay is required. Second, the frequency must either be low, to allow the quasi-static treatment of the magnetic coupling mechanism, or the phase delay at higher frequencies must be incorporated in the receiver's decision strategy (but this is possible only for objects at rest with a constant delay). Finally, synchronous detection requires the maintenance of synchronism between the transmitter and the crystal during the whole receiving phase as well as the absence of any undetermined phase shift due to a detuning between the crystal and the main LC-circuit. The latter requirement can hardly be met in an identification system where several crystals are simultaneously coupled to the main circuit. For an analytical investigation of the free oscillation of the crystal-loaded label we invoke (1). First, Signal Processing

we state that the response to a unit-amplitude timeharmonic excitation of the type exp@o0t) is approximately given by •

A

H(jcoo) ejo°' =-- eJ'°°'+ B ej,O,,,. a

/3

After the excitation has stopped, this steady-state response continuously passes into the free oscillation A

- e

at

eJO~otq_ B e- fit eJW0t.

a

The amplitudes A/ct and B/fl are opposite in sign, while the former is slightly greater in magnitude. This result follows from (7)' A/a_

fl

B/fl

fl - 7

_

Qext

(8)

QL

It is easily visualized by the fact that A / a represents the main resonance in Fig. 1 while B/fl stands for the (opposite and weaker) antiresonance. As agreed, the measurement starts a short time after the source has been switched off. Then, together with a potential pram oscillation, the free oscillation of the main LC-circuit has faded away. Hence the natural oscillation after an excitation with exp(jcoot) ultimately assumes the form y(t) = B e f ' e J ~ ° ' = - A QL e ,:~ eJo'°t. fl aOex,

(9)

Under the assumption that QL/Qext is held fixed (this ratio equals the relative strength of the antiresonance dip), the amplitude of the natural oscillation is proportional to the quality factor Q1 = (Oo/2a of the main circuit, taking into consideration that A is independent of QI. (This follows from the fact that the impulse response h(t)= A e-~t eJ~Oo, (valid for t > O) of the main circuit has an initial value A that depends only on the reactive elements of the system)•

H.J. Butterweck et al. / A passive remote identi[ication system

4. Detection in the presence of noise Due to their open structure all detection and identification systems are in permanent electromagnetic interaction with their surroundings. For that reason electromagnetic interference (EMI) aspects form a fundamental limiting factor in their actual operation and require that basically detection has to be treated as a statistical problem. To be more concrete, we have to limit the radiated power in the transmitting mode to avoid interferences in adjacent electronic equipment and we have to reckon with noisy fields in the receiving mode. Thus the signal-to-noise ratio S/N is doubly bounded: the maximum S is limited by international Rules and Regulations, while the minimum N is determined by thermal noise as predicted by statistical mechanics. (Actually, man-made noise exceeds thermal noise by several orders of magnitude.) From the above it can be concluded that detection errors cannot be completely avoided. Their occurrence becomes less probable when S/N increases. In other words, the reliability of a properly designed system grows with increasing S/N. Summarizing some elementary results of statistical detection theory we first state that the decision between 'Yes' (a label is present) and 'No' (a label is not present) is made dependent on the value of some output signal w of the receiver. If w exceeds a certain threshold W the decision is 'Yes', otherwise it is 'No'. The decision may be wrong: 'False alarm' occurs if the decision 'Yes' is made with no label present, while we speak about a 'miss' if 'No' comes out although a label is present. Generally the threshold W is chosen so that the probability of a wrong decision is minimized. Often, however, different importance is attached to the two error types, which leads to some threshold shifting. As an example, W is chosen relatively large, if false alarm is to be absolutely avoided as is normally required for an anti-shoplifting system. This implies that a 'miss' becomes more likely. Thus, in a certain sense the two wrong decisions are interchangeable.

375

The probability density function of the random variable w in the two system states (label present, label absent) strongly depends upon the signal-tonoise ratio SIN. This function is always centered on some fixed mean value ~, (which is usually normalized to zero or unity in the absence or presence of a label), but its standard deviation decreases with increasing SIN. For S / N ~ ~rv (no noise) the density function assumes impulse character. In this limiting case a wrong decision is impossible and the system becomes completely reliable. Physically this occurs approximately for large-sized labels s which yield such a high signal level at the receiver's input that the influence of the additive noise is negligible. For smaller labels we observe a rapid decrease of SIN which roughly varies proportional to the sixth power of the linear dimensions [6]. Then the system becomes less reliable and asks for some sophistication in the generation of the signal w. Naturally we are interested in an optimum receiver that for a given value of SIN delivers an output variable w with a maximal concentration around ,?, i.e., one with a minimum standard deviation. For uncorrelated white Gaussian noise the solution to this problem is a matched filter, i.e. a filter with an impulse response equal to the mirrored version of the signal to be detected. Here we deal with a decaying exponential signal that is formed from the receiver input signal (9) by demodulation. Further the time of occurrence of the signal is known to the receiver (if at all, the signal arrives in the receiving phase) so that the matched filtering can be replaced with a simpler multiplication of the noisy input signal by the noise-free exponential exp(-t/r) followed by an integration. The implementation in the form of an AD-converter and a digital signal processor requires little hardware. Another means to increase the concentration of the decision-determining output signal is repetition 5 In more general terms this effect occurs for strong coupling between the gate antennas and the label, which can also be achieved by small distances. Vol, 26, No, 3, March 1992

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H.J. Butterweck et al. / A passive remote identification system

label

0

label

W

~

Way

Fig. 4. Probability density function of the averaged output signal way in the presence (absence) of a label.

(signal integration). The above experiment is repeated N times and the different outcomes wi (i = 1. . . . . N) are averaged: 1 N Wav=N

i~l w~.

(10)

The decision is then based on Way instead of the individual outcomes w~. Under the (mild) condition that the w~'s are statistically independent random variables with equal mean ~ and equal standard deviation s , the variable w.v also has the mean value @ but its standard deviation is smaller:

o.v= 1 o.

(11)

The probability density function of Way is thus x/N times more concentrated than that of the w~. The result is shown in Fig. 4. The probability density function of Wav is concentrated around ~, which we arbitrarily normalize to unity in the presence of a label and to zero otherwise 6. Due to the central limit theorem, these density functions are Gaussian for N ~ ~ . For our purposes it is important that their widths decrease inversely proportional to N 1/2 so that the functions pertaining to the two states become less and less overlapping. With W= 1/2 the errors 'false alarm' and 'miss' become equally probable. The probabilities are found through integrating over the tails of the Gaussian functions. The hatched area in Fig. 4 represents the false alarm probability. In the case of envelope (asynchronous) detection, w can assume only positive values, which leads to some modification of the presented reasoning. Signal Processing

Thus, at first glance the detection can be rendered perfectly reliable for N-~ oo. In practice, however, the total measuring time N.6r is limited by the motion of the object through the gate. During the measurement the object should not be displaced so far that the gate transmission changes substantially. Particularly for high object velocities this poses a severe restriction on the total measuring time. Often interference in a detection system occurs due to a deterministic time-harmonic signal from a distant transmitter. Also in that case the described receiver has excellent properties in the sense that only in exceptional cases the detector is misled. Such a situation occurs only if the frequency lies within the bandwidth of the quartz crystal and if it, moreover, does not vary during the measuring time. Another effective means to improve the reliability due to an S/N increase is of a physical nature [1]. Conventionally a coil behaves as a dipole for great distances with a field strength decreasing as the inverse third power of the distance from the coil. More sophisticated coil configurations behave as quadrupoles or even as octopoles with a field decrease according to (distance) -4 or (distance) -5, respectively. Such configurations yield a double benefit: they produce less radiation in the transmitting mode and they receive less noise in the receiving mode. The correlation between these two effects is of a fundamental nature; it is rooted in the law of reciprocity. With respect to the desired reliability the gain due to such special coil systems is quantitatively comparable to that provided by the signal-theoretical means discussed before.

5. Special considerations

Some specific problems arise in systems designed for identification, in which several (anti-) resonances have to be detected simultaneously. Let M denote the number of available crystal frequencies, then M crystal-stabilized oscillators simultaneously

H.J. Butterweck et al. / A passive remote ident!fieation system

feed the transmitting antenna, while at the receiving end M demodulators generate the signals for the pertinent 'Yes No' decisions. The demodulation is performed with the aid of time-harmonic signals whose frequencies equal those of the associated crystals. In the case of asynchronous detection two demodulators are required for each crystal. Their time-harmonic reference signals have to be in quadrature while their outputs are (base-band) matched filtered, squared and subsequently added. Such an envelope detector optimally extracts the decaying exponential of a specific crystal oscillation from the received signal. With a sufficiently fast technology parallel processing with a single digital processor is enabled, which delivers the M determinant output signals Wl. . . . . w~, for the ' Y e s No' decisions. If a label contains an arbitrary nonzero number of crystals from a set of M, we obtain

possible combinations. For M = 10 this leads to a set of 1023 distinguishable labels, which number equals that of all binary words of length 10, excluding 0000000000 (corresponding to an undetectable label without any crystal). Often the use of a larger catalogue (e.g. M~>15) will be advantageous because one can then confine the possible combinations in accordance with some meaningful rules. As an example, a maximum number p < M of crystals can be prescribed in order to limit the price and physical dimensions of a label. Such a strategy also allows some form of error detection/correction, because impossible words can be rejected. As an example, let us consider the combination M = 18, p = 3 which yields

377

are connected to the labels leads to

(19/__9 9 3/ combinations. Here a more pronounced error detection applies, because any number greater or less than 3 is rejected. Such strategies are limited by the maximum number of crystal resonances fitting into the resonance curve of the LC-tag. This number has to be clearly less than the ratio of the quality factors QL and QI of the crystal and the LC-circuit, which is in the order of 100. Also the maximum degree of parallel processing in the digital signal processor might be a limiting factor. A final remark concerns the choice of the frequency band for a detection/identification system. If possible, the frequency should be low enough to allow for the quasi-statical treatment of the magnetic interaction. If this is not the case, the system does not allow synchronous detection. On the other hand, very low frequencies are associated with large time constants r which are only acceptable for slowly moving objects. Also the quality factor Q1 of the LC-tag which determines the sensitivity of the system due to (9) is low in that region. These aspects favour such frequencies for which the gate dimensions are just below the wavelength. For such a choice the quality factor Q1 is determined by ohmic copper losses only and radiation effects do not yet contribute to the damping of the tag. The final decision will, however, be strongly determined by the noise spectrum in the sense that extremely noisy frequency bands have to be avoided.

6. Prototype system description 18)+(18)+(18) 1/

\ 2/

987

\ 3/=

different combinations. In this case we deal with labels of maximally 3 crystals from a set of 18. As an alternative, a set of 19 crystals of which exactly 3

Figure 5 shows a block diagram of a prototype PAssive Remote Identification System (PARIS), operating at a nominal frequency of 27 MHz. Figure 6 depicts the pertinent signals (schematically and not to scale) at various points marked Vol. 26, No. 3, March 1992

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H.J. Butterweck et al. / A passive remote identification system

@ o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

i

__..

Fig. 5. Block diagram of prototype PAssive Remote Identification System (PARIS).

©

@

Fig. 6. Schematic waveformscorresponding to Fig. 5.

in Fig. 5. The identifiers contain exactly three crystals out of a set of M = 20, leading to 1140 identities and a simple identification mechanism. The modulators M1 to M20 each contain a crystal-tuned oscillator for one out of twenty possible resonant frequencies, three of which are present in each identifier. The identifier is indicated symbolically by an i in a circle. The blocks A1 and A2 Signal Processing

provide amplification, buffering and the mode of operation of transmission and reception in opposite phase. The timing is taken care of by the switching block S. The repetition rate is about 1 kHz. This rate is dictated by the time constant r of the natural response of the excited crystals in the identifier. If, as a rule, we receive during a time interval of 3r, to fully observe this response, and

H.J. Bunerweck et al. / A passive remote identification system

transmit during the same amount of time, in order to fully excite the identifier, we arrive at a period of 6r, which is about 1 ms for the prototype. In Fig. 6 only a single frequency is actually drawn, whereas twenty frequencies are in fact superimposed, the identifier responding to exactly three with a sum of damped oscillations. Also in general, this response is much smaller than suggested by waveform 3. Suppression of the stronger transmitter signal leads to the received signal 4. D~ to D20 demodulate this signal to baseband (waveform 5). This is done by mixers that multiply with the frequencies generated by the same crystal-tuned oscillators used for transmission. Nevertheless, demodulation is incoherent or asynchronous, because the identifier signal necessarily has a freerunning phase. So the D-blocks are in fact quadrature demodulators using two different phases (sine and cosine) of the frequency to be mixed down. The correlators C~ to C20 perform a matched filter operation to optimally detect an exponential signal in white noise. Finally, the identification block I selects the three largest of twenty numbers which are the result of correlation and computes the corresponding identity number ranging from 1 to 1 140. The correlators also take care of the signal integration mentioned in Section 4. The number of repetitions N depends on the time duration of exposure of the identifiers. Setting this time to a

379

nominal value of 1/8 s ( N = 125), a signal-to-noise ratio of 0 dB proves to be sufficient for reliable identification (error probability Pe = l 0- 6). Finally, we remark that the correlators and the identification block were actually implemented digitally, using a single digital signal processor.

References [1] H.J. Butterweck, "On magnetic coil configurations with reduced stray fields", Arch. fiir Elektrotechnik, Vol. 74, 1991, pp. 203 211. [2] R.J. Klensch, J. Rosen and H. Staras, "A microwave automatic vehicle identification system", RCA Rev., Vol. 34, No. 4, December 1973, pp. 566 579. [3] J.K. Maier, S.H. Roth and D.R. Sheldon, ' A passive electronic tag system", Proc. 1976 Carnahan Conj. Crime Countermeasures, Lexington, KY, USA, May 1976, pp. 97 102. [4] D.D. Mawhinney, "Microwave tag identification systems", RCA Rev., Vol. 44, No. 4, December 1983, pp. 589 610. [5] R.H. Richardson, "Electronic antipilferage system", Proc. 1978 Carnahan Conf. Crime Countermeasures, Lexington, KY, USA, May 1978, pp. 61 65. [6] J.H.F. Ritzerfeld, "A remote sensing system for the identification of passive coded labels", Internal Report, Department of Electrical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands, February 1992. [7] J.H.F. Ritzerfeld, H.J. Butterweck and A.C.P. van Meer, "'A passive remote identification system", Proc. 6th European Signal Processing Conf. EUSIPCO-92, Brussels, Belgium, August 1992, to be presented. [8] F. Sterzer, "'An electronic license plate for motor vehicles", RCA Rev., Vol. 35, No. 2, June 1974, pp. 167 175.

Vol. 26, No. 3, March 1992