Download An active tagging system using circular

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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
An Active Tagging System Using
Circular-Polarization Modulation
Marcel A. Kossel, Student Member, IEEE, Roland Küng, Member, IEEE, Hansruedi Benedickter, Member, IEEE,
and Werner Bächtold, Senior Member, IEEE
Abstract—An active read/write microwave tagging system using
circular-polarization modulation as a novel modulation scheme
for radio-frequency identification systems is presented. The proposed modulation scheme reduces demodulation complexity and
power consumption on the battery-powered tag. Additional coding of the circular-polarization modulated data reduces transmission errors due to polarization inversion at multipath propagation. In multiple-reader environments, the main jamming threat
occurs from power carriers of different interrogators. A combination of circular-polarization modulation and frequency hopping
is presented that shows an increased immunity against multipath
phenomena for multiple-tag and multiple-reader environments.
Index Terms— Circular-polarization modulation, frequency
hopping, microwave tagging system, RFID.
I. INTRODUCTION
I
N RECENT years, there has been growing interest in the
development of communication systems for the localization
and identification of objects [1], [2]. Examples of major
applications for tagging systems operating in the 2.4- and 5.8GHz industrial–scientific–medical (ISM) bands are security
systems, access control, and identification systems for industrial automation. Microwave tagging systems can be divided
into three types, which are described below.
Remotely powered fully passive tags (type 1) have to be
operated in the near-field region of the interrogator antenna.
Therefore, they are used only for short transmission distances
[3]. A second type of tagging system (type 2) uses batterypowered tags, which possess a lifetime of several years, as
the power supply is only used for the low-frequency signalprocessing unit consuming a few microwatts. Due to power
economy, no RF generator is available on such a tag; hence, a
passive reflex modulator is used for the communication from
the tag to the interrogator. The loss of the passive reflex modulator converts directly into a decrease of transmission distance
[4]. A third type of tagging system (type 3) uses an active
modulator on the tag to increase transmission distance. Since
the active RF modulator consumes much more battery power
than the baseband signal-processing circuits, the decrease in
Manuscript received March 26, 1999; revised July 12, 1999. This work was
supported by the Swiss Priority Program in Micro & Nano System Technology
(MINAST).
M. A. Kossel, H. Benedickter, and W. Bächtold are with the Laboratory for
Electromagnetic Fields and Microwave Electronics, Swiss Federal Institute of
Technology (ETH) Zurich, Switzerland.
R. Küng is with the Wireless Laboratory, University of Applied Science
(HSR), Rapperswil, Switzerland.
Publisher Item Identifier S 0018-9480(99)08415-X.
battery lifetime has to be weighted against an increase in
transmission distance.
This paper presents a novel active tagging system (type 3)
for the 2.4-GHz ISM band using circular-polarization modulation. The advantages of the proposed modulation scheme
are: 1) to lower demodulation complexity on the tag; 2) to
enhance interference resistance at multipath propagation; and
3) to allow the use of an easy-to-manufacture and highly
efficient active modulator.
II. DESCRIPTION
OF THE
TAGGING SYSTEM
A block diagram of the interrogator is depicted in Fig. 1.
Switches SwA, SwB, and SwC are used to select between
transmitter (Tx) and receiver (Rx) operation. The interrogator
is connected to a personal computer (PC) that acts as a
terminal. A microcontroller in the interrogator handles the
serial interface to the PC and performs signal-processing tasks
required to communicate with the remote tag.
Transmission of data from the interrogator to the tag proceeds as follows. First, the binary data, shown in Fig. 2(a), is
coded to be insensitive to polarization inversions by attaching
the data bits to symbols that will be decoded on the tag by
an EXOR relation. For instance, a logical zero is coded by the
symbol “00” and a logical one is attached to the symbol “01”
[see Fig. 2(b)].
After that, the coded data is circular-polarization modulated.
The circular-polarization modulator (CPM) consists of a frequency synthesizer operating in the 2.4-GHz ISM band and
a 50- terminated single-pole double-throw (SPDT) switch.
The output signal of the CPM is fed over two switches that
are used to select between Tx and Rx operation, to a circularly
polarized antenna (Fig. 1). A dual-polarized antenna in conjunction with a quadrature hybrid as polarizer is used to form
the circularly polarized antenna with switchable polarization
sense. As illustrated in Fig. 2(c) a “0” of the coded binary data
is transmitted as a right-hand-side circularly polarized (RHCP)
wave and a “1” is represented by a left-hand-side circularly
polarized (LHCP) wave.
The transmitted circular-polarization modulated waves are
received by the circularly polarized antenna of the tag whose
block diagram is shown in Fig. 3. For the circular-polarization
demodulator, a very simple circuit topology has been chosen. It
consists of a circular polarized antenna that is able to separate
received waves of opposite polarization sense, two identical
passive RF detectors, and a low-power CMOS comparator.
The two RF detectors are connected to the mutually isolated
0018–9480/99$10.00  1999 IEEE
KOSSEL et al.: ACTIVE TAGGING SYSTEM USING CPM
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Fig. 1. Interrogator block diagram. Switches are in the Rx position.
(a)
(d)
(b)
(e)
(c)
(f)
(g)
Fig. 2. Down- and up-link communication scheme (e.g., ASCII code for character “M”). (a) Data sequence. (b) Coding. (c) CPM. (d) RHCP sigal. (e) LHCP
signal. (f) Comparator output. (g) Decoding. (a)–(c) Modulation and coding in interrogator and tag, respectively. (d)–(g) Demodulation and decoding in
interrogator and tag, respectively (with bit inversion).
ports of the antenna polarizer for rectifying the incident waves.
The output voltage of the RF detectors is fed to the input
ports of the comparator whose output signal corresponds to
the coded baseband signal of the transmitted data sequence. A
micro-controller samples the output signal of the comparator.
In order to demonstrate that the system is insensitive to
polarization inversions, it is assumed in Fig. 2(d) and (e) that
the strongest received signal is bit inverted. This means that
the transmitted LHCP waves are received as RHCP signals
and vice-versa. By performing an EXOR relation between the
and
, corresponding to the
sampled data at
first and second bit of the transmitted symbols, the received
data sequence is decoded. Fig. 2(g) shows that polarization
inversion of the circularly polarized signal indeed does not
affect the demodulation due to the chosen coding of the data
sequence.
Compared to a conventional ASK modulation scheme frequently applied to radio-frequency identification (RFID) systems where, in the simplest form, a threshold voltage is
used for decision between logical “0” and “1,” the advantage
of the presented circular-polarization demodulator is that no
measures have to be taken for adjusting a decision threshold
voltage. This is because only the instantaneous power between
two signals is relevant for the demodulation. Therefore, linear
polarized jamming signals are also suppressed when a perfect
circularly polarized antenna is assumed, splitting the linear
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
Fig. 3. Tag block diagram.
polarized signal equally to the demodulation paths of the
left- and RHCP signals. Compared to conventional frequency
shift keying (FSK), circular-polarization modulation has the
advantage of using the whole transmitted power for the information bearing signal. FSK needs an additional carrier when
demodulated by simple RF detectors.
In the following, the interrogation of the tag is described.
Backscatter modulation is applied to read data from the
tag. An active modulator is used on the tag to perform a
circular-polarization modulation of the back-scattered waves.
Compared to a passive reflex modulator, an active modulator
shows conversion gain, which converts into an increase of
transmission distance. As shown in Fig. 3, the active modulator consists of an RF amplifier and two SPDT switches.
For reading or interrogating the tag, a linear polarized
unmodulated RF carrier is emitted by the interrogator and is
received by the tag antenna where it is split equally to the
mutually isolated antenna ports connected to the input and
output ports of the active modulator, as depicted in Fig. 3. Two
SPDT switches controlled by the tag microprocessor, according to the modulating data, switch the RF amplifier either in a
forward or backward direction. By amplifying the incident RF
power appearing at the LHCP antenna port as a RHCP wave
or vice-versa, a circular-polarization modulation is performed.
A coding equal to that used for communication from the
interrogator to the tag has been chosen to be insensitive to
bit inversions [see Fig. 2(b)]. For power saving, the power
supply for the active modulator is only turned on by a battery
switch when the tag is read. To ensure a stable operation,
the cross-polarization isolation of the circularly polarized tag
antenna has to be higher than the gain of the active modulator,
as no further means have been taken to decouple its input and
output ports.
At the interrogator, where enough supply power is available,
a homodyne detection can be used for the demodulation.
However, it is known that the use of one homodyne detector
can cause a cancellation of the received signal for critical
distances between the interrogator and tag. To prevent this
phenomenon, single-sideband (SSB) down-converters are used
at the demodulation path of the LHCP and RHCP signals. The
intermediate frequency signal of the SSB mixers is led to the
input ports of a comparator whose output signal is sampled
by a microcontroller. Further demodulation and decoding is
performed in the same way as already described at the tag
[see Figs. 2(f) and (g)].
III. SYSTEM EXTENSION FOR FREQUENCY-HOPPING
TECHNIQUE
Microwave tagging systems used for managing production
lines and manufacturing processes are operated in environments where channel fading occurs due to multipath propagation. Excess delay spreads up to more than 1 s are possible
in different factory environments. This results in a frequencyselective slow fading channel. Spread-spectrum techniques
may be used to achieve high immunity against multipath
phenomena. Additionally, in multiple-reader environments,
the main jamming threat occurs from RF carriers of other
reader units located at the same place. By randomizing the
carrier frequency of each reader using frequency hopping
(FH) methods, signal destructive fading and jamming can be
reduced to short time intervals. In synchronous FH systems,
jamming can even be avoided completely. Due to the packetoriented transmissions, the FH rate must be relatively fast,
e.g., 1 hop/byte.
Therefore, the combination of circular-polarization modulation and the FH technique is very promising for multiple-tag
and multiple-reader environments. The implementation of the
FH technique for the presented microwave tagging system requires some modifications at the reader side (interrogator). The
RF front-end of the tag and the tag itself remain unchanged,
which is important with respect to power consumption and
price.
Fig. 4 shows the block diagram of the interrogator extended
for FH technique. Compared to Fig. 1, the main modification
concerns the RF generator, which consists of a low-cost
fast hopping synthesizer, which covers the whole 2.4-GHz
ISM band. This synthesizer is composed of three sub units.
First, a direct digital synthesizer (DDS) generates the narrow
KOSSEL et al.: ACTIVE TAGGING SYSTEM USING CPM
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Fig. 4. Block diagram of interrogator extended for the FH technique.
frequency spacing (fine grid) in the hopping pattern, but
over a limited frequency range. Second, a voltage-controlled
oscillator (VCO) controlled by a phase-locked loop (PLL)
circuitry generates the broad frequency spacing (coarse grid) in
the order of several megahertz. Third, a SSB up-converter with
carrier suppression combines the two RF signals to the transmit
signal. The mixed use of the DDS and PLL technique allows
to extend the frequency range to several times the span of
the DDS without reduction of switching speed or phase-noise
performance. This is due to the fact that DDS allows for instant
frequency switching with arbitrary fine frequency spacing, but
over a limited frequency range of several megahertz. The PLL
has to cover only a coarse frequency grid corresponding to
the DDS frequency span and, therefore, can be operated with a
large loop bandwidth. This combination allows FH bandwidths
over 100 MHz with any channel resolution and switching
speeds of less than 50 s. Hopping speeds up to 10 khop/s
have been achieved in a prototype system.
The approach is superior to synthesizers, which multiply
the output of a DDS through the action of the PLL. As those
designs use the DDS as reference in the PLL, all DDS-related
spurs occurring inside the loop are multiplied the same way as
the reference frequency and lead to unacceptable phase noise.
Small loop bandwidths in the PLL, on the other hand, result
in slow switching speeds.
In the transmit mode, the output signal of the FH synthesizer
is fed to the CPM and radiated from the antenna. In the
receive mode, the FH signal is supplied to the linear polarized
antenna that transmits the RF power carrier used for the
backscatter modulation on the tag. The FH signal is also fed
over a power splitter to the local-oscillator ports of the downconverters in the demodulation paths. As depicted in Fig. 4,
the FH process is removed from the received signals of the
circular polarized antenna and also converted to the baseband.
The down-converted signals are sampled after filtering by an
A/D converter. Further demodulation is performed by digital
signal processors. This allows for more complex demodulation
algorithms and correction of errors due to nonideal hardware.
Although the power carrier of the circular-polarization modulated waves is FH modulated, no synchronization of a despreading code is needed on the tag. Therefore, only a broadband impedance match of the tag front-end is required. An
aperture-coupled antenna is used for the tag. By optimizing
the parameters (relative permittivity, substrate thickness) of the
patch layer, the bandwidth requirements for the operation in
the 2.4-GHz ISM band can be fulfilled (4% relative bandwidth
at 2.44 GHz corresponding to a VSWR lower than two). The
input impedance match of the passive RF detector is mainly
limiting the bandwidth of the tag front-end. For a VSWR
of two, a relative bandwidth of only 3% may be achieved
with a high dielectric substrate
. A high dielectric
substrate is preferred for further miniaturization of the tag,
as the dimensions of the RF front-end on the feed layer of
the aperture-coupled tag antenna scale down with increasing
relative permittivity. Thus, the frequency range of the FH
signal is finally limited by the bandwidth of the RF detector
on the tag to about 75 MHz for the 2.4-GHz ISM band.
IV. EXPERIMENTAL RESULTS
First, an aperture-coupled patch antenna with switchable
polarization sense [5], depicted in Fig. 5, has been built for a
prototype tag. The patch antenna is dual polarized and uses a
quadrature hybrid to feed the orthogonal slot apertures with the
required 90 phase shift. To ensure a good radiating efficiency,
a 3.18-mm-thick substrate with a low relative permittivity of
has been chosen for the upper layer containing the
patch. The feed layer consists of a 0.508-mm-thick (20 mil)
.
substrate of
Far-field measurements in an anechoic chamber have been
performed to determine the antenna gain and radiation patterns
(Figs. 6 and 7). The axial ratio and cross-polarization isolation,
i.e., the isolation between RHCP and LHCP waves, have been
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
Fig. 5. Multilayered structure of the tag antenna (right-hand-side circular
polarization: RHCP, left-hand-side circular polarization: LHCP. "r1 = 2:33,
h1 = 3:18 mm, "r2 = 2:22, h2 = 0:508 mm).
Fig. 7. Measurement of cross-polarization ratio and axial ratio of tag antenna
at 2.45 GHz (antenna is turned (angle ) around the axis perpendicular to the
patch surface and ' = 0 ).
Fig. 6. Measured radiation pattern of tag antenna at 2.45 GHz (angle in
azimuthal direction ' and = 0 ).
measured with an automated electromechanical positioner. The
antenna gain is 7 dBi at 2.45 GHz and the measured crosspolarization isolation is better than 20 dB. Also, an axial ratio
lower than 1 dB has been measured.
As the axial ratio and cross-polarization isolation of the
tag antenna determine the performance of the whole tagging
system, the circular polarization has been characterized by
means of an automated -field scanner [6] (Fig. 8). The
ellipticity defined as the ratio of the semiminor and semimajor
axis is about 0.95 in the middle of the patch. The measurement
of the magnitude of orthogonal -field vectors has been
performed at a distance of 0.1 m above the patch surface.
Due to the radiation characteristic the ellipticity increases by
a factor of two at a distance of about two wavelengths from
the patch edges for this measurement distance.
The simulation of the antenna was performed by a planar electromagnetic solver using the method of moments.1
1 “Momentum,” HP Advanced Design System, Hewlett-Packard Company,
Santa Rosa, CA, 1998.
Fig. 8. Visualization (magnitude of orthogonal E -field vectors) of circularly
polarized tag antenna measured with an automated E -field scanner (Dosimetric Assessment System (DASY), Schmid & Partner Engineering AG, Zurich,
Switzerland).
Fig. 9 shows the comparison between measured and simulated
VSWR at the LHCP and RHCP antenna ports. It has been
found that an additional crossed slot (Fig. 6) blocking the surface currents, which flow across the direction of the resonant
components, increases the cross-polarization isolation.
In order to demonstrate the superior performance of
aperture-coupled antennas used for microwave tags, a fully
operational prototype tag (Fig. 10) has been built. Aperturecoupled patch antennas are the preferable antenna type because
they allow to separate the feed layer from the radiating patch,
resulting in a high front-to-back ratio. All RF front-end
KOSSEL et al.: ACTIVE TAGGING SYSTEM USING CPM
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TABLE I
BASIC INTERROGATOR SPECIFICATIONS
Fig. 9. Simulated and measured VSWR at LHCP and RHCP antenna ports
over the 2.4-GHz ISM band.
TABLE II
BASIC TAG SPECIFICATIONS
Fig. 10. Photograph of prototype transponder corresponding to block diagram of Fig. 1 using the aperture-coupled patch antenna of Fig. 5.
A prototype hand-held interrogator (Fig. 11) using commercial components for the RF amplifier, frequency synthesizer, and RF switches has also been built for setting up a
demonstrator tagging system based on the basic specifications
summarized in Tables I and II.
V. CONLUSION
Fig. 11.
RF front-end of interrogator.
electronics is placed on the rear side of the patch on the
feed layer.
The RF front-end of the prototype tag consists of an
aperture-coupled antenna with switchable polarization sense,
RF detectors, and off-the-shelf components for the RF switches
W) and the comparator (
W). For the
(
two identical RF detectors located in the demodulation paths
on the tag, low-barrier zeroth-bias Schottky diodes have been
used, showing a voltage sensitivity of 30 mV/ W at 2.45 GHz
for a narrow-band match. A low-noise amplifier (LNA) has
been used as active device for the active modulator on the
tag. The current consumption of the LNA is 3 mA at a supply
voltage of 3 V.
A novel approach for active read/write tagging systems
has been presented by using circular-polarization modulation
for the bidirectional communication between interrogator and
tag. Circular-polarization modulation allows power-saving demodulation on the tag and increased interference resistivity
compared to a conventional ASK modulation scheme frequently used for RFID systems. A simple active modulator
to increase transmission distance has been presented.
To enhance immunity against multipath phenomena and
channel fading, a spread-spectrum technique can be used. It
has been suggested that a combination of circular-polarization
modulation and FH is a promising concept for microwave
tagging systems used in multiple-tag and multiple-reader environments.
A key component of the presented tagging system is the tag
antenna. An aperture-coupled patch antenna with switchable
polarization sense has been used to build a fully operational
prototype tag. Also, a portable interrogator has been built
to demonstrate the feasibility of the proposed novel tagging
system.
REFERENCES
[1] U. Kaiser and W. Steinhagen, “A low-power transponder IC for highperformance identification systems,” IEEE J. Solid-State Circuits, vol.
30, pp. 306–310, Mar. 1995.
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 47, NO. 12, DECEMBER 1999
[2] C. W. Pobanz and T. Itoh, “A microwave noncontact identification
transponder using subharmonic interrogation,” IEEE Trans. Microwave
Theory Tech., vol. 43, pp. 1673–1677, July 1995.
[3] F. Carrez, R. Stolle, and J. Vindevoghel, “A low-cost active antenna
for short-range communication applications,” IEEE Microwave Guided
Wave Lett., vol. 8, pp. 215–217, June 1998.
[4] T. Ohta, H. Nakano, and M. Tokuda, “Compact microwave remote
recognition system with newly developed SSB modulation,” in IEEE
MTT-S Symp. Dig., 1990, pp. 957–960.
[5] J. Zürcher and F. Gardiol, Broadband Patch Antennas. Norwood MA,
Artech House, 1995.
[6] T. Schmid, O. Egger, and N. Kuster, “Automated E -field scanning
system for dosimetric assessments,” IEEE Trans. Microwave Theory
Tech., vol. 44, pp. 105–113, Jan. 1996.
Marcel A. Kossel (S’99) was born on November 14,
1972. He received the Dipl.Ing. degree in electrical
engineering from the Swiss Federal Institute of
Technology (ETH) Zurich, Switzerland, in 1997,
and is currently working toward the Ph.D. degree
at ETH.
In 1997, he joined the Laboratory for Electromagnetic Fields and Microwave Electronics, ETH.
His current research is in the field of microwave
tagging systems.
Roland Küng (M’86) was born in Switzerland, in
1954. He received the Dipl.Ing. ETH degree from
the Swiss Federal Institute of Technology (ETH),
Zurich, Switzerland, in 1978.
From 1979 to 1983, he was active in the design
of high-frequency communication systems. In 1984,
he founded a research group at Ascom Ltd., whose
interests were in RF communications, digital signal
processing, and VLSI design. In 1992, he was
elected as Professor at the University of Applied
Sciences Rapperswil (HSR), Rapperswil, Switzerland, where he teaches electronic communications systems and heads the
Wireless Laboratory. He is one of the co-owners of Elektrobit AG (a leading
research and development company in the fields of RF and digital signal
processing (DSP) designs and a member of the Elektrobit Group, also located
in Finland and the U.S.), Bubikon, Switzerland, founded in 1995, where he
is responsible for research activities. He has special interests in the fields of
RF architectures, spread-spectrum techniques, algorithm engineering, software
radios, and RF identification, and holds several patents on these topics.
Hansruedi Benedickter (S’81–M’85) was born on
April 20, 1951, in Zug, Switzerland. He received
the Diploma degree in electrical engineering from
the Swiss Federal Institute of Technology (ETH),
Zurich, Switzerland, in 1976.
He has been a Research Assistant and a Senior
Research Associate at the Microwave Laboratory,
Swiss Federal Institute of Technology, and, since
1987, at the Laboratory for Electromagnetic Fields
and Microwave Electronics. His main research interests are microwave, millimeter-wave, and on-wafer
measurement techniques.
Werner Bächtold (M’71–SM’99) received the
Diploma and the Ph.D. degrees in electrical
engineering from the Swiss Federal Institute of
Technology (ETH), Zurich, Switzerland, in 1964
and 1968, respectively.
From 1969 to 1987, he was with the IBM Zurich
Research Laboratory. Since 1987, he has been a
Professor for electrical engineering at the Swiss
Federal Institute of Technology, where he heads
the Microwave Electronics Group, Laboratory for
Electromagnetic Fields and Microwave Electronics.
He has contributed in the fields of small-signal and noise behavior of bipolar
transistors and GaAs MESFET’s, microwave amplifier design, design and
analysis of Josephson devices and circuits, and design of semiconductor
lasers. His group is currently involved in the design and characterization of
GaAs MESFET and high electron-mobility transistor (HEMT) monolithic
microwave integrated circuits (MMIC’s), InP–HEMT device and circuit
technology, and modeling, characterization, and applications of semiconductor
lasers.