chipless rfid bar code of the future
TRANSCRIPT
December 2010 87 1527-3342/10/$26.00©2010 IEEE
Digital Object Identifier 10.1109/MMM.2010.938571
Stevan Preradovic and Nemai Chandra Karmakar
Stevan Preradovic ([email protected]), and Nemai Chandra Karmakar ([email protected]) are with the Department of Electrical and Computer Systems Engineering, Bldg 72 Clayton Campus, Monash University, 3800 VIC Australia.
Chipless RFID: Bar Code of the Future
Radio-frequency identifi cation (RFID) is a wireless
data capturing technique that utilizes radio fre-
quency (RF) waves for automatic identifi cation
of objects. RFID relies on RF waves for data
transmission between the data carrying de-
vice, called the RFID tag, and the interrogator [1], [2].
A typical RFID system is shown in Figure 1. An
RFID system consists of three major components:
a reader or interrogator, which sends the interro-
gation signals to an RFID tag that is to be identi-
fied; an RFID tag or transponder, which contains
the identification code; and middleware software,
which maintains the interface and the software
protocol to encode and decode the identification
data from the reader into a mainframe or personal
computer. The RFID reader can read tags only
within the reader’s interrogation zone. The reader
is most commonly connected to a host computer,
which performs additional signal processing and
has a display of the tag’s identity [3]. The host com-
puter can also be connected via the Internet for
global connectivity/networking.
The vast majority of RFID transponders (or tags)
are usually comprised of an antenna and integrated
circuit (IC) [4]. The IC performs all of the data process-
ing and is powered by extracting power from the inter-
rogation signal transmitted by the RFID reader. These
transponders are called passive due to the fact that they
do not have any on-board power supply. RFID transpon-
ders, which use on-board power supply (such as batteries) are
called active RFID tags. Passive RFID tags offer lower prices at
© DIGITAL VISION
88 December 2010
the cost of shorter reading ranges (up to 3 m) when
compared to the more expensive long-range active
RFID tags (read up to 100 m). Various other transpon-
ders are found on today’s market and are comprehen-
sively presented in [5].
The cost of the entire RFID system is dependent on
the cost of the tag, which is dependant on the cost of its
IC [6]. Therefore, efforts have been put in developing
chipless RFID tags with no ICs, which mean that the
main cost of the tag is being removed. So far, the only
commercially available chipless RFID tag is the surface
acoustic wave (SAW) tag (developed by RF SAW) [7].
This article presents a comprehensive review of chi-
pless RFID tags available on the market and reported
in peer-reviewed journals and conferences. However,
in the quest to be as comprehensive as possible the
authors have also referenced online internet articles
that report novel chipless RFID technologies.
Limitations of Bar Codes and Emergence of Chipless RFID ConceptsBar code labels have been used to track items and
stocks for sometime after their inception in the early
1970s. Though bar codes are printed in marks and
spaces and are very cheap to implement, they pres-
ent undeniable obstacles in terms of their short-range
readability and nonautomated tracking. These limi-
tations currently cost large corporations millions of
dollars per annum [8].
The growing tendency today is to replace bar
codes with RFID tags, which have unique ID codes
for individual items that can be read at a longer dis-
tance. The obstacles of reading range and automa-
tion would be solved using RFID. The only reason
why RFID tags have not replaced the bar code is the
price of the tag. The cost of an existing RFID tag is
still much higher when compared to the price of
the bar code.
The main cost of an RFID tag comes from the chip
embedded as the information-carrying and processing
device in the tag. Significant investments and research
have been focused on lowering the price of the RFID
chip. As a result, the price of the RFID tag has become
lower [9]. However, the price of the RFID tag is still
not competitive when compared to the cost of the bar
code. The recent development of chipless tags without
silicon ICs has lowered the cost of the tags to a level
comparable to that of the bar code. Even though the
technology is still in its infancy, a number of develop-
ments have already been made in the industry, which
we overview here.
Difficulties of Achieving Low-Cost RFIDThe use of RFID instead of optical bar codes has not
yet been achieved due to the greater price of the RFID
tag (US$0.10) compared to the price of the optical bar
code (US$0.5) [10]. The reasons why it is difficult to
produce cheap RFID tags are comprehensively pre-
sented in [11]. Fletcher advocates that application spe-
cific IC (ASIC) design and testing along with the tag
antenna and ASIC assembly result in a costly man-
ufacturing process. This is why it is not possible to
further lower the price of the chipped RFID tag. The
basic steps for manufacturing a chipped RFID tag are
shown in Figure 2.
The design of silicon chips has been standard-
ized for more than 30 years, and the cost of building
a silicon fabrication plant is in the billions of U.S.
dollars [12], [13]. Since silicon chips are fabricated
on a wafer-by-wafer basis, there is a fixed cost per
wafer (around US$1,000). As the cost of the wafer is
independent of the IC design, the cost of the RFID
chip can be estimated based on the required silicon
area for the RFID chip. Significant achievements
have been made in reducing the size of the transis-
tors, allowing more transistors per wafer area [14].
Decreasing the amount of transistors needed results
in an even smaller silicon area, hence a lower RFID
chip price. As a result, great efforts have been made
by the Massachusetts Institute of Technology (MIT)
Global
Network
Host
Computer
RFID
ReaderRFID Tag
Clock
Data
Figure 1. Block diagram of a typical RFID system.
ASIC Design
ASIC
Manufacturing
ASIC Testing
Antenna
Manufacture
Tag Assembly
Conversion to
Label/Package
Figure 2. RFID label/tag manufacturing process.
December 2010 89
to design an RFID ASIC with less than 8,000 transis-
tors. Although this will reduce the price of the sili-
con chip, its miniature size imposes limitations and
further handling costs.
The cost of dividing the wafer, handling the die,
and placing them onto a label remains significant,
even if the cost of the RFID chip were next-to-nothing.
The cost of handling the die increases with the use of
smaller-than-standard chips, simply because the elec-
tronics industry is not standardized for them.
Hence, with highly optimized low transistor
count ASICs, implemented assembly processes and
extremely large quantities (over 1 billion) of RFID
chips sold per annum, a minimum cost of US$0.05 is
the reality for chipped RFID tags.
Given the inevitable high cost of silicon chip RFID
tags (when compared to optical bar codes), efforts to
design low-cost RFID tags without the use of tradi-
tional silicon ASICs have emerged. These tags, and
therefore systems, are known as chipless RFID sys-
tems. The expected cost of chipless RFID tags is below
US$0.01. Most chipless RFID systems use the electro-
magnetic (EM) properties of materials and/or design
various conductor layouts/shapes to achieve particu-
lar EM properties/behavior.
Review of Chipless RFID TagsThere have been some reported chipless RFID tag
developments in recent years. However, most are still
reported as prototypes, and only a handful are consid-
ered to be commercially viable or available. The chal-
lenge for researchers when designing chipless RFID
tags is how to perform data encoding without the
presence of a chip. In response to this problem, three
general types of RFID tags can be identified as shown
in Figure 3.
Based on the open literature, it is possible to catego-
rize chipless RFID tags in three main categories:
• time domain reflectometry (TDR)-based chipless
tags
• spectral signature-based chipless tags
• amplitude/Phase backscatter modulation-based
chipless tags.
Time-Domain Refl ectometry-Based Chipless TagsTDR-based chipless RFID tags are interrogated by
sending a signal from the reader in the form of a pulse
and listening to the echoes of the pulse sent by the tag.
A train of pulses is thereby created, which can be used
to encode data.
The advantages of these tags when compared to
chipped tags are low cost, greater reading ranges, and
their applicability in localization/positioning applica-
tions. The disadvantages of these tags are the num-
ber of bits that can be encoded and high-speed RFID
reader RF front-ends required for generating and
detecting short ultrawideband (UWB) pulses.
Various RFID tags have been reported using
TDR-based technology for data encoding. We can
distinguish between nonprintable and printable TDR-
based tags.
An example of a nonprintable TDR-based chipless
RFID tag is the SAW tag, for example, developed by
RFSAW Inc. [15]. SAW tags are excited by a chirped
Gaussian pulse sent by the reader centered around
2.45 GHz [16]–[20]. A SAW tag is shown in Figure 4.
The interrogation pulse is converted to a SAW using
Chipless RFID Tags
TDR based Spectral
Signature Based
Nonprintable Printable Chemical Planar Circuits
TFTC
Delay-Line-
Based Tags
Nanometric
MaterialsCapacitively
Tuned Dipoles
Space Filling
Curves
LC Resonant
Ink-Tattoo
Chipless RFID
Amplitude/Phase Backscatter
Modulation Based
Left-Hand (LH)Delay Lines
Multiresonator
Based
Stub-LoadedPatch Antenna
Remote ComplexImpedance
CarbonNanotube Loading
Multiresonant
Dipoles
TDR Based
SAW Tags
Figure 3. Classification of chipless RFID tags. TDR: Time-domain reflectometry; SAW: surface acoustic wave; TFTC: thin-film-transistor circuit.
90 December 2010
an interdigital transducer (IDT). The SAW propagates
across the piezoelectric crystal and is reflected by a
number of reflectors, which create a train of pulses with
phase shifts [21]–[28]. The train of pulses is converted
back to an EM wave using the IDT and detected at the
reader end where the tag’s ID is decoded [29]–[38].
Printable TDR-based chipless tags can be found
either as thin-film-transistor circuit (TFTC) or
microstrip-based tags with discontinuities. TFTC
tags are printed at high speed on low-cost plastic
film [39]. TFTC tags offer advantages over active and
passive chip-based tags due to their small size and
low power consumption. They require more power
than other chipless tags but offer more functional-
ity. However, low-cost manufacturing processes for
TFTC tags have not yet been developed. Organic
TFTC could provide a cost-effective solution [40].
One of the institutes working on organic TFTC
development is the National Institute of Advanced
Industrial Science and Technology (AIST) in Japan.
An organic TFTC printed on flexible plastic film is
shown in Figure 5. Another issue is the low electron
mobility, which limits the frequency of operation up
to several megahertz.
Delay-line-based chipless tags operate by using
a microstrip discontinuity after a section of delay-
line, as reported in [41]–[43]. A delay-line-based
chipless tag is shown in Figure 6. The tag is excited
by a short pulse (usually 1 ns) EM signal. The inter-
rogation pulse is received by the tag and reflected
at various points along the microstrip line creating
multiple echoes of the interrogation pulse, as shown
in Figure 7. The time delay between the echoes is
determined by the length of the delay-line between
the discontinuities. This type of tag is a replica of the
SAW tag using microstrip technology, which makes
it printable. Although initial trials on this chipless
technology have been reported, only 4 bits of data
have been successfully encoded, which shows the
limited potential of this technology.
Spectral-Signature-Based Chipless TagsSpectral signature-based chipless tags encode data
into the spectrum using resonant structures. Each data
bit is usually associated with the presence or absence
of a resonant peak at a predetermined frequency in the
Antenna
Interdigital
Transducer
Reflectors
Figure 4. Circuit architecture of a surface acoustic wave tag [5].
Figure 5. Organic-thin-film-transistor circuit printed on flexible plastic film. [Courtesy of National Institute of Advanced Industrial Science and Technology (www.aist.go.jp) (www.aist.go.jp/aist_e/latest_research/2008/20080728/20080728.html), reprinted with permission.]
Antenna TransmissionDelay Line
Place forSensorIntegration
Delay Line
Figure 6. Delay-line-based chipless tag with patch antenna and delay line [43].
Amplitude
Input Signal
Reflected Signal
“Generated ID: 011”
Reflected Signal
“Generated ID: 110”
Pulse Position
Modulation Code
Representation00
000
101
001
1
011
100
101
110
110
111Time
Figure 7. Interrogation and coding of delay-line-based chipless tag [43].
December 2010 91
spectrum. The advantages of these tags are that they
are fully printable, robust, have greater data storage
capabilities than other chipless tags, and are low cost.
The disadvantages of these tags are large spectrum
requirements for data encoding, chipless tag orienta-
tion requirements, size, and wideband dedicated RFID
reader RF components. So far, seven types of spectral
signature-based tags have been reported, and all seven
are considered to be fully printable. We can distin-
guish two types of spectral signature tags based on the
nature of the tag: chemical and planar circuit.
Chemical tags are designed from a deposition
of resonating fibers or special electronic ink. Two
companies from Israel use nanometric materials to
design chipless tags. These tags consist of tiny par-
ticles of chemicals, which exhibit varying degrees of
magnetism, and, when EM waves impinge on them,
they resonate with distinct frequencies, which are
picked up by the reader [44]. They are very cheap and
can easily be used inside banknotes and important
documents for anticounterfeiting and authentication.
CrossID, an Israeli paper company, claims to have 70
distinct chemicals, which would provide unique iden-
tification in the order of 270 (more than 1,021) when
resonated and detected suitably [45]. Tapemark also
claims to have nanometric resonant fibers, which are
5 µm in diameter and 1 mm in length [46]. These tags
are potentially low cost and can work on low-grade
paper and plastic packaging material. Unfortunately,
they only operate at frequencies up to a few kilohertz,
although this gives them very good tolerances to metal
and water.
Ink-tattoo chipless tags use electronic ink patterns
embedded into or printed onto the surface of the object
being tagged. Developed by Somark Innovations [47],
the electronic ink is deposited in a unique bar code
pattern, which is different for every item. The system
operates by interrogating the ink-tattoo tag by a high
frequency microwave signal (>10 GHz) and is reflected
by areas of the tattoo, which have ink creating a
unique pattern which can be detected by the reader.
The reader detection is based on spatial diversity cre-
ated by the presence or absence of ink particles on the
tagged surface. The reading range is claimed to be up
to 1.2 m (4 ft) [48], [49]. In the case of animal ID, the
ink is placed in a one-time-use disposable cartridge.
For nonanimal applications, the ink can be printed on
plastic/paper or within the material. Based on the lim-
ited information available for this technology (which
is still in the experimental phase) we assume that it is
spectral signature based.
Planar circuit chipless RFID tags are designed using
standard planar microstrip/coplanar waveguide/
stripline resonant structures, such as antennas, filters,
and fractals. They are printed on thick, thin, and flex-
ible laminates and polymer substrates. Capacitively
tuned dipoles were first reported by Jalaly [50]. The
chipless tag consists of a number of dipole antennas,
which resonate at different frequencies. The capaci-
tively tuned dipole tag is shown in Figure 8. When the
tag is interrogated by a frequency sweep signal, the
reader looks for magnitude dips in the spectrum as a
result of the dipoles. Each dipole has a 1:1 correspon-
dence to a data bit. Issues regarding this technology
include tag size (lower frequency longer dipole—half
wavelength) and mutual coupling effects between
dipole elements.
Space-filling curves used as spectral signature
encoding RFID tags were first reported by McVay [51].
The tags are designed as Piano and Hilbert curves
with resonances centered around 900 MHz. The tags
represent a frequency selective surface, which is
manipulated with the use of space-filling curves (such
as the Hilbert and Piano curves). The space-filling
curve exhibits an interesting property of resonating
at a frequency, which has a wavelength much greater
than its footprint. This is an advantage since it allows
the development of small footprint tags at UHF ranges.
Figure 9 shows the 5-bit space-filling curve chipless
tag, which comprises an array of five second-order
… First Bit 11th Bit
Laminate (Dielectric)
Dipole (Conductor)
Figure 8. Capacitively tuned dipoles arranged as a 11-bit chipless RFID tag.
–15
–20
–25
–30
–35
–40
–45
–50
RC
S (
dB
)
0.5 0.6 0.7 0.8 0.9Frequency (GHz)
1
y
x
Ey
Figure 9. Five-bit piano-curve-based tag and tag radar-cross-section spectral signature [51].
92 December 2010
Piano curves, which create five peaks in the radar
cross-section (RCS) of the tag. The chipless tag was suc-
cessfully interrogated in an anechoic chamber. Only 5
bits of data have been reported to date. The advantage
of the tag is its compact size due to the properties of the
space-filling curves. However the disadvantage of the
tag is that it requires significant layout modifications
in order to encode data.
LC Resonant chipless tags comprise a simple coil,
which is resonant at a particular frequency. These tags
are considered 1-bit RFID tags. The operating prin-
ciple is based on the magnetic coupling between the
reader antenna and the LC resonant tag. The reader
constantly performs a frequency sweep searching
for tags. Whenever the swept frequency corresponds
to the tag’s resonant frequency, the tag will start to
oscillate, producing a voltage dip across the reader’s
antenna ports. The advantage of these tags is their
price and simple structure (single resonant coil), but
they are very restricted in operating range, informa-
tion storage (1 bit), operating bandwidth, and multiple-
tag collision. These tags are mainly used for electronic
article surveillance (EAS) in many supermarkets and
retail stores [52].
The Multiresonator-based chipless RFID tag was
designed and patented by the authors at Monash Uni-
versity [53]. The chipless tag comprises three main
components: the transmitting (Tx) and receiving (Rx)
antennas and multiresonating circuit. A block dia-
gram of the integrated chipless RFID tag with basic
components is shown in Figure 10.
The multiresonator-based chipless RFID tag con-
sists of a vertically polarized UWB disc-loaded mono-
pole Rx tag antenna, a multiresonating circuit, and a
horizontally polarized UWB disc-loaded monopole
Tx tag antenna [54]–[57]. The tag is interrogated by
the reader by sending a frequency swept continuous
wave signal. When the interrogation signal reaches
the tag, it is received using the Rx monopole antenna
and propagates towards the multiresonating circuit.
The multiresonating circuit encodes data bits using
cascaded spiral resonators, which introduce attenu-
ations and phase jumps at particular frequencies of
the spectrum. After passing through the multireso-
nating circuit, the signal contains the unique spectral
signature of the tag and is transmitted back to the
transmitter using the Tx monopole tag antenna. The
Rx and Tx tag antennas are cross-polarized in order
to minimize interference between the interrogation
signal and the retransmitted encoded signal contain-
ing the spectral signature. Figure 11 shows a 35-bit
tag designed on Taconic TLX-0 (er 5 2.45, h 5 0.787
mm, tan d 5 0.0019).
The main differences between the multiresonator-
based tag and those reported in the previous sections
are that the tag encodes data in both amplitude and
phase (Figures 12 and 13), the tag operates in the UWB
region, the tag supports simple spiral shorting data
encoding [58] and the tag responses are not based
on RCS backscattering but on retransmission of the
cross-polarized interrogation signal with the encoded
unique spectral ID. The chipless tag is designed for
printing on the Australian polymer banknote as an
anticounterfeiting security feature.
The Multiresonant dipole-based chipless RFID tag
is based on a similar concept as the multiresonator-
based chipless tag. However, the tag’s designers seek
to build on the concept of the multiresonator tag by
replacing the stop-band spiral resonators and the sec-
ond tag antenna with a novel multiresonant dipole
antenna [59]. The multiresonant dipole antenna com-
prises a set of parallel loop antennas, which resonate at
different frequencies. Each loop antenna corresponds
to a single bit of data. The multiresonant dipole-based
chipless RFID tag is shown in Figure 14. From Figure
14, it is clear that the tag receives the reader’s wideband
interrogation signal by the Rx UWB monopole antenna
and retransmits only certain frequencies, hence encod-
ing a unique spectral signature in the response signal
sent by the Tx multiresonant dipole antenna.
The multiresonant dipole antenna comprises a
series of folded half-wave dipole antennas. The dipole
arms etched out in the bot-
tom (ground) layer are fed by
a prolongation of the ground
plane with the prolongation
impedance being 50 V. The
half wavelength dipole anten-
nas produce peaks in the
return loss at their resonant
frequencies. By removing
any of the half wavelength
dipoles, the corresponding
resonant peak disappears
without influencing the reso-
nances of the other dipoles.
The main benefit of using the
multiresonant dipole antenna
First
Resonator
Second
Resonator
Third
Resonator
N th
Resonator
UWBMonopole
RxAntenna
UWBMonopole
TxAntenna
Vertical Polarization
Horizontal Polarization
Multiresonator
Figure 10. Chipless RFID tag circuit block diagram.
December 2010 93
is that the size of the entire tag can be reduced and
spatial efficiency is enhanced.
Amplitude-Phase-Backscatter-Modulation-Based Chipless TagsAmplitude/Phase backscatter modulation-based chi-
pless RFID tags are the third type of chipless RFID
tags presented in this article. These tags require less
bandwidth for operation than TDR-based and spectral
signature-based chipless tags. Data encoding is per-
formed by varying the amplitude or phase of the back-
scattered signal based on the loading of the chipless
tag’s antenna. The variation of the loading is not con-
trolled by an on/off switch between two impedances,
but, instead, it is controlled by reactive loading of the
tag’s antenna. The antenna loading influences the RCS
of the antenna [60] in amplitude or phase, which can
be detected by a dedicated RFID reader. The reactance
of the load may vary due to the fact that the antenna
load is an analog sensor or left-handed (LH) delay
line, or that the antenna is terminated by a microstrip-
based stub reflector.
The advantages of this type of chipless tag are
that it operates over narrow bandwidths, and it has a
simple architecture. The disadvantages are the num-
ber of bits that can be detected, and that data encod-
ing is performed by a lumped/chipped component
which increases its cost. Based on the data encoding
antenna loading element we can distinguish between
four types of different backscatter modulation-based
chipless RFID tags.
LH delay line loading of the chipless tags is one of
the most recent developments of chipless tag technol-
ogy. It utilizes analog circuits for phase modulation
and increases the response time of the tag using the
slow-wave effect of LH delay lines [61], which also
minimizes the size of the tag. The operating principle
of the chipless tag is presented in Figure 15.
From Figure 15, it is clear that the chipless tag is
interrogated by a band-limited pulse transmitted from
the RFID reader. The interrogation pulse is received
by the chipless tag antenna and propagates through
a series of cascaded LH delay lines, which represent
periodical discontinuities. The received interrogation
pulse is reflected upon reaching each discontinu-
ity and the information is coded by the phase of the
reflected signal with respect to a reference phase. The
envelope of the reflected signals with encoded data
maintain similar magnitudes (envelopes) while the
phase variation differs due to different G1, G2, and
G3 with phase values w0, w1 and w2, respectively. The
LH delay line-based chipless tag encodes data using
–16
–14
–12
–10
–8
–6
–4
–2
0
3 4 5 6 7Frequency (GHz)
Magnitude o
f S
pectr
al
Sig
natu
re (
dB
)
Figure 12. 35-bit magnitude response of the multiresonator-based chipless RFID tag.
–60
–40
–20
0
20
40
60
80
3 4 5 6 7Frequency (GHz)
Phase o
f S
pectr
al
Sig
natu
re (
°)
Figure 13. 35-bit phase response of the multiresonator-based chipless RFID tag.
Rx UWB
Monopole
Tx Multiresonant
Dipole Antenna
R
Feed
Extension
Spacing
Figure 14. Multiresonant-dipole-based chipless RFID tag [59] (red—top layer, yellow—bottom layer) (© 2009 EuMA, reprinted with permission).
Tag Tx
Antenna
Tag Rx
Antenna
Multiresonator
with 35 Spirals
Figure 11. Photograph of 35-bit chipless RFID tag (length 5 88 mm, width 5 65 mm).
94 December 2010
a higher order modulation scheme, such as quadra-
ture phase shift keying (QPSK), which enables greater
throughput but requires a higher signal-to-noise ratio
for successful tag detection [62]. The QPSK modula-
tor used within the chipless tag is based on a vari-
able reactive element, which minimizes the variation
of the amplitude and maximizes the phase variation.
Remote complex impedance-based chipless RFID
tags comprise a printable antenna, which is loaded/
terminated with a lossless reactance. The tag
antenna is chosen to be a scattering antenna (such
as a patch antenna) instead of a typically used mini-
mum scattering antenna (such as a dipole) [63]. The
difference between scattering and minimum scat-
tering antennas is that, when terminated with an
open or short, the scattering antenna should scatter
back the same power, irrespective of the type of loss-
less termination (including open and short), while
the minimum scattering antenna will scatter almost
no power back in open circuit conditions [64], [65].
This property of scattering antennas is reported by
Mukherjee et al. in [66] to encode data by means
of loading a scattering antenna with microstrip
stubs, which represent different inductances, and
therefore manipulating the phase component of the
antennas RCS and backscattered signal. The chi-
pless RFID system based on remote measurement of
complex impedance can be modeled as a two-port
network where the reader is considered to be the
source while the reactive impedance is considered
to be the load. Figure 16 shows the model of the chi-
pless RFID system. The transmitted interrogation
signal is defined by the S21 parameter while the S12
parameter is the backscattered chipless tag response
signal with phase signature.
By having chipless RFID tags with different induc-
tive loadings of their antennas, it is possible to cre-
ate different phase signatures in the backscattered
signal, which can be used to identify each tag at the
reader end [67]. The reactive loadings are designed
to be microstrip stubs in order to make the tag fully
printable and low-cost. Figure 17 shows the phase sig-
natures of different chipless RFID tags with different
inductive loadings.
Stub-loaded-patch-antenna (SLPA)-based chi-
pless RFID tags reported by Balbin et al. in [68] are
a newer generation backscatter phase signature tags
similar to the remote complex impedance based tag
presented earlier. However, the SLPA-based tags are
more robust and industry-suited since they incor-
porate another degree of diversity, such as cross-
polarization diversity (besides the phase variation
of the backscattered signal due to reactance loading)
and multiple tag antennas. The operating principle
of the SLPA chipless RFID tag is based on basic prin-
ciples of vector backscattered signals from multiple
planar reflectors. The SLPA-based tag is shown in
Figure 18.
The chipless tag antennas are multiple patch
antennas, which are suited due to their scattering
antenna properties as described earlier. The planar
reflectors are in the form of meander stubs in order to
minimize area and cost. The numbers of bits that can
be encoded by the tag vary depending on the number
of patches (n) and the available meander line induc-
tances. The chipless tag is interrogated by transmit-
ting n different continuous wave (CW) signals from
the reader at n frequencies corresponding to the oper-
ating frequencies of each patch antenna. When the tag
Reflection
SectionΓ1 Γ2 Γ3
ϕ1
e jϕ1
ϑ1
ϕ2
ϑ2
ϕ3
ϑ3
T ⋅ e jϕ0 T ⋅ e jϕ
0 Delay Line
Section
Carrier Phase
Carrier
Envelopet
ϕ1 + 2ϕ0
ϕ2 + 2ϑ1 + 4ϕ0
ϕ3 + 2(ϑ1 + ϑ2)+ 6ϕ0
Figure 15. Operating principle of left-hand-delay-line-based chipless RFID tag [61].
RFID Reader
Free Space
Loss
Scattering
Antenna
Inductive/
Reactance
Load
Zfreespace
Zfreespace
S21
S12
S11
S22 Γ
Z0
Figure 16. 2-port model of chipless RFID system based on remote measurement of complex impedance.
60
40
20
0
Pha
se R
ippl
e (°
)
–20–40
–606.9 7.1 7.3 7.5 7.7 7.9
Frequency (GHz)
Figure 17. Variation of the chipless tag’s phase signature with inductance loading [67] (© 2007 EuMA, reprinted with permission).
December 2010 95
is read by directive reader antennas, a bit sequence
can be detected using the relative phase difference of
the backscattered signals. The relative phase refers to
the phase difference between the E-plane and H-plane
signals at the reader, adding another degree of dif-
ferentiation. It is important to notice that this type of
chipless RFID tag requires interrogation and reading
with a directional dual polarized reader antenna and
not circularly polarized due to the tag’s operating
principles. The SLPA-based chipless tag is suitable for
conveyor belt applications due to the cascaded place-
ment of its antennas.
Carbon-nanotube-loaded (CNL) chipless tags
are a novel and unique example of RFID technol-
ogy and nanotechnology combining to create a
novel RFID tag and sensor module. The CNL chi-
pless RFID tag comprises a conformal UHF RFID
antenna and a single-walled carbon nanotube
(SWCNT) designed for toxic gas detection [69].
The CNL chipless RFID tag is shown in Figure 19.
It is important to note that both the antenna and
SWCNT were printed using inkjet printing technol-
ogy for the first time. The chipless tag antenna is a
bowtie meander-line dipole antenna. The SWCNT
is placed between at the input port of the antenna
in order to enable data encoding.
The SWCNT is highly sensitive to the presence
of ammonia (NH3), and its impedance characteris-
tics when placed in air and NH3 are shown in Fig-
ure 20. From Figure 20, it is clear that the impedance
of the SWCNT varies depending on the presence or
absence of NH3 in the environment. The CNL chi-
pless RFID tag operates by varying the amplitude
of the backscattered signal, depending on the con-
centration of NH3, as shown in Figure 21. Ampli-
tude variation of the backscattered signal is due
to the RCS variation influenced by the change of
the impedance of SWCNT. The amplitude varia-
tion of the backscattered power from the tag can be
detected at the reader end and decoded to estimate
the level of NH3.
ConclusionAn overview of reported chipless RFID tags in open
literature and on the market has been presented. As
the requirement for cheaper RFID tags for various
Meandering O/C Stubs
L1 L2 L3
Inset
LengthInset
Width
Element 1Spacing Element 2 Element 3
E-Plane
H-Plane
Figure 18. Stub-loaded-patch-antenna-based chipless RFID tag comprising three patch antennas loaded with meander line stubs [68].
15 m
m
27 m
m
25 mm36 mm
SWCNT
SWCNT
(a)
(b)
118 mm
Figure 19. Carbon-nanotube-loaded chipless RFID tag on flexible laminate with (a) dimensions and (b) actual photograph [69].
0
–5
–10
–15
–200.6 0.7 0.8 0.9 1
Frequency (GHz)
Pow
er
Reflection C
oeffic
ient (d
B)
AirNH3 Flow
Figure 21. Power reflection coefficient of the carbon-nanotube-loaded chipless RFID tag before and after gas flow [69].
125
100
75
50
25
0
75
50
25
0
–25
–50
Resis
tance (
Ω)
Resis
tance (
Ω)
0 0.2 0.4 0.6Frequency (GHz)
0.8 1
Resistance in NH3 Resistance in NH3
Resistance in Air Resistance in Air
Figure 20. Measured impedance characteristics of single-walled carbon nanotube in air and ammonia [69].
96 December 2010
applications grows, there are a greater number of dif-
ferent chipless RFID tags that can be classified in a
wide range of different types. This article reports the
first classification of chipless RFID tags, which classi-
fies 14 different chipless tags in three main categories.
The main classification of chipless tags is based on
modulation techniques, which are TDR-based, spec-
tral signature-based and amplitude/phase backscatter
modulation-based chipless RFID tags. All three types
of tags can be either printable or nonprintable, which
determines their eligibility for certain applications,
robustness and cost.
Although the majority of chipless tags are still in
prototyping stage it remains to be seen whether they
will make it into the mainstream market. However, the
progress of chipless RFID technology in recent years
enthusiastically suggests that the best of chipless RFID
is yet to come.
AcknowledgmentThis work was supported in part by the Australian
Research Council under Discovery Grant DP0665523:
Chipless RFID for Bar code Replacement.
References[1] K. Finkenzeller, RFID Handbook, 2nd ed. New York: Wiley, 2003.
[2] U. Kraiser and W. Steinhagen, “A low-power transponder IC for
high-performance identification systems,” IEEE J. Solid-State Cir-cuits, vol. 30, no. 3, pp. 306–310, Mar. 1995.
[3] S. Preradovic and N. Karmakar. (2009, Aug.). Modern RFID readers. Microwave J. [Online]. Available: http://www.mwjournal.com/ar-
ticle.asp?HH_ID=AR_4830
[4] U. Kraiser and W. Steinhagen, “A low-power transponder IC for
high-performance identification systems,” IEEE J. Solid-State Cir-cuits, vol. 30, no. 3, pp. 306–310, Mar. 1995.
[5] S. Preradovic, N. Karmakar, and I. Balbin, “RFID transponders,”
IEEE Microwave Mag., vol. 9, no. 5, pp. 90–103, Oct. 2008.
[6] J. Collins. (2008, Apr.). Alien cuts tag price. RFID J [Online]. Avail-
able: http://www.rfidjournal.com/article/articleview/857/1/1/
[7] C. S. Hartmann. (2002, Oct.). A global SAW ID tag with large data capacity. Proc. 2002 IEEE Ultrasonics Symp., Munich, Germany, vol.
1, pp. 65–69 [Online]. Available: http://www.rfsaw.com/pdfs/
Global_SAW_ID_Tag_lg.pdf
[8] S. D’Hont. (2006, Mar.). The cutting edge of RFID technology and ap-plications for manufacturing and distribution. Texas Instruments White Paper [Online]. Available: http://www.ti.com/rfid/docs/manu-
als/whtPapers/manuf_dist.pdf
[9] P. Harrop. (2008, May). The price-sensitivity curve for RFID. IDTe-chEx [Online]. Available: http://www.idtechex.com/products/
en/articles/00000488.asp
[10] H. Boek. (2010, May). Some hot North American RFID applications
[Online]. Available: http://www.rfidradio.com/?p=9
[11] R. R. Fletcher. (2002, Sept.). Low-cost electromagnetic tagging: De-sign and implementation. Ph.D. thesis [Online]. Available: www.me-
dia.mit.edu/physics/publications/theses/97.02.fletcher.pdf
[12] D. A. Hodges and H. G. Jackson, Analysis and Design of Digital Integrated Circuits, 2nd ed. New York: McGraw-Hill, 1988.
[13] J. R. Baker, H. W. Li, and D. E. Boyce, CMOS Circuit Design, Layout and Simulation. New York: IEEE Press, 1998.
[14] S. Natarajan, “A 32nm logic technology featuring 2nd–genera-
tion high-k + metal-gate transistors, enhanced channel strain and
0.171µm² SRAM cell size in a 291Mb array,” in Proc. IEEE Int. Elec-tron Devices Meeting 2008 (IEDM’08), San Francisco, Dec. 15–17,
2008, pp. 1–3.
[15] S. Harma, V. P. Plessky, C. S. Hartmann, and W. Steichen, “SAW
RFID tag with reduced size,” in Proc. IEEE Ultrasonics Symp. 2006,
Vancouver, Canada, Oct. 2006, pp. 2389–2392.
[16] Y. Y. Chen, T. T. Wu, and K. T. Chang, “A COM analysis of SAW
tags operating at harmonic frequencies,” in Proc. IEEE Ultrasonics Symp. 2007, New York, Oct. 2007, pp. 2347–2350.
[17] S. Harma, V. P. Plessky, L. Xianyi, and P. Hartogh, “Feasibil-
ity of ultra-wideband SAW RFID tags meeting FCC rules,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 56, no. 4, pp. 812–820,
Apr. 2009.
[18] S. Harma, V. P. Plessky, and X. Li, “Feasibility of ultra-wideband
SAW tags,” in Proc. IEEE Ultrasonics Symp. 2008, Beijing, China,
Nov. 2008, pp. 1944–1947.
[19] P. Brown, P. Hartmann, A. Schellhase, A. Powers, T. Brown, C.
Hartmann, and D. Gaines, “Asset tracking on the international
space station using global SAW tag RFID technology,” in Proc. IEEE Ultrasonics Symp. 2007, New York, Oct. 2007, pp. 72–75.
[20] V. P. Plessky, S. N. Kondratiev, R. Stierlin, and F. Nyffeler, “Saw
tags: New ideas,” in Proc. IEEE Ultrasonics Symp. 1995, Cannes,
France, Nov. 1995, vol. 1, pp. 117–120.
[21] T. Han, W. Wang, H. Wu, and Y. Shui, “Reflection and scatter-
ing characteristics of reflectors in SAW tags,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 55, no. 6, pp. 1387–1390, June 2008.
[22] T. Han, W. Wang, J. M. Lin, H. Wu, H. Wang, and Y. Shui, “Phases
of carrier wave in a SAW identification tags,” in Proc. IEEE Ultra-sonics Symp. 2007, New York, Oct. 2007, pp. 1669–1672.
[23] N. Saldanha and D. C. Malocha, “Design parameters for SAW
multi-tone frequency coded reflectors,” in Proc. IEEE Ultrasonics Symp. 2007, New York, Oct. 2007, pp. 2087–2090.
[24] S. Harma, C. Kim, S. M. Balashov, and V. P. Plessky, “Properties of
narrow metal reflectors used in reflective array compressors and
surface acoustic wave tags,” in IEEE MTT-S Int. Microwave Symp. Dig. 2007, Honolulu, HI, June 2007, pp. 2051–2054.
[25] D. Puccio, D. Malocha, and N. Saldanha, “Implementation of
orthogonal frequency coded SAW devices using apodized reflec-
tors,” in Proc. IEEE Int. Frequency Control Symp. Exposition 2005,
Vancouver, Canada, Aug. 2005, pp. 892–896.
[26] S. Harma, W. G. Arthur, C. S. Hartmann, R. G. Maev, and V. P.
Plessky, “Inline SAW RFID tag using time position and phase en-
coding,” IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 55, no. 8,
pp. 1840–1846, Aug. 2008.
[27] S. Harma, V. P. Plessky, C. S. Hartmann, and W. Steichen, “Z-path
SAW RFID tag,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol.
55, no. 1, pp. 208–213, Jan. 2008.
[28] S. Harma, W. G. Arthur, R. G. Maev, C. S. Hartmann, and V. P.
Plessky, “Inline SAW RFID tag using time position and phase en-
coding,” in Proc. IEEE Ultrasonics Symp. 2007, New York, Oct. 2007,
pp. 1239–1242.
[29] J. Liu and J. Yao, “Wireless RF identification system based
on SAW,” IEEE Trans. Ind. Electron., vol. 55, no. 2, pp. 958–961,
Feb. 2008.
[30] N. Saldanha and D. C. Malocha, “Low loss SAW RF ID tags for
space applications,” in Proc. IEEE Ultrasonics Symp. 2008, Beijing,
China, Nov. 2008, pp. 292–295.
[31] L. Wei, H. Tao, and S. Yongan, “Surface acoustic wave based radio
frequency identification tags,” in Proc. IEEE Int. Conf. e-Business Engineering 2008, Xi’An, China, Oct. 2008, pp. 563–567.
[32] J. Yao, J. Liu, and B. Gao, “The realization of a passive identi-
fication tag transmitter based on SAW,” in Proc. 4th Int. Wireless
December 2010 97
Communications, Networking and Mobile Computing 2008, Dalian,
China, Oct. 2008, pp. 1–4.
[33] J. Liu and J. Yao, “A new wireless RF identification system,”
in Proc. 6th World Congr. Intelligent Control and Automation 2006,
Dalian, China, June 2006, pp. 5191–5195.
[34] S. Schuster, S. Scheiblhofer, A. Stelzer, and A. Springer, “Model
based wireless SAW tag temperature measurement,” in Proc. Asia-Pacific Microwave Conf. (APMC) 2005, Suzhou, China, Dec. 2005,
pp. 4–7.
[35] E. L. Tan and Y. W. M. Chia, “Green’s function and network
analysis of quasi-2D SAW ID-tags,” in Proc. IEEE Ultrasonics Symp. 2000, San Juan, PR, Oct. 2000, pp. 55–58.
[36] D. Enguang and F. Guanping, “Passive and remote sensing based
upon surface acoustic wave in special environments,” in Proc. Microwave and Optoelectronics Conf. 1997, Natal, Brazil, Aug. 1997,
vol. 1, pp. 133–139.
[37] D. Puccio, D. C. Malocha, N. Saldanha, D. R. Gallagher, and
J. H. Hines, “Orthogonal frequency coding for SAW tagging and
sensors,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 53, no. 2,
pp. 377–384, Feb. 2006.
[38] G. Buckner and R. Fachberger, “SAW ID tag for industrial ap-
plication with large data capacity and anticollision capability,” in
Proc. IEEE Ultrasonics Symp. 2008, Beijing, China, Nov. 2008, pp.
300–303.
[39] R. Das and P. Harrop. (2006, Mar.). Chip-less RFID forecasts, tech-nologies & players 2006–2016. IDTechEx [Online]. Available: http://
www.idtechex.com/products/en/view.asp?productcategoryid=96
[40] Advanced Industrial Science and Technology (AIST). (2010,
Mar.). Printing of organic thin-film transistor arrays on flexible sub-strates [Online]. Available: http://www.aist.go.jp/aist_e/latest_
research/2008/20080728/20080728.html
[41] A. Chamarti and K. Varahramayan, “Transmission delay line-
based ID generation circuit for RFID applications,” IEEE Micro-wave Wireless Compon. Lett., vol. 16, no. 11, pp. 588–590, Nov. 2006.
[42] J. Vemagiri, A. Chamarti, M. Agarwal, and K. Varahramyan,
“Transmission line delay-based radio frequency identification
(RFID) tag,” Microwave Opt. Technol. Lett., vol. 49, no. 8, pp. 1900–
1904, 2007.
[43] S. Shretha, J. Vemagiri, M. Agarwal, and K. Varahramyan, “Trans-
mission line reflection and delay-based ID generation scheme for
RFID and other applications,” Int. J. Radio Freq. Identification Tech-nol. Appl., vol. 1, no. 4, pp. 401–416, 2007.
[44] R. Das. (2006, Feb.). Chip-less RFID—The end game. IDTechEx
[Online]. Available: http://www.idtechex.com/products/en/ar-
ticles/00000435.asp
[45] M. Glickstein. (2006, Feb.). Firewall protection for paper documents. RFID J. [Online]. Available: http://www.rfidjournal.com/article/
articleprint/790/-1/1/
[46] J. Collins. (2006, Apr.). RFID fibers for secure applications. RFID J [Online]. Available: http://www.rfidjournal.com/article/arti-
cleprint/845/-1/1/
[47] K. C. Jones. (2009, June). Invisible tattoo ink for chipless RFID safe, company says. EE Times White Paper [Online]. Available: http://
eetimes.eu/industrial/196900063
[48] J. Dowe. (2009, June). SOMARK’s chipless RFID ink tattoo field demo brings the company closer to launch. MoreRFID [Online]. Available:
http://www.morerfid.com
[49] Somark Innovations. (2009, June). Platform technology capabilities. White Paper [Online]. Available: http://somarkinnovations.com/tech-
nology/
[50] I. Jalaly and I. D. Robertson, “RF bar codes using multiple fre-
quency bands,” in Proc. IEEE MTT-S Int. Microwave Symp. Dig. 2005, Long Beach, June 2005, pp. 4–7.
[51] J. McVay, A. Hoorfar, and N. Engheta, “Space-filling curve RFID
tags,” in 2006 IEEE Radio and Wireless Symp. Dig., San Diego, Jan.
17–19, 2006, pp. 199–202.
[52] Tagsense, Inc. (2006, Oct.). Chipless RFID products. Data Sheet [Online]. Available: http://www.tagsense.com/ingles/products/
product_chipless.html
[53] S. Preradovic, I. Balbin, S. M. Roy, N. C. Karmakar, and G. Swieg-
ers, “Radio frequency transponder,” Australian Provisional Patent Application P30228AUPI, Apr. 2008.
[54] S. Preradovic, I. Balbin, and N. Karmakar, “The development and
design of a novel chipless RFID system for low-cost item tracking,”
in Proc. Asia Pacific Microwave Conf. (APMC) 2008, Hong Kong, Dec.
2008, pp. 1–4.
[55] S. Preradovic and N. Karmakar, “Design of fully printable planar
chipless RFID transponder with 35-bit data capacity,” in Proc. 39th European Microwave Week, Rome, Italy, Sept. 2009, pp. 13–16.
[56] S. Preradovic, S. Roy, and N. Karmakar, “Fully printable multi-
bit chipless RFID transponder on flexible laminate,” in Proc. Asia Pacific Microwave Conf. (APMC’09), Singapore, Dec. 2009, pp. 2371–
2374.
[57] S. Preradovic, I. Balbin, N. Karmakar, and G. Swiegers, “A novel
chipless RFID system based on planar multiresonators for bar code
replacement,” in Proc. IEEE Int. Conf. RFID 2008, Las Vegas, NV,
Apr. 2008, pp. 289–296.
[58] S. Preradovic, I. Balbin, N. C. Karmakar, and G. F. Swiegers,
“Multiresonator-based chipless RFID system for low-cost item
tracking,” IEEE Trans. Microwave Theory Tech., vol. 57, no. 5, pp.
1411–1419, May 2009.
[59] I. Balbin and N. Karmakar, “Novel chipless RID tag for conveyor
belt tracking using multi-resonant dipole antenna,” in Proc. 39th European Microwave Conf., Rome, Italy, Sept. 2009, pp. 1109–1112.
[60] K. V. S. Rao, P. V. Nikitin, and S. Lam, “Antenna design for UHF
RFID tags: A review and practical application,” IEEE Trans. Anten-nas Propagat., vol. 53, no. 12, pp. 3870–3876, Dec. 2005.
[61] M. Schuler, C. Mandel, M. Maasch, A. Giere, and R. Jakoby,
“Phase modulation scheme for chipless RFID- and wireless sen-
sor tags,” in Proc. Asia Pacific Microwave Conf. 2009, Singapore, Dec.
2009, pp. 229–232.
[62] C. Mandel, M. Schussler, M. Maasch, and R. Jakoby, “A novel
passive phase modulator based on LH delay lines for chipless mi-
crowave RFID applications,” in Proc. IEEE MTT-S Int. Microwave Workshop Wireless Sensing, Local Positioning and RFID 2009, Cavtat,
Croatia, Sept. 2009, pp. 1–4.
[63] C. A. Balanis, Antenna theory: Analysis and design, 2nd ed. New
York: Wiley, 1982.
[64] P. V. Nikitin and K. V. S. Rao, “Theory of measurement of back-
scatter from RFID tags,” IEEE Antennas Propagat. Mag., vol. 48, no.
6, pp. 212–218, Dec. 2006.
[65] W. Kahn and H. Kurss, “Minimum-scattering antennas,” IEEE Trans. Antennas Propagat., vol. 13, no. 5, pp. 671–675, Sept. 1965.
[66] S. Mukherjee, “Antennas for chipless tags based on remote mea-
surement of complex impedance,” in Proc. 38th European Microwave Conf., Amsterdam, Netherlands, Oct. 2008, pp. 71–74.
[67] S. Mukherjee, “Chipless radio frequency identification by remote
measurement of complex impedance,” in Proc. 37th European Mi-crowave Conf., Munich, Germany, Oct. 2007, pp. 1007–1010.
[68] I. Balbin and N. C. Karmakar, “Phase-encoded chipless RFID
transponder for large-scale low-cost applications,” IEEE Microwave Wireless Compon. Lett., vol. 19, no. 8, pp. 509–511, Aug. 2009.
[69] L. Yang, R. Zhang, D. Staiculescu, C. P. Wong, and M. M. Tentzeris,
“A novel conformal RFID-enabled module utilizing inkjet-printed
antennas and carbon nanotubes for gas-detection applications,” IEEE Antennas Wireless Propagat. Lett., vol. 8, pp. 653–656, 2009.