Temporal Multi-Frequency Encoding Technique for Chipless RFID
Applications
Raji Nair, Student Member IEEE, Etienne Perret, Member IEEE and Smail Tedjini, Senior Member IEEE
Grenoble-INP/LCIS 50, rue Barthélémy de Laffemas - BP 54 26902 Valence Cedex 9 - France
Abstract — A novel temporal multi-frequency encoding technique based on group delay for chipless Radio Frequency Identification (RFID) tag is presented. Cascaded microstrip transmission line sections coupled at alternative ends(C-sections) are utilized to generate the tag Identification (ID).C-sections are dispersive structures, have a group delay maximum at different frequencies which is purely dependent on the length of the C-section. The proposed device is designed, prototyped and experimentally verified for 2 bit coding. The obtained results confirm the concept and its use in chipless RFID applications. Furthermore the transformation of the prototype into chipless tag using simulation results is also incorporated.
Index terms — Chipless RFID, encoding, group delay,
cascaded transmission line, C-sections, Backscattering.
I. INTRODUCTION
The Chipless RFID, owing to its low cost compared to
conventional devices, has opened a new path for low cost and
robust identification system [1]. The principle of information
encoding in chipless tags is based on the generation of a
specific electromagnetic signature. Contrary to frequency
encoding approach, the temporal method of encoding permits
a solution to the problem of FCC (Federal Communications
Commission) frequency regulation since it can work within a
permitted narrow frequency range. For example the SAW tag
developed by RFSAW Inc. [2], in which reflectors are placed
in a piezoelectric material where the signal will reflect with
different time which can be used for the encoding. SAW tag
operates at 2.45 GHz and it doesn’t permit to work at several
frequencies simultaneously. In case of low cost non-
piezoelectric substrate [3], a longer transmission line is
needed to produce a comparable delay. In this case the coding
capacity is increased by increasing the length of the delay line.
However a time domain RFID system using C-sections based
on pulse position modulation (PPM) coding is discussed in [4]
where a train of pulses modulated with different carrier
frequencies are utilized and the concept is proved by the
simulated results of the prototype.
The authors already proved the concept of utilizing the
group delay characteristics of microwave C-sections in
chipless RFID [5]. However in this paper the prototype of the
chipless tag is designed and the encoding is explained using
time domain measurement. Contrary to [4], a proof of concept
is developed for the possibility of using the dispersive nature
of transmission lines to increase the coding capacity by using
multiple frequencies. The structure makes use of the coupling
effect to increase the amount of delay which miniaturizes the
transmission line in comparison to a linear or meandered line.
It facilitates a direct link between the geometry of the tag and
delay time. If the ID is known, the tag geometry can be
obtained directly and hence it is easy to implement in practice.
Finally the transformation of the prototype into chipless tags
has been done by adding two cross polarized antennas and
information encoding is established using the scattering
waveforms from the antenna.
II. OPERATING PRINCIPLE AND STRUCTURE DESIGN
Fig. 1. Proposed Chipless RFID system.
Fig. 1 depicts the operating principle and different
parameters of the C-section, which is formed by shorting the
alternate ends of coupled transmission lines. The frequency
dispersive characteristics of the microwave transmission line
provides different spectral components rearranged in time [6]
and is utilized for encoding of chipless RFID tags .The group
delay indicates time taken by a signal to propagate through a
structure as a function of frequency.
The receiving antenna in the chipless tags receives the
interrogation signal sent by the reader which comprises of two
different frequencies F(l1), F(l2), separated in time,
corresponding to specific lengths of the C-sections l1 and l2
respectively. The C-sections with different lengths modulate
this signal in time and can be used for the information
encoding. This modulated signal will comprise of two
components, the structural mode which is due to the reflection
from the antenna and independent of the length of the C-
sections and the tag mode produced by different lengths of the
C-sections (see Fig.1). This modulated signal will be sent
Fc=F (l1)
Fc=F (l2)
I/P Pulses
Tag Rx Antenna
l2 l1
w1
g1
w
g
Tag Tx Antenna
Prototype
Delayed O/P Pulses Fc=F (l1)
∆t1
Fc=F (l2)
∆t2
978-1-4673-1088-8/12/$31.00 ©2012 IEEE
back to the reader for decoding. The structural mode can be
used as the reference and the time difference Δt1 and Δt2
between the structural mode and different tag modes can be
used for generating different combinations of ID. Δt1 and Δt2
are a function of the lengths l1 and l2 and which is configurable
independently from each other. The lengths l1 and l2 are used
to produce two independent delays at two selected frequencies
and therefore, it gives four different combinations
corresponding to two bit code.
Fig. 2. Structure of the reference tag used for measurement with
length l1=17.87 mm, l2 =8.93 mm, g=0.1 mm, g1=1 mm, w1
=0.1 mm and w=0.7 mm; εr=4.3, h=0.8 mm.
Based on the parametric studies, the prototype is developed
as shown in Fig. 2. The substrate used is FR-4 (εr=4.3,
tanδ=0.025 and thickness=0.8 mm). The width of the line is
kept as 0.7mm (82 ohms) because of the fact that decrease in
the width of the line increases the delay. Additionally, it is not
necessary to design the width for 50 ohms; instead the width
can be designed to match with the input impedance of the
antenna. The frequencies 2.6 GHz and 5.3 GHz are randomly
chosen to provide better isolation between each peak. When
consecutive C-sections are added, a negotiable frequency shift
is observed due to the coupling effect between them [6].
Hence the λd/4 lengths are optimized as 17.87 mm and
8.93 mm. The value of gap g and gap width w1 are assigned as
0.1 mm to provide tight coupling and the gap g1 between each
group has been optimized as 1 mm to minimize mutual
coupling. This tag configuration is chosen as the reference. In
order to produce a measurable amount of delay, more number
of C-sections are used. The overall dimension of the prototype
is 35x25 mm² including the feed line. The proposed device is
modeled and numerical simulations were performed using
CST Microwave Studio 2011.
III. RESULTS AND DISCUSSION
A temporal simulation has been done as per the operating
principle. An impulse signal (signal which can be compatible
with UWB regulations) is used as the excitation signal.
MATLAB processing has been done for the simulated and
measured results to extract the envelope of the signal at
2.6 GHz and 5 GHz which is used to calculate the time delay
between input and output signals. Table 1 explains the delays
obtained along with the encoding method. ∆ l1 and ∆ l2 are
chosen as 2.98 mm and 0.81 mm respectively. For
experimental validation, an impulse with 80 ps pulse width
has been generated using the impulse generator (Picosecond
Pulse Labs-Model 3500) and used as the input for the
structure. The delayed spectrum for the reference tag has been
measured by using the Digital Oscilloscope (Agilent Digital
Oscilloscope DSO91204A). Three other combinations of the
tag for the measurement corresponding to the code 01, 10 and
11 by changing the length of the two C-sections were also
designed. As shown in Fig. 3, a very good agreement has been
obtained between both simulation and measurement results.
Table 1 summarizes these results.
Fig. 3. Comparison of simulated and measured delays for different
IDs a) code 00, b) code 01, c) code10 and d) code 11. The input
signal for simulation and measurement are superimposed.
IV. TRANSFORMATION INTO CHIPLESS TAG
The chipless tag consists of the prototype along with two
cross polarized antennas as shown in Fig. 4 (a). The classical
UWB antenna has been used for this purpose since it can
operate in the two chosen frequencies [1]. In order to enhance
the scattering characteristics of the antenna, Rogers R04003
(εr=3.55, tanδ=0.0027) is used instead of FR-4, because of its
low tangent loss. The high tangent loss of the FR-4 material
degrades the backscattering performance [5]. In this case the
prototype is redesigned for the new substrate by simply
varying the length of the C-section. The gap width w1 is
assigned as 0.7 mm same as the width w of the line in order to
make the whole system with unique impedance which will
reduce the unwanted reflections. The group delay of the tag
antennas has been simulated separately by placing a probe at a
far-field distance, r. Fig. 4 (b) shows the group delays
obtained for two antennas.
Am
plit
ude(V
) Am
plit
ude(V
)
∆t=2.28ns
∆t=2.28ns
∆t=2.28ns
∆t=2.26ns
∆t=1.63ns
∆t=1.65ns
∆t=2.32ns
∆t=2.29ns
Time (ns) Time (ns)
Am
plit
ude(V
)
(c) (d)
Am
plit
ude(
V)
Am
plit
ude(V
)
∆t=2.31ns ∆t=2.32ns ∆t=2.90ns
∆t=2.95ns
∆t=1.74ns
∆t=1.71ns
∆t=2.89ns
∆t=2.94ns
Time (ns) Time (ns)
Am
plit
ude(
V)
(b) (a)
Input signal at 2.6 GHz Input signal at 5 GHz
Simulated delayed signal
at 2.6 GHz
Measured delayed signal
at 2.6 GHz Simulated delayed signal
at 5 GHz
Measured delayed signal
at 5 GHz
978-1-4673-1088-8/12/$31.00 ©2012 IEEE
Fig. 4. Structure of the reference tag with group delays of tag
antennas a) reference tag with various parameters: Wfeed= 0.7 mm,
Wgnd=74.5 mm, L1gnd=38.5 mm, L2gnd=47 mm, Wtag=103 mm,
Ltag=68 mm, and R=13 mm. b) Simulated group delays for the tag
antennas.
Fig. 5. Back scattered signal corresponds to code 00 and 11.
Back scattered signal at a) 2.45 GHz and b) 5 GHz.
The tag is interrogated by a vertically polarized plane wave
using Gaussian modulated signals modulated at carrier
frequencies Fc1=F(ωl1)=2.45 GHz and Fc2=F(ωl2)=5 GHz
respectively, since it gives more delay variations. Fig. 5 shows
the back scattered signal collected from the horizontally
polarized antenna. The time difference between the structural
mode and tag mode is used for the encoding. This time
difference will be the sum of group delay produced by the C-
section and the two antennas. For example for the code 00,
∆t2.45GHz=GD(C-section+Antennas)2.45GHz =2.18 ns+1.21 ns
=3.39 ns and ∆t5GHz=GD(C-section+Antennas)5GHz =2.29 ns +
1.15 ns = 3.44 ns. From the figure it is clear that ∆t obtained
agrees with this calculation. The reflections are due to the
poor impedance matching of the output antenna at the lower
frequency which can be eliminated by a better optimization.
The time at which the structural mode starts depends on the
distance r, ie. ∆tstart=r/2c=0.6 ns, where c is the speed of light.
From the figure it is shown that the time ∆t is directly
proportional to the length of the C-section. Two other
combinations were also simulated and results were verified.
V. CONCLUSION
The present work proposes for the first time, a new
approach of temporal multi-frequency encoding utilizing the
highly dispersive characteristics of the C-section. A very good
agreement between the simulation and measurement validates
this coding concept. It is proved that the addition of antennas
will modify the group delay of the structure and the encoding
approach remains intact. It is also possible to send two mono-
pulses, rather than a single UWB pulse to stay on two ISM
frequencies. Coding capacity can be increased by adding
several C-sections of varied lengths or by considering more
than two states of ∆t for each frequency.
ACKNOWLEDGMENT
The authors would like to acknowledge the French National
Research Agency for financially supporting this project via
the ANR-09-VERS-013 program.
REFERENCES
[1] S. Preradovic, I. Balbin, N. C. Karmakar, and G. F. Swiegers,
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Transactions on, vol. 57, pp. 1411-1419, 2009.
[2] C. S. Hartmann, "A global SAW ID tag with large data
capacity," 2002, pp. 65-69.
[3] A. Chamarti and K. Varahramyan, "Transmission delay line
based ID generation circuit for RFID applications," Microwave
and Wireless Components Letters, IEEE, vol. 16, pp. 588-590,
2006.
[4] S. Gupta, B. Nikfal, and C. Caloz, "RFID system based on pulse-
position modulation using group delay engineered microwave C-
sections," in Asia-Pacific Microwave Conference (APMC),
Yokohama, Japan, 2010, pp. 203-206.
[5] R. Nair, E. Perret, and S. Tedjini, "Chipless RFID based on
group delay encoding," in IEEE International Conference on
RFID technologies and Applications (RFID-TA), Barcelona,
Spain, 2011, pp. 214-218.
[6] S. Gupta, A. Parsa, E. Perret, R. V. Snyder, R. J. Wenzel, and C.
Caloz, "Group-Delay Engineered Noncommensurate
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Processing," Microwave Theory and Techniques, IEEE
Transactions on, vol. 58, pp. 2392-2407, 2010.
l1=21.5mm, l2=9.5mm l1=17mm, l2=10.2mm
(b)
Reflections
Am
plit
ude (
v /m
)
Time (ns)
∆t=3.3ns(code0)
∆t=2.5ns(code1)
∆t=5ns (code1)
∆t=3.42ns(code0)
Structural mode Time (ns)
Tag modes
Am
plit
ude (
v /m
)
∆tstart
(a)
(b) Frequency(GHz)
Gro
up d
ela
y (
ns)
Vertical Antenna
Horizontal Antenna
(a)
TABLE I
4 COMBINATION OF BITS BASED ON TIME DELAY
lengths GD at 2.6 GHz
(ns)
Sim. Mes.
GD at 5 GHz (ns)
Sim. Mes.
co
de
l1 l2 2.31 2.32 2.95 2.90 00
l1+∆ l1 l2 1.71 1.74 2.94 2.89 01
l1 l2+∆ l2 2.28 2.28 2.28 2.26 10
l1+∆ l1 l2+∆ l2 1.63 1.65 2.29 2.32 11
978-1-4673-1088-8/12/$31.00 ©2012 IEEE