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,

"Multiresonator-based chipless RFID system for low-cost item

tracking," Microwave Theory and Techniques, IEEE

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

Transmission Line All-Pass Network for Analog Signal

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