Chipless RFID Based on Group Delay Encoding

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

raji.nair@lcis.grenoble-inp.fr

Abstract— A two bit planar chipless RFID tag based on group

delay encoding has been presented in this paper. This chipless tag

encodes data in the phase signature. The data encoding has been

done by using C-sections formed by coupling the cascaded

transmission line sections at alternative ends. The tag produces

two different delays for two specific frequencies corresponding to

the two different lengths of the transmission line sections. A

measurable amount of delay has been obtained. The new

approach has been validated with simulation results in frequency

and temporal domain and with measurement results in frequency

domain.

Keywords- Chipless RFID, tag, Cascaded transmission line, C-

sections, group delay.

I. INTRODUCTION

Chipless RFID has become very interesting research area in recent years. Even though the conventional RFID has numerous advantages like specificity in information, long read range and mass storage of data [1], its growth has been slow down mainly because of the economical reasons. The development of chipless RFID gives an attractive solution for this [2].

As its name implies, chipless tags do not contain any silicon chip. The principle of information encoding is based on the generation of a specific temporal or frequency signature. The chipless tag usually contains some planar circuits which will reflect the reader‟s signal back. This unique reflected signal can be used as the identifier, a concept very similar to the principle of radar [3]. The data is electromagnetically coded in the amplitude or phase of the wave.

Researchers developed different kinds of tags among which some of them are based on amplitude-frequency signature [4-5]. Resonators are used to encode the data and polarization diversity is used to isolate interrogation signal and the backscattered signal. Since it based on the amplitude, there are possibilities of error in a noisy environment and in this context phase based coding has more significance.

Information encoding by using the phase has been well explained in [6]. A delay line has been used along with the periodical discontinuities. An interrogation pulse has been transmitted to the delay line and a part of the signal is reflected back at the discontinuities. Encoding has been done using the signal phase of the reflected signal with respect to the reference phase.

Another phase based approach is presented in [7] by using the simple microstrip patch antennas. These antennas will re- radiate the backscattered signals when they excite with their resonant frequency signals. This re-radiated signal will have distinct phase characteristics which have been encoded for the chipless tags. The orthogonally polarized backscattered signal has been used as the reference. But only one patch is resonant at a time so that there is a chance of undesired non-resonant backscattering from the other patches.

A backscattered multilayer structure has been well explained in [8] which also uses phase-frequency signature for the encoding. The structure gives a predictable phase variation at pre determined frequencies. But the presence of background signals makes it difficult to implement in the real time environment since it always demands subtraction of background signal from the real signal.

E.G. Cristal [9] investigated the analytical properties of cascaded commensurate (equal length) transmission line C-sections and proposed a procedure for realizing the phase characteristics of a cascaded non commensurate transmission line. The frequency dispersive property of the transmission line is used in [10], which enables to rearrange different spectral components in different time, to find out the group delay characteristics of the cascaded transmission line. A computer design based approach has been utilized to find out Gaussian, linear and quadratic group delays.

A theoretical approach of group delay engineering in RFID system is well explained in [11]. But the novel approach proposed here clearly depicts the encoding technique and presents a direct relation between the structure of the tag and the encoding. Microstrip technology has been used which is easy to realize and the encoding can be done by simply varying the lengths of the C-sections. It utilizes the spectral rearrangement property of microstrip dispersive transmission line and group delay for encoding the information in chipless tags. The reflected signal at the input antenna terminal is used as the reference signal.

II. PRINCIPLE OF UTILIZATION OF GROUP DELAY

IN CHIPLESS RFID

Fig. 1 depicts the principle used here. In this example, the tag consists of two C-sections with two different lengths enabling two bit coding (see Fig. 1(a)).

2011 IEEE International Conference on RFID-Technologies and Applications

978-1-4577-0027-9/11/$26.00 ©2011 IEEE 214

Frequency

Gro

up

Del

ay (

gd)

gd(l1)

gd(l2)

f (l1) f (l2)

Am

pli

tud

e

Time

Ref

eren

ce

Sig

nal

∆t(l2)

∆t(l1)

l1 l2 t

g

w

W‟

l

The input pulse is a combination of the frequencies of interest, in time domain. The receiving antenna in the tag receives this signal. After passing through the tag the frequency corresponding to the length of the C-section will be delayed. The amount of delay can be controlled by the design of the structure. The frequency domain response shows the delay produced by each C-section (Fig. 1(b)). In the time domain response, this amount has been shown as ∆t (Fig. 1(c)). Thus the C-section which produces large group delay in frequency domain will shift the corresponding frequency component more from the reference signal in time domain. The reflected signal at the input antenna terminal can be used as the reference for the delayed signals.

III. STRCUCTURE DESIGN

A group delay in general is the slope of the transmission line phase response. It is obtained by taking the negative derivative of phase with respect to the angular frequency.

(a) (b) Fig.2: Structure of the proposed tag a) Structure of the reference tag b)various parameters of a single C-section g=0.1 mm, w=0.7 mm,

w‟=0.1 mm.

In order to reduce the mutual coupling effect between each group, the gap between two groups of C-sections has been optimized as 1mm as shown in Fig. 2(a).

The group delay corresponding to the longer C-section is almost double that of the shorter one and that is the reason we

are including more number of shorter C-sections. Various

parameters of the C-section are depicted in Fig. 2(b).

It has been found that the group delay curve is periodic in

frequencies with each peaks occur at odd multiple of the frequency of interest. From the parametric studies we had found that group delay increases with an increase in length l

shifting the frequency to the lower band. In contrast decrease in gap g and width w will increase the value of delay without affecting the frequency. Fig. 3 depicts the effect of group delay for various values of gap, g. The gap has also an effect of coupling, lesser the value of g greater will be the coupling and hence more the delay. Decrease in gap will make the delay curve narrower. Here a tag with two different lengths has been utilized as an illustration which gives 4 combinations of bits with a capacity of coding 2 bits.

IV. PRINCIPLE OF ENCODING

The tag with length l1=17.87 mm and l2=8.93 mm is

considered as the reference tag and the corresponding response implies to the code 00. These lengths are chosen to provide a good isolation between each group. The three other combinations of the two bit coding are obtained by merely

(a) (b) (c)

Fig.1: Principle of utilization of group delay in chipless tags. a) structure of the proposed tag b) group delay curve in frequency domain c) corresponding time delay

Fig.3: Group delay response obtained as a function of gap g with

l=8.93mm, w=0.7 mm, w‟=0.1mm, εr=4.3, tanδ=0.025, h=0.8mm.

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changing the length of the C-section by a factor of ∆l. The code

01 and 10 is obtained by reducing either of the lengths l1 or l2

by an amount ∆l1 or ∆l2. Finally reducing both the lengths will

give rise to the code 11.

V. TIME DOMAIN SIMULATION STUDY

CST Microwave Studio and MATLAB have been used as the simulation software. MATLAB is used to produce the modulated time domain signal and also to extract the envelope of the tag response. The input signal comprises of a signal which is modulated with carrier frequency of fc1 =2.6 GHz and fc2=5.3 GHz in time domain. These are the two frequencies at which a distinguishable delay is obtained between each peak.

Fig. 4 represents the temporal simulation results. It is seen that the change in length ∆l1 (corresponds to the code 01)

produces a time delay ∆t of 440 ps compared to the reference tag (corresponds to the code 00) and a change in ∆l2

(corresponds to the code 10) produces a time delay ∆t of 590 ps. In both cases it can be seen that only the group delay corresponding to the C-section, with a change in length, is changes to an appreciable amount. But at the same time the group delay corresponding to the C-section kept constant also gives an unwanted shift of 100 ps and 30 ps. And finally by varying both the lengths „l1‟ and „l2‟ (corresponds to the code

11) gives a delay ∆t of 560 ps compared to the reference tag.

For implementing this structure as a chipless tag, a receiving and transmitting antenna has been added. The entire system has been simulated and the backscattered signal from

the receiving antenna has been used for the encoding purpose. Here the reflected signal at the input antenna terminal has been used as the reference signal for the delayed output signal. The well known disc monopole UWB antenna, an approach similar to [4], has been used for modeling the tag. Fig. 5(a) shows the structure of the monopole antenna, along with the parameters, used for the simulation. The antenna gives the UWB characteristics between 2.3 GHz-10 GHz.

Fig.4: Simulated results of time domain response of the tag for the four different combinations of bits.

(a) (b)

Fig :5 Structure of the proposed UWB antenna with different parameters:

wfeed=1.5 mm, Lgnd=29.5 mm, wgnd=44 mm, R=13 mm, Lgap=0.5 mm, εr=4.3, tanδ=0.025, h=0.8 mm b)Simulated return loss of the proposed UWB antenna

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The parameters of the antenna are optimized to adapt with chipless application. Fig. 5(b) shows the return loss obtained for UWB antenna. Fig. 6 shows the chipless RFID transponder with vertically polarized receiving antenna and horizontally polarized transmitting antenna, simulated in FR4 substrate (εr=4.3, tanδ=0.025, h=0.8 mm).

The structure has been excited by plane wave using CST. E field is applied in the vertical direction of the proposed structure. To verify the response of the structure to each frequency, a single pulse corresponding to 2.6 GHz (first frequency of interest) is applied. It has been found that the reflected signal at the input antenna terminal consists of two components. First component is the reflection from the entire device, which is constant in all cases and is used as the reference signal for the delayed output and the second component is the response of the C-section. Fig. 7 depicts this.

It shows the four combinations of 2 bit coding corresponding to the longer C-section (at 2.6 GHz). It is seen that the tag gives the same response as we already explained. The time ∆t is assumed to be constant when the corresponding C-section kept as constant. Similarly the time ∆t seems to be varying when the

length of the C-section is varied by an amount of ∆l.The value

of ∆t obtained is higher than that of the value obtained by simulating the C-sections without antennas. It is because the reflected signal travels to and fro through the C-section bringing a small amount of delay which will be constant throughout the four configurations.

VI. SIMULATION AND MEASUREMENT RESULTS IN

FREQUENCY DOMAIN

In order to validate the above concept, we had implemented the structure practically by using the frequency domain concept. Rather than connecting real antennas to the tag, two connectors have been connected between the two ports of the tag as shown in Fig. 8.

The measurement has been done by using the Vector Network Analyzer Agilent 8720D. The overall dimension of the tag is 3.5 cmX2.5 cm including the feed line. In order to validate the concept in frequency domain, the S-parameter and the group delay were measured. The measurement results have been compared with the simulation results obtained in frequency domain. Excellent agreement has been obtained between simulation and measurement (Fig. 9) which proves the proposed new concept. Three other combinations corresponding to the codes 01, 10 and 11 were experimented by changing the lengths of the two C-sections. Fig. 9(a) shows the group delay curve corresponding to the code 00 and 11. From the figure it is clear that code 11 is obtained when both lengths of the C-sections are changed by a factor of ∆l. The two other combinations are shown in Fig. 9(b) which is obtained by reducing the length l1 by ∆l1 and l2 by ∆l2. This

frequency domain curve gives the direct information of the amount of time the signal delayed from the reference signal.

The effectiveness of the phase to an additive noise is clearly depicted in [12]. An analytical and experimental evaluation has been done to prove that group delay based systems are more robust than any other conventional systems. Since our structure uses phase level coding rather than

Fig: 6 Structure of the proposed tag with different parameters: L1tag= 58 mm,

L2tag= 48 mm, Wtag=102 mm, R=13 mm, εr=4.3, tanδ=0.025, h=0.8 mm.

Fig :7 Envelope of the reflected wave corresponding to 2.6 GHz obtained by

using the above configuration for the four combinations of tags

Fig: 8 Structure of the reference tag used for measurement with length

l1=17.87 mm and l2 =8.93 mm, g=0.1 mm and w=0.7 mm, εr=4.3,

h=0.7 mm.

RX anetnna

TX anetnna

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amplitude, it is more robust than the conventional systems

which utilize amplitude level coding. These features make it a

novel coding technique based on group delay characteristics in

chipless tags.

VII. CONCLUSION

The present work proposes a new kind of encoding in

chipless tags utilizing group delay. The new approach is

explained with the help of both simulation and measurement

results and the agreement between them is enough to prove the

concept. The structure is designed in an easily available cheap

substrate like FR4. By simply using C-sections with two

different lengths, it is possible to encode two bits. The group

delay can be increased to an appreciable amount if the number

of C-sections is increase. As it is based on the phase signature

encoding, the tag is more robust than that of the conventional

approach.

ACKNOWLEDGMENT

The authors would like to thank Prof. L. Duvillaret, Dr. F.

Garet, Guy Eymin-Petot-Tourtollet and Yann Boutant for their

guidance and fruitful discussions on part of this work and A.

Vena for his help.The authors would also like to acknowledge

the French National Research Agency for financially

supporting this project via the ANR-09-VERS-013 program.

REFERENCES

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[11] S. Gupta, B. Nikfal, and C. Caloz, “RFID System based on Pulse-Position Modulation using Group Delay Engineered Microwave C-Sections,” in Proc. Asia-Pacific Microwave Conf. (APMC), Yokohama, Japan, pp. 203-206, December 2010.

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(a) (b)

Fig: 9 Simulation results compared with measurement results for the four combinations of bits in frequency domain. Group delay response corresponding to the

code a) 00 and 11 b) 01 and 10.

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