Chipless RFID Tag Localization

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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1

Chipless RFID Tag LocalizationRubayet-E-Azim Anee, Student Member, IEEE, and Nemai C. Karmakar, Senior Member, IEEE

Abstract—A novel localization technique for frequency-domainchipless RF identification (RFID) tag is proposed. A short-dura-tion ultra-wideband impulse radio signal interrogates the tags, andmultiple receivers in the interrogation zone capture the backscat-tered signal from the tags. The received signals from the chiplesstags are analyzed for the structural mode radar cross section to de-termine the relative ranges. Using the range information, the linearleast square (LLS) method is employed for accurate localizationof tagged items. The accuracy of the localization method is ana-lyzed by moving the chipless tag within a fixed interrogation zone.The whole system is modeled in CSTMicrowave Studio Suite 2012to comply with a realistic scenario. The post-processing for rangeand tag position estimation through LLS is done in MATLAB. Themethod is also verified in laboratory environment with fabricatedchipless RFID tags andmultiple receiving units. The range and an-gular resolution are 2.1 cm and 3.5 , respectively. The analysis andresults create a strong foundation for chipless RFID tags to be usedin tracking and localization.

Index Terms—Backscattered signal, chipless tag, linear leastsquare (LLS), localization, positioning, ranging, radar crosssection (RCS), RF identification (RFID), round-trip time-of-flight(RTOF).

I. INTRODUCTION

R F IDENTIFICATION (RFID) is a wireless data com-munication technology, which uses an RF wave for

communication between the reader and tag. A block diagram ofa conventional RFID system is shown in Fig. 1 where the tag,contains application-specific integrated circuits (ASICs) fordata storing and processing. The cost of the chipped tag, whichprimarily comes from the ASIC [1], is the main hindrance inmassive low-cost item tagging applications. To address thisissue, significant research is going on for the developmentof low-cost chipless RFID tags. Chipless RFID enjoys thebenefit of on-demand printability on papers or polymers withconventional printing methods and conductive inks, whichmakes it apt for mass deployment in low-cost item tagging forwide range of applications [2]. Among the reported chiplesstags (time [3], frequency [4], phase [5], and image [6], [7]based tags), the frequency-domain tags [8]–[10] have higherdata capacity than their predecessors [3]. Fig. 2 shows the basicoperating method of frequency-domain chipless RFID systems.The tag is interrogated by an ultra-wideband (UWB) signal,

Manuscript received May 08, 2013; revised September 04, 2013; acceptedSeptember 05, 2013. The work was supported by the Australian ResearchCouncil under Discovery Project Grant DP110105606: Electronically Con-trolled Phased Array Antenna for Universal UHF RFID Applications.The authors are with the Department of Electrical and Computer Systems En-

gineering, Monash University, Clayton, VIC 3800, Australia (e-mail: [email protected]; [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMTT.2013.2282280

Fig. 1. Conventional chip-based RFID system.

Fig. 2. Chipless RFID system.

and response from the tag is analyzed in the frequency domain(FD) for extracting the resonance information; hence, the tagID [8].Thus far, the research on chipless RFID mainly focuses on

tag design and identification (ID) [3]–[6], [8]–[13], but in addi-tion to ID, knowing the location of the tag adds extra flexibilityand opens up new application sectors for chipless RFID systems[14]. The significance and applications of localization are sum-marized as follows.• On knowing the location of the tag, the reader can direct theantenna beam toward precise directions and avoid unde-sired reflections and interference from nearby objects andtags.

• Localization facilitates multiple tag reading in chiplessRFID systems [15]. On knowing the location of all thetagged items, the reader can read them one by one bybeam steering.

• Tracking and detection of objects or personnel carryingchipless RFID tags enables automatic responses such as:automatic door operation, turning on/off lights, or trig-gering alarms. Chipless RFID can support these applica-tion within low implementation cost.

• Localization plays an important role in healthcare moni-toring and supply chain management as well.

Chipless RFID can be used for indoor localization. To over-come the current range limitation, a cluster can be formed withmultiple cells within the room and each cell can be equippedwith transceivers for localization and ID of chipless tags. Thus,we can cover more areas and address multiple tags in differentcells simultaneously. However, with improved tag design, radarcross section (RCS) and advanced level signal processing on thebackscattered signal are playing important roles in improvingthe operating range of chipless tags. In [16], 1-m reading range

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2 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES

has been reported and continuous research is going on for fur-ther improvement in the reading range. With increased readingrange, the localization will be particularly important and willallow penetration of chipless RFID into new application do-mains.Tag localization methods for conventional chipped RFID

tags [15], [17], [18] rely mostly on the processing capabilitiesof ASICs [17]. Localization methods used for conventionalchipped RFID tags can be divided into three categories,which are: 1) round-trip time-of-flight (RTOF) estimation—thereader calculates the elapsed time between the interrogationand received pulse [14], [19] and 2) received signal strength(RSS)—the tag–reader distances versus RSS is mapped a priori,and based on this map, the received signal’s strength is matchedand the distance of the tag is estimated [17], [20]. It requireson-site adaptation [15], and 3) phase evaluation method—inthe phase evaluation method, the round-trip phase is measuredto estimate the distance of the tag [14], [21]. However, thelocalization method largely depends on type of transponders,such as active, passive, semi-passive, or chipless tags.The contrasting and challenging issues in chipless RFID sys-

tems are: 1) tags are unable to modify the backscattered signalpower according to the requirement for distance estimation re-quired for RSS method due to the absence of ASIC; 2) they areunable to establish inter-tag communication; 3) a large numberof reference points is required for RSS implementation; and4) tags are incapable of sending beacon signals for localiza-tion. For localization and ID, the chipless RFID system solelydepends on the backscattered signal from the tag. The RTOF-based approach has been investigated and analyzed for surfaceacoustic wave (SAW) chipless tags for localization. A localiza-tion accuracy of 20 cm has been reported using three receivingantennas and the auto-correlation method [19], [22]. Arumugamet al. [22] has reported a sub-decimeter localization accuracy(3.17 cm) for SAW tags. In [23], a unipolar monopole antennahas been studied, but focuses mainly on ID rather than localiza-tion. To the authors’ best knowledge, the FD chipless tags [1],[5], [10]–[12], [24], [25] have not been analyzed for ranging andlocalization.The FD chipless tags are interrogated by an ultra-wideband

impulse response (UWB-IR) signal [25]–[28] and the backscat-tered pulse from the tag is analyzed for ranging, localization,and ID. Ranging and localization with a UWB-IR signal hasbeen reported in much literature. In [29], UWB ranging hasbeen presented for passive UHF RFID tags by employingan extra UWB radar system with the UHF RFID reader. Awirelessly powered RFID tag with an active UWB transmitterhas been studied for localization and 5.4-cm root-mean-squareerror (RMSE) has been achieved within a 1-m observationregion [30]. In [31], a hybrid UWB tag antenna connected witha network analyzer is tested for localization accuracy using aUWB-IR signal, and an average position error of 2 cm has beenreported. Unlike chipless RFID systems, in all of the mentionedliterature, an active tag or a transmitter has been considered.This paper proposes a novel localization technique for print-

able FD chipless RFID tags. The localization is done by ana-lyzing the time-domain (TD) backscattered signal from the tagby a UWB-IR ranging technique. A set of multiple transceivers

Fig. 3. (a) Layout of the chipless tag, mm, mm,mm, and mm on Taconic TLX 8.0. (b) Backscattered RCS

of 4-bit tag (simulation).

are placed at known positions in the interrogation zone. Thetransmitter illuminates the chipless tag with a UWB-IR signal[26], and collocated receivers collect the backscattered signalfrom the tag. The RTOF, and hence, the relative distances be-tween the tag and transceivers, are calculated from the structuralmode response of the tag’s backscattered signal. Subsequently,the tag position is determined using linear least square (LLS) ap-proximation [32]. The accuracy of the method is analyzed firstby placing tags at different known positions. Afterwards, arbi-trary unknown tag positions are estimated with the localizationmethod and known error.This paper is organized as follows. Section II discusses the

backscattered signal from the tag, localization method, andrange of the method. Section III presents the simulation andexperimental results. Section IV concludes this paper withfuture directions.

II. PROPOSED LOCALIZATION FOR CHIPLESS RFID TAGS

This section describes a novel localization method for fre-quency-domain chipless RFID tags. For the proposed method,the RTOF information is extracted from the backscattered signalof the tag; hence, the backscattered signal is analyzed first. Theranging and positioning techniques are then explained in detail.

A. Backscattered Signal From Chipless Tag

In this analysis, an array of multi-bit slot-loaded patches isused as the chipless RFID tag [10], as shown in Fig. 3(a). The FDresponse of the tag is shown in Fig. 3(b). It has four resonancesat 6.6, 7.2, 7.9, and 9 GHz. The tag is interrogated by a UWB-IRpulse, which is expressed as

(1)

Here, is the center frequency of the pulse, is the timeindex for the peak value of the pulse, and is the variance.When interrogated with such a pulse signal, the backscatteredsignal from the tag has two scattering modes, which are: 1)structural mode response and 2) tag mode response .The initial backscattering from the tag depends on the sizeof the tag and is very similar to the interrogation pulse. The de-layed backscattering is the tag mode , which contains the


ANEE AND KARMAKAR: CHIPLESS RFID TAG LOCALIZATION 3

resonance information [26], [33]. Hence, the overall TD RCSresponse from the tag is expressed as

(2)

Here, is the Gaussian random noise. In this expression,the tag’s resonance information is represented as the summationof exponentially decaying signals and the number of exponen-tials depends on the number of resonances present in the tag.

is the complex amplitudes and are poles cor-responding to predefined resonance frequencies [28], [34].is the delay involved in the tag’s backscattering and depends onthe distance between the tag and transceiver. The interrogationpulse and the backscattered responses in the TD from the tag areshown in Fig. 4(a) and (b). As we can see in Fig. 4(a) and (b),the interrogation pulse is centered at 1.2 ns, whereas the struc-tural mode response from the tag is centered at 2 ns. Therefore,

ns represents the RTOF for the tag. Using TD rectan-gular windowing techniques on the tag’s backscattering, shownin Fig. 4(b), two modes are separated and their frequency con-tents are analyzed for tag information. The frequency analysis ofthe separated responses (structural and tag modes) is shown inFig. 4(c). As discussed, the structural mode contains the similarfrequency content as the interrogation pulse, whereas the latetime tag mode contains the resonance information of the chip-less tag. In this case, ns yields the tag’s range of12 cm.The TD-structural RCS signal has the maximum mag-

nitude within the overall RCS from the tag, which can be seen inFig. 4(b). The time delay between the interrogation pulseand the structural mode RCS is inversely proportional tothe fourth order of distance between the tag and receiver. Thisis called the RTOF for a particular tag. The RTOF is identifiedwithin the overall RCS response through a peak detec-tion algorithm [35]. From , the range information of the tag isextracted and used to localize the exact position of it in the in-terrogation zone.

B. Maximum Detection Range

The maximum detection range of the tag is described by thewell-known radar equation [21]. It is expressed as

(3)

Here, is the gain of the transmitting and receiving an-tennas, denotes transmitted power, is the wavelength of thecenter frequency of the operating bandwidth, is the structuralmode RCS of the chipless tag, is the minimum detectablepower of the receiver, and is the maximum detectablerange of the chipless tag. In our setup with a vector network an-alyzer (VNA) and horn antennas, the noise level is measuredas 60 dB. In order to successfully detect and decode the tagID, it has been found from measurement that the backscatteredRCS from the tag need to be 10 dB greater than the noise level.

Fig. 4. (a) UWB-IR pulse for interrogation. (b) TD backscattering from tag.(c) Normalized magnitude spectrum of the structural mode and tag mode re-sponse.

Therefore, is chosen 50 dBm. The center of the oper-ating frequency band is 9 GHz, which gives cm. Thetransmitting and receiving antennas used here have a gain of

dBi. The VNA transmits from 15- to 3-dBm powerover the entire frequency band. Sincewe use an array ofmultipleslot-loaded square patches, for simplicity we consider the tagas a rectangular flat plate and approximate the structural modeRCS from physical optics according to the following expres-sion [36]:

(4)

Here, , and are two sides of the chipless tag,and is the aspect angle. The chipless tag has a dimension of6.5 cm 4.5 cm, which gives a structural mode RCS,dBm at 9-GHz frequency according to (4).Fig. 5 shows the dependency of maximum range of the tag

on the transmitted power and backscattered structuralmode RCS of the tag. For our given setup, with the chip-less tag described here, a maximum of 80-cm detection rangecan be obtained. Thus, we consider two separate setups. In thefirst setup, we placed the transceivers on a circle of 70-cm di-ameter, which gives an observation area of 70 70 cm , where

cm is considered ( dBm). In the secondsetup, the transceivers are placed on the perimeter of a circleof 120 cm, which allows an observation area of 1.2 1.2 m



Fig. 5. Schematic of simulation and experimental setup for localization.


where cm ( dBm). The signal-to-noise ratio(SNR) also plays an important role in the accuracy of the rangeestimation according to Cramer Rao lower bound (CRLB). TheCRLB indicates the lower bound of the range estimation errorby the following expression [37]:

Delay estimation error, (5)

Here, is the bandwidth, which is 6 GHz in our setup, andis the speed of the electromagnetic (EM) wave (3 10 m/s).Fig. 6 shows the CRLBs for ranging errors in terms of SNR fordifferent bandwidths. As the bandwidth increases, the rangingerror decreases, and for a particular bandwidth, it improves withthe improvement of SNR. However, for 6-GHz bandwidth, thetheoretical lower bound of ranging error varies from 0.2 to 1.8cm for different SNRs.

C. Localization of Tag

For unambiguous localization in a 2-D plane, a minimum ofthree transceivers are required. Fig. 7 illustrates the analysissetup and coordinate system for chipless tag localization. Thetransceivers are placed at equal radial distance from the cen-troid of the interrogation zone (here, a square room is consid-

Fig. 7. Coordinate system for trilateration for chipless RFID systems.

ered; hence, the centroid is the intersection point of the diago-nals). The angular separation between adjacent transceivers isexpressed as

(6)

Here, is the number of transceivers. If three transceiversare used, then the angular separation between adjacent trans-ceivers is 120 according to (6). When a tag enters in the inter-rogation zone, the backscattered signal from the tag is capturedby the transceivers placed around the interrogation zone and an-alyzed for the time of arrival for the structural mode to calcu-late the RTOF, and hence, the range of the tag. After the relativeranges from multiple transceivers are calculated, the position ofthe tag is estimated using LLS approximation.For estimating the estimation error of the method for local-

izing the chipless tag, first the tag is placed at various known po-sitions denoted by , and the position is estimated usingthe localization method. Afterwards, it is compared with theactual tag position to estimate the estimation error. The tag isplaced at six different angular positions (30 , 90 , 150 , 240 ,270 , and 330 ) with respect to for each of the eightradial distances (5, 10, 15, 20, 30, 40, 50, and 55 cm) from thecenter of the interrogation zone.The localization starts by sending a UWB-IR signal for inter-

rogation. The chipless tag backscatters the signal, which is re-ceived by the receiving antennas. The localization technique forthe chipless tag is broadly divided into two parts: ranging andpositioning of the tag. During ranging, the relative distances ofthe tag from the receivers are calculated from the RTOF of thereceived signals at different receiver positions. These distancesare used to estimate the location of the tag in the inter-rogation zone during positioning.1) Ranging of Tag: Due to spatial differences, the signal

from the tag to the receiving antennas suffers from measurable



Fig. 8. UWB-IR received signal for tags at reference position and arbitraryposition.

delays. For localization purposes, the delay between the trans-mitted and received signals is calculated. The response receivedfrom the tag has two parts, as described in Section III-A: struc-tural mode response and tag-mode response . It hasbeen established in [26] and [28] that the structural mode RCShas larger signal amplitude compared to the tag-mode responseand possesses similarity with the interrogation UWB-IR pulse.The cross-correlative peak detection method can be used bycorrelating the backscattered time-series with the interrogationpulse shape to identify the delay of the structural-mode RCS.The delay can also be calculated from the envelope of the TDbackscattered response signal. As can be seen from the enve-lope of the signal shown in Fig. 4(b), the structural RCS has thehighest magnitude within the overall response. Therefore, themaximum in the envelope of the TD RCS response is used todetect the delay . However, instead of using actual RTOF,we have used a relative delay and range measurement methodin our analysis. The advantages are: 1) we do not need to knowthe exact time index for the interrogation pulse enabling us touse the -parameters captured by the VNA [38]; 2) we cancalibrate the center of the interrogation zone as the referencepoint; and 3) we can simply apply a relative range measurementmethod. Here, the transceivers are fixed in their positions on theperimeter of the circle. Therefore, the center is used as a ref-erence position. After calibrating the reference position, a ref-erence locus can be considered for each of the transceivers, asshown in Fig. 7.Fig. 8 depicts a backscattered signal from the tag for the refer-

ence position and an arbitrary position . How-ever, for calculating the envelope of the TD signal, the receivedsignal is converted to an analytic signal bya Hilbert transform [39]. The peak value is extracted from theenvelope of the signal [40], [41]. The time of arrival of the struc-tural mode is given by the following expression:

(7)

Here, is the time instant where the tag’s backscatteredsignal has its maximum magnitude and is equal to the valueof , which maximizes . From Fig. 6, we can seethat the times of arrival of the structural mode (maximum in theenvelope of the backscattered signal) for the reference and tagsignals are and , respectively. Therefore, the ranges ofthe tag can be calculated from the following expression:

(8)

2) Positioning of Tag: The position of the tag can be deter-mined once the distance from known receivers is estimated. Thesolution is actually finding the intersection points of several cir-cumferences. If -number of receivers are used, the tag positionis estimated by solving nonlinear equations. For estimatingthe tag position, the LLS, singular value decomposition (SVD)[38] or nonlinear least squares (Newton) method [38], [42] canbe used. In this paper, the LLS method is used to estimate thetag position.Based on the tag and the antenna positions shown in Fig. 5,

the relative distance between the tag at and thetransceiver at is represented by the following expres-sion:

To avoid complexity of the nonlinear solution, the system ex-pressions are converted to a linear system by using a lineariza-tion tool [38], [43]. Here, the th expression is used to linearizethe system and the expressions are

Expanding and rearranging the terms of this expression leadsto the following expression:

(9)

Here, is expressed as

(10)

Thus, for number of receivers, the linearized system ofexpressions are

...

These system equations are written in matrix form as [32],[42], [44]

(11)

where



and

Here, is known as the fixed transceivers’ positions, whichare known a priori. is calculated by (6) using and valuesobtained from ranging, and is the unknown tag position. Now,is determined such that the mean square error for (11) has

minimum value. This condition is expressed as [45]

(12)

Solving (11) such that it validates the condition expressed by(12), the unknown tag position is determined.

III. RESULTS AND DISCUSSION

A. Simulation Environment

In order to obtain a backscattered signal close enough to therealistic scenario, as shown in Fig. 7, a 3-D model is constructedwith a chipless tag described in Section II-A andmultiple probesas the receivers in CST Microwave Studio Suite 2012, and full-wave EM simulation is performed by moving the tag at dif-ferent positions within the interrogation zone. AUWB-IR signalof 6-GHz bandwidth and 1.2-ns duration is used to interrogatethe tag. In our simulation and laboratory experimental setup, wehave used three transceivers at three known positions in the in-terrogation zone. The chipless tag is movedwithin the interroga-tion zone at different positions denoted by , as alreadydescribed in Section III-C.From the CST simulation model, the tag response signals at

the probes are extracted for RTOF calculation. The RTOF data isused to estimate the relative distance between tag and receivingantennas. For positioning, the mathematical formulation of thelocalization problem is done in MATLAB as a system of non-linear equations, as presented in Section III-D. The nonlinearsystem is linearized using a linearization tool to an approxi-mated linear system. The linear system is then solved by ap-plying LLS approximation [see (1) and (12)] for estimating theposition of the tag.

B. Experimental setup

The experimental validation of the proposed localizationmethod has been carried out at Monash Microwave, Antenna,RFID, and Sensor Laboratory (MMARS), as shown in Fig. 9.The experimental setup consists of an Agilent VNA PNAE8361A as the reader electronics with a pair of horn antennasas reader antennas and fabricated chipless tags.The horn antennas operate from 6 to 12 GHz with an almost

flat gain of 11 dBi over the band. The tag is fabricated on TaconicTLX-0. The measurement and post-processing for localization


is presented in the flowchart shown in Fig. 10. As can be seenfrom the flowchart, three measurements are taken for the taglocalization purpose of: 1) background measurement; 2) ref-erence/calibration measurement; and 3) tag measurement. Thebackground measurement is performed with the receivers only,without any tag. Assuming static clutter contributions, the sub-traction of background measurement from the tag measurementremoves the clutter, antenna coupling, cable effects, and back-ground noise. The reference measurement is taken to calibratethe center of the interrogation zone for relative range measure-ment, as described in Section III-C. To remove the interferencefrom nearby receivers and undesired reflections fromwalls, timegating is used. The TD backscattered RCS from a chipless tagcan be directly obtained by a sub-nanosecond sampler or othermethods [46]. However, in a laboratory environment, we haveused a VNA as the reader system. Therefore, the FD RCS datais stored in the VNA and converted to the TD RCS through in-verse fast Fourier transform (IFFT). From the TD RCS, the timeof arrival of the structural mode RCS is calculated and the rela-tive range is estimated.

C. Results and Discussion

This section presents the simulation and experimental resultsfor the proposed localization method. Their close resemblancevalidates the proposed localization technique for chipless RFIDtags. First, the result of error analysis is presented. Afterwards,some unknown tag positions have been estimated with the pro-posed method considering the known error probability.1) Accuracy: The localization accuracy has been analyzed

from the perspective of linear tag distance and angularposition of the tag. The simulation and experimentalbackscattered response signals are imported to MATLAB forpost-processing for ranging and positioning of the chiplesstag. For each data set, the ranging and tag position areestimated through peak detection and the LLS approximationtechnique ,respectively, from system equations. The deviationsof measured and from the actual angular and linear posi-tions are represented through root mean square error (RMSE). Itis calculated for both and . The radial and angular positionestimation errors are denoted as and , respectively.



Fig. 10. Measurement and post-processing steps for proposed localizationmethod.

An accurate estimation of with a minimal error leadsto successful localization of the chipless tag in the interrogationzone.

a) Accuracy in Angular Position Estimation: The resultsfor angular tag position measurement are presented in Figs. 11and 12. The measured angular tag position for different lineardistance of the tag from origin is shown in Fig. 11. Both the sim-ulation and experimental results are included in the graph. Themeasured angular position of the tag is close enough to the ac-tual angular positions with some exceptions. As for andcm, and , the measured value differs for almost 10

from the actual tag position. This is the maximum deviation ob-served from simulation data. However, in experimental results,a maximum of 13 is observed for cm and .Fig. 12 shows the RMSE for angular position estimationfrom the actual angular position of the tag for different valuesof . For simulation, a maximum average deviation of 7 is ob-served for . However, in all other angular positions, the

Fig. 11. Measured angular tag position with varying linear distance.

Fig. 12. RMSE with respect to actual angular position.

deviation remains below 3 . In actual measurement, the devia-tion varies from maximum 5 to 1.3 . The 5 variation occursfor and for all other angular positions the deviationremains below 3.5 . However, other than some discrepancies insome cases, for most of the angular positions for all values of ,the measured angular tag position matches closely to the actualangular position.

b) Accuracy in Linear Tag Position Estimation: The mea-sured results of linear tag distance estimation are presented inthis section. Fig. 13 presents the measured result for linear tagdistance estimation with respect to actual tag distance for dif-ferent values of . For cm, the measured linear distancevaries within 1 cm from the actual value. For some angular tagpositions, as for , it deviates a maximum of 2 cm, forall other tag positions, it remains within 1.3 cm. Fig. 14 shows



Fig. 13. Measured linear tag position with varying angular positions.

Fig. 14. Average deviation from actual tag position.

the RMSE for tag range from the actual tag range for var-ious values of . The simulation and experimental values areclose to each other with a minimum of 0.5 cm and maximumof 2.7-cm deviation, whereas the experimental result deviatesan average of 1 cm for most values of , except in two caseswhere it shows 3-cm deviation. On average, 2-cm variation isobserved in estimated tag linear distance from the actual posi-tion.Table I shows the summary of the result of error analysis. In

the tag’s radial distance estimation, varies by 0.4 cm betweensimulation and measured results. However, in angular positionestimation , the RMSE is the same for simulation and mea-sured results. The estimation error clearly states that the methodis capable of estimating the chipless tag’s position with 2 cm and3.5 error in and , respectively.

D. Unknown Tag Localization

In the first step, the estimation error for the localizationmethod by placing tags at known positions is calculated andthe result has been presented in Section III-C. Next, the method

TABLE IERROR IN POSITION ESTIMATION OF CHIPLESS TAG

Fig. 15. Estimated positions of tags in interrogation zone.

TABLE IISOME ARBITRARY TAG POSITIONS ESTIMATION

is used to localize randomly placed tags using the proposedmethod. First the relative ranges are calculated, followed byapplication of the LLS method for tag position estimation.Table II represents the result of tag position estimation forsome of the arbitrary positions. Fig. 15 depicts the tags in theinterrogation zone. However, from the earlier analysis, wehave concluded that the estimated errors in radial distance andangular position estimation are 2.1 cm and 3.5 , respectively.Therefore, the estimated tag positions vary within 2.1 cm and3.5 in radial distance and angular positions, respectively. As

shown in Table II, for Tag position 1, the estimated tag radialdistance is 20.3 cm, considering the error, the radius can be18.2 or 22.4 cm. Therefore, it will remain within 4 cm of lineardistance, whereas the angular position is estimated as 254 , it



can vary from 250.5 to 257.5 , within 6 of angular variation.Similar results are obtained for all other arbitrary positions. Theproposed method yields simple, but accurate localization of thechipless tag in the interrogation zone within acceptable RMSE.

IV. CONCLUSION AND FUTURE DIRECTIONS

A comprehensive review for both chipped and chiplessRFID tag localization and explanation of a novel method forlocalization of frequency signature based chipless RFID tagshave been presented in this paper. Tag localization benefits theRFID system by improving the reliability of the tag readingprocess, tracking of objects, and even collision detection andmitigation of the collision scenario by DOA estimation usinga beamforming algorithm. A unique RTOF calculation fora frequency signature based chipless tag from the UWB-IRinterrogation signal and time gating analysis has been pre-sented here. A unique signal conditioning method is used forrepresenting the TD response of the chipless tag with structuraland tag mode RCS together with an ambient noise model. Thestructural mode RCS is the most dominant response and easilydistinguishable, and the delay of this response compared tothe interrogation pulse gives the RTOF and relative range ofthe tag. The tag mode RCS yields the ID information. A TDwindowing for both of the RCS modes yields precise range andID information. This finding is unique in the chipless RFIDtag localization and has not been reported before. A set oflinear equations have been developed based on the trilaterationlocalization method and has been applied for tag position andRMSE estimation. The theory is validated through extensivesimulation both in EM solver CST and in-house developedMATLAB codes and experimental setup. In the measure-ment setup, a set of calibration processes were developed tosubtract background noise and obtain the reference data fortag reference and RTOF reference. Based on the calibratedinformation, the relative ranges of the tag for three transceiversand position data were extracted. This unique set of calibrationnot only removes the ambient random noise, but also convergeto accurate determination of range and position . Theanalysis, synthesis, and application of extracted data revealsvery accurate reliable tag localization for six valid situations ofinvestigation for 5–80-cm read range (with being 5–55 cm)and 30 –330 angular resolution. The analysis based on RMSEre-validated the accuracy with 2.1 cm and 3.5 of res-olution. Finally arbitrary tag position investigation has yieldedvery accurate results.The adaptation of the proposed backscattered signal based

localization method is applicable to different types of fre-quency-domain chipless tag and does not require any modifi-cation within the tag structure. Therefore, it can be employedwith existing chipless RFID systems with the post-processingfor localization implemented in the back-end IT layer. Thereare some challenges that may affect the performance of themethod. In a multi-tag scenario, response from multiple tagsmay overlap and makes it challenging to calculate the RTOFfor any of the tags. This will affect the accuracy of the local-ization method. To capture the short duration, a high-resolutionresponse from the tag, a high sampling ADC, or sequential

sampling with low sampling rate need to be used [46]. Thesechallenges will be investigated further in future research to-gether with practical implementation.

REFERENCES

[1] S. Preradovic and N. C. Karmakar, “Chipless RFID: Bar code of thefuture,” IEEE Microw. Mag., vol. 11, no. 7, pp. 87–97, Dec. 2010.

[2] R.-E. A. Anee, S. M. Roy, N. C. Karmakar, R. Yerramilli, and G. F.Swiegers, “Printing techniques and performance of chipless tag de-sign on flexible low-cost thin-film substrates,” in IGI Global Chiplessand Conventional Radio Freq. Identification: Syst. Ubiquitous Tag-ging, 2012, pp. 175–195.

[3] A. Ramos, D. Girbau, A. Lazaro, and S. Rima, “IR-UWB radar systemand tag design for time-coded chipless RFID,” in 6th Eur. AntennasPropag. Conf., 2012, pp. 2491–2494.

[4] S. Preradovic, I. Balbin, N. C. Karmakar, and G. F. Swiegers, “Mul-tiresonator-based chipless RFID system for low-cost item tracking,”IEEE Trans. Microw. Theory Techn., vol. 57, no. 5, pp. 1411–1419,May 2009.

[5] I. Balbin and N. C. Karmakar, “Phase-encoded chipless RFIDtransponder for large-scale low-cost applications,” IEEE Microw.Wireless Compon. Lett., vol. 19, no. 8, pp. 509–511, Aug. 2009.

[6] R. Das, “Chip-less RFID—The end game,” IDTechEx, Cam-bridge, MA, USA, (2006, Nov. 9, 2012). [Online]. Available:http://www.idtechex.com/products/en/articles/00000435.asp

[7] M. Zomorrodi, N. C. Karmakar, and S. G. Bansal, “Introduction ofelectromagnetic image-based chipless RFID system,” in IEEE 8thInt. Intell. Sens., Sensor Networks, Inform. Process. Conf. , 2013, pp.443–448.

[8] S. Preradovic and N. C. Karmakar, “Design of fully printable planarchipless RFID transponder with 35-bit data capacity,” in Eur. Microw.Conf., 2009, pp. 013–016.

[9] I. Balbin and N. Karmakar, “Radio frequency transponder system,”Australian Provisional Patent DCC, Ref:30684143/DBW, Oct. 20,2008.

[10] M. A. Islam and N. C. Karmakar, “A novel compact printable dual-polarized chipless RFID system,” IEEE Trans. Microw. Theory Techn.,vol. 60, no. 7, pp. 2142–2151, Jul. 2012.

[11] M. S. Bhuiyan, R. Azim, and N. Karmakar, “A novel frequency reusedbased ID generation circuit for chipless RFID applications,” in Proc.Asia–Pacific Microw. Conf., 2011, pp. 1470–1473.

[12] M. A. Islam and N. Karmakar, “Design of a 16-bit ultra-low costfully printable slot-loaded dual-polarized chipless RFID tag,” in Proc.Asia–Pacific Microw. Conf., 2011, pp. 1482–1485.

[13] S. Preradovic and N. C. Karmakar, “Multiresonator based chiplessRFID tag and dedicated RFID reader,” in IEEE MTT-S Int. Microw.Symp. Dig., 2010, pp. 1520–1523.

[14] R. Miesen et al., “Where is the tag?,” IEEE Microw. Mag., vol. 12, no.7, pp. S49–S63, Dec. 2011.

[15] M. Bouet and A. L. dos Santos, “RFID tags: Positioning principles andlocalization techniques,” in 1st IFIP Wireless Days, 2008, pp. 1–5.

[16] S. Preradovic and N. Karmakar, “Fully printable chipless RFID tag,”in Advanced Radio Frequency Identification Design and Applications,S. Preradovic, Ed. Rijeka, Croatia: In Tech, Mar. 2011, pp. 131–154.

[17] T. Sanpechuda and L. Kovavisaruch, “A review of RFID localiza-tion: Applications and techniques,” in 5th Int. Elect. Eng./Electron.,Comput., Telecommun., Inform. Technol. Conf., 2008, pp. 769–772.

[18] J. Zhou and J. Shi, “RFID localization algorithms and applications—Areview,” J. Intell. Manuf., vol. 20, pp. 695–707, 2009.

[19] T. F. Bechteler and H. Yenigun, “2-D localization and identificationbased on SAW ID-tags at 2.5 GHz,” IEEE Trans. Microw. TheoryTechn., vol. 51, no. 5, pp. 1584–1590, May 2003.

[20] C. Alippi, D. Cogliati, and G. Vanini, “A statistical approach to lo-calize passive RFIDs,” in Proc. Int. Circuits Syst. Symp., May 2006,pp. 21–24, “,” .

[21] M. I. Skolink, Radar Handbook, 3rd ed. New York, NY, USA: Mc-Graw-Hill, 2008.

[22] D. D. Arumugam, V. Ambravaneswaran, A. Modi, and D. W. Engels,“2D localisation using SAW-based RFID systems: A single antennaapproach,” Int. J. Radio Freq. Identification Technol. Appl., vol. 1, pp.417–438, 2007.

[23] H. Sanming, Z. Yuan, L. C. Look, and D. Wenbin, “Study of a uni-planar monopole antenna for passive chipless UWB-RFID localizationsystem,” IEEE Trans. Antennas Propag., vol. 58, no. 2, pp. 271–278,Feb. 2010.



[24] T. Singh, S. Tedjini, E. Perret, and A. Vena, “A frequency signaturebased method for the RF identification of letters,” in IEEE Int. RFIDConf., 2011, pp. 1–5.

[25] A. Vena, E. Perret, and S. Tedjini, “High-capacity chipless RFID taginsensitive to the polarization,” IEEE Trans. Antennas Propag., vol.60, no. 10, pp. 4509–4515, Oct. 2012.

[26] P. Kalansuriya and N. Karmakar, “Time domain analysis of a backscat-tering frequency signature based chipless RFID tag,” inProc. Asia–Pa-cific Microw. Conf., 2011, pp. 183–186.

[27] A. Vena, E. Perret, and S. Tedjni, “Novel compact RFID chipless tag,”in Progr. Electromagn. Res. Symp., Marrakesh, Morocco, Mar. 20–23,2011, pp. 1062–1066.

[28] P. Kalansuriya and N. Karmakar, “UWB-IR based detection for fre-quency-spectra based chipless RFID,” in IEEE MTT-S Int. Microw.Symp. Dig., 2012, pp. 1–3.

[29] D. Arnitz, U. Muehlmann, and K. Witrisal, “UWB ranging in passiveUHF RFID: Proof of concept,” Electron. Lett., vol. 46, pp. 1401–1402,2010.

[30] Z. Zhuo et al., “Design and demonstration of passive UWB RFIDs:Chipless versus chip solutions,” in IEEE Int. RFID Technol. Appl.Conf., 2012, pp. 6–11.

[31] C. C. Cruz, J. R. Costa, and C. A. Fernandes, “Hybrid UHF/UWB an-tenna for passive indoor identification and localization systems,” IEEETrans. Antennas Propag., vol. 61, no. 1, pp. 354–361, Sep. 2013.

[32] X. Xiaochun, S. Sahni, and N. S. V. Rao, “On basic properties of local-ization using distance-difference measurements,” in 11th Int. Inform.Fusion Conf., 2008, pp. 1–8.

[33] S. Jang et al., “Exploiting early time response using the fractionalFourier transform for analyzing transient radar returns,” IEEE Trans.Antennas Propag., vol. 52, no. 11, pp. 3109–3121, Nov. 2004.

[34] M. Manteghi, “A novel approach to improve noise reduction in thematrix pencil algorithm for chipless RFID tag detection,” in IEEE Int.Antennas Propag. and CNC-USNC Symp./URSI Radio Sci. Meeting.,Toronto, ON, Canada, Jul. 11–17, 2010.

[35] P. Kalansuriya, N. C. Karmakar, and E. Viterbo, “On the detectionof frequency-spectra-based chipless RFID using UWB impulsed in-terrogation,” IEEE Trans. Microw. Theory Techn., vol. 60, no. 12, pp.4187–4197, Dec. 2012.

[36] R. A. Ross, “Radar cross section of rectangular flat plates as a functionof aspect angle,” IEEE Trans. Antennas Propag., vol. AP-14, no. 3, pp.329–335, May 1966.

[37] C. W. Cheol and H. Dong-Sam, “An accurate ultra wideband (UWB)ranging for precision asset location,” in IEEE Ultra Wideband Syst.Technol. Conf., 2003, pp. 389–393.

[38] H. Karl and A. Willig, Protocols and Architectures for Wireless SensorNetworks. New York, NY, USA: 2006.

[39] R. V. Koswatta andN. C. Karmakar, “A novel reader architecture basedon UWB chirp signal interrogation for multiresonator-based chiplessRFID tag reading,” IEEE Trans. Microw. Theory Techn., vol. 60, no.9, pp. 2925–2933, Sep. 2012.

[40] F. Blanchard, L. Razzari, and H.-C. BanduletG. Sharma, R. Moran-dotti, J.-C. Kieffer, T. Ozaki, M. Ried, H. F. Tiedje, H. K. Haugen, andF. A. Hegman, “Generation of 1.5 J single-cycle terahertz pulses byoptical rectification from a large aperture ZnTe crystal,” Opt. Exp., vol.15, no. 20, pp. 13212–13220, Oct. 2007.

[41] Y. Depeng, A. E. Fathy, L. Husheng, M. Mahfouz, and G. D. Pe-terson, “Millimeter accuracy UWBpositioning system using sequentialsub-sampler and time difference estimation algorithm,” in IEEE RadioWireless Symp., 2010, pp. 539–542.

[42] F. Izquierdo, M. Ciurana, F. Barcelo, J. Paradells, and E. Zola, “Perfor-mance evaluation of a TOA-based trilateration method to locate termi-nals in WLAN,” in 1st Int. Wireless Pervasive Comput. Symp., 2006,pp. 1–6.

[43] W. Murphy and W. Hereman, “Determination of a position in threedimensions using trilateration and approximate distances,” ColoradoSchool of Mines, Golden, CO, USA, Tech. Report MCS-95-07, 1995.

[44] C. Dionisio, “Detection and localization of lost objects by SAR tech-nique,” in Int. Geosci. Remote Sens. Symp., 1996, vol. 1, pp. 28–30.

[45] F. Reichenbach, A. Born, D. Timmermann, and R. Bill, “A distributedlinear least squares method for precise localization with low com-plexity in wireless sensor networks,” presented at the 2nd IEEE Int.Distrib. Comput. Sensor Syst. Conf., San Francisco, CA, USA, 2006.

[46] N. C. Karmakar, R. V. Koswatta, P. Kalansuriya, and R. E. Azim,“Time domain based chipless RFID reader,” in Chipless RFID ReaderArchitecture. Norwood, MA, USA: Artech House, 2013, pp. 93–118.

Rubayet-E-AzimAnee (S’11) received the B.Sc. de-gree in electrical and electronics engineering fromthe Bangladesh University of Engineering and Tech-nology, Dhaka, Bangladesh, in 2009, and is currentlyworking toward the Ph.D. degree in electrical engi-neering atMonash University, Melbourne, VIC, Aus-tralia.Her research interest includes chipless RFID tag

design, digital electronics, signal processing, and an-tennas.

Nemai C. Karmakar (S’91–M’91–SM’99) receivedthe Ph.D. degree in information technology and elec-trical engineering from the University of Queensland,St. Lucia, QLD, Australia, in 1999.He possesses 20 years of teaching, design, and

research experience in smart antennas, microwaveactive and passive circuits, and chipless RFIDs inboth industry and academia in Australia, Canada,Singapore, and Bangladesh. He is currently an Asso-ciate Professor with the Department of Electrical andComputer Systems Engineering, Monash University,

Melbourne, VIC, Australia. He has authored or coauthored over 230 refereedjournal and conference papers, 24 refereed book chapters, and three edited andone coauthored book in the field of RFID. He has two patent applications forchipless RFIDs.

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Chipless RFID Tag Localization