FSS inspired polarization insensitive chipless RFID tag

This article has been accepted and published on J-STAGE in advance of copyediting. Content is final as presented. IEICE Electronics Express, Vol.*, No.*, 1–6

FSS inspired polarization insensitive chipless RFID tag Muhammad Sohail Iqbal1 , Humayun Shahid1a) , Muhammad Ali Riaz1 , Shahid Rauf1 , Yasar Amin1 , and Hannu Tenhunen2,3 1

ACTSENA, Department of Telecommunication Engineering, University of Engineering and Technology, Taxila-47050, Punjab, Pakistan 2 iPack VINN Excellence Center, Royal Institute of Technology (KTH), Isafjordsgatn 39, Stockholm, SE-16440, Sweden 3 TUCS, Department of Information Technology, University of Turku, Turku-20520, Finland a) [email protected]

Abstract: A polarization insensitive, compact, fully-passive bit encoding structure exhibiting 1:1 resonator-to-bit correspondence is presented. Inspired by frequency selective surface (FSS) based microwave absorbers, the structure readily operates as a chipless radio frequency identification (RFID) tag. The unit cell is composed of several concentric hexagonal loops. Finite repetitions of the unit cell constitute the proposed RFID tag in its entirety. The required bit sequence is encoded in the frequency domain by addition or omission of corresponding nested resonant elements. A functional prototype is fabricated on a commercial-grade grounded FR4 substrate, occupying a physical footprint of 23 × 10 mm2 while offering a capacity of 14 bits. The proposed tag boasts a minuscule profile, and demonstrates polarization insensitivity as well as stable oblique angular performance. Keywords: chipless RFID, radar cross section, data encoding circuit Classification: Microwave and millimeter wave devices, circuits, and systems References

©IEICE 2017 DOI: 10.1587/elex.14.20170243 Received March 12, 2017 Accepted March 28, 2017 Publicized April 28, 2017

[1] U. H. Khan, et al.: “Novel chipless displacement sensor circuit using spurline resonator,” IEICE Electron. Express 13 (2016) 20161008 (DOI: 10.1587/elex.13.20161008). [2] H. Huang, et al.: “RFID Tag Helix Antenna Sensors for Wireless Drug Dosage Monitoring,” IEEE J. Transl. Eng. Health Med. 2 (2014) 1 (DOI: 10.1109/JTEHM.2014.2309335). [3] P. Soboll, et al.: “A Multitude of RFID Tags: A Broadband Design for Stackable Applications,” IEEE Microw. Mag. 18 (2017) 107 (DOI: 10.1109/MMM.2016.2616196). [4] M. A. Islam and N. C. Karmakar: “A Novel Compact Printable DualPolarized Chipless RFID System,” IEEE Trans. Microw. Theory Techn. 60 (2012) 2142 (DOI: 10.1109/TMTT.2012.2195021). [5] M. W. Gallagher, et al.: “Mixed orthogonal frequency coded SAW RFID tags,” IEEE Trans. Ultrason., Ferroelect., Freq. Control 60 (2013) 596

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(DOI: 10.1109/TUFFC.2013.2601). [6] M. A. Islam and N. Karmakar: “A compact printable dual-polarized chipless RFID tag using slot length variation in ’I’ slot resonators,” IEEE Microwave Conference (EuMC), European (2015) 96 (DOI: 10.1109/EuMC.2015.7345708). [7] F. Costa, et al.: “A Chipless RFID Based on Multiresonant HighImpedance Surfaces,”IEEE Trans. Microw. Theory Techn. 61 (2013) 146 (DOI: 10.1109/TMTT.2012.2227777). [8] S. Rauf, et al.: “Triangular loop resonator based compact chipless RFID tag,”IEICE Electron. Express 14 (2017) 20161262 (DOI: 10.1587/elex.14.20161262). [9] M. Martinez and D. V. Weide: “Compact slot-based chipless RFID tag,” IEEE RFID Technology and Applications Conference (RFID-TA) (2014) 233 (DOI: 10.1109/RFID-TA.2014.6934234). [10] A. Vena, et al.: “Chipless RFID tag using hybrid coding technique,” IEEE Trans. Microw. Theory Techn. 59 (2011) 3356 (DOI: 10.1109/TMTT.2011.2171001).

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Introduction

Chip-based Radio Frequency Identification (RFID) tags find enormous utility in a number of contemporary applications including wireless sensing [1], medicine tracing [2] and smart logistics [3]. This market dominance is an outcome of numerous benefits such as non-line-of-sight communication, simultaneous tag detection, longer read range and higher interrogation rate. Yet, chip-based RFID tags remain expensive for widespread deployment in commonplace scenarios [4]. Chipless variants of RFID tags offer an affordable alternative for deployment in low-end applications, replacing barcode technology. Chipless RFID tags are categorized under two main classes namely time domain and frequency domain-based tags. Within time domain, Surface Acoustic Wave (SAW) tags have been proposed. Complex manufacturing process inhibits mass market adoption of SAW-based tags [5]. In frequency-domain, chipless RFID tags use radiating structures to transform ID information into a distinct frequency signature. The radiating structures typically include metallic scatterers and reflective strands, including I-shaped [6], rectangular [7], triangular [8] and circular resonators [9]. The performance of chipless RFID tags is generally hampered by presence of harmonic resonances and existence of direct proportionality between encoding capacity and tag size [6]. Chipless RFID tags based on I-shaped and triangular resonators are not optimized for arbitrary polarization of interrogating electromagnetic wave [6, 8], whereas rectangular and circular resonator based tags remain low on bit density [7, 9]. This letter proposes a novel frequency-domain based chipless RFID tag that offers higher bit density while retaining polarization insensitivity and design compactness. The proposed tag encodes ‘1’ and ‘0’ bits by means of absorbing and reflecting properties of the resonators’ peculiar structure, making it operationally identical to a finite-sized frequency selective surface.

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Resonant Element Design

The proposed fully passive chipless RFID tag is the functional equivalent of an FSS-based microwave absorber. However, unlike conventional FSS structures, only a few repetitive instances of the unit cell are sufficient to constitute the tag in its entirety. The tag is etched out of a copper sheet realized over a thin substrate layer, and is also provisioned with a metallic ground plane.

Fig. 1. Resonator design and surface current distribution

A regular hexagon shaped resonator, shown in Fig. 1(a), is evolved into a multi-resonant data-encoding structure functioning as a fully passive, chipless RFID tag. The surface current distribution of a single hexagonal resonator, when illuminated by a horizontally polarized wave at 10.35 GHz, is illustrated in Fig. 1(b). The distribution depicts maximum concentration of surface current around edges A and B, indicating the existence of inductive characteristics. The same tends to be minimal around vertices C and D, signifying the presence of capacitive effects. The simultaneous existence of inductive and capacitive components distributed along a single hexagonal loop generates resonance at a particular value of frequency. The resonance is readily identifiable as a pronounced dip in the radar cross section (RCS) response, representing a ‘1’ data bit. When there is no resonance associated with the structure, total reflection of the impinging electromagnetic waves takes place signifying a ‘0’ data bit. By meticulously designing a multi-resonant circuit based on this phenomenon, numerous information bits can be embedded in the same structure without the need of a dedicated silicon-based chip.

Fig. 2. Design symmetry and polarization insensitivity

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The resonator element geometry offers three prime advantages: compactness of tag design, ease of structural nesting and incorporation of geometrical symmetry. As shown in Fig. 2(b), a regular hexagonal resonator, being both equilateral and equiangular, exhibits six reflection and six rotational symmetries. The peculiar symmetry imparts polarization insensitivity characteristics to the resonator. The RCS response of a single hexagonal resonant element with l =4.6mm for 1 × 2 configuration is depicted in Fig. 2(a). It can be observed that the frequency signature of the hexagonal resonator is readily identifiable for different polarization values of the interrogating wave using H and V probes. The geometrical symmetry of the hexagonal resonator renders the resulting tag insensitive to polarization characteristics of the impinging electromagnetic waves. 3

Encoding Circuit Design and Tag Optimization

A loop-based design strategy is the key to store multiple information bits within a small physical tag footprint. The geometrical dimensions, as well as the optimal number of nested resonant loops, are determined iteratively R R R using CST MICROWAVE STUDIO (CST MWS ). The optimal number of nested resonant loops per unit cell is seven. The dimensions obtained after parametric optimization of the hexagonal resonators are enlisted in Table I. Table I. Optimized dimensions of 7-bit encoding circuit (mm)

L1 5.2

L2 4.6

L3 4.0

L4 3.4

L5 2.8

L6 2.2

L7 1.6

Both the thickness of the hexagonal resonators and the spacing between them are kept at 0.26 mm. The overall dimensions of the tag are 22 × 10 mm2 . The unit cell is replicated twice, forming a 1 × 2 configuration. The proposed tag is realized on a commercially available FR4 substrate having a thickness of 1.6 mm provisioned with a metallic ground plane. The fabricated prototype, placed beside a one euro coin and a meter rod for the purpose of size comparison, is shown in Fig. 13(a). As evident from the illustration, the proposed chipless RFID tag offers a compact physical footprint.

Fig. 3. 7-bit and 14-bit tag design and size comparison

A method for increasing bit-carrying capacity of the data encoding cir4


cuit is employed. An altered version of the tag is developed by positioning hexagonal resonators side-by-side, this time around with an offset equal to 0.3 mm. The formulated technique essentially doubles the informationbearing capacity of the previously designed tag, without a noticeable increase in the physical footprint. The capacity-enhanced tag is depicted in Fig. 3(b) and dimensions are specified in Table II. With the modified tag, seven additional resonances have been accommodated in the same spectral range. The capacity-enhanced version of the tag occupies overall dimensions of 23 × 10 mm2 , and remains both conveniently compact and portable. Table II. Optimal dimensions capacity-enhanced 14-bit tag (mm) (mm)

M1 5.5 M8 3.4

M2 5.2 M9 3.1

M3 4.9 M10 2.8

M4 4.6 M11 2.5

M5 4.3 M12 2.2

M6 4.0 M13 1.9

M7 3.7 M14 1.6

The capacity can be increased further by adding more resonant elements to the exterior of the nested loops, at the expense of an increase in tag size. 4

Results and Discussion

This section presents an account of electromagnetic performance descriptors for the formulated chipless information encoding circuit. Computer-based R R simulations have been performed using CST MWS .

Fig. 4. Angular stability and RCS for two configurations

Fig. 4(b) demonstrates the computed RCS response for two variants of the same 7-bit tag when impinged upon by electromagnetic waves over a common spectral range. The comparison is performed by overlaying the RCS response of the same tag obtained for the 1×2 and 2×2 configurations. The increase in unit cell repetitions results in a larger RCS being obtained. The phenomenon can be capitalized upon to enhance the interrogation distance of the proposed tag, at the cost of larger overall tag dimensions. The chipless RFID tag is investigated for its RCS performance at different angles of incidence in Fig. 4(a). The tag demonstrates stable oblique angular performance up to 60 degrees, allowing for interrogation in a slanted orientation. 5


Fig. 5. Measured vs Computed RCS for 14-bit tag The proposed 14-bit tag is probed for its real world performance. Experimental setup for carrying out measurements is the same as devised in [10], and involves two similar linearly-polarized horn antennas, vector network R analyzer (VNA) R&S ZVB-20 and multiple tag prototypes under test.The measured back-scattered RCS response of the fabricated tag prototypes, encoding both repetitive and alternating bit sequences, is presented in Fig. 5. The simulated and measured results exhibit good overall agreement. Table III. Comparison with other shaped resonators Resonator Bit density shape (bit/cm2 ) Hexagon 6.08 I slot [6] 4.10 Rectangular [7] 0.55 Triangular [8] 1.21 Circular [9] 3.80

No. of bits 14 16 05 10 09

Tag Size (cm2 ) 2.30 3.90 9.00 8.25 2.36

Average measured RCS (dBsm) -27 -27 -21 -22 -28

Table III compares the proposed tag with other shaped resonators. The proposed tag encodes a maximum of 14 data bits while occupying a compact physical footprint of 2.3 cm2 . The resulting bit density is, therefore, convincingly higher. Furthermore, with an average RCS of -27 dBsm, the proposed tag offers comparable RCS response. Moreover, the proposed tag also exhibits characteristics of polarization insensitivity. 5

Conclusion

A novel, compact, fully-passive, FSS-inspired chipless RFID tag is proposed. The peculiar geometrical symmetry of the resonant elements renders the resulting tag polarization insensitive. Data carrying capacity of 14 bits has been achieved over compact dimensions of 23 × 10 mm2 . The tag offers 1:1 resonator-to-bit correspondence and is operable at various incident angles. 6

Acknowledgment

We thank UET, Taxila for ACTSENA research fund and NIE, Islamabad.

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