Design of a Cobweb Shape Chipless RFID Tag
A nested octagon passive chipless radio frequency identification (RFID) tag with an encoding capacity of 12 bits has polarization diversity and 1:1 slot-to-bit correspondence. Its compact 23 × 23 mm footprint yields a bit density of 2.26 bits/cm2. The tag is fabricated on a flexible Rogers RT/duroid® 5880 laminate.
RFID is an evolving technology1 that uses wireless data capturing techniques for identifying and tracking objects.2 Due to its qualities such as inexpensive fabrication, non-line-of-sight communication3 and swift reading capability, it is largely replacing barcode technology.4 RFID is penetrating the retail industry for intelligent and smart tracking5 and for the creation of smart entities in IoT environments.6
An RFID system consists of transponders generally known as tags, also called labels, and an interrogator or reader connected to RFID middleware.7 RFID tags are further classified as chip-based and chipless. Chip-based tags require dedicated silicon ICs for encoding information,8 while chipless tags need no external IC or power source, reducing the total cost of production and maintenance. Chipless tags are classified mainly as either frequency or time domain dependent. Surface acoustic wave tags are examples of time domain tags. Their manufacturing involves a complex and costly submicron photolithographic fabrication process, requiring economies of scale.9 Frequency domain tags store information in the form of frequency signatures using metallic or directly printable radiating and conducting structures.10
Due to characteristically low-cost, robustness and miniaturized footprints, frequency domain chipless RFID tags have been of most interest to researchers. These structures may vary in their shapes, geometrical aspects and underlying technologies. Different shaped radiating structures have been reported, including spurline resonators,10 rectangular resonators,11 L- and I-shaped resonators12 and triangular patterns.13 Nijas et al.14 used stepped-impedance resonators. Frequency selective surface (FSS) inspired tags with an extensive physical dimension and with a ubiquity of active elements have been reported in the literature.15 Due to the existence of higher order spectral harmonics and a direct relationship between encoding capacity and physical size, electromagnetic performance is compromised. This hampers the use of a wide frequency band, since higher order harmonics may interfere with high frequency resonances.
In this article, a compact frequency domain dependent, polarization independent, chipless RFID tag with an encoding capacity of 12 bits is described. The design offers a 1:1 slot-to-bit resonance in its radar cross section (RCS) response. This provides an encoding capacity of 12 bits or 212 distinct possible combinations, for a total of 4096 items that can be uniquely tagged.
The tag occupies a small 23 × 23 mm footprint containing 12 discrete octagonal slot-based structures. A single resonating element is shown in Figure 1. The length of a side is L, the slot width is M and the gap width at the defected vertices is G. The tag is fabricated on an ungrounded 0.508 mm thick Rogers RT/duroid 5880 substrate. The RCS response of a conventional octagonal slot resonator with no gaps has an unwanted resonance, as shown in Figure 2a. The defective gaps G on alternate vertices suppress this response while enhancing the desired resonance (see Figure 2b).
The dimensional parameters were chosen after iterative parametric analyses. For example, as L is changed from 9 to 11 mm, the resonant frequency shifts down, where the slot width M varies the absorption level of each corresponding resonant peak. Changing the defective gap G has no effect on the RCS response, although it helps reduce mutual coupling between resonating elements. The surface current distribution and RCS response of a single element illuminated at 6.87 GHz are shown in Figure 3. The surface current distribution at opposite defective points G’ is maximum, indicating inductive effects and minimum at the other vertices G”, indicating capacitive characteristics. Their simultaneous presence indicates resonance. Geometrically, the tag is equiangular and equilateral, making it polarization insensitive (see Figure 4). The frequency signature of a single resonant element is identifiable with illumination from both E and H probes.
Symmetrical nesting of the elements results in a cobweb, polarization independent, chipless RFID tag (see Figure 5a). The FSS is designed to resonate at multiple frequencies, corresponding to the resonance of each slot. A resonant peak in the RCS response signifies a 1 at a given frequency; a 0 is represented by the absence of a resonant peak. Optimal dimensions of the structure were obtained iteratively through simulation using CST MICROWAVE STUDIO®. G and M for all resonators were kept equal. Parameters L2 through L12 were computed using L1 and M, resulting in the values shown in Table 1. Fabricated samples of the tags are shown in Figure 5b.
The tag measurement setup comprised a Rohde & Schwarz R&S®ZVB-20 vector network analyzer (VNA), a pair of linearly polarized transmit and receive horn antennas and fabricated prototypes of the tag (see Figure 6).16 A resonant peak at a particular frequency because of the presence of a resonating element is interpreted as a 1, signifying complete absorption; in the case of complete reflection, i.e., no resonating element, a 0 is encoded. Adding and removing resonating elements leads to the formulation of distinct bit combinations (see Figure 7). A graphical comparison of the numerically computed and experimentally measured results shows good agreement. Repeating, alternating and random bit sequences from tags on a 0.508 mm thick Rogers RT/duroid 5880 laminate are shown in Figure 8. Table 2 lists the properties of the laminate. Copper with a thickness of 0.035 mm was used as the radiator. The encoding capacity is determined by the number of resonating slot elements to obtain an equal number of resonances in the resulting frequency band. Figure 5a shows 12 octagonal resonating elements in a cobweb shape, corresponding to the RCS response in Figure 8a for a sequence of all 1s. Each cobweb shaped resonator represents a bit, resulting in a total capacity of 12 bits. Within the operating band from 6.5 to 18 GHz, there are 12 resonances with a 1:1 resonance-to-slot correspondence. The examples shown in Figure 8 illustrate the capability of the design to encode any 12-bit sequence. The RFID tag has a small 5.29 cm2 footprint, achieving a bit density of 2.26 bits/cm2. It is insensitive to a variety of polarization angles and is viable for use on non-rigid substrates.
A comprehensive comparison of this work with other reported results is summarized in Table 3. The table includes encoding capacity, bit density, polarization insensitivity, the flexible nature of the laminate, spectral bit capacity and spatial bit density. Encoding capacity is the number of bits stored for encoding data, which is 12 in this design. Bit density is the number of bits per unit area, usually measured in bits/cm2. The readability of the tag at different orientations with reference to the XY plane is indicated by polarization insensitivity. Spectral bit density is the number of bits per GHz, and spatial bit density relates to the number of bits per λ2, where λ is the wavelength.
A passive, polarization independent, chipless RFID tag with a compact size of 23 × 23 mm and 12-bit encoding capacity provides a bit density of 2.26 bit/cm2. The design mitigates high order harmonic components and possesses 1:1 slot-to-bit correspondence, enabling a total of 4096 items to be uniquely tagged.
This work was financially supported by Vinnova, the Swedish Governmental Agency for Innovation Systems, and the University of Engineering and Technology in Taxila, Pakistan, through the Vinn Excellence Centers program and ACTSENA research group funding, respectively.
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