The frequency domain coding resides on modifying the chipless RFID transponder frequency response for a given bandwidth, allocating specific signatures to every individual chipless RFID transponder that the reader should be able to analyze and successfully identify. As per the time domain case, two different types of structures can be found in literature: frequency resonating circuits with antennas or scatters.
2.3.2.1 Chipless RFID Transponders Based on Resonating Circuits
Preradovic et. al. presented in [25] a concept considering the amplitude and phase of the spectral signature of a multi-resonator circuit, assigning a 1:1 correspondence between resonators and data bits. The geometry of the chipless RFID transponder is shown in Fig. 2.9a, it theoretically encodes a total of 6 bits, and is composed of six microstrip spiral resonators, and two microstrip UWB disc loaded monopole antennas for cross-polarized transmission and reception. By varying each spiral resonator dimensions, a new different stopband is generated, and therefore, six different frequency dips within the 2 – 2.5 GHz frequency band are achieved, which are separated around 100 MHz from each other, starting from 2 GHz.
The coding technique consist of an on-off-keying (OOK) and is illustrated in Fig. 2.9b, if a resonance is generated, it is considered as a digital 0, if the resonance is removed, then it becomes a digital 1. The resonance removal is achieved by
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short-circuiting the spiral resonators, a short circuit will generate a shift in the frequency dip locating it outside the designed chipless RFID transponder operational range, forming a digital 1, where the original resonance should have taken place [25].
The multi-resonators circuits and the antennas are fabricated on Taconic TLX-0 substrate and the chipless transponder frequency response is measured using a vector network analyzer (VNA). Another 35-bits chipless RFID transponder composed of a 35 spiral resonators circuit is also fabricated in [25], working under the same coding principle. Later in [26] and [27], the same author proposed the reader architecture illustrated in Fig. 2.10, is fabricated and used to detect two 23-bit chipless transponders based on the same spiral resonators principle. For transmission, the micro-controller generates a sequence of bits that are converted to voltages by the analog-to-digital converter (ADC) and fed to a Teledyne YIG oscillator, which generates a frequency
a)
b)
Fig. 2.9: Chipless RFID transponder based on spiral resonators: a) geometry, b) principle of encoding [25] Rx antenna Tx antenna Spiral resonators 15 mm 15 mm 75 mm -8 -7 -6 -5 -4 -3 -2 -1 0 1.5 2 2.5 3 3.5 4 4.5 5 5.5 In se rt io n Lo ss ( dB ) Frequency (GHz)
Unshorted spiral Shorted spiral Short between spirals
0 1
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sweeping signal (7 and 10.7 GHz) with 15 dBm constant power that is sent through the 10-dB coupler to the antenna to interrogate the chipless RFID transponder. The received signal through the antenna is down-converted by the mixer and sent to the gain/phase detector to be compared with the down-converted version of the transmitted one. The comparison voltages are converted to bits by the digital-to-analog converter (DAC) and sent to the microprocessor. The detection is performed using a simple peaks detector algorithm, and variations on the amplitude and phase of one single chipless transponder were detected up to 15 cm away from the horn antennas in a noise free environment.
A more compact structure based on open stubs in a microstrip transmission line and two cross-polarized transmitting and receiving disc monopole antennas is proposed in [28] and its geometry shown in Fig. 2.11a, eight different resonators are designed to produce equal number of resonating dips and bits. It is based on the OOK coding principle or a further analysis of the group delays. The chipless transponder was measured in an anechoic chamber with a programmable network analyzer (PNA) and the results are shown in Fig. 2.11b, a measurement considering only the transmission line directly connected to the PNA (no antennas) is also illustrated.
The chipless RFID transponders based on resonating circuits have the disadvantage, that they additionally required the fabrication and design of antennas, which increases
Fig. 2.10: Proposed UWB reader architecture to read chipless RFID transponders based on spiral resonators [27]
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their size and fabrication cost. Therefore, another type of structures based on scatters are also being investigated and are presented below.
2.3.2.2 Chipless RFID Transponders Based on Scattering Structures
In 2005, Jalaly and Robertson presented two works on RF barcodes using multiple frequency bands inspired by frequency selective surfaces (FSS) and the barcode technology: one approach considering between others, a chipless RFID transponder consisting of an arrays of five opaque metallic microstrip dipoles with different lengths and widths [29]. And the second one, considering an array of eleven identical split dipoles [30], both in the 5.8 GHz frequency band, fabricated on Taconic’s TLY-5 substrate, and using the same coding principle: OOK. The geometry of the work presented in [29] is
a)
b)
Fig. 2.11: Chipless RFID transponder based on open stub resonators: a) geometry, b) measured response, modified from [28]
50 mm Rx antenna Tx antenna Open stubs 30 mm 15 mm 15 mm -90 -70 -50 -30 -10 2 2.5 3 3.5 4 S21 (d B ) Frequency (GHz)
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illustrated in Fig. 2.12a, the dipole-like structures behave in a similar ways as the previous discussed structures, generating a resonant band pass or band stops frequency response. However, the author does not explain clearly how the coding is implemented physically on the UWB chipless RFID transponder (short-circuit or complete removal of a specific dipole). The network analyzer measurement results for two transponders with codes 11111 and 11010 are shown in Fig. 2.12b. The presence of a resonance dip represents a one, and its absence a zero.
The geometry of another scattering structure proposed by Vena et. al [31] is shown in Fig. 2.13a, it is based on 20 C-sections without antennas or ground plane and fabricated on FR-4 substrate. The author explores the OOK and the group delays as coding principle. To configure the chipless RFID transponders, each resonator is replaced by a conductive strip of the same dimensions. As illustrated in Fig. 2.13b, the chipless transponder produces 20 resonance peaks between the 2 – 4 GHz frequency band, which should
a)
b)
Fig. 2.12: Chipless RFID transponder based on RF barcode principle: a) geometry, b) measurement results for codes 11111 and 11010, modified from [29]
-34 -32 -30 -28 -26 -24 -22 -20 5.4 5.42 5.44 5.46 5.48 5.5 5.52 5.54 5.56 5.58 5.6 S21 (d B ) Frequency (GHz) 11111 11010
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correspond to 20 bits. Finally, the chipless transponder are measured with the help of a VNA and radar cross-section (RCS) response, which is a measure of the detectability of an object, is calculated from the measured scattering parameters and a calibration method. The RCS concept will be further explained in subsection 4.1.1.
Another similar structure based on open conical resonators is proposed by Nair et. al. in [32], a picture of the prototype is shown in Fig. 2.14a. The chipless transponder is fabricated printing silver ink on PET, it produces 12 peaks in the 2.5 – 9.5 GHz frequency
a)
b)
Fig. 2.13: Chipless RFID transponder based on C-section like scatters a) geometry and coding principle, b) simulated |RCS| for codes 11111111111111111111 and 10111111110111111101, modified from [31] 0 0 0 1 1 1 70 mm 25 mm -55 -50 -45 -40 -35 -30 |R C S | d B m 2 Frequency (GHz) 11111111111111111111 10111111110111111101 2 2.5 3.0 3.5 4.0
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band, its calculated |RCS| from the VNA measured scattering parameters and a calibration method, as well as the coding principle based on PPM is shown in Fig. 2.14b. A peak placed at a specific frequency represents a digital zero and if placed in another frequency represents a digital one. The chipless RFID transponder physical coding is achieved by the changing the length of the resonators.
Thus, different geometries consisting of squares [33], rings [34], and a very interesting approach using genetic algorithms to generate a change in the frequency response [35], are being investigated to encode the chipless transponders using any desirable characteristics: amplitude, phase, group delay, etc. The maximum coding capacity claimed for frequency domain transponders are: 35 bits for transponders based on resonating circuits plus antennas [25], and of 42 bits for scattering structures [34]. However, these capacity claims are mostly based solely on the visual inspection of the chipless transponders generated peaks or dips, obtained through a front-end or measurement equipment without the proper implementation of a computer-based detection algorithm under normal working conditions or taking the influence of the
a)
b)
Fig. 2.14: Chipless RFID transponder based on open conical resonators: a) geometry, b) calculated |RCS| from measured scattering parameters [32]
-60 -50 -40 -30 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 |R C S | d B m 2 Frequency (GHz) 000000000000 10000000000 01
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communication channel into consideration. Furthermore, under normal working conditions, the chipless RFID transponder may experience frequency response degradations due to its handling while being placed in the interrogation zone, which compromises a detection technique based solely on the position of the peaks or the dips, and the need for a more robust coding and detection techniques becomes evident [33].
Finally, a block diagram containing the classification of chipless RFID transponders according to their coding technique and structures is illustrated in Fig. 2.15.