9.2.1 Tag Design and Fabrication

Figure 9.2 shows the photograph of a 4-bit tag designed at the 21–27 GHz ultrawide band (UWB).The tag is comprised of four sets of distinct resonators thoughtfully located so that nearby resonances are not interfered. The tag is designed to occupy 4 bits in the 21–27 frequency band. The resonance frequencies are selected as 22.5, 23.5, 24.5, and 25.5 GHz as simulation shows that the 3-dB bandwidth of each resonator response at this frequency band is about 1 GHz. The diameter of the circle is inversely proportional to the resonance frequency. The diameters for the above resonance frequencies are found and displayed in Table 9.1.

The specific frequency resonators are repeated to increase the backscattered signals of the individual resonators [1]. The tag is printed in-home using MMARS Laboratory's SATO printer. This monochrome printer has a resolution of 600 dpi. The substrate is 0.09-mm-thick glossy paper with a bulk silver coating of 10 μm on top. The design is thermally transferred to the paper substrate and the fully printable silver tag is created on the paper substrate.

9.2.2 Experimental Setup

In this section, the experimental setup used to verify the performance of the tags was designed. A chipless RFID reader that reads the magnitude of the tag response was used to read the tag designed in the previous section. An experiment was set up as shown in Figure 9.3. The reader transmits a narrow banded sinusoid signal using a transmit horn antenna and as shown in the figure the receiver receives the tag response for that frequency using a receive horn antenna. The magnitude of the received signal is recorded along with the frequency of the signal sent. Similarly, the frequency is swept across the 21–27 GHz band and the complete tag response is recorded.

Figure 9.4 shows the tag response ([1111]) recorded by the chipless RFID reader. It can be seen that the four resonances occur at the designed resonating frequencies. The presence (ON) and absence (OFF) of the resonators can be selected to form 16 different tag types for a 4-bit tag as shown in Table 9.2 The circular patches having the same diameter increase the effective radar cross section and hence form a bigger frequency dip at the resonance frequency. The four resonator sets are selected such that tags with all 16 combinations can be designed. These 16 tag types are fabricated and used for testing the detection techniques derived in Chapter 10.

9.3.1 Power Divider Design

The power divider needs to divide the power equally into two branches over a broadband of 400 MHz centered at 2.4 GHz. Therefore, a symmetrical T-junction power divider is designed. The design parameters are the length and the width of each transmission line presented in the divider. The designed and fabricated T-junction power divider is shown in Figure 9.5. The power divider is designed on TLX8 substrate with a relative permittivity () of 2.4, loss tangent () of 0.004, and thickness () of 0.5 mm. Both the simulated and the experimental S parameter magnitudes versus frequency for the power divider are shown in Figure 9.6. It is clear that both simulated and measured S21 and S31 are similar and close to the required value of (half power division). The measured and simulated S11 in decibels (return loss) versus frequency is also shown in the figure. There is a difference in the two curves and this is possibly due to fabrication defects, where it shifts the operating frequency little higher. As far as the performance is concerned, the fabricated power divider still has a bandwidth of over 500 MHz at centered around 2.4 GHz. So the power divider performance is acceptable and any further tuning is not attempted. Once the satisfactory performance of the T-junction power divider is obtained, the next component designed is the monopole antenna.

9.3.2 Monopole Antenna Design

A monopole antenna is designed to operate at 2.4 GHz with a bandwidth of 400 MHz. The monopole antenna is selected due to its figure-of-eight bream radiation pattern and distinct polarization so that vertical and horizontal antennas can be used to reduce the cross talk between the transmit and receive chains of the MIMO chipless RFID tag. A microstrip-based antenna with a circular patch is used for size optimization [2]. Figure 9.7 shows the monopole antenna design in CST as well as the fabricated antenna. The design parameters are the radius of the circular patch, gap between the ground plane, and disc edge and the dimension of the feedline [3].

The performance of the monopole antennas is presented next. Figure 9.8 shows the return loss versus frequency of the antenna obtained from both simulated and measured data. It can be seen that they both agree and the antenna has an operating bandwidth of over 400 MHz at .

Figure 9.9 shows the antenna radiation pattern at 2.4 GHz, which is a figure-of-eight omnidirectional radiation pattern with the maximum gain of the main lobe with 2.7 dB, which is acceptable.

The realized gain of the antenna on the main lobe was analyzed over a bandwidth of 400 MHz centered around 2.4 GHz and the results are shown in Figure 9.10. It is clear that the realized gain is above 2.5 dB over the frequency band. These results verify the successful operation of the monopole antenna at a bandwidth of 400 MHz centered around 2.4 GHz. After obtaining satisfactory performance from the designed antenna, the spiral resonators are designed.

9.3.3 Spiral Resonator Design

A set of spiral resonators are designed to operate at the resonance frequencies given by Table 9.1. Design parameters are the transmission line lengths, widths, and the gap between the microstrip lines [3]. Figure 9.11 shows both the CST designs on spiral resonators and one of the fabricated resonators.

A spiral resonator response obtained from CST simulations is shown in Figure 9.12. It is clearly seen that both the amplitude and the phase contain the data and existing techniques mostly rely on only one of them. However, the proposed detection techniques use the information available in both amplitude and phase. A photograph of the fabricated MIMO tag with all “0” bits (no multiresonator) in branch connected to and all “1” bits in the branch connected to is shown in Figure 9.13.

9.3.4 Experimental Setup

The schematic of the experiment conducted to measure the MIMO tag response using an arbitrary waveform generator (AWG) and an oscilloscope with a high sampling rate is shown in Figure 9.14. However, antennas are replaced using cables as the sole purpose of this work is to verify the validity of the ML-based detection method.

Figure 9.15 shows the CST-generated tag response and the measured tag response for tag bits [1010]. measurements across the resonator are recorded using a performance network analyzer and converted from log scale to linear scale for comparison with the CST-generated response. It can be seen that they are closely matched.

So far the design of a MIMO-based chipless RFID tag is discussed. Its individual component performances are analyzed using the CST-simulated results and experimental results. It is clear from Figure 9.15 that there are some anomalies between the theoretical and experimental results. This may be because of the coupling between the two branches in the MIMO tag. However, the proposed MIMO-based chipless RFID system has an inbuilt mechanism to overcome this challenge. Any coupling between the antennas can be incorporated into the MIMO channel matrix. This can be performed using a calibration tag. Now the system is free from any coupling between the two branches, and with the known channel, it is possible to perform tag detection.