most common techniques used for codifying the information in chipless frequency-domain sensors, however, other phase- and polarization-based codifications have been also reported [40, 41, 42]. All these codifying approaches can be considered as analog, to mean that the sensing vari- able, that is, the RCS, changes its value continuously with respect to the variation of the monitored parameter. Nonetheless, any of these codification techniques could also be turned into digital as well, for example, based on threshold detection [43, 44].
From the interrogator point of view, the FC approach seems to be more robust than AC, since the interrogator does not need to keep a precise amplitude reference in the long-term. On the other hand, FC requires to sweep over the whole frequency range for interrogating the sensor, whereas in AC a narrower band can be employed once the exact working frequency of the sensor is defined.
Figure 1.2: Main information codification types for frequency-based chipless sensors: Amplitude-based Codification (AC) and Frequency-based Codification (FC).
f1 f2 f
f
1 2
AC
FC
Source: The author.
In order to interrogate frequency-domain chipless sensors based on the backscattered response after EM illumination, reader’s transmitter emit either a Continuous Wave (CW) or Ultra-Wide Band (UWB) pulses, as illustrated in Fig. 1.3. Readers based on CW interrogation employ common radar transceiver architectures, such as Frequency-Stepped Continuous Wave (FSCW) or Frequency-Modulated Continuous Wave (FMCW) radars [36]. In both architectures, the frequency of the signal carrier is swept and the backscattered wave from the tag is measured at each frequency step. Conversely, UWB pulsed interrogation is based on narrow RF pulses whose spectral power density usually extends from 3 to 10 GHz. In this way, the EM signature of the sensor is retrieved
from a single interrogation pulse, which is advantageous from the point of view of the total reading time [45]. Nonetheless, UWB interrogation require, in general, finer electronic components such as pulse generators and high-speed analog-to-digital converters, which generally makes them unsuitable for low-cost implementations [46], and are also prone to jitter and synchronization issues [45].
Figure 1.3: Continuous-Wave (CW) vs Ultra-Wide Band (UWB) pulsed interrogation
CW Pulse
Source: The author.
Besides the reader architecture, antennas also play an important role on the reading strategy of the whole system. Provided an applica- tion, either bistatic or one- or two-antenna monostatic configurations might be specified. In addition to this, the antenna type is selected according to the main design parameters such as gain, bandwidth, and size. Moreover, antenna polarization must be compatible to the tag polarization-related strategy for information recovery. One can classify these strategies into: co-polarization [47], dual polarization [48] and cross-polarization [49]. Among the three, co-polarization interrogation is the most common reading strategy reported in frequency-domain chipless tags. In the case of dual polarization interrogation, the tag is illuminated twice, with two orthogonally-polarized EM waves, and the information is obtained with co-polarization interrogation from both.
This strategy is usually employed for doubling the amount of informa- tion retrieved from the tag, while this must be designed appropriately for this purpose. Finally, cross-polarization reading is used for reducing the clutter interference [50]. Among these three polarization modes, co- polarization interrogation is inherently compatible with single-antenna monostatic configuration, which can be favorable to low-profile compact readers. On the other hand, cross-polarization and dual-polarization reading are often implemented with two antennas, although circular polarization [51] or carefully designed two-port dual polarized antennas may be a viable solution for reducing the reader size [52].
On the tag side, miniaturization and cost may be more easily
achieved through the usage of planar structures [53]. Planar chipless sensors are also suitable for additive fabrication processes which can reduce the fabrication cost, specifically in uniplanar (single-layer) designs [54]. Although, uniplanar tags may be more vulnerable to material- related drifts and environment compared to “grounded” structures, this issue might be controlled by considering these factors during the design stage or by tuning techniques [55]. In addition to this, additive processes allow the usage of flexible and low-cost substrates, which is favorable for fully disposable tags or even promotes the integration of the sensor with the object under monitoring [56].
Miniaturization is also directly related to the design approach adopted. According to their radiation properties, two main approaches can be distinguished. The first one refers to the ReTransmission (RT) tags [37]. Its design consider separated structures for the reception/
transmission antennae and the sensing element. As the name reveals, after reception, the incident EM wave is guided along a transmission line and modified on its way by the sensing element before it is retransmitted back to the reader. The reception and the transmission are usually accomplished by two separated cross-polarized antennas, thus requiring cross-polarizing interrogation [57].
The tags regarding the second approach are known as radiofre- quency Encoding Particle (EP), whose name refers to the fact of in- tegrating in the same structure both the antennae and the sensing element [58]. In this manner, the EPs are more suitable for miniatur- ized structures, even though there is an inherent trade-off between size reduction and RCS magnitude which may limit the reading range of the tag. Examples of chipless sensors presented in literature which followed RT and EP approaches are shown in Fig. 1.4(a) and (b), respectively.
All of the subjects discussed above regarding chipless technology are summarized in the tree diagram of Fig. 1.5. The highlighted terms in this diagram point out the specific focus of this research, e.i., frequency- domain frequency-coded uniplanar chipless sensors designed with the encoded-particle approach.In particular, this research is aimed at working with this type of sensors looking forward implementing miniaturized chipless sensors by enabling the use of low-cost materials and additive fabrication processes.
Figure 1.4: Chipless sensors following the (a) Retransmission (RT) [59] and (b) the Encoding Particle (EP) [39] approaches.
Source: The author.
Figure 1.5: Tree diagram showing the diversity of subjects regarding chipless technology and highlighting the scope of this research.
Source: The author.