## 3 Communication Issues and Solutions
### Backscatter Communication and Measurable Parameters  
The EPC C1G2 standard defines communication between RFID readers and tags in the UHF band [73]. As determined by this protocol, the communication between the readers and tags is reader initiated [74]: The reader first sends out continuous wave (CW) to activate a subset of the tags in its interrogation region and then a query (downlink) asking the tags to respond with their IDs; for the uplink (assuming that the tag IC remains powered), the tag chip alters the reflection coefficient of tag antenna by varying its internal impedance (\(\mathrm{Z_{L}}=\mathrm{R_{L}}+\mathrm{jX_{L}}\)) so as to enable re-radiation of the readers CW signal (backscatter modulation). The configuration of a passive antenna sensor and system based on this mechanism is shown in Figure 4. In order to maximize the efficiency of WPT, the tag antenna is designed to be conjugately matched with the input impedance of tag chip at its centre frequency. The reflection coefficient, \(\Gamma\), which accounts for the impedance mismatch between the tag chip \((\mathrm{Z_{L}}=\mathrm{R_{L}}+\mathrm{jX_{L}})\) and the tag antenna \((\mathrm{Z_{A}}=\mathrm{R_{A}}+\mathrm{jX_{A}})\) with \(\mathrm{Z_{A}^{*}}\) being its conjugate, is given by:  
\[\Gamma=\frac{\mathrm{Z_{L}}-\mathrm{Z_{A}^{*}}}{\mathrm{Z_{L}}+\mathrm{Z_{A}}}. \tag{1}\]  
The tag's antenna reflects an amplitude or phase shifted version of the incident signal, where the amount of shift is governed by the antenna's loading [75]. Assuming the antenna load's reflection coefficient is \(\Gamma_{0}\) or \(\Gamma_{1}\) respectively corresponding to bit '0' or bit '1', the captured signal in the reader due to the variation of RCS can be denoted as RCS\({}_{0}\) or RCS\({}_{1}\). The antenna can be a regular antenna fabricated with conventional dielectric materials or coated with functionalized materials in the passive antenna sensor system. The defect directly or indirectly changes the electric property of the antenna sensor, corresponding to its impedance variation. The reader (interrogator) can actively and wirelessly monitor the antenna parameters via wireless channel based on RCS. Then, features are extracted from the backscattered signal and used to detect and characterize the defect. The main purpose of the modulator is therefore to modulate the interrogation signal received by the tag antenna so that the signal backscattered by the tag antenna, i.e., the antenna backscattering, can be separated from the signals backscattered by the surrounding structures, i.e., the structural backscattering [27]. _This is also the major difference between chipped and chipless antenna sensors._  
In order to provide a physical insight about the above interference, the influence of sensing signal via communication and coherent I/Q demodulation was analytically studied with respect to the power and phase measurements in [76]. The derivation procedures are thereby neglected, and the results are directly given out. One can directly measure both power and phase of the received tag signal as follows:  
Figure 4: A passive antenna sensor system based on backscatter communication. \[\Delta\text{P}_{\text{received}}=\frac{\text{I}_{\text{AC}}^{2}+\text{Q}_{\text{AC}}^ {2}}{\text{Z}_{0}},\ \varphi=\arctan\frac{\text{Q}_{\text{AC}}}{\text{I}_{\text{AC}}}, \tag{2}\]  
where \(\text{I}_{\text{AC}}\) and \(\text{Q}_{\text{AC}}\) are the difference signal in the period \(T\), \(\text{Z}_{0}\) is the input impedance of the receiver, e.g., 50 \(\Omega\).