## 3 Communication Issues and Solutions
### Backscatter Communication and Measurable Parameters
One indirect measurable parameter, i.e., the differential RCS or \(\Delta\text{RCS}\), can be expressed as:  
\[\Delta\text{RCS}=\frac{\Delta\text{P}_{\text{received}}}{\text{P}_{\text{in}} \text{G}_{\text{R}}^{2}}\frac{(4\pi)^{3}\text{d}^{4}}{\lambda_{0}^{2}\eta_{ \text{P}}}. \tag{3}\]  
Here, \(\text{P}_{\text{in}}\) is transmitted power input to the terminal of the reader antenna, \(\text{G}_{\text{R}}\) is the gain of the reader antenna, \(\lambda_{0}\) is the free space wavelength at operating frequency, \(d\) is the distance between the reader and tag antennas, and \(\eta_{\text{P}}\) is the polarization mismatch between the two antennas. Assuming the precision (number of bits) of analog-to-digital converter (ADC) is B, the \(\Delta\text{P}_{\text{received}}\), also known as received signal strength indicator (RSSI), at the antenna connection can be given out by [77]:  
\[\text{RSSI}=10^{-\frac{\text{C}_{\text{rf}}}{10}}\frac{1.2567\times 10^{4} \text{V}_{\text{c}}^{2}}{(2^{28})\text{R}}\left(\frac{1}{\text{N}}\sum_{n=0}^ {\text{N}-1}\left|\text{Y}_{\text{I or Q}}[\text{k},\text{n}]\right|\right)^{ 2}\text{mWatt}. \tag{4}\]  
Here, \(R\) is the input resistance of ADC and \(\text{V}_{\text{c}}\) is the input chip level, \(\text{G}_{\text{rf}}\) is the analogue gain from antenna connector to ADC input, and \(\text{Y}_{\text{I or Q}}[\text{k},\text{n}]\) is the \(n\)-th sample at the ADC output of \(I\)- or \(Q\)-branch within single \(k\). Meanwhile, the forward power to activate tag, i.e., \(\text{P}_{\text{in}}^{\text{to}}\), can be expressed as:  
\[\text{P}_{\text{in}}^{\text{to}}[\Psi(\theta,\ \phi)]=\left(\frac{4\pi \text{d}}{\lambda_{0}}\right)^{2}\times\frac{\text{P}_{\text{th}}}{\text{G}_{ \text{R}}(\theta,\ \phi)\text{G}_{\text{T}}[\Psi(\theta,\ \phi)]\tau[\Psi]\eta_{\text{P}}}, \tag{5}\]  
where \(\text{P}_{\text{th}}\) is the minimum incident power needed to activate the tag chip (also called read sensitivity), \(\text{G}_{\text{T}}\) is the gain of the tag antenna, \(\tau=1-\left|\Gamma\right|^{2}\) is power transmission coefficient, and \(\Psi\) represents the defect variable. Here, \(\Gamma\) is the reflection coefficient at the matching state. It is worthy to note that both the \(\text{G}_{\text{T}}\) and \(\tau\) are dependent on the defect while the former depends on the orientations as well. Typical passive RFID systems suffer from round-trip path loss; specifically signal-to-noise ratio (SNR) at the receiver drops with the fourth power of reader-to-tag distance, for a two-ray propagation model [78]. Compared with the counterpart of near-field communication (NFC), UHF antenna sensors use standing wave (or evanescent mode) for sensing and propagation mode for communication. The direct usage of the direct measurement quantities, e.g., amplitude [79] and phase [80], is inevitably influenced by the wireless channel. Furthermore, because of the limited receiver's sensitivity as well as ADC's resolution, the resolution of the passive antenna sensor systems decreases as an increase of the read range. The challenges and related solutions will be introduced in Section 3.2. At the same time, people are trying to reconstruct the antennas' parameter by combining several quantities together and obtain the impedance or other robust sensing variables. This part will be introduced in Section 4.3.