## 4 Sensing-Oriented Issues and Solutions
### Sensing Variables and Measurement Uncertainties  
The RF signals carrying sensed information are backscattered into the wireless channel and passive antenna sensors with the combination of sensing and communication in the system need addressing the RF channel to mitigate path loss and multipath effects [132]. In spite of analogue RFID sensors are gaining increasing attention from academic and industrial domains, their true applicability in the real world is still in question, since it is not clear whether and in which conditions the variation of the measured signals related to the sensing activity may be distinguished from the measurement uncertainties [133]. The defect-induced changes in antenna properties vary the amplitude and phase of a tag's response and sensing data can be directly or indirectly measured via the RFID reader. Similar to a pulsed eddy current technique [134], the time domain measurement, e.g., transient envelope of tag's response, has been explored to characterize corrosion in the LF and HF RFID sensor systems [49; 50]. This procedure is fast and accurate in near field range but cannot be directly used in UHF band since the extraction of such a transient information needs a high sampling rate, which is not practical to be implemented for a cost-effective receiver. Furthermore, the influence from environments becomes severe because of the scatters in the path of forward and backscattered signals. As coherent receivers can directly provide both amplitude and phase information [135], power and phase become the mostly used sensing data in the UHF band. In particular, to obtain a better consistency and communication range, one of the major challenges for wirelessly monitoring defects is to mitigate influences from the test setup and environment. For power-based measurements, sensing capability is realized at the expense of the mismatch of the tag antenna impedance and of the decrease in efficiency [136]. Good resolutions in power can be achieved by improving the quantization resolution in the receiver's ADC, but it is not feasible with low-cost readers. The tradeoff issue between sensing and communication is a major impetus for recent efforts in defect characterization via phase [137]. However, the measurement of phase heavily depends both on the propagation channel and on modulating properties of the tag which can be both frequency- and power-dependent. The modulation RCS is a concise and effective application of the mature and proven RCS concept, but some challenges arise in its use [138]: The use of modulation RCS in typical indoor deployment environments is complicated by fading that is not studied in the mature radar literature. The tag's backscatter modulation efficiency of the tag chip is also nonlinear, tending to fall sharply with increasing incident power, so the RCS must also be a function of the incident field strength. Interestingly, benefiting from the power and frequency dependent characteristics of tag chips, a differential RCS measurement significantly improves the sensitivity and increases immunity from the environment [44]. Without prior information about the tag-reader mutual position, multiple measurements can be applied for separation and reductions of multiple influences and also improving the repeatability. The drawback using power measurement has been partially solved by combining the forward power and backscattered power and a sensing variable named AID was invented for this purpose [18]. In fact, AID is only related to impedance rather than antenna gain [100]. By means of ad-hoc test-beds, it was demonstrated that backscattered power, or RSSI, exhibits a combined uncertainty ranging from 0.5 to 2 dB, with a deep dependence on the measurement instrumentation, which implies only sensors with large dynamic ranges could be used in real applications [136]. Meanwhile, for a confidence level of 95%, the measurement uncertainty on the \(\Delta\)RCS is calculated and found to be 2.27 dB or 29.8% [139].