## 5 Future Trends and Perspectives
### More Simple and Reliable  
The issue is of interest for existing real-world systems for the following question: How far can the tag signal be heard and correctly decoded in a real environment? It has obvious implications for privacy and security of current RFID deployments but is also an important input for the design of novel distributed systems based on low-cost Rx-only devices [190]. The reader design is based on COTS components-notably the Universal Software Radio Peripheral (USRP) and the GNU Radio signal processing toolkit. The USRP is a low-cost, general purpose RF front end for SDR development that interfaces with a standard PC via USB, with nearly signal processing being performed on the host using GNU Radio software [74]. The usage of this platform makes the access of physical layer and integration with other spectrum easy [191].  
Chipless RFID tags and systems are not new [192]. A chipless RFID tag can be fabricated on flexible substrates by printing technologies using conductive inks because it does not include bulky chips but only a metal pattern as an antenna so that fabrication cost goes much further down [193]. Meanwhile, printability of the tags on stretchable substrates is also desired to enable the RFID tag to be conformable to any surface [42]. Sensor-based chipless structure rolling as a monolithic construction can be mounted (or implanted) on safety critical structures as a smart-skin. A major challenge for the chipless tags is the generation of UID. The frequency division, time division, spatial division, even phase division can be used to generate the ID, each of which has its own advantages and disadvantages. Furthermore, 2D structures (patterns), e.g., meta-surface [194], frequency selective surface (FSS), even absorber based chipless antenna and their printing manufacturing are developed for this purpose. However, while some bits of the ID code are used to transmit the value of the sensed parameter [195], the performance for RCS measurement is dependent on several factors, e.g., environment, polarization [196], calibration. These limit the achievable read range and reliability [197]. Above all, the metal-mountable design and anti-collision for multiple chipless RFID tags are still big challenges. In addition to the advantages of chipped sensors, this type of sensor has potential for future integrated smart multi-sensing and monitoring because of its ultra-low-cost and ability to work in extreme environments.  
The sensing ability of hazardous and flammable substances in the environment has received much attention because of the demands of various application fields, such as disaster prevention, home automation, healthcare, and advanced traceability systems [198]. At the same time, passive antenna sensor technology at an absence of electronic device allows for the inspection and monitoring in areas that are dangerous for humans to carry out activities, for example, energy systems (e.g., oil and gas,nuclear plants, off-shore renewables, etc.) and infrastructure (e.g., bridges, roads, and rail). Based on the fact that the dielectric constant of a ceramic material monotonically increases versus temperature, a chipless RFID tag was applied to design temperature sensor reliably working in harsh environment, e.g., inside the combustion chamber of gas turbines with a temperature as high as 1000 \({}^{\circ}\)C [199]. Using a high-Q Zr0.85n0.2 TiO4 (ZST) dielectric resonator and without patterned electrodes or metallization, a sensitivity of \(-4500\) ppm for the resonant frequency shift was achieved at the range of 200-700 \({}^{\circ}\)C in a 1.2-m distance [200].