# Methane Gas Density Monitoring and Predicting Based on RFID Sensor Tag and CNN Algorithm
## 2 RFID Sensor Design
### Wireless Sensor Design  
The block diagram of the proposed wireless sensor is illustrated in Figure 3a. It consists of three parts: a communication section, an RF front-end section, and a digital section. The communication section contains the antenna and RFID tag chip, which are employed to perform the transmission and reception of wireless signals, modulation, and demodulation. The RF front-end section, composed of a matching network, a four-stage rectifier, and a low dropout (LDO) voltage regulator, is adopted to provide the stable supply voltage for other parts. The matching network, containing an RF inductor (\(L_{m}\)) and an adjustable ceramic trimmer capacitor (\(C_{m}\)), is employed to realize the maximum energy  
Figure 1: Schematic diagram of radio frequency communication.  
Figure 2: Comparison of sensor data storage method; (**a**) traditional method; (**b**) proposed method.  
transferring efficiency. The power harvested by the antenna is an ultra-high frequency signal, which cannot be used to power the remaining part of the RFID sensor directly. Therefore, as shown in Figure 3b, a four-stage rectifier is introduced to rectify and boost the harvested power to the required direct current (DC) voltage. The LDO voltage regulator is then used to generate a stable DC voltage for other parts. The digital section, consisting of a micro-controller unit (MCU) and a methane gas density sensor, is employed to achieve sensor data and control the overall operation of the wireless sensor based on EPC G2 protocol.  
Figure 4 shows the prototype of the exploited RFID sensor tag. Aiming at the low-cost application, a dipole antenna is adopted for both communicating and energy harvesting. The antenna is etched on the FR4 substrate, the dielectric constant and thickness of which are 4.5 and 1.5 mm, respectively. The RFID chip chosen is Monza X-8K, with an impedance at 915 MHz of 19-172 \(\Omega\). The rectifier adopts a zero-bias Schottky diode SMS7630 as the switch diode for high rectifying efficiency. The NCP583 is selected as the LDO voltage regulator, which can generate 1.8 V output voltage within the input range from 1.65 to 1.83 V. The MCU adopts MSP430 FR6964, which can provide 2526 kB FRAM and a 12-bit 200-ksps Analog-to-Digital Converter (ADC). This MCU only consumes 118 \(\upmu\)A/MHz at 1.8 V supply voltage in the measurement mode. The employed methane gas density sensor is MQ-4, which can detect the methane gas density within the range of 300-10,000 ppm.  
Figure 4: The introduced four-stage rectifier.  
Figure 3: Proposed radio frequency identification (RFID) sensor tag: (**a**) architecture of the RFID sensor tag; (**b**) proposed four-stage rectifier.  
Figure 5 shows the whole procedure of monitoring and predicting methane gas density. The proposed RFID sensor tag is adopted to collect the methane gas density data, and the CNN is employed to extract robust features from the raw data. After that, the least squares support vector regression (LS-SVR) is established based on the extracted features. Then, the established LS-SVR is used to predict the methane gas density.