## 2 Detection System Architecture and Signal Characteristics  
### System Architecture of Vehicle Detection  
The vehicle detection system is composed of a LoRaWAN (LoRa Wide Area Network) gateway, vehicle detectors and a cloud server. The vehicle detector is made up of a battery ER34615M produced by Ultralife company, Shenzheng, China, a geomagnetic sensor PNI11096 produced by PNI company, Santa Rosa, CA, USA, a FM radio module SI4708 produced by Silicon Labs company, Austin, TX, USA, a LoRa module SI1278 produced by Semtech company, Camarillo, CA, USA, and a microcontroller STM32L052 produced by STMicroelectronics company, Geneva, Switzerland. The detection system architecture and installation diagram are shown in Figure 1.  
The PNI11096 is a low cost magnetic measurement chip produced by U.S. PNI company and connects with the microcontroller through a SPI interface in our circuit board. The chip can control and measure three independent magneto-inductive sensors and each sensor is individually selectable for measurement. The power consumption is only 1 \(\upmu\)A at idle mode and 1.5 mA at sampling mode [15]. Therefore, the chip is very suitable for the power-constrained wireless vehicle detectors.  
The SI4708 is a tiny FM radio receiver chip produced by Silicon Labs company, Austin, TX, USA. It requires only 6.25 mm\({}^{2}\) of board space and a minimum component count. The SI4708 has highly flexible functionality catering to the subjective nature of audio preferences and variable FM broadcast  
Figure 1: Vehicle detection system architecture and installation diagram.  
environments worldwide. Two-wire slave-transceiver and three-wire interfaces are provided for the microcontroller to read and write the control registers of the chip. The RSSI (Received Signal Strength Indicator) level will be available by reading from the bits RSSI [7:0] of register 0 AH when the seek operation completes [16].  
The SX1278 transceivers feature the long range modem that provides ultra-long range spread spectrum communication and high interference immunity whilst minimizing current consumption. The chip can achieve a sensitivity of over \(-148\) dBm using a low cost crystal and bill of materials. The high sensitivity combined with the integrated +20 dBm power amplifier yields industry leading link budget making it optimal for any application requiring range or robustness [17]. The LoRa modulation technique also provides significant advantages in both blocking and selectivity over conventional modulation techniques, solving the traditional design compromise between range, interference immunity and energy consumption. Therefore, the chip is widely used in IoT (Internet of things) devices. In our application, it is programed by the microcontroller STM32L052 through SPI (Serial Peripheral Interface) interface.  
The ultra-low-power STM32L052 microcontroller with the high-performance ARM Cortex-M0+ 32-bit RISC core and the maximum operating frequency can up to 32 MHz [18]. The microcontroller not only undertakes the collection of geomagnetic signals and electromagnetic wave signal strength also undertakes the analysis and processing of these data to form the vehicle detection results and transmits these results to the LoRaWAN gateway through SX1278 chip.  
The LoRaWAN gateway, as a core device, is installed within the effective communication distance of the vehicle detectors. The LoRaWAN gateway will transmit the collected vehicle detection results to the cloud server through the Internet. Finally, the results will be displayed on various terminals.