## 3 The Integrated Sensing and Communication Platform  
### Sensor Node Architecture  
The sensor node was fully custom designed to fulfill the requirements of the application scenario and to comply with the constrains in terms of physical dimensions, measurement accuracy and energy consumption. Its architecture is reported in Figure 1 and it encompasses 3 gas sensors, two electrochemical amperometric gas sensors [23] and a catalytic gas sensor. The sensors used in this application, all manufactured by Alphasense, Braintree, UK, are the CO-A4 for the measurement of carbon monoxide concentration (CO) and the O2-A1 for the measurement of oxygen concentration (O\({}_{2}\)), while the catalytic sensor, for the detection of potentially explosive atmospheres, is the CH-A3.  
Electrochemical and catalytic gas sensors, as well as many other chemical sensors, behave linearly in the working range. The average response time (t90) for all the used sensors is less than than 15 s from producer specifications. From tests reported in Figures 2-4 we verified the sensor responses. The response times, for the \(CO\) sensor tested from 0 ppm to 28 ppm of CO in air, for the O\({}_{2}\) sensor tested from 20.9% to 10% of O\({}_{2}\) in nitrogen and for the explosive gas sensor tested with steps of 0.2% of methane in air, do not exceed what expected and for all the tested sensors is in general around 10 s. This induces a time delay which is relatively small and aligned with commercial gas detecting systems performance. Considering the application scenario, for example a small room or an operator entering into a tank, the sensor response time is smaller than the time required to a gas to fill the room volume or the time an operator takes to enter into a tank. Hence, this aspect does not represent a critical issue for the developed architecture as it is actually a delayed or unreliable data transmission.  
Figure 1: Sensor node architecture.  
Figure 2: Catalytic Sensor test.  
The digital part of the node is based on a low power ARM microcontroller from STMicroelectronics (STM32 LQT5), that embeds 12 bits Analog to Digital Converters (ADCs) and Digital to Analog Converters (DACs) used to acquire signals from the sensors front end and to provide the correct biasing of electrochemical sensors. The microcontroller can save and read data from an Secure Digital (SD) card memory storage for logging purposes and to load sensors calibration parameters. The node sends data through the LoRa radio channel exploiting an RFM95 transceiver by HopeRF, interfaced through an SPI bus.  
Figure 4: O\({}_{2}\) Sensor test.  
Figure 3: CO Sensor test.  
In Figure 5, the current absorption of the sensor node microcontroller and LoRa transceiver parts is shown. The measurement was taken during a sensors data acquisition phase followed by a 20 bytes LoRaWAN packet transmission using SF = 7. From this plot, it is possible to measure the maximum time delay from the sensor data acquisition phase end and the radio transmission start. The measurement was obtained by clocking the microm controller at 32 MHz, acquiring data from the ADC channels and processing them. The acquisition and processing time (few milliseconds) is negligible with respect to the LMIC stack packet elaboration, however, this time delay is below 50 ms.  
The final prototype of the node with gas sensors is reported in Figure 6.