## 2 Structure of the Monitoring and Control System
### Wireless Communication  
WSN communication protocol plays a fundamental role in data collection and feedback control. In the selection of a wireless module, low power consumption and stability which is specific as link budget, are two considerable factors. The core component selected in the wireless module is Si446x (Eptfe, Silicon Labs, Austin, TX, USA), including Si4460/61/63/64, with a working frequency range from 119 MHz to 1050 MHz and a link budget up to 146 dB. In consideration of cooperation with other research groups, Si4463 working in 433 MHz was the final option, with a maximum output power of 20 dBm (100 mW) and transmission current consumption of 70 mA. The optional multi-band switching makes the ad hoc networks possible in future development. Under the maximum transmission power of 100 mW, the corresponding ideal transmission distance is nearly 1000 m. However, the power should be adjusted reasonably according to the RSSI value. Although, the RSSI value, which implies the link quality, is vulnerable to the application environment, such as humidity, temperature and plant shelter conditions, and reliable transmission distance. Therefore, in consideration of a comprehensive influence in the greenhouse environment, link quality, co-channel interference, and plant shelter, the RSSI value was set in the range from \(-\)70 dBm to \(-\)90 dBm. For sensor nodes, the inherent transmission parameters of wireless modules, including configuration of the module channel, transmission power, and Identification (ID) number, can be adjusted by the micro controller.  
The detailed communication process is shown in Figure 3. In most cases, the sensor nodes are kept in listening mode, waiting for the call from the coordinator, to save power, except for special real-time monitoring required conditions. With a received command, such as data measurement and transmission power adjustment, a sensor node should complete it, and then sleep again, as shown in Figure 3a. However, the coordinator node does not collect the environmental information directly and only acts as a transmission part to coordinate the data transmission between the terminal PC and sensor nodes. The related control command is also transmitted through the coordinator node as shown in Figure 3b.  
The data measured by sensor nodes were collected by the coordinator node as the input of the software platform. The control parameters determined by the related algorithm was transmitted to  
Figure 3: The flow charts of (**a**) communication process of sensor node; and (**b**) communication process of coordinator.  
the controller facility to regulate the CO\({}_{2}\) concentration. Then, the regulated CO\({}_{2}\) concentration was measured by the sensor nodes again as the second round input. The interval of this closed-loop process was adjustable according to practical requirements. A short period indicates a relative timely feedback and accurate CO\({}_{2}\) concentration regulation. As a comparison, a long period is able to decrease the power consumption. The trade-off between the control performance and power consumption should be taken into account according to large amounts of historical data and special requests of managers.