## 2 Background and Related Work
### WSN Deployment
#### 2.4.3 Coverage and Connectivity  
Two concepts are fundamental when talking about deployment: coverage and connectivity. Coverage is the ability of a network to monitor an entire area of interest and can be considered a quality measure in the WSN [37]. In the mining context, this means ensuring that every RoI of the mine is within range of at least one sensor node (e.g., work front and transport tunnels). Connectivity refers to the ability of nodes to communicate with each other and sink nodes. Coverage and connectivity are essential issues in the WSN [38] and IIoT. Therefore, the optimal sensor deployment strategy aims to achieve a network connection while optimizing coverage. By optimizing coverage, deployment strategies ensure that sensor coverage covers the optimal areas of the sensor field, as the fundamental application requires. By ensuring that the network is connected, sensor information can be transferred to other nodes and possibly to centralized base stations to make decisions [38]. Previously, the communication range was obtained by carrying out campaign measures in a real mine. The objective of the experiment was to evaluate how the error metrics PER (packet error rate), BER (bit error rate), and RSSI (received signal strength indicator) as a function of distance are impacted in an underground mine tunnel environment. The measurement campaign was executed in the 433 MHz band because it is within the ISM band (industrial, scientific, and medical) to avoid interference with the radio communications equipment and alarm systems; in addition, the United States Mine Safety and Health Administration (MSHA) suggests using frequencies between 150 MHz and 6 GHz for communications in coal mines. This recommendation is because high-frequency signals face more significant attenuation than low-frequency signals [14; 15]. We propose to design, develop, and implement an IIoT system, shown in Figure 2, that allows the monitoring of environmental variables in mining environments. The solution comprises static nodes in different mine locations and mobile nodes that the miners would carry. The nodes would communicate using a WSN and be sent to external monitoring centers over the Internet. The monitoring centers will be located in safety entities and mine administrators. For the experiment, a WSN was designed with a point-to-point linear topology. The WSN comprises a receiver node and three transmitter nodes, which emulate the deployment of a network for monitoring and supervising gases in an explosive atmosphere in a coal  
Figure 2: Methodology general process. mine. The action-taking starts when the receiving node sends a start signal to the sending nodes, and they respond to the packets in turns according to a timer set to 3 s. Packets are transmitted to the node closest to the receiver, then the second node in range, and finally, the third node at a maximum distance of up to 5 m. Each node transmits a total of 20 packets of 264 bytes in size. The packets are received by the receiving node and forwarded to a computer to be processed and stored. For each location, 100 measurements were taken at the receiver, for a total of 2000 readings averaged to represent the level of the parameter of interest. Measurements were collected under the line of sight (LOS) and NLOS conditions and divided into three parts. The first part of the measurement campaign was carried out between 15 and 25 m because the initial tests at the mine found a reliable transmission range of up to 15 m. Some environmental factors were identified from this distance, representing limitations in the range between nodes. The measurements showed a packet and bit error rate of 0% and a RSSI between \(-\)83 and \(-\)99 dBm. For the second part, nodes located at 30, 35, and 40 m, significant losses were observed in the third node. Due to the above, locating nodes in the 35 to 40 m range was decided. In the third test, nodes situated at 36, 37, and 38 m, a PER of 15%, 25%, and 90%, respectively, were observed, as well as the BER of 7.1%, 3.45%, and 59.51%.