# A Routing Protocol for Multisink Wireless Sensor Networks in Underground Coalmine Tunnels
###### Abstract  
Traditional underground coalmine monitoring systems are mainly based on the use of wired transmission. However, when cables are damaged during an accident, it is difficult to obtain relevant data on environmental parameters and the emergency situation underground. To address this problem, the use of wireless sensor networks (WSNs) has been proposed. However, the shape of coalmine tunnels is not conducive to the deployment of WSNs as they are long and narrow. Therefore, issues with the network arise, such as extremely large energy consumption, very weak connectivity, long time delays, and a short lifetime. To solve these problems, in this study, a new routing protocol algorithm for multisink WSNs based on transmission power control is proposed. First, a transmission power control algorithm is used to negotiate the optimal communication radius and transmission power of each sink. Second, the non-uniform clustering idea is adopted to optimize the cluster head selection. Simulation results are subsequently compared to the Centroid of the Nodes in a Partition (CNP) strategy and show that the new algorithm delivers a good performance: power efficiency is increased by approximately 70%, connectivity is increased by approximately 15%, the cluster interference is diminished by approximately 50%, the network lifetime is increased by approximately 6%, and the delay is reduced with an increase in the number of sinks. multisink; power control; coalmine tunnels; routing protocol +
Footnote †: journal: Sensors  
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hailage roadway). Coalmining tunnels can be categorized according to their function: haulage roadways, main return airways, development roadways, and preparation roadways. Haulage roadways are used for transportation and act as the main ventilation source for the entire mine, and their dimensions and circumstances are superior to those of other tunnel types; therefore, WSNs can be used. This paper therefore focuses on the haulage roadway as the research objective. If a tunnel has only one sink, information collected by the sensor nodes is transmitted from inside the tunnel to the sink at the tunnel entrance; the sink then transmits all of the data to the control center on the ground. However, the WSN used in underground coalmine tunnels is different from that of normal WSN in several ways. First, tunnels contain various types of complicated elements, such as methane, dust, and explosive gases so it is very difficult to transfer data reliably. In addition, it is impossible to change the batteries of sensor nodes in tunnels; therefore, the development of prolonged network lifetime is a primary research focus. Second, the coalmine tunnel is usually more than several kilometers long; therefore, if only one sink is located at the entrance of the tunnel, it cannot be guaranteed that the cluster head nodes, which are located far from the sink, will be able to transmit data to the sink. Furthermore, these cluster head nodes have large energy consumption and thus require a large amount of energy, which decreases the network's lifetime. Third, in relation to the cluster head selection, the principle of rotation needs to be adopted to avoid premature damage to the cluster heads. In addition, the distance between the nodes and the sink needs to be considered with respect to residual energy to ensure that the sensor nodes that are located near the sink (and which have higher energy) can become the cluster heads. Existing studies on the use of WSNs in coalmines have focused on the single-sink network structure. However, there is a major problem inherent in this type of structure: the sensor nodes, which are located far from the single-sink, are unable to promptly transmit data.