| Power supplies for sensor nodes are limited and hard to replace in wireless sensor networks(WSNs), so energy efficiency has become one of the most key issue for extending the network lifetime. Because of the unbalanced energy consumption over different parts of WSNs, the entire network becomes disconnected that is major causes of affecting the network lifetime. To prolong the lifetime of WSNs, we can use mobile nodes to balance the energy expenditure of sensor nodes. In practical environment, the monitored region may contain obstacles of any shape and size. A research challenge is how to efficiently dispatch mobile nodes to finding an obstacle-avoiding shortest path. This paper considers the energy-efficient scheduling problem of mobile nodes in WSNs, especially in WSNs with obstacles. The main contributions are summarized as follows:(1) According to the LEACH(low-energy adaptive clustering hierarchy) protocol, this paper proposes an energy-efficient routing mechanism based on the cluster-based method for the mobile node in WSNs. In the cluster-based algorithm, all sensor nodes in the network are divided into two categories: cluster heads and cluster members. In general, cluster heads consume more energy than cluster members. We can balance energy consumption of sensor nodes by using the cluster-based method for the mobile node. Therefore, compared with the LEACH protocol, the cluster-based method for the mobile node can prolong the network lifetime significantly. By applying the improved nearest neighbor algorithm, the mobile node can find a shortest route and collect data from cluster heads. Simulation results verify the effectiveness of our method.(2) Due to the complexity of the scheduling problem in the monitored region with obstacles, the paper proposes a cluster-based method for the mobile node based on spanning graphs. In physical environments, all kinds of obstacles could exit in the monitored region. Therefore, a research challenge is how to efficiently dispatch the mobile sink to find an obstacle-avoiding shortest route. By using the grid-based technique, we obtain regulation shape of obstacles on which the spanning graphs of the cluster-based method are constructed. Here, it points out that the shape of obstacles cannot be changed really. The essential reason for the regularization shape of obstacles is that the moving trajectory of mobile nodes will be changed. It is convenient to efficiently dispatch mobile nodes when they move in a regular graph. With the search space of the mobile node from all grid cells to the spanning graph, the scheduling for the mobile node will become more efficient. Hence, we can finally find an obstacle-avoiding shortest route for the mobile node by the algorithm of one by one revision of two sides. Through simulation, the validity of our method is verified.(3) This paper presents the grid-based scheduling for the mobile node in WSNs with obstacles. According to the grid-based technique, we divide the monitored region into the same size grid cells. When the mobile node stays the geometric center of the grid cell, it can collect all sensing data from sensor nodes inside the same grid cell. Based on the spanning graph from grid cells, we can find an obstacle-avoiding shortest route for the mobile node. Through simulation, the network lifetime of the grid-based scheme is longer than that of the cluster-based scheme.(4) For mobile sensors with limited energy, this paper proposes an efficient scheduling mechanism based on connection graphs. In a hybrid WSN, static sensor nodes monitor the environment and report where events appear in the monitored region. Then, mobile nodes are dispatched to these event locations to perform in-depth analysis. With the progress of energy harvesting technology, the lifetime of static sensor nodes is assumed long enough. Therefore, the network lifetime is determined by the lifetime of mobile nodes. To prolong the network lifetime, mobile nodes must be efficiently dispatched to find an obstacle-avoiding shortest path. To remedy this issue, we propose an efficient scheduling mechanism based on connection graphs in this paper. Simulation results verify the effectiveness of our method based on connection graphs. |
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