## 1 Introduction  
With the help of the rapid development of micro-electro-mechanical systems (MEMS), various types of sensors and actuators can be forged into an incredibly small size. Together with the state-of-the-art wireless communication technologies, wireless sensor networks (WSNs) are now playing an indispensable role in environment monitoring, target tracking, disaster rescuing and industrial process control [1]. Wireless sensor networks deployed in coal mines are assigned to gather environment temperature, humidity, seismic, gas leak information, _etc._ With this information at hand, we are able to evaluate the safety issues more precisely and provide workers with more reliable working conditions. However, due to the built-in nature of wireless sensor nodes, they are prone to different types of failures. Therefore, it is still not an easy task to ensure that these sensor nodes work properly over a long time. Since sensor nodes deployed in an underground environment may suffer from certain unattended ordeals, such as high temperature and humidity, the reasons accounting for failures of them may vary a great deal. Amongst all of these reasons, running out of energy supply takes up a large percentage of the reasons for node failure incidents. Sensor nodes deployed in coal mines may be installed on the ceilings of mine tunnels or in walls; it is somehow difficult, if not impossible, to replace their batteries when their energy is depleted. Meanwhile, the energy harvesting techniques introduced by researchers may not be applied easily, since light, wind and ambient radio are all scanty commodities in an underground environment. Many other research works have been conducted in order to spend every bit of energy more wisely and efficiently to balance the communication load among all sensor nodes and to prolong the lifetime of WSNs. All of these works will be discussed in Section 2. In this article, we discuss the corresponding issues of wireless rechargeable sensor networks that are implemented in coal mines with wireless energy transfer technique to prevent sensor nodes from energy depletion. As for wireless rechargeable sensor networks, this means that sensor nodes are equipped with certain wireless energy-receiving devices, such that they are able to receive energy charging remotely from a wireless power transferring source. The wireless energy transfer is not a brand-new concept. Early in the 20 th century, this technique was advocated by a great inventor, [PERSON]. At the turn of the 21 st century, the world witnessed the tremendous development of the inductive wireless power transfer technique. [PERSON] _et al._ from the Massachusetts Institute of Technology (MIT) proposed a new way to transfer energy remotely in a non-radiative way by means of the same magnetic resonance frequency among the transferor and the transferees. This technique is pretty suitable for wireless sensor networks in coal mines due to several reasons. Firstly, the way of wireless energy transfer is inductive, which will reduce the chance of fire accidents. Secondly, this type of wireless energy transfer nearly has no interference with this communication process. The main contribution of this article is that we discuss the optimal cross-layer optimization problem for wireless sensor networks implemented for coal mines with the wireless energy transfer technique. The sensor nodes installed in coal mines are recharged remotely by mine locomotives passing through the tunnels with wireless energy transfer devices mounted on them. In order to make these sensor nodes immune to insufficient energy supply, the compatible working schemes for wireless sensor nodes and locomotives for recharging tasks are developed in this article. In order to obtain these schemes, two optimization problems are formulated and solved in this paper.