## 1 Introduction  
There is a high demand for multimedia monitoring such as video and environmental parameters in underground coal mine production operations [1; 2; 3; 4]. However, the existing wired monitoring systems cannot meet the requirements of safe production in underground coal mines, since there are a large number of blind monitoring areas. An underground wireless sensor network (WSN) can be an important supplement to a wired monitoring system due to its characteristics of flexible and rapid deployment. Therefore, the development of wireless sensor network (WSN) technology which is adaptive to underground channel conditions has an important theoretical and practical value for safety monitoring in underground coal mines. Compared with wireless channels on the ground, underground wireless channels experience much more serious multipath fading along confined tunnels and the transmission conditions are much worse [5]. In underground coal mines, there are numerous challenges for applying WSNs such as long distances and unreliable data transmission [6]. The transmission rate of the WSN based on ZigBee is low, and it cannot satisfy the requirements for multimedia monitoring of an underground coal mine. Besides, with ZigBee the transmission distances are short and the reliability is low in underground coal mines [7; 8]. Different from the wireless communication on the ground, the space, time and frequency resources in an underground coal mine are open, and they can be utilized efficiently for wireless transmission based on MC-CDMA to improve the quality of wireless transmission in underground coal mine tunnel [9]. The confined space of underground tunnel appears to be a banded structure [1; 2; 3]. Due to the different transmission distances to the sink node, it turns out that the channel conditions are better for some wireless sensor nodes while they are poorer for other wireless sensor nodes. A cooperative transmission strategy in which sensors with good channel conditions assist sensors with poor channel conditions is proposed to achieve space diversity gain which will improve the overall wireless transmission performance [10; 11]. Therefore, a WSN based on the time-frequency coded cooperative MC-CDMA method is constructed for underground coal mines to improve the wireless transmission performance and reliability. The channel fading problem is different for sensors at different locations. Therefore, the orthogonality between the MC-CDMA subcarriers is damaged to some extent, which will lead to MAI and influence the BER performance of the WSN. In order to solve this problem, multi-sensor detection (MSD) is adopted to improve the BER performance of the WSN for underground coal mines. Optimal multi-sensor detection (O-MSD) requires the detection of all possible bit sequences at the receiving sensor to find the best sequence that maximizes the objective function [12; 13; 14]. Theoretically, the O-MSD detection performance is optimal. However, the complexity of O-MSD increases exponentially with the number of sensors, which is contrary to the energy saving requirements of wireless sensors. Decorrelating multi-sensor detection (D-MSD) is a suboptimal detection approach where the receiving sensor demodulates the sending message of each transmitting sensor by using the channel state information (CSI) and spreading codes of transmitting sensors [15]. The complexity of D-MSD is lower than that of O-MSD, but its BER is much higher than that of O-MSD [16, 17]. In order to make the detection performance of the time-frequency coded cooperative MC-CDMA WSN close to that O-MSD with relatively low complexity, a kind of multi-sensor detection with particle swarm optimization based on D-MSD (D-PSO) was proposed which optimizes the output of D-MSD at cooperative sensors and sink nodes with particle swarm optimization [18, 19, 20]. With this method, the BER performance of the proposed WSN for underground coal mines is improved significantly with relatively low complexity.