# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 2 Overall Design of the WMSNs for Underground Coalmine and Related Works  
### Network Structure  
According to demands for comprehensive monitoring, the wireless multimedia monitoring system for underground coalmine based on Multi-Band Orthogonal Frequency-Division Multiplexing UWB (MB-OFDM-UWB) WMSNs mainly provides the following functions: (1) video surveillance for underground environments; (2) wireless voice communication; (3) multi-parameter monitoring for underground coalmine; and (4) localization of sensor nodes and targets. On the basis of the previous research and experiment works of [PERSON] et al. [11] and [PERSON] and [PERSON] [7], the structure of the wireless multimedia monitoring system for underground coalmine based on MB-OFDM-UWB WMSNs is shown in Figure 1. Details about the network structure will also be discussed in this section. As shown in Figure 1, WMSNs, consisting of wireless and interconnected smart devices such as cameras, phones and sensors, are mainly deployed in branch tunnels, and communicate with Ground Monitoring and Dispatching Center (GMDC) via optical fiber backbone network. Different from main tunnels' landforms, landforms in some of the branch tunnels, mining areas and mined-out areas, are narrow and irregular. In this condition, WMSNs, as a great supplement to CMS and WSNs, are mainly deployed where the CMS is difficult to deploy, bringing in more flexibility and abilities of comprehensive surveillance. Specifically, for branch working zone, there are two basic methods of mining coalmines: longwall mining and room-and-pillar mining [12]. In longwall mining, coal is excavated by slicing along a long wall, and the mined-out areas are allowed to collapse. Basically, the longwall mining is a continuous operation involving using self-advancing hydraulic roof supports, coal-shearers, and armored conveyors. In contrast, the working space for room-and-pillar mining is laid out in a checkerboard of mining rooms and supporting pillars. For both mining methods, the working zones are dynamically changing. For instance, the roof of mined-out areas will collapse when hydraulic roof supports are removed, thus it is uneconomic and inconvenient to use a cable monitoring system that is stationary. Therefore, WMSNs are good alternative for achieving deploying flexibility and monitoring quality. Additionally, WMSN has obvious advantages in emergency applications such as disaster relief in virtue of simple networking, easy expansibility, and stable communication. WMSNs are deployed in different local regions, and can be easily added in or removed off at any time according to varieties of monitoring requirements. Local regions have the characteristics of regional decentralization and small relevance to each other, and thus need to be monitored separately. Besides, monitoring contents and nodes configurations may vary dramatically. For instance, from the perspective of monitoring scale, the number of sensor nodes deployed in some local regions is small while that of other regions may be very large. Furthermore, the deployment of WMSNs for underground coalmine is first planned and fulfilled by workers, and that means it is a deterministic deployment instead of a random one which are usually used by WSNs. To ensure the safe operation of the WMSNs and the whole monitoring system, the system needs to be maintained and updated periodically. Figure 1: Network structure of the monitoring system for underground coalmines. Sensor nodes are the foundation of constructing WMSNs and all the protocols and algorithms concerning about sensor networks make sense when applied on sensor nodes. At present, the wireless sensor nodes designed or produced are generally classified into two categories: the first are nodes which take the general-purpose microprocessors as core components and use the design methods of embedded systems; the second are nodes designed using the platforms for designing specified device like FPGA and ASIC, such as PicoRadio [13], Multi-Radio WSN platform [14], and so on.