# 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
Most sensor nodes are designed using the first method. The difference between the hardware of sensor nodes in WMSNs and that in traditional WSNs is that the sensor nodes of WMSNs are equipped with CMOS sensors that can ubiquitously capture images and videos, and processors that have stronger processing capacities. However, the basic components of sensor nodes in WMSNs and those in WSNs are basically the same, which can be divided into four parts as shown in Figure 2: a sensing unit, a processing unit, a transceiver unit and a power unit [4; 15]. The mobilizer unit in Figure 2, which allows movements or manipulation of objects or a sensor node itself, is optional according to the requirements for monitoring. Ignored by other surveys for WSNs and WMSNs, the electrical explosion proof issue for sensor networks is emphasized here, since the environment condition of a coalmine is usually harsh and dangerous and deserves more attention and design efforts. In addition, the issues of safety, immunity, protection, size, design, and reliability need to be considered for communication devices and equipment. In the design of hardware, the prevalent technology of applying extended interfaces to connect sensors is used, which enhances the universality and flexibility of the hardware platform. From the perspective of processors, the sensor nodes can be divided into two categories. The first category uses the high-end processors represented by ARM, mostly supports dynamic voltage scaling (DVS) or dynamic frequency scaling (DFS), and has strong video processing capabilities. The Stargate [16] developed by Inter and manufactured by Crossbow is one of the typical examples of this category. In contrast, the other category, represented by Cyclops [17] based on Micaz platform, uses low-end processors that have low processing capabilities and could only finish the tasks of capturing simple images. The wireless transceiver unit uses 802.15.4 compliant Chipcon CC2420 radio, whose maximum transmission rate is 250 kbps, seeming powerless for transmitting video streams. Then, in some application scenarios that have high real-time requirements, 802.11b wireless cards are used. However, this increases the energy consumption and seems impractical. Further research work and techniques need to be introduced to solve the problem of high bandwidth demand and resource constraints. With different sensing modules embedded into sensor nodes, variable parameters including temperature [18; 19], humidity, gas and smoke concentration [11; 18] and even seismic wave [20] can be collected. In addition, through UWB localization, which will be introduced later, we can also get the locations and movement trajectories of mines. Furthermore, with multimedia sensing modules, multimedia information like real-time videos and pictures of coalmines can also be acquired. The parameters that WMSNs can collect are not limited to these mentioned above. The schedule for choosing proper sensing units is based on the requirements of monitoring systems, and those sensing units should be compatible with other units. Figure 2: The components of a sensor node.