# 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
### Main Challenges for Designing  
In recent years, the techniques of applying WSN into underground coalmines have been investigated. In [21, 22], multi-parameters monitoring and early warning systems for coalmine environment based on mesh structured WSN and hierarchy WSN were proposed by [PERSON], making the integrated monitoring of gas, coal dust, temperature and so on come true. In these works, the mode of multi-parameters collection, i.e., periodic monitoring routine combined with interrupt service, was proposed. The supporting techniques such as routing mechanism, principles of slot allocation for wireless end device and data aggregation were also designed. A routing method, combing least-hop routing protocol, which is remaining energy aware, and tree protocol, was proposed. Besides, node address recycling strategy was also proposed. The experiment results carried out both in the laboratory and underground coalmine show that the designed system can monitor coalmine environment parameters effectively. Parts of the system functions have been applied in the actual projects [11]. However, the data transfer rate of the system is very low and only some simple scalar parameter can be collected. Thus, our work helps consider to use UWB to enhance the performance of existing monitoring systems. Other researchers have also contributed a lot in different directions of this area. Their work can be referred in [18, 23, 24]. At present, as mentioned in the previous section, the research work and application of WSN for underground coalmine are mainly based on ZigBee protocol in IEEE 802.15.4 standard. The basic requirement of the data rate for transferring CIF video streams is still higher than the date rate of ZigBee. Obviously, the maximum transmission rate of ZigBee could not satisfy the higher requirements of conducting multimedia surveillance in coalmine. On the other side, introducing Wi-Fi into coalmine seems a good choice for performing video surveillance, taking the advantages that Wi-Fi has a long transmission distance and high data rates [25]. Nevertheless, the high power consumption and poor multipath performance cannot guarantee the lifetime and transmission performance of sensor nodes. Therefore, UWB is a good solution which achieves good tradeoffs between diverse network performances. A comparison among UWB, Wi-Fi, and ZigBee from the aspects of typical transmission distance, frequency, data rate, power consumption and complexity is presented in Table 1[26, 27, 28]. As shown in the table, UWB outperforms Wi-Fi and ZigBee in data rate and normalized energy, which means UWB suffices for providing capabilities to transfer video stream with the resolution of even 1080p (1920 \(\times\) 1080 pixel) while guaranteeing great energy consumption efficiency. Although the maximum transmission distance of UWB is not as long as the other two, we can actually deploy more sensor nodes in the network to make up the deficiency and to provide more flexibility for the environment like the room-and-pillar working zone. In Figure 3, the normalized energy consumption [27] of the three candidates in shown. As evident from this figure, UWB has great advantages in the normalized energy consumption which indicates that UWB is more efficient when transferring a large amount of multimedia information. In addition, except for the high data rate with low power, the great accuracy in location tracking and no additional requirements for repeater nodes compared with the other two schemes, make UWB technology very promising for underground mine communication and monitoring [29, 30].