# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 3 Key Supporting Technologies for the WMSNs
### Coverage-Enhancing Algorithms  
Due to the extreme and changing conditions of coalmines, deploying conventional wired networks is difficult and costly. For WMSNs, a successful deployment could enable tremendous savings in operational expenses because of enhanced communications and quality-of-service (QoS). Nevertheless, a hostile radio frequency environment (e.g., non-line-of-sight and multipath problems caused by heavy moving machinery and rock faces) and interference generated by heavy equipment are still challenges for engineers when considering deployment or coverage problems.  
Coverage-enhancing issues, which are about how to deploy sensor nodes to maximize the coverage range with the guarantee of QoS, are basic problems for researching WMSNs [39]. Measuring the coverage performance of the network can help us find monitoring holes, thus guiding us to adjust the deployment of sensor nodes or add new nodes to improve the coverage performance of the system. Therefore, it is not simply an issue of deployment, but an issue concerning about QoS.  
Based on the monitoring requirements, the sensor nodes of the WMSNs for underground coalmine can be classified into three types: video or image sensors, voice communication sensors and environment parameter monitoring sensors. Thus, the WMSNs for underground coalmine, made up by these sensors, are heterogeneous and multifunctional coverage networks. Currently, the research work on the coverage issues of WMSNs is concentrating on coverage-enhancing algorithms of homogeneous networks. Area coverage, target coverage, barrier coverage and connectivity coverage are main topics for the coverage issues. The coverage issue of heterogeneous networks, by contrast, is more difficult for research because we should consider the heterogeneous characteristics of sensor networks such as the different sensing models, the different information content of monitoring data and the difference between sensors' resources or capacities [40]. Further studies are still necessary for the coverage issue of heterogeneous networks, considering that the existing research achievements of that are still less.  
The deployment problem of the monitoring sub-net mainly lies in the video surveillance sensors and environment parameter monitoring sensors. The voice communication sensors can be deployed according to the requirements of voice communication, thus not being our focus. Owing to the difference between the functions and the qualities of these two sensors, research work should be conducted separately.  
Figure 4: Main research issues for underground coalmine WMSNs.  
Preliminary research work on the deployment problem of WMSNs in confined space like underground coalmine have been shown in [7; 8]. As shown in Figure 5, [PERSON] and [PERSON] [7] defined the effective coverage issue for underground coalmine WMSNs, considering characteristics of underground mine roadways and sensing models, and then enhanced the coverage performance by using improved virtual potential field method. Although the research work was aimed at homogeneous WMSNs, it still makes sense since the QoS of underground coalmine is boosted by focusing on coverage issues. In short, the problem of combining video surveillance sensors and environment parameter monitoring sensors to form an optimized data forwarding network structure under the requirements of coverage and QoS, need further studies for the sake of improving the efficiency and performance of networks. Providing safety guarantee and full-scale monitoring and communication service are the basic requirements for coverage issues.