# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 3 Key Supporting Technologies for the WMSNs
### MAC Protocols  
Guaranteeing the bandwidth for transmitting multimedia data over wireless links, while ensuring the high transmission quality with limited sensor capabilities, are key factors for the network design of WMSNs. Media access control protocols, as the direct controller of sending and receiving all the data packets and the controlling packets over wireless channels, define the regulations of how to share media between the sensors to ensure the satisfactory network performance. Therefore, applying MAC protocols to efficiently use wireless channels are one of the crucial factors for guaranteeing the communication of WMSNs.  
MAC protocols for WMSNs can be mainly categorized into three types: contention-based protocols, contention-free single channel protocols, and contention-free multi-channel protocols [4; 41]. Contention-based protocols, using random timers and a carrier sense mechanism to avoid collisions, generally cannot guarantee the real-time requirements and support multimedia real-time data transmission. On the other hand, the main idea of contention-free protocols is assigning non-interference time slots or channels for each sensor, thus avoiding the collisions caused by sharing the same wireless channel. Contention-free protocols can achieve high link-level throughput and fairness. Unfortunately, most of the existing protocols, are not designed or improved for multimedia services transmission to offer the guarantee of QoS.  
Some of the existing studies [11; 42; 43; 44] on solving the MAC issues in coalmine WSNs do not take the real-time requirement into consideration, since only scalar data flow in the networks and the functionality of emergency multimedia communication may not be included in the initial design. Contention-based protocols were adopted in these works. To the author's knowledge, there is little information available in the literature regarding MAC protocols for coalmine WMSNs; thus, further studies are still essential.  
Recent years have seen many newly designed MAC protocols for WMSN that made effort to support multimedia services and satisfy the QoS requirements of users. In the paper [45], a new QoS-based sensory MAC protocol, which not only adapts to application-oriented QoS, but also  
Figure 5: Effective coverage issue for WMSNs of coalmine and the directional adjustable sensing model.  
attempts to conserve energy without violating QoS-constraints, is proposed. In comparison to other existing sensory MAC protocols, the proposed protocol is capable of providing lower delay and better throughput, at the cost of reasonable energy consumption.  
When designing MAC protocols for coalmine WMSNs, tradeoffs between the additional overhead of contention-free protocols and the energy savings realized through collision elimination of contention-based protocols need to be explored to definitively ascertain whether a single MAC protocol or a hybrid one should be used [46]. Meanwhile, the real-time requirement is also an important factor in WMSNs since there are more multimedia applications in the network. Therefore, according to the structure characteristics of the underground coalmine WMSNs, and the ways of channel assignment for MB-OFDM-UWB and factors mentioned above, further research work need to be done to design efficient MAC protocols that adapt to the QoS requirements of WMSNs for underground coalmine.