# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 3 Key Supporting Technologies for the WMSNs
### Cross-Layer Design  
In traditional WSNs, the transmission strategies supporting multimedia streams, proposed by different layers, are generally confined to one single layer and ignore the interaction between layers. The network resource is restricted mutually by the physical layer, MAC layer and network layer: in physical layer, the interference at the receivers affect the multi access of nodes in wireless channels; the bandwidth for forwarding nodes assigned by MAC layer, has influence on the performance of detecting useful signals in physical layer; low bandwidth and high packet delays introduced by transmission schedules compel networks to change their routing, and the selection of routing protocols alter link scheduling schemes, thus affecting the performance of MAC layer. Therefore, cross-layer design is needed to improve the network performance, such as QoS, energy consumption and channel utilization rate.  
The cross-layer design of WMSNs is a set of methodologies which combine the optimizing strategies of different layers and make use of potential improvements of exchanging information between different layers of the communication stack, to efficiently transmit multimedia data streams, though with the constraints of network resource and QoS. In [51], a new cross-layer communication architecture based on the time-hopping impulse radio ultra-wide band (TH-IR-UWB) technology is introduced, whose objective is to reliably and flexibly deliver QoS to heterogeneous applications in WMSNs, by leveraging and controlling interactions among different layers of the protocol stack according to applications requirements. In [52], a novel cross-layer framework for QoS support in WMSNs is proposed, which maximizes the capacity of the deployed network to enhance the number of video sources by means of mapping application requirements on joint operations of application, network, link and MAC layers to achieve the desired QoS.  
Hence, according to the characteristics of multimedia content and applications for underground coalmine WMSNs, adopting cross-layer designing techniques will significantly enhance the performance of underground coalmine WMSNs. For instance, the coal dust concentration collected for application layer, can also be used by lower layers, to adjust radio output power to eliminate the interference incurred by coal dust and choose better routes. On the contrary, the channel properties can sometimes reflect the environmental parameters. For prolonging network lifetime, routing protocols will generally select the links with lower transmit powers. This information is collected by the physical layer, processed by cross-layer controller and utilized by the network layer, thus achieving power savings.  
With the object to make cross-layer designing techniques more practical, the models for cross-layer design can follow the principle of using a cross-layer controller to uniformly manage the operations and interactions among different layers of the protocol stack, although the network structure is still a hierarchical one. To minimize energy consumption and offer QoS guarantee for multimedia streams, the optimizing strategy for coding rate, data rate, and access control and so on, should alter in the light of the status of independent functional modules like UWB transceiver, source or channel encoder, and QoS scheduler.