# MB-OFDM-UWB Based Wireless Multimedia Sensor Networks for Underground Coalmine: A Survey
## 3 Key Supporting Technologies for the WMSNs
### Multimedia Encoding in Application Layer  
Multimedia encoding techniques are important techniques in WMSNs which are directly related to video surveillance. The basic objectives of their design are as follows: (1) achieve high compression efficiency to effectively limit bandwidth and energy consumption, with the guarantee for the quality of video surveillance; (2) have low complexity and power consumption to enable multimedia encoders to be embedded in sensor devices and to prolong the lifetime of sensor nodes; and (3) provide robust and error-resilient coding of source data. For typical state-of-the-art video compression standards, such as MPEG, H.263 and H.264, which employ the motion estimation functionality at encoders, the encoder is five to ten times more complex than the decoder [53]. Thus, the low-complexity and low-power requirements for the sensor nodes in WMSNs could not be satisfied.  
Distributed source coding (DSC) adopts the technique of combining intra-frame encoding with inter-frame decoding, which leverages knowledge of the source statistics at the decoder and reduces the complexity of coding at the encoder, making itself extremely suitable for WMSNs [54]. DSC has the characteristics of easy encoding and complex decoding, and are highly complementary with the traditional video encoding whose characteristics are reversed. In recent years, the research focus is on the distributed source encoding techniques based on Wyner-Ziv encoding, such as Pixel-domain Wyner-Ziv encoding and Transform-domain Wyner-Ziv encoding [53; 55]. A rate compatible punctured turbo code (RCPT), which is often used in Wyner-Ziv encoder and decoder, is proposed in [56]. In [57], a distributed video coding method for constraining the number of feedback requests to a fixed maximum number of N requests for an entire Wyner-Ziv frame is proposed, achieving great trade-offs between time delay of the system and the complexity of encoding. Other research works on distributed source coding in sensor networks can be found in [54].  
Though great effect has been achieved by these distributed video coding techniques, most of the recent research papers are still confined to discussing the theory of coding and decoding itself, leaving lots of problems to be explored and discussed in the area of combining the practical application of WMSNs.  
Since video distortions, felt by end users, depend on source coding and channel coding, multimedia encoding techniques for coalmine WMSNs should take the factors like channel coding techniques for coalmine environment into consideration. Besides, the environments of coalmines are generally with poor light and more coal dust, which have great influence on the quality of video surveillance, video streams in coalmines present different statistic features from traditional surficial applications, and these features should be utilized by encoding techniques. In addition, network topology may also effect the implementation of the Wyner-Ziv video coding algorithm [58].  
Therefore, the video surveillance techniques should be associated with the MB-UWB wireless transmission technique, the transmission characteristics of wireless channels in coalmine tunnel, and the temporal and spatial relationship between video sensors in coalmine. Moreover, the distributed video coding techniques should be designed to meet the requirements of video surveillance for underground coalmine.