# 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
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **UWB** & **Wi-Fi** & **ZigBee** \\ \hline IEEE Standardization & 802.15.3a [1] & 802.11 & 802.15.4 \\ Maximum Transmission distance & Up to 10 m & 200 m & 10–100 m \\ Frequency range & 3.1–10.6 GHz & 2.4 GHz, 5 GHz & 868/915 MHz, 2.4 GHz \\ Data rate & 480 Mbps & 250 Mbps & 250 kb/s \\ Energy consumption & High & High & Low \\ Normalized energy consumption & Low & Low & High \\ Complexity & Medium-High & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of UWB, Wi-Fi and ZigBee. Above all, constructing wireless multimedia monitoring network for underground coalmine based on MB-OFDM-UWB wireless transmission technology is a promising and ideal way to realize coalmine multimedia surveillance, making full use of UWB's characteristics of high speed, low power consumption and so on, and underground coalmine's characteristic of open frequency resource. As a new-generation wireless communication technique, the ultra-wide band (UWB) is the key technique to realize high speed wireless communication [31]. UWB provides wireless connection solutions for consumptive electronic devices. It enables a variety of applications to run on a general platform, realizing a high-speed and interoperable wireless multimedia communication. The data transmission rate of UWB is extremely high, from 100 Mbps to 500 Mbps within ten meters, and can even reach the level of Gbps [32]. At the same time, UWB also has varieties of characteristics, such as high spectrum efficiency, robustness to multi-path, low transmit power, high system security, simple structure, low costs and so on. The initial UWB technique did not use the carrier but the ns-ps level non-sinusoidal pulses with very short duration for data transmission [31]. Because of the high peak energy of narrow pulse, UWB using narrow pulses is not suitable for dangerous environments, such as the underground coalmine [33]. At present, two mainstream solutions of UWB are Multi-Band Orthogonal Frequency-Division Multiplexing (MB-OFDM) and Direct-Sequence Code Division Multiple Access (DS-CDMA). These two solutions are both at the bottom layer i.e., physical layer of the whole UWB structure, but each has its own characteristics [34]. The MB-OFDM solution, led by Intel, Texas Instruments (TI) and other enterprises, and supported by the WiMedia Alliance which is made up by more than 200 enterprises all over the world including HP, Microsoft, Nokia, and Philips, is especially suitable for a variety of wireless multimedia application. On the other hand, the DS-CDMA solution, also known as the DS-UWB solution, is mainly led by Motorola and Freescale, and pushed by the UWB forum. Based on the MB-OFDM solution, the WiMedia UWB platform has become the first international UWB commercial standard. Figure 3: Comparison of UWB, Wi-Fi and ZigBee in normalized energy consumption by choosing four typical wireless products from [27]. The MB-OFDM-UWB solution divides the whole bandwidth into several sub-bands, which will enable data processing over a much narrower bandwidth. Additionally, it has also inherited the advantages of the multi-carrier modulation, such as the abilities to efficiently eliminate the inter-symbol interference (ISI) owing to multi-path propagation, and the complexity of the receivers. Therefore, the MB-OFDM-UWB solution is well-suited for the multi-path propagation environment like coalmine. The communication environment for underground coalmine is a typical representation of the confined space. The walls of laneways bring in plenty of refraction and scattering paths, resulting in the complex multi-path propagation of signals in laneways [35]. The delay spread caused by multi-path propagation can be seen as a selective fading of frequency in the frequency domain.