Wireless Underground Communication Networks (WUCNs) constitute one of the promising application areas of the recently developed wireless networking techniques. The WUCNs consist of wireless devices that operate below the ground surface. These devices are either (i) buried completely under dense soil or (ii) placed within a bounded open underground space such as underground mines and road/subway tunnels. In the former case, networks of wireless nodes are buried underground and communicate through soil.
Description of Underground Wireless Communication using Magnetic Induction
Underground wireless communication enables a wide variety of novel applications, including soil condition monitoring, earthquake and landslide prediction, underground infrastructure monitoring, sports-field turf management, landscape management, border patrol and security, etc. However, underground is a challenging environment for wireless communication. The propagation medium is no longer air but soil, rock and water where the well-established terrestrial wireless communication do not work well.
Using of electromagnetic waves in underground has several disadvantages. Magnetic induction(MI) seems a better choice. A detailed analysis on the path loss and bandwidth of the MI communication channel in underground environments was analysed. Then based on the analysis, we develop the MI waveguide technique for WUSNs, which can significantly reduce the path loss, enlarge the transmission range and achieve practical bandwidth for MI communication in underground environments. Here some small coils are deployed between the transmitter and the receiver as relay points, which form a discontinuous waveguide that transmit the MI waves.
Traditional wireless communication techniques using electromagnetic (EM) waves encounter three major problems in underground environments: the high path loss, the dynamic channel condition and the large antenna size. First, EM waves experience high levels of attenuation due to absorption by soil, rock, and water in the underground. Second, the path loss is highly dependent on numerous soil properties such as water content, soil makeup (sand, silt, or clay) and density, and can change dramatically with time and space. Consequently, the bit error rate (BER) of the communication system also varies dramatically in different times or positions. Third, large size antenna is necessary for the efficient propagation of EM waves. Path loss can be reduced if lower operating frequencies are used.
Magnetic induction (MI) is a promising alternative physical layer technique for underground wireless networks in deep burial depth. It can address the problems on the dynamic channel condition and the large antenna size of the EM waves techniques. The MI channel conditions remain constant. More over transmission and reception are accomplished using small coil of wire. However, MI is generally unfavourable for terrestrial wireless communication. As the transmission distance r increases, magnetic field strength falls off much faster(1/r 3 )than the EM waves (1/r 2 ) in terrestrial environment. The total path loss of the MI system is lower than the EM wave.
Taking into account all the limitations mentioned above, MI waveguide technique is developed for WUSNs, which can significantly reduce the path loss, enlarge the transmission range and achieve practical bandwidth for MI communication in underground environments.
Unlike the simple transmitter receiver model of MI communication, in MI waveguide several coils are placed between transmitter and receiver which forms a continuous waveguide.