Wireless Signal Propagation Using Magnetic Induction in Underground Sensor Network

Abstract

The wireless underground sensor networks (WUSNs) and the terrestrial wireless sensor networks differ in the aspect of signal propagation medium. The underground sensor network is a challenging environment for wireless communications since the propagation medium consists of soil, rock and water. The former signal propagation techniques use the EM wave suffers from high path loss, dynamic channel condition and large antenna size. To overcome these disadvantages we go for a MI (Magnetic Induction) technique. Based on the channel analysis, the MI waveguide technique is used to reduce the high path loss. The MI transmission technique requires low operating frequencies and lesser power than the ordinary RF frequencies. But the lower operating frequencies requires high antenna size used to transmit and receive signals at 300 MHz .Based on the analysis we increase the transmission range to a larger extent than the existing range with the help of low antenna size. In this paper, we first provide a detailed analysis on the path loss and the bandwidth of the MI communication channel in underground environments. 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. In particular, the MI transmitter and receiver are modeled as the primary coil and secondary coil of a transformer. Multiple factors are considered in the analysis, including the soil properties, coil size, the number of turns in the coil loop, coil resistance, operating frequency. The analysis shows that the ordinary MI systems have larger transmission range but lower bandwidth than the EM wave systems. However, neither the ordinary MI system nor the EM wave system is able to provide enough communication range for practical WUSNs applications. Motivated by this fact, we develop the MI waveguide technique to enlarge the communication range. In this case, some small coils are deployed between the transmitter and the receiver as relay points, which form a discontinuous waveguide. It shows that the MI transmission is not affected by soil type, composition, compaction, or moisture content, and requires less power and lower operating frequencies than RF transmission. However, the theoretical/experimental results show that the communication range is no more than 30 inches (0.76 m). Moreover, the bandwidth of the MI system is not considered in the paper.