Link quality analysis of wireless underground sensor networks

Abstract

Wireless Sensor Networks have received significant attention due to their capability for distributed sensing at relatively low cost, in applications which range from environmental to industrial monitoring. More recently, wireless underground sensor networks have been proposed for applications such as personnel tracking in underground mines and moisture monitoring for precision agriculture. Wireless underground sensor networks are typically categorised into wireless sensor networks for mines and tunnels (which communicate overthe- air) and Subsoil Wireless underground sensor networks (which communicate wirelessly through soil). For Subsoil wireless underground sensor networks specifically, it is well known that the soil medium introduces a number of challenges. Firstly, the path loss in soil is very high. Secondly, propagation characteristics are dependent on soil conditions and properties, which can change due to irrigation or rain. Thirdly, communication in Subsoil Wireless underground sensor networks takes place over three different types of channels: underground-to-underground, aboveground-to-underground and underground-toaboveground. Therefore, communication protocols developed for over-the-air wireless sensor networks are not suitable for wireless underground sensor networks. Although some studies on wireless underground sensor networks have focussed on channel characterization, many aspects related to link characteristics have not been thoroughly investigated. Understanding the link behaviour in wireless underground sensor networks is a fundamental building block for protocol development for medium access, topology management and routing. The aim of this research is to gain insight into the link quality in wireless underground sensor networks which can aid in the development of efficient and reliable communication protocols. To this end, the link quality in the three wireless underground sensor network communication channels is characterized empirically for dry and wet soil conditions. This characterization is performed using the received signal strength, link quality indicator and packet reception rate. The results show that links in the underground-to-underground channel are very stable (in terms of received signal strength) and exhibit high symmetry and high packet reception rate, even for received signal strength values near the receiver sensitivity, but the communication ranges are limited due to the very high attenuation. On the other hand, links in the aboveground-to-underground /underground-to-aboveground channels are typically asymmetric and have longer communication ranges. For most links in all three channels, it is observed that the link quality indicator is highly variant and is not correlated with received signal strength and packet reception ratio. Furthermore, an increase in the soil moisture also affects the link asymmetry and the width of the transitional windows in the aboveground-to-underground/underground-to-aboveground channels. The results show that efficient communication protocols for wireless underground sensor networks will have to be highly adaptive/reactive to soil conditions, and link quality estimation has to be robust to the asymmetry present in most links.