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.
Dissertation (MEng)--University of Pretoria, 2015.