Wireless Underground Sensor Networks

Supported by NSF CAREER Grant CNS #0953900

Wireless Underground Sensor Networks (WUSNs) constitute one of the promising application areas of the recently developed wireless sensor networking techniques. WUSN is a specialized kind of WSN that mainly focuses on the use of sensors at the subsurface region of the soil. For a long time, this region has been used to bury sensors, usually targeting irrigation and environment monitoring applications, although without wireless communication capability. WUSNs promise to fill this gap and to provide the infrastructure for novel applications. The main difference between WUSNs and the terrestrial WSNs is the communication medium. In fact, the differences between the propagation of electromagnetic (EM) waves in soil and in air are so significant that a complete characterization of the underground wireless channel was only available recently.

Potential applications of WUSNs.

Despite its potential advantages, the realization of WUSN is challenging and several open research problems exist. The main challenge is the realization of efficient and reliable underground wireless communication between buried sensors. To this end, underground communication is one of the few fields where the environment has a significant and direct impact on the communication performance. More specifically, the changes in temperature, weather, soil moisture, soil composition, and depth directly impact the connectivity and communication success in underground settings. Hence, characterization of the wireless underground channel is essential for the proliferation of communication protocols for WUSNs.

Communication Channels of WUSNs

Types of Communications in WUSNs.

Although its deployment is mainly based on underground sensor nodes, a WUSN still requires aboveground devices for data retrieval, management, and relay functionalities. Accordingly, three different communication links exist in WUSNs based on the locations of the transmitter and the receiver:

• Underground-to-underground (UG2UG) Link: Both the sender and the receiver are buried underground and communicate through soil. This type of communication is employed for multi-hop information delivery.

• Underground-to-aboveground (UG2AG) Link: The sender is buried and the receiver is above the ground. Monitoring data is transferred to aboveground relays or sinks through these links.

• Aboveground-to-underground (AG2UG) Link: Aboveground sender node sends messages to underground nodes. This link is used for management information delivery to the underground sensors.

To characterize the communication channels in WUSNs, we have been conducting experiments to measure channel path loss at different test beds and under different soil moisture conditions. We have a test bed at SCAL center of the University of Nebraska-Lincoln in addition to a small test bed at the city campus. The data gathered there help us to understand the attenuation in soil as well as the impact of soil moisture.

Channel Model Experiments

Antenna Design for WUSNs

The antennas designed for over-the-air communications are not suitable for WUSNs. We design antennas specifically for the soil medium to improve the performance for the communication.

Autonomous Irrigation with WUSNs

One promising application of WUSNs is precision agriculture. Sensor motes are buried underground to provide soil condition information, such as soil moisture, temperature, to center pivot system, which can then adjust water usage in irrigation. Thanks to the help from UNL's Biological Systems Engineering professor Dr. Suat Irmak, we are conducting experiments with a center pivot system in a corn field at South Central Agriculture Lab in Clay Center, NE.

A Mote Attached to a Center Pivot System.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.