Energy efficient and robust communication in wireless underground sensor networks for autonomous irrigation management

Precision agriculture (PA) refers to a series of practices and tools necessary to correctly evaluate farming needs. The accuracy and effectiveness of PA solutions are highly dependent on accurate and timely analysis of the soil conditions. In this dissertation, wireless underground sensor networks (WUSNs) are employed in precision agriculture to monitor soil moisture in real time. To this end, the challenges in WUSNs, including the lossy communication medium due to the high attenuation of soil, the variation of the medium overtime, and the extreme energy limits of the network, are addressed. To improve the quality of underground communication, the channels are first modeled mathematically, including the communications between an underground node and an aboveground node and the communications between underground nodes. These models capture the effects of the environment, especially soil moisture, on the received power strength of the communication. Field experiments are conducted to validate the developed models. Based on the characteristics of the channel, the underground antenna is designed accordingly, considering both the dielectric properties of the soil and the reflection of the soil-air interface. It is shown that with the designed antenna, the communication distance increases more than 200%. Based on the channel model and the antenna model, the channel capacity of wireless underground communications is analyzed. It is shown that to improve the channel capacity of the system, a cognitive radio, which can adjust its operation frequency at a wide range is suitable for wireless underground communication. To prolong the lifetime of the buried network, the limited energy capacity of the underground motes is also addressed. To this end, the error control schemes and communication schemes are studied. Specifically, for the error control scheme in wireless underground communication, rate compatible low-density parity-check (LDPC) code and adaptive transmit power control scheme are analyzed. It is shown that when the transmit power of the underground node can be adjusted in a wide range, adaptive transmit power control can reduce packet error rate and increase the energy efficiency of the mote. Moreover, due to the fact that the communication is impacted by local factors, especially soil moisture, the underground node can adjust its error control scheme based only on local information. For the communication schemes, the connectivity of WUSNs is first modeled. Employing this model, communication schemes are developed to reduce energy consumption while at the same time maintain the connectivity of the network. In addition, a proof-of-concept prototype of employing WUSNs in smart irrigation is also described in the dissertation. It is shown that the communication distance of the system is sufficient for irrigation management. The results of this thesis highlight that WUSN based soil moisture sensors will be an important technology in precision agriculture