Agnelo R. Silva

Empirical Evaluation of Wireless Underground-to-Underground Communication in Wireless Underground Sensor Networks

Many applications for irrigation management and environment monitoring exploit buried sensors wired-connected to the soil surface for information retrieval. Wireless Underground Sensor Networks (WUSNs) is an emerging area of research that promises to provide communication capabilities to these sensors. To accomplish this, a reliable wireless underground communication channel is necessary, allowing the direct communication between the buried sensors without the help of an aboveground device. However, the significantly high attenuation caused by soil is the main challenge for the feasibility of WUSNs. Recent theoretical results highlight the potential of smaller attenuation rates with the use of smaller radio frequencies. In this work, experimental measurements are presented at the frequency of 433MHz, which show a good agreement with the theoretical studies. We observe that (a) a decrease of the frequency of the wireless signal implies a smaller soil attenuation rate, (b) the wireless underground communication channel presents a high level of temporal stability, and (c) the volumetric water content (VWC) of the soil is the most important factor to adversely affect the communication. The results show the potential feasibility of the WUSNs with the use of powerful RF transceivers at smaller frequencies (e.g., 300-500MHz band). We also propose a classification for wireless underground communication, defining and showing the differences between Subsoil and Topsoil WUSNs. To the best of our knowledge, this is the first work that reports experiment results for underground to underground communication using commodity sensor motes.

Communication with Aboveground Devices in Wireless Underground Sensor Networks: An Empirical Study

Wireless Underground Sensor Networks (WUSNs) consist of wirelessly connected underground sensor nodes that communicate untethered through soil. WUSNs have the potential to impact a wide variety of novel applications including intelligent irrigation, environment monitoring, border patrol, and assisted navigation. Although its deployment is mainly based on underground sensor nodes, a WUSN still requires aboveground devices for data retrieval, management, and relay functionalities. Therefore, the characterization of the bi-directional communication between a buried node and an aboveground device is essential for the realization of WUSNs. In this work, empirical evaluations of underground-to- aboveground (UG2AG) and aboveground-to-underground (AG2UG) communication are presented. More specifically, testbed experiments have been conducted with commodity sensor motes in a real-life agricultural field. The results highlight the asymmetry between UG2AG and AG2UG communication with distinct behaviors for different burial depths. To combat the adverse effects of the change in wavelength in soil, an ultra wideband antenna scheme is deployed, which increases the communication range by more than 350% compared to the original antennas. The results also reveal that a 21% increase in the soil moisture decreases the communication range by more than 70%. To the best of our knowledge, this is the first empirical study that highlights the effects of the antenna design, burial depth, and soil moisture on both UG2AG and AG2UG communication performance. These results have a significant impact on the development of multi-hop networking protocols for WUSNs.

Development of a testbed for wireless underground sensor networks

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 Wireless Sensor Network (WSN) that mainly focuses on the use of sensors that communicate through soil. Recent models for the wireless underground communication channel are proposed but few field experiments were realized to verify the accuracy of the models. The realization of field WUSN experiments proved to be extremely complex and time-consuming in comparison with the traditional wireless environment. To the best of our knowledge, this is the first work that proposes guidelines for the development of an outdoor WUSN testbed with the goals of improving the accuracy and reducing of time for WUSN experiments. Although the work mainly aims WUSNs, many of the presented practices can also be applied to generic WSN testbeds.

(CPS)^2: integration of center pivot systems with wireless underground sensor networks for autonomous precision agriculture

Precision agriculture (PA) refers to a series of practices and tools necessary to correctly evaluate farming needs and a high density of soil sensors is an essential part of this effort. The accuracy and effectiveness of PA solutions are highly dependent on accurate and timely analysis of the soil conditions. Traditional soil measurements techniques, however, do not provide real-time data and hence, cannot fully satisfy these requirements. Moreover, the use of wired sensors, which usually must be installed and removed frequently, impacts the deployment of a high density of sensor nodes for a certain area. In this paper, a novel cyber-physical system (CPS) is developed through the integration of center pivot systems with wireless underground sensor networks, i.e., (CPS)2 for precision agriculture (PA). The Wireless Underground Sensor Networks (WUSNs) consist of wirelessly connected underground sensor nodes that communicate untethered through soil. A CP provides one of the highest efficient irrigation solutions for agriculture and the integration of WUSNs with the CP structure can provide autonomous irrigation capabilities that are driven by the physical world, i.e., conditions of the soil. However, the wireless communication channel for the soil-air path is significantly affected by many spatio-temporal aspects, such as the location and burial depth of the sensors, the soil texture and moisture, the vegetation canopy, and also the speed of the center pivot engine. Due to the high number of real-time parameters to be considered, a cyber-physical system (CPS) must be developed. In this paper, as a proof-of-concept, the results of empirical experiments with these components are provided. The main characteristics of a precision agriculture CPS are highlighted as a result of the experiments realized with a WUSN built on top of a real-life center pivot system. The experiment results show that the concept of (CPS)2 is feasible and can be made highly reliable using commodity wireless sensor motes. Moreover, it is shown that the realization of (CPS)2 requires non-trivial management due to stochastic real-time communication constraints. Accordingly, guidelines for the development of an efficient (CPS)2 solution are provided. To the best of our knowledge, this is the first work that considers a CPS solution based on WUSNs for precision agriculture.

Power-Management Techniques for Wireless Sensor Networks and Similar Low-Power Communication Devices Based on Nonrechargeable Batteries

Despite the well-known advantages of communication solutions based on energy harvesting, there are scenarios where the absence of batteries (supercapacitor only) or the use of rechargeable batteries is not a realistic option. Therefore, the alternative is to extend as much as possible the lifetime of primary cells (nonrechargeable batteries). By assuming low duty-cycle applications, three power-management techniques are combined in a novel way to provide an efficient energy solution for wireless sensor networks nodes or similar communication devices powered by primary cells. Accordingly, a customized node is designed and long-term experiments in laboratory and outdoors are realized. Simulated and empirical results show that the battery lifetime can be drastically enhanced. However, two trade-offs are identified: a significant increase of both data latency and hardware/software complexity. Unattended nodes deployed in outdoors under extreme temperatures, buried sensors (underground communication), and nodes embedded in the structure of buildings, bridges, and roads are some of the target scenarios for this work. Part of the provided guidelines can be used to extend the battery lifetime of communication devices in general.

Communication Through Soil in Wireless Underground Sensor Networks – Theory and Practice

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, that is, the top few meters 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. This chapter presents advanced channel models that were developed to characterize the underground wireless channel considering the characteristics of the propagation of EM waves in soil and their relation with the frequency of these waves, the soil composition, and the soil moisture. Additional important aspects such as the burial depth, the reflection, the refraction, and multi path fading effects on the EM waves are also considered. The results from the field experiments in conjunction with the simulation results, both considering the path loss and the bit error rate, prove the feasibility of WUSNs. The chapter concludes with an outlook on potential research topics that are essential for the realization of WUSNs.

Strategic frequency adaptation for mid-range magnetic induction-based Wireless Underground Sensor Networks

Some classes of sensing applications target scenarios where the overall system, including antenna and energy sub-system, is placed under the ground, through concrete, or under-the-debris (disaster scenario). A real-time soil moisture sensing system deployed at the root zone of a crop area for precise irrigation and an oil pipeline leak detection (PLD) system are particular examples of interest discussed in this work. Communication solutions for these scenarios have been recently investigated in the Wireless Underground Sensor Networks (WUSN) literature and magnetic induction (MI) technology is usually regarded as the best candidate for low-power and mid-range (i.e., 15..30m) wireless underground communication. Nonetheless, underground MI systems are still significantly impacted by changes at the electrical properties of the medium surrounding the MI nodes, such as due to the soil moisture variability. The design of a MI system for the worst-case scenario (e.g., high soil moisture) is a strategic approach but it may significantly reduce the application bandwidth. Therefore, the dynamic frequency adaptation has the potential of balancing robustness and higher bandwidth for MI-WUSNs. In this work, a novel design procedure is proposed for the optimization problem of selecting the proper operational frequency for MI-WUSN nodes according to the medium conditions.

WSN-SA: Design foundations for situational awareness systems based on sensor networks

WSN-SA is a Wireless Sensor Networks (WSN) framework that is been proposed to support rescue operations in collapsed structures, such as houses and buildings, after natural and human-made disasters. The sensors and actuators are deployed prior to the disaster and ideally such sensor network is a required infrastructure of any building. Such SA systems can help to indicate how to distribute the rescue resources and how to identify imminent risks for rescuers and survivors. So far, few situational awareness (SA) systems based on WSNs have been proposed and no long-term realization of such systems has been reported. The goal of this work is to provide the foundations to change this reality so it would be possible to see a SA system accompanying each existing fire system. To this end, a strong emphasis is given to a) the achievement of low total cost of ownership (TCO), b) reliability, c) and expandability for the adopted SA solution.

Ripple-2: a non-collaborative; asynchronous; and open architecture for highly-scalable and low duty-cycle WSNs

The design of Ripple-2, a wireless in-situ soil moisture sensing system is presented in this paper. The main objective of such system is to collect high fidelity and fine grained data both spatially and temporally compared to radar remote sensing, which is the more traditional way of capturing soil moisture, and to use the former to validate and calibrate the latter. To do so, the in-site sensor network must cover a sufficiently large area, on the order of at least a few square kilometers. At the same time, cost constraints (both in deployment and in maintenance) puts a limit on the total number of sensor nodes, resulting in a very sparse (on average) network. The main challenge in designing the system lies in achieving reliability and energy efficiency in such a sparse network. For instance, in our pilot deployment, a 200mx400m area is covered by 22 nodes (average inter-node distance > 50m). Traditional WSN technology typically calls for many more nodes to be deployed in such an area. Ripple-2 is introduced as a non-traditional WSN architecture where (1) the network is physically and logically segmented into isolated clusters, (2) a regular node (or end device, ED) only communicates with the cluster head (CH) of its segment, and (3) the ED-CH communication is distinct from the CH-sink (or CH-Data Server) and both links can use virtually any kind of point-to-point wireless technology. We use both simulated and empirical results to demonstrate the effectiveness of Ripple-2; it proves to be ideal for low duty-cycle data collection applications due to its exceptional small network overhead (typically smaller than 1%) and its robustness to the size of the network.

Operating frequency selection for low-power magnetic induction-based wireless underground sensor networks

The possibility of employing sensor nodes that wireless communicate under the ground, through concrete, or under-the-debris (disaster scenario) has been recently highlighted at the Wireless Underground Sensor Networks (WUSN) literature. Nonetheless, the best operating frequency for such systems is still an open research aspect. In this work, we address this question for mid-range distances (e.g., 15..30m) by proposing a soil path attenuation model for an underground magnetic induction (MI)-based system involving a pair of nodes. The model is empirically validated and based on simulation results it is possible to conclude that for mid-range MI systems it is strategic to adopt a dynamic frequency selection scheme where audio frequencies are chosen whenever high soil moisture levels are detected.