Event detection and recognition are important for environmental monitoring in the Internet of things and cyber-physical systems. Low power wide area (LPWA) networks are one of the most powerful wireless sensor networks to support data gathering; however, they do not afford peak wireless access from sensors that detect significant changes in sensing data. Various data gathering schemes for event detection and recognition have been proposed. However, these do not satisfy the requirement for the three functions for the detection of the occurrence of an event, the recognition of the position of an event, and the recognition of spillover of impact from an event. This study proposes a three-stage data gathering scheme for LPWA. In the first stage, the access limitation based on the comparison between the detected sensing data and the high-level threshold is effective in reducing the simultaneous accessing sensors; thus, high-speed recognition of the starting event is achieved. In the second stage, the data centre station designates the sensor to inform the sensing data to achieve high accuracy of the position estimation of the event. In the third stage, all the sensors, except for the accessing sensors in the early stage, access the data centre. Owing to the exhaustive gathering of sensing data, the spillover of impact from the event can be recognised with high accuracy. We implement the proposed data gathering scheme for the actual wireless sensor system of the LPWA. From the computer simulation and experimental evaluation, we show the advantage of the proposed scheme compared to the conventional scheme.

In order to improve crop production and efficiency of farming operations, an IoT (Internet of Things) system for remote monitoring has been attracting a lot of attention. The existing studies have proposed agricultural sensing systems such that environmental information is collected from many sensor nodes installed in farmland through wireless communications (e.g., Wi-Fi, ZigBee). Especially, Low-Power Wide-Area (LPWA) is a focus as a candidate for wireless communication that enables the support of vast farmland for a long time. However, it is difficult to achieve long distance communication even when using the LPWA because a clear line of sight is difficult to keep due to many obstacles such as crops and agricultural machinery in the farmland. In addition, a sensor node cannot run permanently on batteries because the battery capacity is not infinite. On the other hand, an Unmanned Aerial Vehicle (UAV) that can move freely and stably in the sky has been leveraged for agricultural sensor network systems. By utilizing a UAV as the gateway of the sensor network, the gateway can move to the appropriate location to ensure a clear line of sight from the sensor nodes. In addition, the coverage area of the sensor network can be expanded as the UAV travels over a wide area even when short-range and ultra-low-power wireless communication (e.g., Bluetooth Low Energy (BLE)) is adopted. Furthermore, various wireless technologies (e.g., wireless power transfer, wireless positioning) that have the possibility to improve the coverage area and the lifetime of the sensor network have become available. Therefore, in this study, we propose and develop two kinds of new agricultural sensing systems utilizing a UAV and various wireless technologies. The objective of the proposed system is to provide the solution for achieving the wide-area and long-term sensing for the vast farmland. Depending on which problem is in a priority, the proposed system chooses one of two designs. The first design of the system attempts to achieve the wide-area sensing, and so it is based on the LPWA for wireless communication. In the system, to efficiently collect the environmental information, the UAV autonomously travels to search for the locations to maintain the good communication properties of the LPWA to the sensor nodes dispersed over a wide area of farmland. In addition, the second design attempts to achieve the long-term sensing, so it is based on BLE, a typical short-range and ultra-low-power wireless communication technology. In this design, the UAV autonomously flies to the location of sensor nodes and supplies power to them using a wireless power transfer technology for achieving a battery-less sensor node. Through experimental evaluations using a prototype system, it is confirmed that the combination of the UAV and various wireless technologies has the possibility to achieve a wide-area and long-term sensing system for monitoring vast farmland.