# An Autonomous Wireless Device for Real-Time Monitoring of Water Needs
## 2 Wireless Sensor Networks in the Context of Smart Agriculture  
We can see different solutions for monitoring crops using WSNs. Many of these proposals have only been tested in the laboratory and still have to be developed and implemented on real crops. A low-cost, small-scale system to remotely monitor and control a greenhouse was created in [9]. In this study, the information was collected by an Arduino Mega and sent to a web page through ethernet communication. In [10], a system based on short message service (SMS) communication using a global system for mobile (GSM) module was developed to measure parameters such as soil moisture, pH, and atmospheric pressure. With these data, irrigation could be controlled and the farmer informed of the state of his crops. Similarly, the authors of [11] developed a WSN system with a soil moisture sensor and a temperature and humidity sensor. One of the most widely used technologies for creating wireless sensor networks in agriculture is the ZigBee technology. In [5; 12; 13], a WSN was presented for monitoring crops by measuring parameters such as soil moisture, pH, conductivity, salinity, and ambient light. General packet radio service (GPRS) technology can also be used in agriculture to create WSNs [6]. In [14], the authors developed an automated irrigation system to optimize the use of water in crops by combining the ZigBee and GPRS technologies. By means of this system, the humidity and temperature of the land can be measured. Furthermore, in [15], an IoT-based irrigation system was presented using soil moisture sensors controlled by ATMEGA 328P on an Arduino UNO board, along with a GPRS module. NarrowBand IoT (NB-IoT) is an LPWAN technology based on narrow band radio technology that can coexist with GSM and long-term evolution (LTE) under licensed frequency bands. NB-IoT is standardized by the 3 rd Generation Partnership Project (3 GPP) and its specifications were released in June 2016; thus, additional time is needed before its network is established [16]. SigFox is a new technology used in IoT applications. In [17], a geolocation system was developed to locate herds of cattle. As advantages of the SigFox technology, we can highlight its wide range of coverage and low consumption, which makes it very appropriate for IoT solutions. With SigFox, LoRa technology has started to be used in precision agriculture in recent years as an alternative to previous protocols [18; 19]. It is a long-range and low-power wireless communication system whose objective is to be used in long-term devices [20]. Based on the LoRaWAN technology, the MoniSen end-device [1] used the ATMEGA324P processor and the RN2473 chip to establish LoRa communication, to measure parameters such as ambient temperature and humidity, soil temperature and humidity, and light intensity received by the plant. In a smart irrigation system [19], the end-device achieved a distance of 7 km in the field tests. Table 1 shows a comparison of the different communication technologies discussed above for the creation of WSNs [8; 16; 21; 22]. If we analyze the data shown in the table above, we can see that GPRS technology, apart from its high cost, is not entirely suitable for the communication of wireless sensors due to its high energy consumption, which means it is little used in the creation of high autonomy nodes. Currently, the ZigBee wireless communication protocol is considered one of the best technologies in precision agriculture, given its low consumption. If we compare it with the LoRa and Sigfox technologies, we can see that all three have low consumption and low cost, making them ideal for the creation of WSNs. However, one of its main disadvantages is its short reach between nodes--100 m in open spaces, as shown in Table 1--which is both lower than the other wireless communication protocols and a great limitation for agriculture. NB-IoT is preferred for applications that require guaranteed quality of service, whereas applications that do not have this constraint could choose both LoRa or Sigfox.