# Smart Irrigation Systems from Cyber-Physical Perspective: State of Art and Future Directions
## 3 Problem Space
### Cost-Effective Dimension
#### 3.3.2 Energy Efficiency  
IoT devices need continuous power to work properly. One way to extend the battery's lifetime is to turn the system into sleep mode once we do not need it. Such kind of scheduling assumes sensors have two operation modes [99]: (i) _Active Mode_, in which sensing, communication, and computation can be carried out; and (ii) _Sleep Mode_, which refers to the energy-saving mode with a small amount of energy being used. To this end, [PERSON] et al. [100] suggested irrigating different types of crops in turns. That means that even during the active time, only partial sensors will be activated to save more energy. The risk associated with the sleep mode is the possibility of losing data when turning off the node. In addition, activation might consume time and extend the data transmission time. [PERSON] et al. [101] proposed clustering schemes similar to sleep mode to increase the network lifetime and efficiency of data transmission. Each cluster has its cluster head in the clustering schemes, which gathers and forwards the data collected from its sensors to the base station or access point as a data collector. Another way is to use green energy, such as solar energy, which is a renewable energy source that can eliminate the process of changing or charging a battery. It can also reduce the emission of carbon dioxide and improve the overall environment. [PERSON] et al. proposed a photovoltaic pumping irrigation system with solar energy [102]. The solar photovoltaic pump module provides the partial solar energy to the device for operations, and the remainder is stored in the battery. Thus, even in the evening, with the saved battery, there are no concerns about resource shortage without the sun. [PERSON] and [PERSON] [45] proposed connecting a solar panel with a solar charger with a 12 V Direct Current (DC), which can shut down the system for 4 h to save energy once it detects wet soil. The total solar energy generated depends on the solar panel power, efficiency, number of daily sunlight hours, and losses. Thus, it is unstable in providing stable and non-stop energy. Innovative approaches are explored to enhance solar energy utilization. For instance, [PERSON] et al. [58] designed a system attaching one servo motor to one side of a solar panel and placed two LDR sensors on the other side to detect the sunlight. These devices are attached to an Arduino microcontroller. When the sunlight is within LDR sensor 1's measurement range, 90 clock degrees will be moved. When the sunlight is within LDR sensor 2's measurement range, 90 reverse clock degrees will be performed. In this way, the solar panel can turn in the direction of sunlight to absorb more energy, similar to a sunflower. [PERSON] et al. [47] compared the water savings with an irrigation system with a solar panel. According to their test result, it saves 64% of the water and 50% of the energy. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cost Range** & **Features** & **Hardware** & **Strengths** & **Limitations** & **Paper ID** \\ \hline \multirow{3}{*}{\(<\)$500} & WiFi/ZigBee, soil moisture & Soil NoMedCU/Arduino & \(<\)$30/unit, simple setup & Limited range (\(<\)100 m), single sensor & [34, 38, 40, 52, 59, 60, 62] \\ \hline \multirow{3}{*}{\(<\)$500} & WiFi/ZigBee, soil moisture, environmental metrics & Up to 4 sensors, NodeICU/Arduino Uno & \(<\)$40/unit, multi-sensor data & Limited range (\(<\)100 m) & [35, 36, 41, 44, 5, 48, 50, 54, 56, 59, 98] \\ \hline \multirow{3}{*}{\(\$500\)–\(\$2000\)} & LoRaWAN/NB-IoT, soil moisture & LoP34, Arduino/Raspberry Pi & Extended range & Reliability of LPWAN & [37, 39, 43, 51, 53] \\ \hline \multirow{3}{*}{\(\$500\)–\(\$2000\)} & WiFi/BLE/ZigBee, environmental metrics & 6 sensors, camera, NodeICU/Arduino/Pi & Crop-monitoring capability & Limited range (\(<\)100 m) & [32, 52, 64] \\ \hline \multirow{3}{*}{\(>\)$2000} & LPWAN/WiFi/BLE/ZigBee, field & Multiple sensors, UASs & Real-time field data & Extended range, but less efficient & [31, 52] \\ \hline \hline \end{tabular}
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