# Energy-Efficient Wireless Sensor Networks for Precision Agriculture: A Review
## 3 Agriculture-Based Energy-Efficient Schemes in Literature
### Agriculture-Based Power Reduction Techniques
#### 3.1.1 Sleep/Wake Strategy  
Wireless transceivers consume a significant amount of power relative to processors. The energy of the wireless nodes is mostly dissipated during the reception and transmission processes. The radio of sensor nodes can enter sleep mode via the _sleep/wake_ strategy to reduce the power consumption of the RF components of WSNs. No data communication occurs during the sleep mode. The sensor nodes wake up to collect and transmit data within a certain period and then return to sleep mode to conserve energy. In agricultural applications, the _sleep/wake_ strategy is implemented via (i) duty cycling, (ii) medium-access control (MAC) protocols, and (iii) topology control. _Duty cycle_ is proposed in several studies to lower the power consumption for different agricultural applications based on WSN. [PERSON] et al. [69] proposed a WSN-based system to monitor soil moisture and analyzed the temporal and spatial variability of soil moisture for variable irrigation. To reduce the power consumption of the soil sensor nodes, the soil moisture sensors were always in sleep mode and activated only when data were to be collected. [PERSON] et al. [70] designed a WSN for PA in Malawi and called it WiPAM. This system was intended to automate irrigation and implement an irrigation management system (IMS). The authors investigated soil moisture variability. The sensor nodes were homogeneous, and each was composed of a ZigBee end device (ZED) and a ZigBee coordinator (ZC). In the star topology, three sensors nodes were configured as ZED (in field-nodes) and one as a gateway node. Another node was configured as ZC for aggregating data and actuating the irrigation valves. The power source to sustain the operation of the network was solar power. This approach was complemented by an energy-efficient management approach. In other words, sensor nodes went to sleep when idle and woke up to transmit data when required. The irrigation valves opened or closed depending on the values stored in the coordinator node. [PERSON] et al. [71] presented an energy-efficient WSN for soil moisture monitoring in an irrigation system. The new platform, DZ50, consists of an RFM12b transceiver and an ATmega328P microcontroller. This platform consumes very low power, and much energy can be saved in sleep mode, in which the transceiver goes inactive (sleep mode) for long periods. Comparison showed that the performance of DZ50 is better than that of TelosB and MicaZ. Specifically, the battery lifetime of DZ50 is seven times that of TelosB and MicaZ based on a 10 s sampling period. Monitoring of environmental conditions, such as wind direction, wind speed, humidity, temperature, conductivity, rain level, pH, and water level, in crop fields was presented in [72]. A solar cell was used to power several sensor nodes deployed at a distance of 500 m to 5 km. WSNs based on IEEE 802.15.4 (C1120 transceiver) and GPRS/3G were adopted to transmit the rural environments to the base station. Energy was saved through low duty cycling (3.3%) for the RF transceiver; the saved energy is equivalent to 30 s of wake up time every 15 min. In addition, a 2-watt solar cell was used to supply power to the overall system, whose total average power consumption was 207 mW. _MAC protocols_ were also proposed in previous research on agriculture WSNs [39; 73; 74; 75; 76]. [PERSON] et al. [39] proposed an energy-efficient TDMA-based algorithm for wireless sensor communications in an automatic irrigation system. Their experiment involved a header node that collected data directly from all the nodes in the field and passed these data to the base station (BS). This network arrangement was a star single-hop topology. These techniques attain power savings of 10% with a high communication distance of 1 km between the sensor node and the base station. However, when this distance increases, the power consumption also increases. [PERSON] et al.