# Energy-Efficient Wireless Sensor Networks for Precision Agriculture: A Review
## 3 Agriculture-Based Energy-Efficient Schemes in Literature
### Agriculture-Based Power Reduction Techniques
#### 3.1.2 Radio Optimization  
Previous studies show that the power is mostly dissipated in the RF components of WSNs than in data processing units, such as microcontrollers and microprocessors [80]. A number of scholars have applied various radio optimization schemes or techniques, including (i) transmission power control (TPC), (ii) modulation scheme, and (iii) cognitive radio, to reduce the power consumption of the RF components of agriculture sensor nodes. In the _TPC scheme_, the sensor nodes modify the transmitted power to save energy, stimulate avoiding interference, and establish a communication link [81]. TPC can be used in the agricultural field [82; 83], where the RF transmitted power of the sensor nodes can be modified to reduce their power consumption based on the measured distance between the sink node and the sensor nodes. [PERSON] et al. [82] have investigated the application of TPC in reducing the power consumption of the sensor nodes in precision agriculture-based WSNs. Several power levels and different receiver sensitivity levels are considered to perform the said process. The network layers and MAC protocol in a WSN (based on CC1110) for the agriculture domain are adopted to further lower the power consumption of the sensor node. Results show that the power saving that employs multiple modes of transmitted power can be improved by approximately 10% relative to the traditional mode. By contrast, [PERSON] et al. [83] prolonged the lifetime of the sensor node by more than 8.5% by modifying the transmitted power of the CC2420 communication device. A _cognitive radio_ is an intelligent wireless communication network [84] in which the wireless communication channel in the spectrum band can be selected efficiently [85]. The transmission metrics (e.g., transmitted power, carrier frequency, and modulation scheme) can be adjusted accordingly. A cognitive radio requires more energy than conventional devices do because it includes sophisticated and complex functions [86]. Therefore, an energy-efficient cognitive radio network poses challenges, especially with regard to the use of battery energy. In wireless communication, the electromagnetic spectrum is fully occupied, with several bands being partially used or unused. This situation supports the utilization of a cognitive radio, in which spectrum holes can be utilized. [PERSON] et al. [19] fused fuzzy logic control (FLC) with a ZigBee wireless sensor actor network (WSAN), which can sense the surroundings and respond in ideal performance without interference from agricultural crops. FLC assists the specialists in managing complex systems more compared with traditional control methods, which are ineffective in terms of flexibility, energy, productivity, and labor interference. Two significant greenhouse environment parameters (i.e., humidity and temperature) are monitored during daytime and nighttime. The results show that the intelligent technique-based FLC can significantly improve the WSAN lifespan. Therefore, the WSAN can work for 14 years with a 210 mAh battery power based on the FLC merged with WSAN. The power consumption of networks can be directly affected by the _modulation scheme_[87]. The modulation parameters can be adjusted to their optimal values to preserve the minimum power consumption of radio modules. [PERSON] et al. [88] investigated the Frequency-shift keying (FSK) and Minimum-shift keying (MSK) modulation strategies to minimize the total power consumption of the sensor node required to transmit given data packets. Through their extensive study, the authors concluded that MSK outperforms FSK in terms of power consumption. However, several modulation schemes cannot be used in the ZigBee wireless protocol that operates at 2.4 GHz because this protocol supports a single modulation scheme. Specifically, the ZigBee wireless protocol works with offset quadrature phase-shift keying (OQPSK) [89]. Table 3 shows the comparison of power reduction technique based on radio optimization schemes for previous research in agricultural applications.