# Energy Sustainability in Wireless Sensor Networks: An Analytical Survey
## 3 Hardware-Based Energy Sustainability in WSNs
### Hardware-Based Methods for Energy Sustainability
#### 3.2.1 Energy Saving Methods Applied in Submodules  
When referring to the main submodules of nodes (i.e., sensors, processors and transceivers) the utilization of low-power MEMS is necessitated in order to achieve energy saving [1; 2; 3; 26]. Moreover, the power of a sensor node can be managed by hardware scaling methods, which are used to handle the settings and the configuration of the hardware in nodes' submodules. When engaging with such methods, the voltage, the frequency, and the rate can be adjusted according to the application's requirements to limit energy consumption. Furthermore, methods such as system power optimization, aim at putting the node in sleep mode while not in operation in order to avoid energy depletion. Actually, several methods may be applied in each one of the submodules of nodes:  
* While designing the Sensing Unit, the type of the application WSN is intended to be used in, needs to be considered in order to choose the appropriate sensors and converters [21; 24]. * The selection of low power sensor units contributes to the energy conservation of the overall sensor node;
* The ability to promptly control the operations of sensors (e.g., turning on and off), as well as its quick response time to irritations and its low duty cycle can lead to energy saving;
* Additionally, instead of active sensors, passive sensors may be used. Such devices do not contain any piece of active circuits. For this reason, they use not exterior energy supplies. Actually, they are not powered at all. Instead, they receive incoming signals that they are reflected backwards along with the sensed information [27]. * The design of the central Processing Unit is related to the choice of the optimum microprocessors and microcontrollers (MCUs) [19; 21]. * Low-power processors offer low frequency clock choices, consume lower currents and are able to operate using lower voltages. In addition, it is critical to avoid implementing a huge number of features and peripherals, since the greater the amount is, the higher the power consumption becomes;  
Figure 4: Categorization of hardware-based methods for energy sustainability in WSNs. Microprocessors, mostly support different modes of operation, such as, active, idle and sleep mode for clearer power management objectives; Furthermore, dynamic voltage scaling (DVS) method frequently applies in processors during their operation in active status in order to lessen the energy consumption levels [28; 29]. Usually, microprocessors do not operate continually at their highest computational power, due to the fact that the work load of each task varies. Thus, the use of DVS method provides energy efficiency to sensor nodes by adjusting both the voltage of the processor and operating frequency dynamically according to the demands of the momentary processing tasks. * The selection of appropriate transceivers to be integrated in the communication unit of the sensor nodes is extremely helpful in order to achieve energy conservation. * The use of low power transceivers is extremely helpful in order to reduce energy consumption [19]; Putting the transceiver in sleep mode while there are no communication needs, or using Adaptive Transmission Power Control can also save energy; The use of Cognitive Radio (CR), i.e., an intelligent radio that enables the dynamic selection of the most suitable radio channel can lead to a network energy conservation [21]. This selection depends on the transmit power, the data rate, the duty cycle, and the modulation required by the existing conditions; In the so-called Adaptive Transmission Power Control method, the power required for data transmission is estimated based on the distances among nodes [19]. Additionally, the power levels of the transmitter are adjusted according to the needs of each application, in order to limit the energy consumption [24]; In addition, directional antennas may be used. Such antennas are able to both send and receive signals in one direction.