# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods  
Section 3 served as a starting point for the study of IoT technologies in terms of energy and power consumption, where various energy and power consumption models were presented for each IoT technology. The focus of this section is to present a classification of proposed techniques and methods aiming at increasing the energy efficiency of IoT technologies. In particular, we give insights on common methods for different technologies, while also highlighting peculiarities of same methods which are tailored on specific LPSAN and LPWAN technologies.  
Overall, we categorize the energy efficiency methods in:  
**Sleep/wake-up techniques.** These techniques are based on the ability of an IoT node to adaptively switch from sleep to active modes, and vice versa, in order to efficiently conserve energy. Where there is no information to exchange, the node goes into sleep mode. In this state, it continues to be registered to the network but does not monitor the status to check whether there is information to be received. These techniques are used by various IoT technologies. On the one hand, in the context of LPSANs, they have been often proposed for RFID systems, and in particular for an energy-efficient functioning of active tags. On the other hand, in the context of LPWANs, these techniques find large usage in cloT systems. In particular, LTE-M inherits the so-called Discontinuous Reception (DRX) method from LTE/LTE-A, as also mentioned in Section 3, while NB-IoT and EC-GSM-IoT propose further enhancements, such as PSM and extended Discontinuous Reception (eDRX), which are further detailed later.  
**Data Reduction techniques.** The main focus of these techniques is to achieve energy efficiency by reducing the transmitted data. As discussed later in detail, these techniques have found a large application for improving the efficiency of NB-IoT systems, compared to the other technologies. It is worth mentioning that the use of such techniques also has a significant impact on the Quality of Service (QoS) provided by the system, for example, in terms of latency.  
**Network techniques.** These are among the most widely used techniques in IoT technologies, as they can be deployed on top of most technologies. Network techniques can be further divided in (a) _clustering techniques_, which use _clusters_ (groups) of nodes, and aim at selecting and using appropriate cluster heads (CH) in order to receive/transmit data, ultimately impacting scalability and robustness of the network, and (b) _routing techniques_, aiming at transferring data minimizing system overhead and congestion. Network techniques have found large use in both LPSANs (RFID, Zigbee, Bluetooth/BLE) and LPWAN (NB-IoT), as discussed in Sections 4.1 and 4.2, respectively.  
**Resource Allocation techniques.** These techniques are mainly used in LPWANs, in particular in LTE-M systems, where both the reduction of PRBs allocated for transmission and the reduction of the transmission power are proposed as ways to improve energy efficiency, and in NB-IoT, where the scheduling of uplink and downlink transmissions is explored in order to increase energy efficiency and reduce power consumption. More details are given in Section 4.2.2.  
A mapping between the above categories of energy efficiency techniques and IoT technologies is provided in Figure 4.