# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 1 Introduction
### Related Work and Contribution  
A primary target for all the above technologies is to deliver their services while taking into account energy efficiency and power consumption aspects, at both device and network levels. These aspects are extremely important for the IoT development, and for this reason recent years have seen a rising research-and-development interest toward (a) the design and implementation of techniques and mechanisms for energy efficiency, including power saving modes at the device level and cooperation schemes across the network, and (b) the derivation of theoretical and empirical models for the power consumption and battery lifetime of the above classes of devices. The peculiarity in terms of constraints and requirements of the IoT systems and scenarios requires significant extensions of mechanisms and theoretical analyses already implemented and derived for non-IoT wireless communication technologies, which however provide a reliable and valid starting point of analysis, as will be also discussed in the following [23; 24; 25; 26; 27; 28]. This paper presents an extensive literature survey on the methodologies and approaches used and proposed to increase the energy efficiency and thus lower the power consumption of the most widespread IoT technologies. Moreover, the paper also provides a deep analysis of proposed energy and power consumption models for the same technologies, and attempts to create a classification of said methods. This categorization of techniques will help provide a basis for the already existing various works that study the IoT technologies as singular entity, and at the same time attempting to create a full picture that includes all the technologies, as part of one big IoT family. At the same time, it creates a landmark for future work that aims to study or improve energy efficiency, giving clear indications of previous work done in that context, and that can serve as starting point for new research. The first aspect has been already addressed, to some extent, by several studies. Some of the most notable ones are briefly discussed in the following, in order to highlight the main differences and clarify the motivation of the present work. One of the first surveys targeting energy-related aspects was presented in [23], with focus on WSNs and the main techniques to reduce the energy consumption of sensors. A classification of such techniques was also proposed, comprising three main categories:  
* _Duty cycling approaches_, based on the idea that the radio transceiver should be turned off, in a status generically identified as sleep mode, if it has no more data to send and/or receive (i.e., in contrast to so-called active mode). Therefore, sensor nodes alternate between active and sleep modes to conserve energy. The current state of a node is defined by the network activity. This process is called duty cycling, with a duty cycle determined as the fraction of time that nodes spend in active state during their lifetime;
* _Data-driven approaches_, designed to reduce the amount of data exchanged by the sensors while maintaining an acceptable level of sensing accuracy, as required by the application the sensors are used for. These methods reduce the energy consumption of the nodes in two ways: first, all unneeded sampled data gathered by the nodes are not communicated at the network-side; second, by reducing the data to be sampled by the sensing subsystem;
* _Mobility approaches_, tailored for mobile WSNs, aiming at prolonging the lifetime of mobile WSNs. These methods rely on mobile nodes in the network to pass information to static nodes. The latter ones wait for a mobile device to pass nearby, and route the messages and data they have gathered to the mobile device. The communication happens between nodes that are very close to each other, helping the nodes to preserve energy, that in other situations would be spent to send messages over long distances. Apart from the above classification, open challenges and issues related to each approach were also highlighted.