# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 1 Introduction
### Related Work and Contribution
Energy efficiency in WSNs was also discussed in [24], which extended [23] by providing a description of the metrics used to evaluate energy efficiency. In particular, energy-efficient Radio Resource Management (RRM) methods were analyzed, focusing on energy saving mechanisms in case of low traffic and making use of application-specific Quality of Service (QoS) requirements. Another aspect considered in [24] is the energy efficiency for heterogeneous networks (i.e., networks of devices produced by different manufacturers or different computers running with different operating systems) and relay communications, that were not widely studied before. State-of-the-art schemes for addressing energy efficiency in case of networks adopting MIMO and OFDM schemes were also discussed. Energy efficiency techniques taking into account application-level requirements were also surveyed in [25], where an in-depth description of the main applications for WSNs was introduced first, moving then on presenting standards and energy efficiency mechanisms for these latter. The mechanisms were classified into the following categories:  
* _Radio optimization approaches_, focused on the reduction of power consumption at the radio module;
* _Data reduction approaches_, aiming to propose techniques for reducing the amount of exchanged data;
* _Sleep/wake-up approaches_, aiming to optimize sleep and active modes of the devices;
* _Energy-efficient routing approaches_, targeting energy savings by optimizing the adopted routing paradigms;* _Charging approaches_, related to the use of energy harvesting and wireless charging techniques to boost battery charging. Energy efficiency was also studied in [26] in the context of smart grids, highlighting mechanisms making use of renewable energy, in order to improve operational practices and deploy so-called green data centers. Being related to smart grids, the work covered aspects related to wireless, wired, and optical communications. Electric Power and Energy Systems (EPES) were analyzed in [27], where a description of EPES was given first, moving then on reviewing IoT-based EPES applications and services. In this context, a technical description and assessment of smart home applications was given, also highlighting the impact that IoT-based EPESs have on economy, society and environment. Challenges and possible solutions for IoT-based EPESs were also discussed. Energy efficiency vs. coverage trade-offs for 5G cellular networks were discussed in [28]. After providing the definition of green communications, several schemes for energy efficiency were reviewed and classified in four categories:  
* _Resource allocation_, aiming at improving the energy efficiency through careful allocation of radio resources;
* _Network planning and deployment_, aiming at deploying infrastructure nodes that can maximize the energy efficiency in the the covered area [28];
* _Energy harvesting and transfer_, which focuses on harvesting energy from the environment, and using it to operate the communication systems;
* _Hardware solutions_, which focuses on development and design of hardware that explicitly accounts for energy consumption. In particular, the proposals include cooperative management between LTE and 5G systems, resulting in intelligent and dynamic on/off switching of 5G base stations, aiming to decrease the power consumption at the infrastructure side. In this paper, we follow the line drawn by the above works, offering a novel and in-depth investigation in the context of short-area and wide-area IoT technologies. In particular, the present work provides:  
* A review of the metrics used to evaluate energy efficiency;
* A taxonomy of methods and techniques addressing energy efficiency challenges for LPSANs and LPWANs, and corresponding technologies;
* A review of power and energy consumption models proposed for LPSANs and LPWANs, and corresponding technologies;
* A discussion of limitations and open issues for the aforementioned methods.