# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Energy Efficiency in LPWANs  
In the following subsections we move on LPWAN technologies, reporting in-depth descriptions of proposed energy efficiency methods. Technology-agnostic energy efficiency methods are preliminary described; then, specific works on particular technologies are analyzed, ranging from LoRa/LoRaWAN to 3 GPP cloT standards, including EC-GSM-IoT, LTE-M and NB-IoT.  
A general approach for evaluating the energy consumption of LPWAN technologies is presented in [136]. It is based on the fact that energy consumption is composed by the energy consumed by the different consuming elements. Therefore, in order to properly calculate the energy consumed, it is key to efficiently calculate the energy consumed by each element, moving from the observation that the sensor unit operates in two modes: _Non-Active Period_, which consists of sleep/idle mode, and _Active Period_, which includes information processing and data exchange operations (i.e., Connected mode). Reference [136] verified the validity of their approach on different LPWAN devices using a Keysight N6705B DC Power Analyzer [137] and the Joulescope Precision DC Energy Analyzer [138]. Experiments show that LoRaWAN has higher energy efficiency and longer battery lifetime, compared to NB-IoT and Sigfox devices. A few other works in the literature provided a comparison between two or more LPWAN technologies in terms of energy efficiency; details are provided in Table 4.  
\begin{table}
\begin{tabular}{p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline
**Paper (s)** & **Technologies** & **Methods** & **Results/Conclusions** \\ \hline \multirow{4}{*}{[81]} & Sigfox, LoRaWAN, NB-IoT, LTE-M & \multirow{4}{*}{Simulations} & LTE-M has the highest energy consumed per transmitted message. For distances below 2 km, the technologies have similar energy consumption. For distances above 2 km, the energy consumption depends on temperature, payload size, and other parameters. \\ \cline{3-4}  & & NB-IoT has better energy efficiency for small data lengths. LTE-M offers better energy efficiency and longer lifetime for large data lengths and good coverage. NB-IoT is a better fit for simple sensors and low data rate applications in medium to poor coverage. \\ \hline \multirow{2}{*}{[136]} & LoRaWAN, Sigfox, NB-IoT & Experiments & LoRaWAN has lower energy consumption compared to Sigfox and NB-IoT. Sigfox is better in terms of coverage but it has the highest energy consumption. \\ \cline{3-4}  & & NB-IoT has better energy efficiency and longer lifetime for large data lengths and good coverage. NB-IoT is a better fit for simple sensors and low data rate applications in medium to poor coverage. \\ \hline \multirow{2}{*}{[136]} & LoRaWAN, Sigfox, NB-IoT & Experiments & LoRaWAN has lower energy consumption compared to Sigfox and NB-IoT. Sigfox is better in terms of coverage but it has the highest energy consumption. \\ \cline{3-4}  & & NB-IoT has better energy efficiency and longer lifetime for large data lengths and good coverage. NB-IoT is a better fit for simple sensors and low data rate applications in medium to poor coverage. \\ \hline \end{tabular}
\end{table}
Table 4: Existing comparative analyses of Low-Power Wide-Area Networks (LPWANs) in terms of energy efficiency and consumption.