# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
## 3 Energy Efficiency Analyses of Radio-and-Fiber Networks
### Cooperating Power-Saving Techniques in the Optical and Wireless Domain of FiWi Networks
It was shown that the EE of the DEC TDMA FiWi network was equivalent to ECO-FiWi, but in terms of transmission delay, DEC TDMA outperformed the ECO-FiWi scheme. In order to achieve energy conservation and ensure satisfactory QoS, a collaborative sleep technique between optical and wireless nodes was developed in ref. [49] (Table 1). This technique consists of two sub-techniques. The first one is a technique based on load balancing for achieving optimal planning of transmission route selection, and the second one is a cooperative sleep technique based on load transfer using priority-based rerouting. The simulation results showed that this technique can achieve high throughput and EE with low latency for high-priority services. In ref. [50], the authors proposed a framework that simultaneously focuses on ensuring the EE and survivability of the FiWi networks using optimization algorithms for ONU placement (Table 1). Several different optimization algorithms were compared in extensive simulations, namely, genetic algorithm, teaching-learning-based optimization, spiral update positioning and encircling prey mechanism. The authors concluded that the genetic algorithm provides the best performance regarding EE, with an average performance in terms of the FiWi network survivability. \begin{table}
\begin{tabular}{c c c} \hline \hline
**References** & **PS Method** &
\begin{tabular}{c} **Summary of Contributions Related to the Improvement** \\ **of FiWi Network Energy Efficiency** \\ \end{tabular} \\ \hline
[60] & \
\begin{tabular}{c} Number and directivity of antennas and \\ antenna position optimization \\ \end{tabular} &
\begin{tabular}{c} Method for improving EE of a DAS by increasing the \\ number of antennas and optimizing the antenna position. \\ \end{tabular} \\ \hline
[61] & Antenna unit output power optimization &
\begin{tabular}{c} Comparison of the EE of optimized narrowband \\ single-service and broadband multi-service DAS solutions. \\ \end{tabular} \\ \hline
[62] & \
\begin{tabular}{c} Antenna position optimization and the \\ selection of the optimal number \\ of antennas \\ \end{tabular} &
\begin{tabular}{c} Investigation of the EE of the RoF DAS technologies by \\ measuring the power consumption of 802.11 APs and \\ smartphones. Proved the existence of the optimal number \\ of distributed antennas for a given indoor \\ environment topology. \\ \end{tabular} \\ \hline
[63] & \
\begin{tabular}{c} Selection of the optimal number of \\ antennas with data frame \\ aggregation mechanisms \\ \end{tabular} &
\begin{tabular}{c} A method for evaluating and optimizing energy \\ consumption in 802.11n RoF DAS systems. Proved the \\ existence of an optimal number of distributed antennas for a \\ given scenario, and that the data frame aggregation \\ mechanisms can further improve EE. \\ \end{tabular} \\ \hline
[64] & \
\begin{tabular}{c} Centralized \\ RoF architectures using dedicated RoF \\ links for each cell \\ \end{tabular} &
\begin{tabular}{c} An EE model for RoF networks confirmed better network \\ EE when designed with small cells and when the energy \\ usage of the remote units surpasses a certain threshold. \\ \end{tabular} \\ \hline
[65] & \
\begin{tabular}{c} Analyses of the impact of E/O/E \\ conversion, number of services and \\ wireless network capacity on EE of the \\ RoF links \\ \end{tabular} &
\begin{tabular}{c} Confirmed that E/O/E losses degrade EE on an optical link \\ using the A–RoF technique. Confirmed that the D–RoF link \\ shows the degradation of EE at higher Nyquist zones due to \\ RF signal reconstruction. Confirmed that the wireless \\ bandwidth can improve the EE of both, the A–RoF and \\ D–RoF connections and that the amount of energy savings \\ in the presence of multiple services depends on the specific \\ wireless environment. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of the related works on PS methods in RoF networks.