# Evaluation of Green Strategies for Prolonging the Lifespan of Linear Wireless Sensor Networks
## 5 Energy-Balancing Techniques
### Energy-Efficient Routing Protocols  
Energy-efficient routing protocols are needed for large-scale battery-powered WSNs to ensure uniform energy consumption and load balancing. Moreover, they also need to achieve reliable and real-time data forwarding to the sink. This has led to many research efforts devoted to the design of energy-efficient routing protocols and/or enhancement of existing ones [63]. In the literature, two prominent strategies, cluster-based routing and energy-aware routing, are particularly effective in addressing the challenge of energy imbalance. Figure 7: Taxonomy for classifying energy-balancing techniques in WSNs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Energy-Balancing Technique**} & \multicolumn{1}{c}{**Strategy Implemented for Achieving Energy Balancing**} & \multicolumn{1}{c}{**Target**} & \multicolumn{1}{c}{**Suitability**} \\  & & & **in LWSN** \\ \hline \multirow{8}{*}{Energy-efficient routing} & \multirow{4}{*}{Load balancing (cluster-based routing)} & Organizes the network into clusters, where each cluster is managed by a selected node known as the cluster head (CH) and balances energy consumption among sensor nodes via CH rotation. The selection is dynamic, and it is based on the residual energy. The node with the highest residual energy is selected as CH. & Network energy & No \\ \cline{2-5}  & Energy-aware routing & Balances energy consumption among nodes by considering the residual energy when selecting the next hop during the setup path phase. & Network energy & No \\ \cline{2-5}  & Multi-path routing & Balances energy among nodes by alternating forwarding nodes. & Network energy & No \\ \hline \multirow{4}{*}{Mobility-based approach} & Mobile sink & Balances the load between nodes by using a mobile base station, which moves around the network to collect node information. & Network energy & Yes \\ \cline{2-5}  & Mobile relay & Balances energy among nodes by introducing special mobile nodes to offer the service of message relaying. & Network energy & Yes \\ \hline \multirow{4}{*}{Topology control} & \multirow{4}{*}{Optimal node placement} & Improves energy balance between nodes via optimal placement of nodes through even distribution by adding a few relay nodes with enhanced capabilities. & Network energy & Yes \\ \cline{2-5}  & & Balances energy consumption among nodes by allowing nodes to dynamically adapt their transmission power levels without losing connectivity. Nodes can increase or decrease their transmission power based on factors such as proximity to the destination and the necessity to reach neighboring nodes. & Network energy & Yes \\ \hline \multirow{4}{*}{Energy transfer} & \multirow{4}{*}{Wireless charging} & Balances energy among nodes by wireless transmitting energy from energy-harvesting sources or nodes with high residual energy to nodes with low residual energy. & Network energy & Yes \\ \cline{2-5}  & Energy-neutral operation & Balances energy consumption by achieving the desired network performance that can be supported by the energy harvested from the required energy sources (_i.e._, solar, vibration, and RF) and the network-wide operations (_i.e._, routing, clustering, and duty cycling) over longer periods of time. & Network energy & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Rationale behind energy-balancing techniques for extending WSN lifetime.