# Joint Balanced Routing and Energy Harvesting Strategy for Maximizing Network Lifetime in WSNs
## 2 Related Work
### Wireless Power Transfer  
With recent energy harvesting models, optimizing data routing is a common approach to reduce energy consumptions of the nodes and improve the lifecycle of WSNs. Considering the power transfer task, the chargers construct power beams [28] to improve the wireless power transfer (WPT) efficiency, such that more power could be harvested by the sensors. To maximize the power reception by the sensor, the present approaches are based on the dominant channel eigenvector of energy beam [29; 30]. Even the energy beamforming is optimized; if the nodes consume energy inefficiently, the power would be wasted and the network lifecycle performance will be degraded. To maximize the network lifecycle and achieve the immortality of the WSNs, the authors propose to reduce the energy consumption of sensors by optimizing the data routing among the sensors and to improve the WPT efficiency by optimizing the energy beamforming of the chargers [31].  
Energy harvesting and WPT are capable of relieving the battery limitation of sensors, an amplify-and-forward relay network (AF-RN) is proposed in [32]. The strategy is a joint power control and energy transfer scheme to maximize the throughput by considering the energy causality constraints. In [33], an AF-RN with energy harvesting (EH) source and relay nodes is investigated. To consider the energy arrival profiles and the energy cooperation between EH nodes, a joint power control and transfer is designed to maximize the total data rate, subject to energy causality and battery storage constraints. With the total source transmission power budget and energy-causality constraints, the authors in [34] formulated an energy efficiency maximization (EEM) optimization problem for the multi-user multicarrier energy-constrained amplify-and-forward (AF) multi-relay network. Under the simultaneous wireless information and power transfer (SWIPT) model, each forwarding node is solely powered by the source nodes, employing an energy harvesting time-switching (EHTS) scheme to harvest the energy via the radio-frequency (RF) signal transmitted from the source nodes. In addition, a suboptimal and best relay selection algorithm is also examined to trade-off between complexity and performance.  
The above energy balance approaches seem to be comprehensive; however, they are just passive and late-remedy solutions based on multiple forwarding factors to determine an appropriate routing path. On the other hand, the above efficient wireless charging schemes consider the efficient routing node selection instead of the balanced routing node determination. To jointly consider the design issues on the balanced routing and the effective WPT, a SPT-EBR is proposed to improve the load balance and thus improve the WPT efficiency. The SPT-EBR is an early-intervention method to make the load balancing in the SCA layer and all the other intra-layers. The proposed method can also avoid the frequent routing update messages and the excessive load concentration in the following maintenance phase. After load balance in each intra-layer, the energy harvesting scheme can be applied to efficiently charge for the hotspot nodes.