# Adaptive Energy Balanced Routing Strategy for Wireless Reschargeable Sensor Networks
## 1 Introduction  
With the wide applications of sensor networks, the demand for long-term operation of the networks in various occasions is increasing, which makes the lifetime of wireless sensor networks become an increasingly important research point. A great part of the recent literature [1; 2; 3; 4; 5] has focused on this problem from different aspects, such as charging process, efficient routing, dynamic user association, etc. Among them, routing and charging technology are two common ways. Routing energy-saving algorithms [6; 7; 8; 9] are used to reduce nodes' energy consumption by all means. However, the method cannot fundamentally solve the energy constraint problems; that the capacity of sensor battery is always small and needs to be replaced frequently for long-term operation. It is costly to replace battery for nodes in time when they deplete energy, especially for nodes in remote areas. The wireless energy transmission technology, allowing fixed or mobile energy supplier to transfer energy to nodes wirelessly, becomes a significant solution to charge nodes timely. Its high replenishment efficiency and strong controllability guarantee broad application prospects. In wireless sensor network (WSN), the sensor nodes send packets to the sink node in the form of multi-hop. However, unreasonable routing strategy can easily lead to energy waste or uneven energy consumption, thus shortening the network lifetime. Before the emergence of charging technology, research mainly focused on optimizing routing strategies to reduce network energy consumption and balance network energy [10; 11; 12; 13; 14]. Among them, the minimum path algorithm [10] tends to minimize the total energy consumption of network. Distributed energy balanced routing (DEBR) [11], one of themost effective energy aware routing protocol, takes the node's residual energy into consideration to balance the energy utilization rate and residual energy. The energy aware routing protocol is extended to address big data routing in [12], and the quality of service and priority levels are considered for routing selection. The authors in [13] divided the network into clusters and a mobile collector to collect data from chosen cluster heads, and the concept of fuzzy if-then rules is adopted to choose the cluster head. The energy optimized dynamic clustering (EODC) [14] also considers nodes' location, energy, and link quality. And particle swarm optimization (PSO) and manhattan distance are used for the clustering of nodes, so as to optimize the consumption of energy. Afterwards, the charging technology is applied to WSN, and wireless rechargeable sensor networks (WRSN) emerges. In WRSN, nodes are able to harvest energy from the radio frequency signals sent by the mobile wireless charging vehicle (WCV), which is the mainstream energy supplier. For charging strategy, one key issue is how to allocate the charging time and to adjust the charging sequence, which has a great impact on the charging effect. In [15], the nodes, whose energy below a certain energy threshold are identified as the target charging nodes, and the charging time is distributed proportionally according to their energy requirements, so as to maximize the energy utilization. A double warning thresholds with double preemption (DWDP) charging scheme is proposed in [16], in which double thresholds are used to adjust charging priorities of different sensors, so as to improve the charging throughput of the network and charging efficiency of WCV. In [17], sensor nodes with charging requirements are firstly screened out, and those with low residual energy will be given priority to charge. After determining the charging sequence, the charging path is planned to extend the total charging time by reducing the moving time, so as to supplement more energy for the network. However, WCV in these literatures only charges for nodes whose energy are lower than the threshold in each charging cycle. Thus, the situation that most nodes need to be charged at the same cycle may occur after a period of time.