# La-CTP: Loop-Aware Routing for Energy-Harvesting Wireless Sensor Networks
## 2 Related Work  
There have been a great amount of routing protocols or algorithms [22; 23; 24] proposed for WSNs, but they cannot fit in well for practical EH-WSNs, especially being incapable of handling loops [25; 26]. Similar to the energy harvesting scenarios, duty-cycled WSNs [18; 27; 28] allow nodes to switch their status from working to sleeping, and back to working. In the duty-cycled scenario, the node switches its status according to a working schedule or a duty cycle that is usually configured in system initialization. The working schedule consists of active slots and sleeping slots, and the span of a single slot is about tens of or hundreds of milliseconds. Dividing the operation time into slots allows the node to reduce the energy consumed by the idle listening of channels, while not impacting the transmission delay too much. There are three main differences between duty-cycled WSN and EH-WSN. First, compared to the duty-cycled nodes, the EH-WSN nodes could leave the network for energy replenishment for a longer time, say, hundreds of seconds or even much longer, which depends on the capacity of energy harvesting per se. Second, in duty-cycled scenario, the node can be waked up by the routing protocol to work, if it is required to participate into packet forwarding. For the EH-WSN scenario, however, a node cannot re-join the routing process in any way, until it completes battery recharging. Third, in practice, the recharging period of an EH-WSN node is likely, but far from certain, to be accurately estimated [29], because the environmental energy source in use is time-varying in output power and hard to be modeled in advance. In duty-cycled WSNs, the nodes often depend on global time synchronization such that all nodes operate strictly by their working schedules and each node can precisely know the working schedules of its neighbors. In recent, researchers have studied or designed routing protocols and algorithms for emerging EH-WSNs. CHESS [30] (Communication using Hybrid Energy Storage System) is an early routing metric for EH-WSNs, which considers the efficiency of energy storage and usage and focuses on the management of energy recharging. In [31], an opportunistic routing protocol, called EHOR, was proposed. EHOR first divides the nodes into different groups and considers the residual energy in forwarder selection. EHOR was evaluated with a simulator. In [32] an opportunistic routing protocol (AOR) was presented for EH-WSNs. However, AOR depends heavily on the geographic information of the deployment, without considering the possible loops. [PERSON] et al. [33] designed a scheme for fair and high throughput data collection in the case of involving the EH-WSN nodes. [PERSON] et al. [34] designed a stability-aware geographic routing for EH-WSNs; they aimded to improve the network throughput while considering the harvesting capacity and the residual energy. [PERSON] et al. [35] proposed a gradient information-aware, hierarchical clustering algorithm. The authors believe that no loops will occur, as the forwarder selection always refers to the gradient information that should keep increasing along the delivery path to sink. However, their numerical experiments did not consider the effect of dynamic wireless channel and random recharging durations. [PERSON] et al. [36] collected data from energy-harvesting nodes that were organized into a single-hop network and constructed the models for throughput optimization. Several other routing approaches (summarized in [37]) were proposed for EH-WSNs and they were dedicated to improving the energy efficiency by balancing the energy consumption. Most of the existing works put more effort on the energy efficiency improvement and on the recharging regulation during packet routing. Furthermore, being evaluated by simulation experiments, they were usually designed as a pure routing layer, without the support from or the compatibility with other indispensable low-level communication layers. Therefore, they might not suit the real-world EH-WSNs easily.