# Data Gathering Techniques in WSN: A Cross-Layer View
## 5 Routing for Data Gathering in WSN
### Data Aggregation Routing Protocols for Energy Harvesting WSN
#### 5.2.1 Cluster-Based Routing Protocol That Relies on EH  
Several studies have suggested various adaptations for LEACH WSN cluster-based routing protocol (e.g., [106]) for EH-based WSN (EH-WSN). Recall that LEACH's cluster-head selection mechanism randomly selects sensor nodes as cluster-heads to distribute the energy consumption between them evenly, which can seemingly cope with the EH constraints. However, note that even when the nodes' platform is exactly the same, the potential of different nodes to harvest energy, dependent on their specific ambient conditions, can be very different. Furthermore, the node's location with respect to the sink can highly influence the amount of data it needs to relay toward the sink, causing high discrepancies between the energy utilized by various nodes. To cope with these discrepancies, [PERSON] et al. [143] modify LEACH's cluster-head selection mechanism and define a new metric termed \"energy potential function\" to measure each node's capability to harvest energy. The paper devises a cluster-head selection strategy that prioritizes nodes with higher expected stored energy (based on the currently available energy and their potential energy) to become cluster-heads, regardless of the number of instances that the node was selected as cluster-head in the past. To address the imbalance between the energy expected to be consumed by cluster-heads (CHs) that are closer to the sink and are expected to spend more energy on relaying packets from farther clusters, and CHs that are further from the sink and expected to relay less traffic, [PERSON] et al. [144] suggest an unbalanced clustering mechanism. In particular, cluster sizes are determined according to the distance (hop count) to the sink to balance the energy consumption of the CHs. Accordingly, clusters closer to the sink, which are expected to relay more inter-cluster traffic, will be smaller, so that they collect less intra-cluster traffic; clusters further away from the sink and expected to relay less inter-cluster traffic will comprise more nodes, so that they collect more intra-cluster traffic. The mechanism suggested to attain this balance is partitioning the network conceptually into concentric rings around the sink with linearly increasing radii. Each ring comprises nodes with the same hop distance to the sink. Clusters within the same ring will have the same size. CH selection is designed to balance the loads of each ring considering the available nodes' energy, which is evaluated based on the EH rate. Following a similar approach, [PERSON] et al. [145] assume a highly symmetric circular sensor network in which the sink (BS) is located at the center, and the sensors are distributed evenly in a disc around it. As with [144], the sensor field is divided into concentric rings; however, in this study, the rings are determined to have an equal area such that the number of sensors in each ring is expected to be the same. Under the given model, Ref. [145] analyzes the energy consumption of intra- and inter-cluster data transmission and derives the energy neutrality constraints, which guarantee that each node consumes less energy than the amount of energy it has harvested. The authors further devise a constraint formula of the number of clusters required in each layer (ring) that balances the average energy consumption of nodes in different layers. The energy neutrality constraints and the cluster parameters are used to obtain the parameters (number of rings, number of clusters in each ring, and minimum network data transmission cycle) that minimize the data transmission cycle. Based on the attained parameters, the cluster-based routing protocol is derived. The protocol consists of an initialization phase and repeated cycles divided into topology formation and data-gathering phases. [PERSON] and [PERSON] (2016) suggest two separate enhancements. The first, termed cooperative transmission strategy, enables nodes to serve as relays to relay undelivered packets from cluster members to CHs and from CHs to the sink node.