# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
In EHE-LEACH nodes located close to the BS, i.e., those whose distance from the BS is below the distance threshold value, do not form clusters and communicate directly with the BS while the others operate within a cluster with an elected cluster head each. In order to reduce energy consumption, there are nodes with two energy levels, normal and advanced, with advanced nodes having higher energy reservoir than normal ones. Furthermore, for improved energy efficiency and network stability, EHE-LEACH gives higher probability during cluster head election to advanced nodes rather than normal ones. On the other hand, non-clustered nodes do not implement a sleeping schedule, thus consuming more energy. Improved-LEACH (I-LEACH) [53] is another modification of the LEACH protocol that improves the processes of cluster head selection and cluster formation. I-LEACH, during cluster head election, implements a threshold and takes into account three parameters which namely are:  
1. Number of Neighbors. 2. Residual Energy. 3. Ratio of the average distance of nodes from the BS and the distance between each individual node. In order for a node to be elected as a cluster head, it needs to have a high number of neighbors, greater residual energy than the average nodes of the network, and be close to the BS. The result of the above three parameters is a number ranging from 0 to 1. In case the number is under the threshold, the node can be elected as cluster head. During cluster formation, a selection priority is used, based on the distance between cluster head and BS, to reduce energy consumption during packet transmission of remote cluster heads. Moreover, to improve the network's performance, the BS is located in a remote point of the sensor node covered area. The use of I-LEACH assures network stability and reduces energy consumption. However, there is lack of a data aggregation mechanism. LEACH-Expected Residual Energy (LEACH-ERE) [54] is another clustering protocol for homogeneous WSNs, based on LEACH. It introduces the Expected Residual Energy (ERE) value and can be suitable for WSNs having the BS far away from the node deployment field. The ERE value is the predicted remaining energy in case the node is elected as a cluster head. In LEACH-ERE, cluster head election happens at the end of each round using a random number generation function between 0 and 1 and a predefined threshold, which is the percentage of the tentative nodes. Cluster nodes with bigger generated numbers of the threshold become cluster heads and calculate the chance, utilizing a fuzzy inference system, with inputs the residual energy and the ERE value. A bigger chance results in a higher possibility to be elected as a cluster head. LEACH-ERE achieves stable performance. On the other hand, it provides no consideration of distance to the BS, resulting in early death of faraway nodes. The Three Level Hierarchical Clustering LEACH Protocol (TLHCLP) [55] is another clustering protocol based on LEACH, for homogeneous WSNs that sets a radius around BS, to include some nodes within it and exclude the others. TLHCLP tries to distribute cluster heads evenly and divides the network into three layers:  
1. Layer 1, which includes nodes collecting data and sends them to layer 2. 2. Layer 2, which includes the cluster heads that gather data from layer's 1 nodes. 3. Layer 3, which includes nodes and cluster heads inside the radius of the BS. During data transmission, only cluster heads within the radius of the base station are allowed to send data to it directly, the cluster heads outside the radius forward data to the cluster heads within the radius. The use of TLHCLP extends network lifetime. However, data redundancy is high close to the BS. _Dominating Set based modified LEACH using Ant Colony Optimization for data gathering_ (LEACH-DS-ACO) [56] is an improvement of THLCLP used to overcome the problem of redundancy of TLHCLP.