# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.1 Hierarchy Based
_Quadrature-LEACH_ (Q-LEACH) [48] is a clustering protocol for homogeneous WSNs, based on LEACH, which divides the network into four equal, parts based on location information and uses an innovative method for cluster head election. For cluster head election in Q-LEACH, every sensor node chooses a random number between 0 and 1, while in every sector there is a threshold between 0 and 1. In the case that the random number of a sensor node is less than the threshold of the network part, which has been set, then the node becomes the cluster head, and broadcasts it to the network. Normal nodes select their cluster head within their own sector based on the Received Signal Strength Indicator (RSSI) of the broadcasting and communicate with it using their assigned TDMA slot. In Q-LEACH, cluster head election does not consume energy and improved network lifetime is achieved. However, the energy holes of the network may be increased. _Universal-LEACH_ (U-LEACH) [49] is a clustering routing protocol that combines Improved LEACH operation and the chain formation of PEGASIS. U-LEACH elects cluster heads by taking into consideration node residual energy and forms clusters based on their position on an \(x\)-axis. After clustering, it uses the chain formation process of PEGASIS to construct a chain starting from the distant cluster heads within every cluster and ending at the BS. Cluster heads transmit their data to the master cluster head, which aggregates data and forwards them to the BS. In U-LEACH, greater network lifetime is achieved due to the reduction of the energy consumption. On the other hand, the distance between master cluster head and the BS are not taken into consideration. _Hybrid-LEACH_ (H-LEACH) [50] is a protocol used to face the problems of LEACH and HEED protocols, such as energy holes and the inability to elect cluster heads based on different energy levels. To deal with these issues, it blocks low energy nodes to be elected as cluster heads. H-LEACH takes into account the maximum energy of a node, its remaining energy, the energy required for data transmission, and the probability of energy usage, and implements a threshold value. The nodes below the threshold are considered dead. Furthermore, to evaluate the routing paths, it uses the distance between transmitter and receiver. Finally, cluster heads transmit data to the BS by utilizing TDMA. By using H-LEACH, the network lifetime is extended. On the other hand, the number of unused nodes which are below the threshold is high. _Three-Layer LEACH_ protocol (TL-LEACH) [51] is another clustering protocol for WSNs, based on LEACH, which reduces the number of nodes that communicate directly to the BS with a similar cluster head election as in LEACH. The network is divided into three layers as follows:Layer 0 sensor nodes. 2. Layer 1 cluster heads elected from Layer 0 nodes. 3. Layer 2 cluster heads elected from Layer 1 cluster heads. Layer 0 nodes sense the environment and send their packets to layer 1 cluster heads; data packets are forwarded there to layer 2 cluster heads and eventually to the BS. In every round, nodes elect themselves randomly as layer 1 cluster heads, while layer 2 cluster heads are elected based on the residual energy of layer 1 cluster heads. Furthermore, in the next rounds, cluster heads are changed randomly, while information from layer 1 cluster heads are fused in layer 2 cluster heads to be sent to the BS later. With the use of TL-LEACH, the number of sensor nodes communicating with the BS is low and the network lifetime is prolonged. On the other hand, the three-layer scheme used creates overhead. Enhanced Heterogeneous LEACH (EHE-LEACH) [52] is an enhanced version of LEACH for heterogeneous WSNs. EHE-LEACH combines the BS to node direct communication and the BS to cluster communication, based on a fixed distance threshold value.