# Energy Efficient Routing in Wireless Sensor Networks: A Comprehensive Survey
## 3 Network Structure
### Hierarchical
#### 3.2.3 Fuzzy Logic Based
MOFKA calculates cluster head competition radius and takes into account three parameters:  
1. Distance to the base station,
2. Node's residual energy, and
3. Density of the cluster. Also, fuzzy logic is used to overcome the uncertainties that may occur during WSN operation. With the use of MOFKA, higher energy efficiency is achieved comparatively with other fuzzy logic based protocols. In addition, scalability is attained. Moreover, mobility of nodes without increasing energy consumption is supported. On the other hand, performance declines in dense networks. SEP based on Fuzzy Logic (SEPFL) [112] protocol is an improved version of the SEP protocol, which utilizes fuzzy logic control during cluster head election. During the setup time, the base station collects information about the nodes and then utilizes fuzzy logic to further operate. It uses three fuzzy parameters, battery level, distance and density to assign a value to every node representing its probability to be elected as a cluster head. After that, the protocol calculates two threshold values, one for normal nodes and one for advanced and stores them. Then, SEPFL considers these two values for each node in order to elect cluster heads. By using SEPFL, network lifetime is extended and great stability is attained. Nevertheless, overhead is caused due to the complexity of the algorithm. Fuzzy Logic-TEEN (FL-TEEN) [113] is a version of TEEN that implements the fuzzy inference system during the cluster head election process. The fuzzy inference system utilizes three parameters:  
1. Battery level, which is the residual energy of the nod. 2. Factor of node density, which shows the number of neighbor nodes or in case of a cluster head it is an indirect indicator of the amount of collected data. 3. Data frequency, which is the data satisfied by the two thresholds of the previous rounds and indicates the quantity of data collected from the corresponding area. At the start of each round, nodes gather the values of these three parameters of themselves. Routing takes place by considering these values. With the use of FL-TEEN, network lifetime is prolonged and, comparatively to TEEN, the election of cluster heads is more sophisticated. In addition, FL-TEEN improves network stability and uniformity of energy However, FL-TEEN needs more variables to consider in order to achieve better cluster head elections. Hybrid Hierarchical Clustering Approach (HHCA) [114] is a protocol that utilizes a three-layer approach and the centralized Fuzzy C-Mean (FCM) approach. HHCA forms grids that include clusters and separates the network into three layers:  
1. Layer three is the base station,
2. Layer two is the Grid Heads (GH),
3. Layer one is the cluster heads, and
4. Layer zero is the Sensor Nodes (SNs). Data packets are transmitted from the SNs, in layer zero, to the upper layers until they reach the base station. In order to form grids and to elect GHs, in every round, nodes send their residual energy and position information to the base station, which performs the FCM algorithm. Then, a distributed LEACH approach with energy considerations is performed to form clusters and to elect cluster heads. With HHCA, great energy balancing improved credibility is attained. On the other hand, reliability issues take place. A synopsis of the characteristics of the aforementioned zone based hierarchical routing protocols is presented in Table 9.