# A Compact Bat Algorithm for Unequal Clustering in Wireless Sensor Networks
## 2 Related Work  
### Unequal Clustering in Wireless Sensor Network (WSN)  
Clustering and unequal clustering techniques have been used for an energy efficient WSN. Clusters in WSNs are carried out by grouping sensor nodes into a partitioned set. One of the nodes in the partitioned set is selected to be a cluster head (CH) that collects information from the cluster member nodes (CNs) [16; 29]. Energy consumption of the CHs near the base station (BS) is more than the CHs farther from the BS. This means CHs close to the BS have a heavy traffic load because of intra-cluster traffic from its cluster members, data aggregation, and inter-cluster traffic from other CHs for relayingdata to the BS. This disrupts the network connectivity, and the clusters close to the BS cause coverage issues that are called hot spot problems. The unequal clustering technique is one of the most efficient ways to deal with hot spot problems because it can be used for load balancing among the CHs [30].  
The purpose of the unequal clustering method is the same as the equal clustering approach with adding functions. The clustered nodes are in WSN for different meanings based on the application demand. Energy saving and preventing hot spot problems are the major objectives. Unequal clustering arranges the clusters according to size with the size reduced closer to the BS. This means that the cluster size is directly proportional to the distance of the CHs from the BS. A smaller cluster near the BS indicates a smaller number of cluster members and less intra-cluster traffic. Therefore, the smaller clusters can concentrate more on inter-cluster traffic, and the cluster head does not drain out of energy as quickly. The more the distance to the BS increases, the more the cluster size increases. If the groups hold a more significant number of member nodes, they will spend more energy on intra-cluster traffic. As the clusters become farther from the BS, they have less inter-cluster traffic and do not need to expend more energy for inter-cluster routing. Unequal clustering forces all the CHs to spend the same amount of energy consumption, and therefore the CHs near the BS and the CHs farther from the BS consume an equal amount of energy.  
Additionally, the cluster formation process may generate a two-level hierarchy with higher and lower levels [31; 32]. The CH nodes form the upper-level cluster, and the member nodes create the lower level. The sensor nodes periodically transmit their data to the corresponding CH. The CHs aggregate the data and transmit them to the BS either directly or through the intermediate CH nodes. The CH performs data aggregation of all data received from its cluster members and forwards the aggregated data to the BS via a single hop or a multi-hop. The CH contains the information of its cluster members such as node id, location, and energy level. When a node dies or moves to another cluster, the changes are registered immediately and the CH informs the BS and reclustering occurs to maintain the network topology effectively. The number of transmissions and also the total load of the network are significantly reduced.