## 2 Related Work  
In LEACH [6], clustered hierarchical networks are used to increase the energy efficiency of WSNs. The clustering method selects a place for data collection (i.e., CH) per cluster. In addition, a probabilistic approach is used for CH selection, but the information of the cluster nodes, such as remaining energy, is not considered. To utilize the information of other nodes, it should be transmitted, but sending and receiving such information on wireless networks is difficult. LEACH-C [7] uses time-division multiple access to address this problem. The BS informs each CH about the CH selection results and synchronizes transmission. The CHs also communicate with neighboring nodes and send schedules to eliminate data time lags. LEACH-C performs CH selection at other network components than the sensor nodes, which have limited computational resources. Because computations for CH selection are handled by the BS and other components, high computing power can be leveraged. In recent years, protocols outperforming LEACH-C have been proposed, as CH selection that considers more data is feasible with improvements of computing power. In particular, swarm intelligence has been adopted in several protocols due to the high computing power currently available. PSO-C was the first protocol to utilize PSO in WSNs [8]. After finding the locations of the optimal CHs, the nodes closest to these locations are defined as CHs. As search using PSO can provide remarkable results, optimally locating the CH increases both energy efficiency and the lifetime of the WSN. However, PSO-C selects the CH without considering the distance to the BS, possibly reducing energy efficiency during CH-BS data transmission. A PSO based Energy efficient Cluster Head Selection (PSOECHS) [9] addresses this problem via an objective function that considers the CH-BS distance and extends the network lifetime by varying the way each node selects a CH, which allows for the control the number of nodes belonging to each cluster. Thus, energy consumption can be controlled during data reception from each CH. However, selecting a CH per sensor node demands high computational power and increases energy consumption, aspects disregarded during PSOECHS simulations. In [19], the amount of received data and number of nodes were adjusted by considering the coverage area of each CH, assuming that most sensor nodes are evenly distributed in the WSN. Thus, if the coverage is similar across CHs, they receive data from a similar number of nodes. Therefore, even if a node does not select a CH, the amount of received data can be adjusted. PSO-EC calculates energy distribution by fixing the coverage area and selects the energy centers as CHs [10]. By selecting the node with the highest energy among surrounding nodes as CH, it improves energy efficiency. However, this method relies on energy distribution, undermining its performance at the initial state when node energy is evenly distributed. SMO-C [16] is a protocol that adopts SMO [15], and like PSO-C, optimally locates the CH that is assigned to the nearest node. The objective function consists of two fitness values, namely node-CH distance and the energy consumed by nodes and CHs. However, energy consumption when sending data from a node to the CH is determined by the distance between adjacent nodes. Therefore, more calculations than in other protocols are required to obtain the fitness values, and the results have not shown a considerable improvement over similar protocols. In fact, the results in [16] showed that SMO-C is not much better than LEACH. Alternatively, SMOEC [17] has been shown to improve SMO-C by specifying the transmission protocol between CHs. Though the network lifetime is improved, stability remains an issue because certain nodes early deplete their energy. When using clustering based on PSO and SMO, a specific location is first determined for a CH. Then, the node closest to this location is defined as CH. In SMOTECP [18], CH selection is directly optimized to avoid this unnecessary computation.