# A PSO-Based Uneven Dynamic Clustering Multi-Hop Routing Protocol for Wireless Sensor Networks
## 2 Related Work  
### Classical Routing Protocols  
Low energy adaptive clustering hierarchy (LEACH) [6] is a well-known clustering protocol. An important contribution of LEACH is the introduction of the concept of clustering hierarchy for load balancing among sensor nodes. The operation of LEACH is divided into a number of rounds. Nodes take turns as CH nodes to balance the energy consumption of the nodes. The main drawback of the LEACH algorithm is that random selection of CH nodes may result in uneven distribution of them and cause the imbalance of energy consumption. Moreover, the single-hop transmission of LEACH causes the CH nodes to quickly consume energy and die sooner. There have been many improvements [17] to the shortcomings of LEACH. CogLEACH (Cognitive LEACH) [18] improved the LEACH deficiency to make the CH distribution more reasonable. Aiming at the problem of uneven energy consumption of CogLEACH, [PERSON] et al. [19] introduced a centralized cognitive LEACH (CogLEACH-C). In the protocol, besides idle channels, nodes' residual energy is also used as a parameter for the CH selection to balance the energy load of nodes. However, it is not suitable for large-scale networks because it uses the single-hop transmission during routing. [PERSON] et al. [20] proposed a new idea called EE-LEACH (Energy-Efficient LEACH protocol). It provides an optimal cluster formation and efficient data aggregation, which saves a significant amount of energy. However, combination of several technologies increases the complexity of the algorithm.  
Grid-based protocols, such as EEBCDA (Energy Efficient and Balanced Cluster-based Data Aggregation algorithm) [14] and Multi-hop EEBCDA [15], can efficiently address the hotspot problem. Multi-hop EEBCDA is an improvement based on EEBCDA. It divides the network area into multiple rectangles where each rectangle has an unequal number of grids and the nodes in each grid form a cluster. This approach significantly reduces the amount of energy utilization and increases the network lifetime. Since the number of grids per rectangle is inconsistent, there is unnecessary forwarding between the layers. There may be a case where the upper layer nodes died and the lower layer nodes cannot transmit data, which means a lot of energy waste. EEMRP (Energy-Efficient Multi-hop Routing Protocol) [16] proposed a grid clustering algorithm for creating clusters and introduced communication management (CM) nodes to relay multi-hop data transmission. The method effectively balances the energy consumption of the network because the CM nodes share the workload of the CH nodes. However, the division of each grid needs to be determined in advance, which limits the scalability of the algorithm. [PERSON] et al. proposed the CAMP (Cluster Aided Multi-Path) routing protocol [21]. The protocol divides the network area into equal virtual areas. Each virtual area is a cluster. In each round, a node can be either a cluster member or a separate node. The nodes perform multi-hop transmission to achieve full coverage of the network. However, CAMP divides the network area into equal areas without taking the hotspot problem into account during data transmission. [PERSON] et al. [22] proposed a fault-tolerant and energy-efficient routing protocol for a virtual three-dimensional wireless sensor network. The protocol can run on any type of 3D WSNs. However, the coordinate system divides the sensor network area into equiangular wedges (or sections), which may cause hotspot problem in the network.