# Non-Uniform Clustering Algorithm for UWSNs Based on Energy Equalization
## 2 Related Works  
UWSNs have been under research over the last decade and researchers have proposed different routing algorithms to extend the network lifetime and reduce high energy consumption. In [12], a classical depth-based routing (DBR) protocol, also known as a locality-free protocol, is proposed. This algorithm uses a greedy flooding strategy, in which sensor nodes calculate the depth using depth sensors, and high-depth sensor nodes transmit data packets to low-depth sensor nodes until they reach the sink. The biggest advantage of this algorithm is that data can be transmitted to the sink based on the simple routing principle. However, this protocol also has certain drawbacks. First, data may be sent in different ways, possibly leading to high energy consumption. The protocol has a low data transfer rate in sparse networks. Second, due to convergence behavior, failures occur earlier in sensor nodes close to the sink. In [13], the proposed EEDBR is an extension of the DBR protocol, which selects the optimal forwarding node while combining the residual energy parameter and depth. The sensor node checks the depth and residual energy of the receiving node with itself. Among the neighboring nodes, the node with the smallest depth value and the largest residual energy becomes the next hop destination. Although this algorithm equalizes the network energy consumption to some extent, there is still a poor energy-saving effect. All of the above are planar routing protocols. The principle of planar routing protocols is usually simple, and the energy consumption problem of the network can be solved to some extent when considering the energy factor. However, cluster routing protocols have more advantages than planar routing protocols [14; 15]. Firstly, cluster routing protocolsconsume less energy and have an evenly distributed energy consumption, which is effective in prolonging the network lifetime and balancing the network load. Secondly, cluster routing protocols are based on some cluster formation strategies that choose to produce a more stable subnetwork, thus reducing the impact of topological changes on the routing protocols. Moreover, cluster head nodes manage nodes within the cluster and can easily communicate various information about the nodes to the base station, such as energy, security, failure, etc. In addition, the base station can effectively send commands to other nodes in the network through the head node, which cannot be effectively achieved by flat routing. In [16], the adaptive low-power LEACH algorithm is proposed, which is a classical clustering algorithm that uses a wheel-based sensor network for topological reconstruction and uses a probability function to choose the cluster head. This algorithm uses a single-hop transmission mechanism and fails to consider the uneven distribution of cluster heads and the different sizes of each cluster, which can easily lead to the premature death of cluster heads due to uneven energy consumption. In [17], the authors improved the LEACH algorithm by proposing the DEEC algorithm. This algorithm introduces the residual energy of nodes in the cluster head selection process to increase the probability of nodes with higher residual energy to become cluster heads. It can further extend the network life cycle but does not consider the impact of node density on the network life cycle. The cluster heads are also used for single-hop transmission with poor scalability, which is not applicable to large wireless sensor networks. In [18], a multi-hop non-uniform clustering algorithm (EEMUC) is implemented in UWSNs. Based on the distance between the node and the sink, the whole network can be divided into an uneven arc-shaped layered model centered on the sink. The nodes in each layer select cluster heads according to the integrated attribute values and reduce the clusters close to the sink. The multi-hop data transmission is used between the clusters to effectively equalize the energy consumption of cluster heads.