# Towards Efficient Sink Mobility in Underwater Wireless Sensor Networks
## 2 Related Work  
[PERSON] et al. presented a location-free depth base routing scheme (DBR) for UWSNs [7]. In DBR, depth is a selection criteria for the data forwarding node. Sensor nodes send their depth along with sensed data. Nodes with low depth are the one forwarding data. This scheme is suitable for networks where sensor nodes are deployed close to each other. Initially, sensor nodes in DBR are densely deployed; however, due to unbalanced energy consumption, energy holes are created. Energy hole creation degrades the performance of DBR resulting in low throughput. In [5], the authors proposed routing protocol called weighting depth and forwarding area division depth base routing protocol (WDFAD-DBR). It reduces the void holes problem and achieves reliability in sparse deployment. The energy consumption is balanced in this protocol through division criteria of forwarding regions. Forwarding regions are divided on the basis of node density and the condition of channel. In [11; 16], SEEC is proposed. In SEEC, clustering is implied on dense regions while mobile sinks are sent to a sparse region to gather data. Clustering in dense regions made the protocol energy efficient. The stability period is also increased as compared to an energy efficient depth based routing protocol (EEDBR) [18]. Mobile sinks in sparse regions are not utilized efficiently, which causes low throughput in SEEC. In [19], the authors proposed energy efficient multilevel cluster selection scheme in which a cluster head is selected on the basis of less radius from sink and high residual energy of the nodes. The novelty in the proposed scheme of [19] is the different level of cluster and competition stage for selecting the clustering head. To become a member of the competition stage, nodes of clusters must have residual energy above the calculated threshold energy and must be in a maximum radius of neighbor nodes. The unique method of clustering in [19] made the scheme energy efficient. [PERSON] et al. [20] presents the sink mobility assisted opportunistic and geographic routing protocol to maximize the network lifetime in UWSNs. They divide the network into logical cubical regions to avoid the interference. Moreover, they use a specific number of forwarded nodes in each region to forward the packets to the destination. However, interference can be further minimized by using a single forwarder instead of multiple forwarders in each region. The authors in [21] proposed and simulated a cluster-based reliable and energy-efficient routing protocol for UWSN. They prove that using a clustering technique helps to reduce the overall energy consumption of a sensor network. Consequently, the network lifetime is improved. A hybrid approach for the selection of cluster heads in static and dynamic clustering in terrestrial wireless sensor networks is presented in [12]. They have minimized the energy holes and maximized the network lifetime. Network coverage and cluster load balancing are achieved by the dividing network field into rectangle and square shapes. Cluster heads (CHs) with adaptive clustering is discussed for terrestrial wireless sensor networks in [13]. The clustering scheme arranges a group of sensor nodes into a cluster. One node from each cluster is appointed as CH. The CH forwards the data sent by sensor nodes to the sink. In this way, energy consumption of nodes that are far from the sink is reduced. Two steps have been proposed for the selection of CHs. First, the election of CH depends on the threshold value of energy. After that, selection is done on the basis of optimal distance between elected CHs. This technique maximizes throughput along with the network lifetime. The authors proposed a distributed data gathering protocol (DDG) in [15]. The proposed scheme uses an autonomous underwater vehicle (AUV) along with clustering. AUV collects data from selected CHs by visiting them. This approach reduces transmission overhead on sensor nodes and, hence, network lifetime is increased.