# Towards Efficient Sink Mobility in Underwater Wireless Sensor Networks
## 1 Introduction  
With the advancement in technologies being used underwater, there has been massive increase in research activities related to underwater wireless sensor networks (UWSNs) by both academia and industry [1; 2; 3]. They are comprised of sensor nodes and sink(s) that are deployed on the seabed through ships. The deployed sensor nodes transmit the sensed data to the sink through a specific routing mechanism [2; 4; 5; 6; 7]. UWSNs are used as substitutes to traditional exploration and monitoring of oceans. Many applications such as harvesting renewable energy, environmental monitoring, oceanographic data collection, and exploration of underwater hidden resources require proper infrastructure and timely maintenance. The high-pressure underwater environment restricts human ability to work there for a long time. Terrestrial Wireless Sensor Networks (TWSNs) is a well-researched area with many routing strategies to choose from. The traditional method of using radio signals for communication in TWSNs cannot be applied in UWSNs because these signals are heavily attenuated in the aqueous environment. Thus, acoustic signals are widely used for such purposes, as they have a low absorption rate and are feasible for underwater communications [8; 9; 10].  
The network topology of UWSNs is dynamic i.e., the nodes' position keeps changing due to change in water current [5; 6; 11]. Furthermore, maintaining the network lifetime in UWSNs ischallenging because the sensor nodes are powered by the batteries that cannot be replaced after underwater deployment.  
In a nutshell, existing routing protocols for TWSNs cannot work for UWSNs due to long propagation delay, high mobility, limited bandwidth, energy-constraint and non-uniform topology structure caused by the node movement. Many routing protocols have been proposed for UWSNs. Some of these maximize the network lifetime on the cost of throughput and some improves throughput by compromising on packet delay [6]. Among these protocols, clustering is reported as a best technique for load balancing in UWSNs where sensor nodes are arranged in multiple groups. These groups are headed by one sensor node known as a cluster head [6; 11; 12; 13]. Similarly, the use of a mobile sink has been observed as an efficient way of improving throughput [14; 15]. Mobile sinks can visit each sensor node individually or the entire cluster to gather data.  
In [11], authors employed two mobile sinks to gather data in sparse regions. The throughput of network is affected when both sinks take adjacent positions as they are covering the same sensor nodes [11; 16]. Recent research proves that the optimum number of mobile sinks with efficient path movement maximizes network performance [14; 15; 17].  
Based on the above discussion, a routing protocol has been proposed that uses two mobile sinks, in a way that they cannot cover the same transmission area. The focus of the proposed scheme is to minimize the packet drop ratio by efficient utilization of mobile sinks. Apart from the routing scheme, a new metric; \"Mobile Sink Utility Ratio (MUR)\" is also introduced in this paper. MUR represents the data packets collected by the mobile sink to the total packets sent from the sensor nodes in any specific round. The higher value of MUR shows the efficient usage of mobile sinks and vice versa.  
The rest of the paper is organized as follows: in Section 2, related work is discussed. Section 3 describes the limitations in the sparsity aware energy efficient clustering routing protocol (SEEC) routing scheme.  
Section 4 presents the proposed scheme in detail. Performance evaluation of the proposed routing protocol and its comparison with existing routing schemes is discussed in Section 5. Conclusions and future work is presented in Section 6.