# Self-Adjustment Energy Efficient Redeployment Protocol for Underwater Sensor Networks
## 1 Introduction  
Underwater Sensor Networks (UWSNs) constitute an emergent technology that has caught the interest of many researchers in the last few years. The diversity of applications supported by the UWSNs explains the success of this type of network. These networks serve a multitude of purposes, ranging from the study of marine ecosystems and climate models to supporting offshore industries like oil and gas exploration, maritime security, and underwater infrastructure maintenance [1, 2]. To enable network communication in the harsh underwater environment, underwater sensors mostly employ acoustic signals using acoustic modems [3, 4]. However, acoustic signals still have many challenges and difficulties, including high propagation delay, high attenuation, and limited bandwidth [5]. Furthermore, the Doppler effect is another problem due to the movement of the sender and the receiver caused by the water currents, which affects the signal intensity [6, 7]. Much research focuses on solutions for UWSNs, such as network deployment, energy efficiency, network connectivity, routing protocols, etc., while taking into consideration the different characteristics and constraints of the UWSNs [8]. In this work, we are interested in network deployment, which is a critical step to achieve the application objective since it affects all other UWSN aspects. In fact, the UWSN is composed of interconnected underwater sensors deployed in the underwater aquatic area. The objective of these sensors is to collaboratively monitor and gather information in the network area according to the requirements of the network application. The collected data are then sent to a surface sink node on a multi-hop path [8]. However, the initial random deployment is not efficient: it does not ensure either the connectivity or the coverage of the network area that needs to be monitored. Many deployment solutions [9] based on the swarm [10], graph coloring [11], virtual forces [12], etc., are proposed for the deployment of the underwater sensor networks. Most of these solutions suppose an ideal environment in which the sensor can move from one position to another without accounting for external forces and the effect of these forces on the movement of the sensor, its energy, and its final position. One of the most important external forces exerted on these sensors is the water currents. In fact, for the underwater sensor network, the water currents directly affect the network deployment. Our target is to propose a redeployment protocol that enhances network coverage and connectivity and takes into consideration the water currents in the underwater environment. On the other hand, the energy constraint in underwater sensors, manifested by the limited amount of energy in the sensors' batteries and the difficulty in replacing or recharging these batteries [13], is considered one of the most challenging points when designing solutions for UWSNs. For these reasons, the proposed solution should take into consideration the energy constraints in the UWSN and reduce the energy consumed during the network deployment. Our contributions in this research work are:  
1. Study the impact of the water currents on the deployment protocol. 2. Propose a solution to achieve the deployment while considering the water currents. 3. Exploit these currents to reduce the energy consumed during the deployment. 4. Evaluate the performance of the proposed protocol using a network simulator. Even though our solution can be applied to any proposed deployment protocol, in this research, we chose the distributed virtual forces algorithm (DVFA) [14] for our discussion. The rest of the paper is organized as follows: Section 2 summarizes the research related to the focus of this paper. Section 3 describes the assumptions of our solution. Section 4 presents the distributed virtual forces algorithm used as the deployment protocol reference in this paper. Section 5 discusses the impact of the water currents' force on the network deployment.