IEEE SENSORS JOURNAL, VOL. 16, NO. 11, JUNE 1, 2016 4431 On Energy Hole and Coverage Hole Avoidance in Underwater Wireless Sensor Networks Kamran Latif, Nadeem Javaid, Member, IEEE, Ashfaq Ahmad, Student Member, IEEE, Zahoor Ali Khan, Senior Member, IEEE, Nabil Alrajeh, and Majid Iqbal Khan Abstract—Due to the limited battery capacity of sensor nodes, a minimization of energy consumption is a potential research area in underwater wireless sensor networks (UWSNs). However, energy hole and coverage hole creations lead performance degra- dation of UWSNs in terms of network lifetime and throughput. In this paper, we address the energy hole creation issue in depth- based routing techniques, and devise a technique to overcome the deficiencies in existing techniques. Besides addressing the energy hole issue, the proposition of a coverage hole repair technique is also part of this paper. In areas of the dense deployment, sensing ranges of nodes redundantly overlap. Our proposed technique takes a benefit of redundant overlapping and repairs a coverage hole during network operation. Simulation results show that our two techniques cohesively conserve nodes’ energy, which ultimately maximizes the network lifetime and throughput at the cost of increased delay. Index Terms— Coverage hole, routing, energy efficiency, underwater wireless sensor networks, energy hole. I. I NTRODUCTION G IVEN the persistent and critical water resource challenges in [1], applications of information and communication technologies can achieve water sustainability. In this regard, UWSNs play an important role as these deploy underwater sensors (nodes) to monitor the parameters of interest. The sensed information is then sent to off-shore sink(s) where it is interpreted to take proper actions; underwater communication is carried via acoustic modems and communication above the surface of water is carried via radio modems. In addition, recent research has shown manifold interest in underwater environment due to expanded application horizon including: detection of underwater oilfield reservoirs, exploitation of undersea minerals, providence of tsunami warnings, monitor-ship of undersea deployed expensive equipments, mine reconnaissance, [2], etc. Irrespective of the application, nodes are exposed to harsh underwater environment that pose many challenges subject to long term operation of nodes. In UWSNs, the radio Manuscript received September 30, 2015; revised November 23, 2015 and February 12, 2016; accepted February 16, 2016. Date of publication February 24, 2016; date of current version April 26, 2016. The associate editor coordinating the review of this paper and approving it for publication was Prof. Mehdi Javanmard. (Corresponding author: Nadeem Javaid.) K. Latif, N. Javaid, A. Ahmad, and M. I. Khan are with the COMSATS Institute of Information Technology, Islamabad 44000, Pakistan (e-mail: klatif0077@gmail.com; nadeemjavaidqau@gmail.com; ashfaq_comsats@ yahoo.com; majid_iqbal@comsats.edu.pk). Z. A. Khan is with the Faculty of Engineering, Internetworking Program, Dalhousie University, Halifax, NS B3H 4R2, Canada (e-mail: zahoor.khan@dal.ca). N. Alrajeh is with the Biomedical Technology Laboratory, College of Applied Medical Sciences, King Saud University, Riyadh 11633, Saudi Arabia (e-mail: nabil@ksu.edu.sa). Digital Object Identifier 10.1109/JSEN.2016.2532389 signals undergo rapid attenuation and high absorption rate. More specifically, the radio signals propagate in seawater only in extra low frequency range (i.e., 30 - 300 Hz). However, this range requires high transmit power and large antennae. For example, the reported transmission range of Berkeley Mica 2 Motes is 120 cm at 433 MHz [2]. Data transmission or/and reception is thus not feasible via these signals. As an alternative, acoustic signals are typically used in such environments due to relatively low absorption rate. However, the acoustic signals have featured low bandwidth and relatively high end-to-end delay as underwater sensor nodes are sparsely deployed. The acoustic channel is highly impaired due to multi path and fading. Thus, high bit error rates are experienced. Furthermore, the underwater sensors are prone to failures due to fouling and corrosion. All these characteristics make the UWSN routing task very difficult. In addition, the UWSNs have highly dynamic topology that requires regular and frequent information exchange between network entities (nodes) if proper network operations are desired. However, these frequent and regular updates lead to significant routing overhead. Energy constrained nature of the nodes further demand for network lifetime maximization. Thus, network lifetime maximization, reliability improvement, efficient data gathering, and end-to-end delay minimization are always among the desired objectives when routing protocols are designed for UWSNs [3], [4]. In [5], authors identify three subsystems in an underwater sensor; (i) sensing subsystem, (ii) processing subsystem and (iii) wireless communication subsystem. The first subsystem is used to acquire data, the second subsystem is used to process data and the third subsystem is used for communicating data. According to [6], these sensors perform collaborative monitoring tasks that involve high degree inter-sensor com- munications which lead to their high energy consumption cost. Authors in [7] point out the highly dynamic network topology of UWSNs which requires frequent exchange of messages (high overhead) among the sensor nodes for proper network functioning. Being small in size, the underwater sensors operate on tiny batteries (limited energy resource) to perform collaborative tasks [8]. Since most of the UWSN applications require large number of deployed sensors in harsh and highly unpredictable underwater environment, either infeasibility or almost impossibility to replace or recharge the batteries of these sensors further demand for network lifetime prolongation [6], [7]. Thus, improvement of energy efficiency is a potential research area in UWSNs because limited energy source of nodes is one of the major constraints in the long term operation of UWSNs. Therefore, intelligent and efficient 1558-1748 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.