# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 1 Introduction  
With a large area of the earth (more than 2/3) covered by water [1, 2], investigating the underwater environment and exploiting the UWSNs in various areas of underwater studies have become imperative due to the increasing human requirements and needs. Applications in different human activities in the underwater environment have become very important and opened a new field for investigators interested in this area. Many researchers such as [3, 4, 5, 6, 7, 8, 9, 10, 11, 12] have proposed solutions to fulfill human requirements and needs in such a harsh environment for industry (detecting chemical pollution, pipeline monitoring, biological phenomena, and seismic studies), government (military applications and maintaining the coast), and nature (hazard events, marine farms, ecological monitoring, and contamination studies). The underwater sensor design in such applications ranges from simple to complex [13]. However, this UWSN research area is very challenging, and most work that has been conducted using terrestrial wireless sensor networks (TWSNs) cannot be directly implemented into UWSNs because different communication channels are used and the characteristics of underwater environments are unique [3, 14, 15, 16]. Moreover, underwater sensor nodes are expensive battery devices, and they need better protection of their hardware to resist the water characteristics [17, 18, 19]. One of the main tasks that faces researchers in the networks is determining how to route the information collected by sensor nodes to reach the sink(s) while minimizing routing costs (minimum delay, energy cost, number of hops, and shortest path) and ensuring network connectivity (void area problem). That is, to assure the network continues to function for as long as possible. OR is one of the routing techniques used to transmit data in UWSNs. It is an emerging routing technology that was proposed to overcome the drawback of unreliable transmission, especially in UWSNs. OR uses the broadcast nature of wireless communication to forward data packets to reach the sink(s) through one-hop or multi-hops. It addresses the major challenges of UWSNs, such as energy efficiency, void avoidance, reliability, and network stability. This OR approach takes into consideration the limited resources (battery and memory) of the underwater sensor nodes, and over the years, many forwarder methods in OR were proposed to prolong the network lifetime and increase the chance that every node has a direct or indirect link to the sink(s). Therefore, OR extends the lifetime of the UWSNs. However, based on previous research, it has been determined that there is still space for development in this area [20]. Many survey papers for UWSNs have been published, such as [21; 22; 23; 24; 25; 26] which cover different directions of studies in the UWSN area, and [2; 27; 28; 29; 30; 31; 32] summarize existing UWSN routing protocols. However, in order to present a more specific survey paper from the perspective of routing, we aim through this survey paper to collect and present a comprehensive overview of the state-of-the-art of routing protocols for UWSNs that focuses on addressing the void area problem by utilizing OR. We also predict future trends and challenges that remain unexplored in order to bring these to researchers' attention. The main contributions of this survey paper are:  
* We explain how the void area problem in UWSNs can be addressed by utilizing the OR technique, and we review up-to-date OR protocols. To the best of our knowledge, our survey paper is the first one that reviews up-to-date void-avoidance OR protocols for UWSNs. * We classify up-to-date void avoiding OR protocols for UWSNs based on their most important characteristics and features. * We identify some of the open issues and challenges in UWSNs, which can assist the designers of OR protocols for UWSNs. The rest of this paper is structured as follows: Section 2 presents the routing protocols in general.