# Void Avoidance Opportunistic Routing Protocol for Underwater Wireless Sensor Networks
## 1 Introduction  
Water covers more than two thirds of the earth's surface. This environment is very important for human life since it plays an important role as a transportation medium and affects the earth's climate as well as global production because of its richness in natural resources. Due to these reasons, lately, researchers have been giving more attention to Underwater Sensor Networks (UWSNs) to investigate and discover the unexplored submerged underwater areas and empower numerous applications such as resource exploration, oceanographic data collection, pollution monitoring, tactical surveillance, oil/gas spills monitoring, etc. [1, 2, 3, 4, 5, 6, 7]. To make such applications realistic, effective communication protocols are fundamentally required to complete the communication procedure between the underwater devices successfully. Many communication protocols have been proposed to address different Terrestrial Wireless Sensor Networks (TWSNs) issues [8, 9], which make such networks widely investigated. On the other hand, many UWSN issues remain open and need more investigation. UWSNs have different characteristics and features when compared to TWSNs [10]. These differences can be seen in many aspects. First, UWSNs use acoustic signals for communication instead of radio signals used by TWSNs. Second, the UWSNs' topologies are more dynamic than the topologies of TWSNs. Third, the underwater deployment is unattended and comparatively sparse as compared to TWSNs. Fourth, node localization in UWSNs is hard compared to node localization in TWSNs. Fifth, underwater sensor nodeshave more costly hardware than terrestrial ones, and they are resource limited (i.e., memory and energy). Moreover, it is hard to replace or recharge their batteries after deployment. As we mentioned above, UWSNs use acoustic signals while TWSNs normally use radio signals. While the radio signals do not propagate well and suffer from attenuation in underwater environments, the acoustic signals perform wireless communication in underwater environments with satisfactory range, a smaller amount of attenuation and higher reliability [11]. The acoustic signal is affected by underwater characteristics such as strong attenuation and ambient noise, time-varying multipath propagation, and low-speed sound propagation (speed of sound in water is \(\approx\)1500 m/s, which is five orders of magnitude lower than the radio signals used in TWSNs) [7; 12]. These underwater properties result in high delay and error rate, temporary loss of connectivity, limited bandwidth capacity, and high-energy communication cost. Since UWSNs use acoustic signals in their communication, the straightforward application of the traditional TWSN protocols to UWSNs reduces network performance [2; 3; 11]. Consequently, several underwater protocols were proposed to improve communication in underwater networks. These proposed protocols for UWSNs have considered various underwater parameters and addressed different problems. Improving network lifetime is a significant objective in UWSNs since replacing or recharging batteries of underwater nodes is a very expensive and difficult task in the harsh underwater environment. Two or more nodes may become unreachable as node battery energy depletes, causing a topology partition that results in the void area problem. This issue has attracted the attention of many researchers. In addition to the void zone problem, acoustic signal fading degrades routing protocol performance. Opportunistic Routing (OR) has been proposed as a novel technique to improve network function by mitigating high bit errors and losses due to shadow zones, limited bandwidth, high power consumption, and signal spreading [6]. It is known that in wireless networks the energy expended by sensor nodes in transmitting a data packet is more than the energy expended in receiving it.