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The most unexplored environment on our planet is Ocean, vast for covering around 140 million square miles that contains nearly the 71% of the earth surface (Ayaz, Baig, Abdullah, & Faye, 2011). It is essential part of human existing being a massive source of nourishment production. It is also a major origin of sea foods, natural reserves, plays a key role for shipment, and practises for experimental purposes. Industry is already diving deeper in ocean waters to discover the oil, gas, plenty of natural resources and minerals from the sea floor. As fast changes in technology has brought unique and better approaches for observing underwater conditions, oceans got importance in economic, social, geological and environmental manners (Draz et al.; Lloret, 2013). The ocean is massive and have average bottom of 2.5 KM, that’s why researches face many challenges during its exploration. Even with all the human history and efforts, approximately 10% of the entire ocean volume has been explored at yet. The possible application of UWSNs are oil gas field exploration, mines exploration, surveillance, oceanic disaster monitoring etc (Haque, Kabir, & Abdelgawad, 2020; Mirsadeghi, Rafsanjani, & Gupta, 2020). Implementation of wireless sensor networks for underwater communications are different in characteristics from traditional wired or even terrestrial sensor networks, so existing terrestrial networks cannot be applied directly in oceanic environment (Akyildiz, Pompili, & Melodia, 2005; Anum et al.; H. Wang, Li, Li, Gupta, & Choi, 2020). Underwater Wireless Sensor Networks (UWSNs) are evolved by unmanned underwater vehicles to form up a network that comprises of sensors which collaboratively communicate and able to generate or sense the demanded scientific information to explore the underwater environment.
UWSN or more specifically under water acoustic communication is required acoustic signal to share and exchange information among the various nodes. The characteristics of wireless sensors for underwater communications are different from terrestrial sensor network in different ways. For example, acoustic signal is considered the only suitable medium of communication as radio communication cannot be used inside water due to its extremely limited propagation. This shift of medium leads to many issues for underwater wireless sensor networks. Firstly, the acoustic signal uses sound waves rather than electromagnetic waves hence speed of communication shifted to the speed of sound, approximately 1500 m/s. Although, it is four times faster than speed of sound in air but five orders of magnitude slower than speed of light which uses electromagnetic waves. Secondly; in case of acoustic signal, network data rate is directly affected by distance as there exists an inverse relationship between both of these. Thirdly, acoustic signal consumes higher energy than radio signal which makes the underwater acoustic networks energy critical than terrestrial sensor networks. Such limitations of UWSN leads to arise the challenges like multi path propagation delay, high bit error and limited available bandwidth. Along these communication issues, underwater sensor networks also face the unique environment challenges such as continuous node movement, 3D-netwok topology and unavailability of node location information (Ali, Ayaz, Jung, Draz, & Shaf, 2017; Tewari & Gupta, 2020).
While developing the routing schemes for underwater wireless sensor networks, researches have to face many hurdles due to both, acoustic medium and unique environment. UWSN are deployed in deep water where consistent node motility and 3-d sensor topology are main complications imposed by the host environment. As the UWSN nodes can move 1-3m/sec due to water currents. Accordingly, due to the nature of acoustic communication, although it can propagate up to 5 km, but it started to face communication degradation over the range of 1000m (Abbas, Bakar, Ayaz, & Mohamed, 2018; G. Khan & Dwivedi, 2018; Panda & Naik, 2018). Further, Doppler scatter and long attenuation causes may decrease in bandwidth transmission capacity. As in underwater with short range only low frequency acoustic signal can be used it will diminish the bandwidth which implies that reduces bandwidth produces more errors. Due to these factors and many other similar reasons, UWSN opens a new platform in support of revising the current structure of traditional protocols.