Recent Trends in Underwater Visible Light Communication (UVLC) Systems

In recent years, underwater visible light communication (UVLC) has become a potential wireless carrier candidate for signal transmission in highly critical, unknown, and acrimonious water mediums such as oceans. Unfortunately, the oceans are the least explored reservoirs in oceanogeographical history. However, natural disasters have aroused significant interest in observing and monitoring oceanic environments for the last couple of decades. Therefore, UVLC has drawn attention as a reliable digital carrier and claims a futuristic optical media in the wireless communication domain. Counterparts of traditional communications, the green, clean, and safe UVLC support high capacity data-rate and bandwidth with minimal delay. Nevertheless, the deployment of UVLC is challenging rather than terrestrial basis communication over long ranges. In addition, UVLC systems have severe signal attenuation and strong turbulence channel conditions. Due to the fact that, this study provides an exhaustive and comprehensive survey of recent advancements in UVLC implementations to cope with the optical signal propagation issues. In this regard, a wide detailed summary and future perspectives of underwater optical signaling towards 5G and beyond (5GB) networks along with the current project schemes, channel impairments, various optical signal modulation techniques, underwater sensor network (UWSN) architectures with energy harvesting approaches, hybrid communication possibilities, and advancements of Internet of underwater things (IoUTs) are concluded in this research.

applications such as undersea monitoring, observing marine life, detecting oil and natural gas sources, early warning of tsunamis, floods, and underwater location coordinates for navigation purposes [4]. Under these conditions, underwater wireless communication (UWC) has become an essential approach to exploring undersea sectors and plays a significant role in order to provide the natural phenomenal underwater activities. The state-of-the-art UWC in coastal securities, fishing farms, marine transportation, and pollution control have been implemented for several decades [5]. UWC networks are necessarily vital approaches in inaccessible and unmanageable regions of the deep ocean, which are highly sought-after research areas among scientists, researchers, and academics. A comprehensive and detailed discussion on UWC as a potential method of wireless signal transmission is recorded as in [5]- [9]. In addition, the proposed deployable underwater communication technologies in existing surveys support the collection of the oceanographic data, observation of water pollution, undersea monitoring, immediate warning of floods & natural disasters, and the phenomena of melting ice sheets due to global warming caused by rising ocean levels are also discussed in [9].
Recent studies encourage exploring the oceanic environment by deploying UWC technologies such as electromagnetic waves (EM) in terms of RF, acoustic, and optical waves propagation. As RF waves highly attenuate by the seawater and change the properties conductor to dielectric at 250GHz frequency ranges [10]. Therefore, RF carriers are not feasible and cover short ranges of communication in an aquatic medium. As claimed by the existing underwater architectural system in literature, the frequency ranges of EM waves in underwater propagation is extremely low between 30 − 300Hz for military applications [11]. A large sized antenna is required in UWRF signaling for better performance in underwater mediums [12]. Similarly, RF waves are also operated as ultra-low frequencies (ULF) to proceed with low attenuation but at a very high budget and at a low data rate. it provides low data rates [13]. Most recently, a joint technical WMO-IOC Commission for Oceanography and Marine Meteorology (JCOMM) as an observing network of the Global Ocean Observing System (GOOS) has proposed a high-frequency radar network (HFR) in 2017s [14]. In [14], the proposed communication system has drawn an attention of HFR system, that is continuously observed and map of ocean currents up to 200km offshore site in the coastal area. It supports detecting the floating objects on the sea surface, such as fishing ships, buoys, floating nodes, boats, yachts, etc., and extremely helpful for rescue operations. In the HFR networking system, the signal propagation is held from the radar and the signal reception by the conduction ocean waves, measuring the Doppler shift and giving direction and velocity. A pictorial view is depicted the function of HFR by Fig.16.
At a glance, EM waves were initially used as lowfrequency ranges to transmit signals on terrestrial-based communication for reliable communication to achieve high bandwidth. Though, in an underwater environment, RF waves propagate differently compared to terrestrial-based signaling and totally depend on water mediums. The advantageous feature of RF waves propagate with high velocity (similar to the optical waves) provide efficient communication among the network nodes. Underwater wireless RF (UWRF) experiences signal fading and easily attenuates over very short distances in underwater. It is challenging to transmit signals through RF waves in a highly turbid water medium. An impressive survey work is proposed on UWRF based communication in seawater, where the authors widely discussed the recent advances of RF waves for coastal and military applications [11].
Besides UWRF, the acoustic waves are received the utmost interest in signal propagation over long distances in underwater medium with the limited bandwidth [15]. The traditional acoustic wireless carriers propagate over long distances with a low speed of approximately 1500m/s, and a meager data rate [12]. Generally, acoustic waves propagation speed is influenced by channel impairments. Conversely, the speed of acoustic waves increases tremendously due to fluctuation of the temperature and salinity of the water [8]. The affecting factors such as scattering, reflection, and absorption by suspended particles corrupt acoustic signals. Therefore, the main concern of UWC is the establishment and architectural design of underwater sensor networks (UWSNs) for obtaining proper communication among nodes. An exhaustively detailed review study of UWSN is summarized in [16]. Additionally, the shipbuilding industries are also increasingly interested in widespread communication through sensor network architectures. The possible UWSNs applications are over-viewed in various underwater applications such as observing strong influences of the aqueous medium, undersea nutrient production, natural oil retrieval, underwater transportation, and the impact of water climate regulation on ecological system [17]. In these corresponding applications, the sensor nodes such as autonomous underwater vehicles (AUVs), remotely operated underwater vehicles (ROVs) support the data collection with the onshore nodes (sub-station) [18]. The cluster of underwater sensor networks is depicted in Fig.1, Fig.14 and sensor network architecture in Fig.16.
Due to the limited data rate and bandwidth of UWRF and acoustic waves, an alternative optical signaling approach has become the most significant and reliable wireless carrier candidate for digital signaling in underwater [19]. For instance, in the first stage of optical signaling, fire, fire-smoke, fire beacons, torches, and reflecting sunlight were introduced. In the starting phase of optical signaling about 800 BC, the ancient Romans and Greeks used to transmit optical signals for communication purposes over long ranges [20]. During 1880s, the optical wave-based telephone (photophone) was invented by the famous scientist Alexander Graham Bell for signal transmission over 210m [21]. In this photophone, a parabolic mirror with a selenium cell was used as an optical signal receiver. In the end of the 19 th century, the continuous development of optical signaling, Heliographs were firstly 22170 VOLUME 10, 2022 proposed to deliver messages through optical waves in the battlefield for military applications [22]. Generally, the heliographs were assembled with a dual-layer of mirrors and directly used to control the light beam propagation. For further development of optical communication, lasers were introduced as an alternative for signal transmission to serve wireless carrier over long distances [12]. On the other hand, the demand for ocean exploration is hiking up for a couple of decades. For instance, a real-time data streaming fiber optics signaling is applicable for several underwater applications and proposed during 1970s [23]. However, the main drawback of fiber optics is highly cost setups and unpleasant for undersea species. Moreover, in connection with the limited propagation coverage and high attenuation, underwater signaling is highly challenging to fill the gap of the desired communication.
For consistent development of optical signaling, He-Ne laser (λ=632.8nm) was investigated as an optical signal carrier over the 30 − 40km range during 1960s [23]. In the early ages of laser development, the very initial foundational laser link was established to control commercial traffic in Japan by Nippon Electric Company during 1970s [21]. This deployed laser link was firstly used for signal transmission over 14 km. Later on, full-duplex communication Free Space Optics (FSO) enabled He-Ne laser at 0.6325µm was introduced for communication between the cities of Yokohama and Tamagawa in Japan [21], [23]. To keep evolution going, FSO communication technology was proposed to transmit high data-rate signals with minimal delay over long distances among communication nodes, which is extensively discussed in [12] and [24]. Later, in the end of the 20 th century, the numerous developments have been documented for optical signaling. Kahn and Barry extended Gfeller and Bapst's work, where they demonstrated a laser of wavelength 780 − 950nm with high data rates 50 − 70Mbps [21], [25] [26].
In laser counterparts, the implementation of Light-emitting diodes (LEDs) in VLC are widely used as transmitters through an illumination nature that provides an ultrahigh bandwidth to achieve a high data rate. For optical signal transmission and receiving, LDs and LEDs are the potential tools and widely use in optical communication over very short, short, medium and long ranges as per the requirement of the applications [27]. Therefore, these LEDs are considered the main sources of optical signal contributors. Moreover, the various light sources, photosensors transmit and receive optical signals such as a laser diode (LDs), Avalanche photodiode (APD), p-n diode, etc. The consumption of LED seems to be a novel approach cause of its dual nature. LED turn-on and turn-off ability turns a new communication paradigm formally known as the visible light communication system. Although, the invention of LEDs received a remarkable extent of research for high data transmission of its dual nature as illumination and signal carrier [28]. The illumination phenomenon occurs based on the visible light wave spectrum at the wavelength 380 − 750nm, which is used as a transmitter and an image sensor at the receiver end [29]. Historically, VLC was proposed by the Visible Light Communications Consortium Japan (VLCCJ), Japan Electronics & Information Technology Industries Association (JEITA) CP-1221, and CP-1222 during 2003s to 2007s [21]. For continuous development of VLC, the Visible Light Communications Associations (VLCA) was established in Japan for further standardization [30]. As Japan Traffic Management Technology Association (JTMTA) and VLCC conducted an experiment where they achieved 4800 kbps data rate over a distance of 300m. Overall, Japan had contributed the initiatives for the development of products based on VLC signal modulated technologies. Additionally, in 2012s, Casio Japan developed the new Apple application ''Picapicamera'' for users where the images could be sent instantly through VLC technology [27]. In contrast, the modulation schemes and dimming phenomenon supports possible VLC communication that has been widely investigated by IEEE 802.15.7 standard [31]. The optical wireless communication (OWC) IEEE 802. 15.7r1 standardization is used for short-range communication as VLC implementation [32]. The new era of optical communication has become a more significant trade for digital wireless signaling media with the integration of different techniques, which are mentioned in Table 1. Subsequently, the optical camera communications (OCC) system is used as image sensors at the receiver end to provide triple functions simultaneously, i.e., illumination, data transmission, and wireless energy transfer [20].
All the above matter facts for real-time data streaming and high bandwidth signal propagation approaches are needed. Therefore, underwater optical wireless communication (UWOC) in terms of visible light, particularly the blue-green spectrum of wavelength (450-532)nm [20], has been chosen and is widely used for high data transmission underwater communication. In the UWOC, the hydrological optical beam experiences signal absorption due to electron transition and converted into another form of energy. It is noticeable that water offers transparency with bluegreen light spectrum [20] rather than with other light spectrum wavelengths. Moreover, the ocean water offers less attenuation for the blue light spectrum while the green light spectrum is attenuated by coastal water [33]. The optical signal attenuation depends on the inherent optical properties (IOPs), and apparent optical properties (AOPs) of water medium [34]. Additionally, the physio-chemical properties of the water channel supports fading contribution due to absorption, optical scattering, and reflection caused by turbidity, pressure, temperature, and existing suspended material within the water environment [35]. As a result, underwater visible light communication (UVLC) has become a complementary approach compared to traditional acoustic communication in high data rates ranging from hundred Mbps to Gbps over short range applications. Even UVLC has received superiority over traditional underwater acoustic and EM signaling. As compared to the existing traditional FIGURE 1. An overview of RF-VLC hybrid communication system where the oceanographic data collects by distinct types of underwater sensor network nodes architecture (1D, 2D, 3D, and 4D types of UWSNs architecture). The extracted data has been transferred to the offshore station for further analysis. In this type of scenario, the different internet of underwater things (IoUT) are included to communicate with each other by utilizing the latest wireless network or beyond. As depicted, the floating nodes collect the information along with the network nodes at the specified depth in the underwater medium.
UWC techniques, UVLC technology has advantages in harsh environments as it is handy to use, easy to deploy with low cost, and highly qualitative communication media [36]. A broad detailed scenario of UVLC and connecting nodes optically in an underwater environment is depicted in Fig.1.
Underwater signaling is entirely different from the terrestrial radio environment due to its energy costs and signal propagation phenomena. As a result, it is challenging to provide energy to power-hungry devices in underwater mediums. To cope with the energy harvesting (EH) issues, Simultaneously Wireless Information and Power Transfer (SWIPT) [37] and Simultaneously Light-Wave Information and Power Transfer (SLIPT) techniques [38], [39] are proposed in the literature in preference to powered up UWSNs. SWITP, wireless power transfer (WPT), and SLIPT will be widely discussed in further sections in this study. At a glance, EH is an optimum solution for power supply to activate the internet-enabled devices. Although, very few existing EH techniques are capable of supplying energy to the internet of underwater things (IoUTs). Several EH techniques are proposed in the open literature to power up underwaterbased sensor nodes through RF and acoustic waves, which are mentioned in section IX.
Considering the EH scheme in acoustic communication, the Microbial Fuel Cell (MFC) and Acoustic Piezo-Electric (APE) techniques are the key enabling solutions for self-energizing nodes [40]. WPT and SWIPT techniques are investigated and introduced to provide energy with information transmission simultaneously over the wireless network [37]. Similarly, SLIPT has been announced as a futuristic wireless carrier candidate for EH and information transmission through light wave [41]. A more detailed summary and comparison between SWIPT and SLIPT is recorded in the literature in [42]. Furthermore, enabling the next-generation wireless network proliferates the communication system quality and efficiency. 5G enabled underwater wireless communication system will be the futuristic communication networking system that has been introduced recently in [35]. The authors in [43] and [44] are investigated the Filter bank multi-carrier (FBMC) and generalized frequency division multiplexing (GFDM) schemes which are considered as the latest reliable networking techniques for 5G applications in underwater. Improving the system quality of communication setup within the higher signal modulation techniques also significantly achieves a high data rate. The higher modulation techniques support a high degree of signal transmission between transceivers with minimum delay. The different types of optical signal modulation techniques are summarized in [45] and [21]. The hybrid links established for the long ranges of communication in different channel environmental conditions are used to deploy according to the requirement. A hybrid RF-UWOC system is investigated in [46]- [52]. Therefore, throughout this study, we present the various optical signal modulations, demodulations techniques, and their implementations in various wireless systems with recent trends in the underwater environment.

A. RELATED SURVEY LITERATURE
The various existing works are recorded based on UWC techniques and investigated with numerous emerging development methods in this domain. The existing related works are summarized by Table 4. For several decades, accelerating UWC to explore the underwater environment has become an alternative communication media that attracts human activities in an aqueous medium. Plenty of underwater wireless applications based on acoustic waves are investigated in existing works to explore the oceanic environment. However, the limited signal propagation speed and bandwidth recorded in underwater sound waves caused communication delay among nodes [12]. Therefore, the requirement for underwater acoustic communication setups for real-time monitoring and to access underwater applications are typically challenging. Based on underwater acoustic communication (UWAC), the most promptly military applications, especially in underwater acoustic communications among nodes such as submarine to the base station, submarine to ship, and submarine with military divers, are widely summarized in [53]. On the other hand, underwater optical signaling propagation reliably supports high bandwidth and high data-rate with minimal delay and latency. This is particularly appealing for real-time underwater applications [54]. To mitigate of its limitations, a hybrid mixed communication system design and evaluation of underwater optical-acoustic networks are investigated as in [55]- [59]. Similarly, for real-time scenarios, the performance of RF underwater communications operating system at frequency ranges 433MHz to 2.4GHz is investigated [60]. A range of existing hybrid underwater links are summarized in Table 4. Due to high signal propagation delay and very few kbps data rate in UWAC, counterparts of UWRF could be a complementary solution VOLUME 10, 2022 to receive high data rate and bandwidth range. For short distances, the RF conduction-based system design for voice communication in an aqueous medium has been recorded in [61].
Prior works are designed on theoretical propositions as a function of the physical properties of channels such as pressure, temperature, salinity, etc. A recent survey on UWOC is widely discussed about signal propagation-related issues based on the physical layer and localization technique in [13]. The authors in [62] investigated the path loss performance of a vertical UVLC link with the physical properties of the water channel considering n th water layers. A UVLC system model considering LOS & non-LOS vertical configurations and the variation of refractive index is also presented in [63]. The survey [12] is exhaustively enlisted the critical factors of optical signals propagation in an underwater medium where the authors modeled the UWOC channels. However, the survey is provided a detailed summary of optical signal modulation and demodulation techniques according to the experimental setup of the system. UWOC wireless carriers as FSO and UVLC technology have been compared and widely discussed in [64]. The higher modulation techniques of UVLC have become attention for developing optical signals in the underwater environment. Due to this, a 16-QAM modulation scheme for underwater signaling was proposed in [65]. The authors in [65] are investigated the four types of different geometrical shaped 16-QAM constellation experimental approaches for improving the system efficiency. The most recent work based on the MIMO-OFDM scheme utilizing VLC in underwater is proposed as in [66] and [67], where the authors proposed the BER performance of the system under consideration of weak turbulence channel conditions. Another study is observed the BER performance over NOMA-based assisted UVLC link as in [68]. In hybrid communication, EH is an issue concerning nodes in active modes. An EH hybrid acoustic-optical underwater sensor networking system was investigated in [69]. The authors in [70], are introduced the possible underwater applications and the differences between the terrestrial-based wireless network (TWSNs) with the underwater sensor network nodes (UWSNs).
In the previous work [4], we reviewed the comparison among the three most significant UWC technologies and concluded the various advantages of UWOC over traditional wireless carriers. In this presented survey, we studied the recent advent of UVLC within different channel conditions. The following complementary sub-topics are widely summarized in this research as follows; 1) UVLC channel model is considered within the combined influence of weak to strong turbulence channel conditions along with the pointing error between transceivers; 2) The performance of underwater hybrid communication is obtained in different water types along with the simulation results; 3) The distinctive VLC higher modulation schemes support to improve channel performance and quality of services (QoSs); 4) The latest current project schemes to cope with the various underwater applications in marine industries all around the world; 5) A diverse study of IoUTs and implementation in different underwater locations for required applications are also summarized; 6) Integration of UVLC towards 5G and 5GB networking in UWC domain is concluded; 7) A SLIPT assisted scheme, based on time-switching and power switching methods to powered up UWSNs is highlighted in this research.

B. MOTIVATIONS AND CONTRIBUTIONS
Since humans footed on earth, oceanic reservoirs are least explored and are far-fetched to envisage the undersea environments. We tend to know more about the moon and mars rather than oceans existed on our earth. The interest growth of human activities in underwater is proposed the new research advances become familiarization with the natural oceanic phenomenal activities. Numerous UWC systems demand plenty of underwater applications for new research perspectives in this particular domain due to the immense development of industrialization. Traditional acoustic signaling is widely used for various underwater applications over long ranges. Nevertheless, a novel optical communication wireless media has been proposed recently to fulfill the requirement of desired data rate. Therefore, optical wireless signaling has become a potential wireless candidate for information transmission, and energy harvesting technology. However, UVLC is in its early age to cope with signal transmission and energy harvesting issues wirelessly in highly turbid water mediums. This study carries an exhaustive survey of UVLC towards 5GB futuristic networking system, the latest and current projects are running through the globe, channel modeling, UWSNs architectures, IoUTs enabled applications, and their contributions are perspectives. The main contributions and recent trends of UVLC through this study are as follows; • An extensive range of underwater applications requires communication techniques with high performance and reliability. Due to this, most of the significant channel impairments and challenges are widely studied. However, absorption, scattering, misalignment of transceivers (Pointing Error), and turbidity are the main influencing factors overlooked in single-hop, dual-hop, and hybrid multi-hop wireless communications; • A wide detailed summary of VLC modulation, and demodulation techniques towards 5G enabled FBMC, and GFDM techniques are briefly summarized. UVLC enabled 5GB supports a proper connection among underwater-based internet-connected elements with the satellites or unmanned aerial vehicles (UAVs) for the sake of expanding human activities [1]; • This study is visualized the most distinctive type of UWSN architectures to power up the nodes through SWIPT and SLIPT schemes; the UWSNs architecture in both academic and industrial perspectives are majorly discussed; • The discussion of this survey study provides a forum among the students, researchers, and engineers. In addition, the key fundamentals of this proposed survey are associated with professionals communication academics.

C. PAPER STRUCTURE
The remainder of the paper is organized into XV sections. An underwater wireless communication overview enabled with different communication media is presented in section II. We are discussing the fundamentals and propagation terminology of UVLC along with the types of waters, channel impairments in section III. The necessity of signal transmitter and receivers are summarized by section IV. Further, an exhaustive survey relating to the current project schemes and future deployments are widely summarized in section V. In section VI, there are plenty of computational methods discussed to provide a problematic solution of bulk data in optical signal processing. To cope with the channel impairments and signal fading factors have taken into account. Therefore the various UVLC system models are proposed and their overview is given in VII.

II. UNDERWATER WIRELESS COMMUNICATION AS AN OVERVIEW
UWC networks are challenging and become a significant approach to exploring an inaccessible unguided sector in the deep ocean with a wired and wireless connection. The deployable technologies allow to get familiar with the oceanic environment. Therefore, UWC accommodates an opportunity for oceanographic data collection and analysis, water pollution, underwater environmental monitoring, changes in currents, early warning of flood or tsunami, and observing the phenomena of ice sheet melting due to global warming cause of rising water level of oceans [4]. Nowadays in UWC, the three most significant distinctive technologies are employable for transmitting the information. These wireless carrier candidates are based on optical, sound, and electromagnetic waves based [5], [13]. The signal transmission is dependent on signal strength which is affected by the brutally chemical properties of the channel. The optical signal could be absorbed, scattered, and reflected caused by the chemical vitality of the channel, and suspended particles in a water environment [10]. Considering UWRF, which is used for short ranges, the signal decay happens by the hydrological properties and water channel. These water channel properties known as permittivity, conductivity, and permeability are directly responsible for the contribution of RF signal attenuation while increasing frequency ranges [37]. Since the saline seawater allows to propagate RF waves in few meters at extra low-frequency range of (30-300Hz), which is enabled to the transmission of few characters per minute to submarine [11]. Moreover, RF waves at the frequency range MHz to GHz accomplishes work efficiently in hybrid scenarios [4]. However, for shallow water scenarios, RF-enabled communication is called buoyant communication [5]. The UWRF system requires a costly and large sized information transmission antenna along with high energy-consuming transceivers [71]. In hybrid UWRF signaling, it offers a significant platform for the connection between underwater and terrestrial-based nodes. It also permits the exploration of the forbidden field of ocean and underwater environment. Extremely low frequencies (ELF) are used for long ranges underwater communication while the high frequencies (HF) strongly attenuate the cause of signal losses [72]. On the other hand, the frequency ranges as ultra-high frequency (ULF) to medium frequency (MF) are mainly used for a variety of underwater environments [73].  As far as long ranges in underwater signaling, acoustic waves are widely used and challenging communication technology to deploy in an underwater environment [74]. Nevertheless, in UWAC propagation, the speed of sounds deficient with high delay over long ranges [75]. According to [9] and [76], an impressive feature of UWAC offers an incremental speed propagation on raising the temperature of the water. An impressive increment (approximately 4m/s) has been recorded on rising 1 • C of water temperature. Moreover, the salinity is also another cause of signal fading, and acoustic waves speed in enhanced by 1.4m/s while rising salinity 1 practical salinity unit (PSU). In respect to receiving signals over long distances under consideration of plane wave model or spherical wave model, emerging technology is widely known as UWSNs, which is alternative and a complimentary communication media solution, especially for military and naval tactical, operational applications [77].

III. FUNDAMENTALS OF UNDERWATER VISIBLE LIGHT COMMUNICATIONS
UVLC has become a unique advantageous technology in the new era of wireless signaling that has drawn attention as a dominant signal propagation over short ranges in the harsh underwater medium. Towards the next generation of 5GB wireless networking system, UVLC will be applicable for numerous undersea applications such as geographical mapping, underwater navigation, naval operations, dataretrieving among nodes, and high quality of UWSNs are also considered [119]. Additionally, UVLC has superiority including high data-rate, real-time steaming data transfer with high security compared with its counterpart traditional underwater signaling acoustic and RF signaling. Despite all of these advantages, several factors affect the signal propagation, such as absorption, scattering, and turbulence due to the IOPs and AOPs properties of water channel and nature of light [5], [9], [12], [34], [79]. The affecting factors cause signal fading, limiting a limited communication range for UVLC systems. Another degradation of signal fading is multi-path propagation that generates an inter-symbol interference (ISI). In this context, the fading-free impulse response (FFIR) of UVLC channel, which is a MIMO-UVLC system, has been recently investigated in [105]. In [105], the authors presented a thorough approach to UVLC channel characterization, performance analysis, and effective transmission along with the reception methods of the proposed system model. Moreover, in another study [120], the authors proposed a CDMA-UVLC approach based on assigning unique Optical Orthogonal Codes (OOC) for each underwater mobile user. An underwater cellular optical CDMA network and related applications of serial relaying mechanism has been recently proposed in [121] and [122]. The wireless signal transmission underwater is challenging, especially when optical transceivers are deployed. Acoustic and VLC are complementary deployable communication technologies to fulfill the desired communication gap in the wireless communication domain. However, acoustic link suffers signal attenuation and high signal propagation latency. VOLUME 10, 2022  Furthermore, it has also mentioned the channel modeling, impairments, signal affecting factors, and the most widely deployable underwater applications.

TABLE 4. (Continued.)
The table is summarized the most recent existing similar works along with the related contributions from the perspective of future directions. Furthermore, it has also mentioned the channel modeling, impairments, signal affecting factors, and the most widely deployable underwater applications.
The implantation of acoustic transceivers is expensive, bulky (highly challenging to deploy rather than optical setup), and high power consumption. The VLC link is considered as an alternative viable underwater signaling technology VOLUME 10, 2022 over moderate distances compared with acoustic and EM waves. To mitigate all the above facts, optical communication systems are defined as horizontal and vertical or slant links according to their implementation direction [123]. The horizontal links are proposed to improve the system performance and minimize the turbulence effects [63]. On the other hand, in the underwater vertical or slant link scenario, the transceivers are arranged in LOS link configuration [100].

A. UNDERWATER VLC CONFIGURATION LINKS
UWC is expected to achieve desirable receiving information in the underwater environment significantly. The design of a VLC link can be classified into three most significant categories such as i) UVLC Line of sight (LOS) link, ii) Retro-reflector assisted LOS link, and iii) Non-LOS configuration link.

• UVLC Line of Sight Communication (LOS) Link:-
Generally, in optical communication LOS links are widely used; the LOS scenario is depicted by Fig.2.
In this scenario, the deployable transceivers should be aligned and directly linked to one another in the line of direction of signal propagation. This type of combination supports and enhances the power efficiency, as well as immunity of the environmental distorting effects such as ambient and artificial light source [124]. Additionally, the LOS scenario is the most reliable approach for an underwater wireless communication system in terms of high data rate, better BER performance, improving system efficiency, and system performance. However, the drawback of LOS links is the mobility of transceivers due to water currents and the random flow of water waves. Even the received optical signals depend on the physical parameters of the experimental setup. According to [124], the LOS communication link by using laser diode wavelength 470nm is capable of covering 200m distance in the deep ocean that allowed 5Mbps data-rate transmission. On the other hand, in coastal ocean water, yellow-green LEDs are the optimal solution to deal with the signal scattering, and field of view (FOV) a wide-angle of beam causes increased attenuation. The propagation loss under Beer Lmabert's law can be defined as follows [125], where i, j are denoted the transmitter and receiver nodes, the euclidean distance between the transceivers and the total sum of absorption and scattering as extinction coefficient are defined as d ij and c(λ), respectively. The propagation losses depend on the path loss component as well as the geometrical parameters of the setup. Generally, the geometrical gain in optical LOS scenario can be written as [79], where the receiver aperture area of nodes and the beam divergence angle of i th transmitters are represented as A i and θ i , respectively. Furthermore, the channel conditions, which are the contribution of the path-loss and geometrical losses, are defined as follows [101], The channel conditions h ij is modified by replacing (1) and (2) into (3) given as follows, .
Furthermore, for approximation of the maximum achievable distance to achieve a specified bit-error-rate is investigated in [101], which can be defined as, where W , D r , T , and θ F represent the Lambert-W function, the aperture diameter, the correction coefficient for data fitting with the simulation results, and divergence angle for full-width transmitted beam, respectively. For the LOS of laser diode, the divergence angle is generally planned for few milliradians, whereas LEDs fixed a divergence angle below 140 milliradians for diffusing light to a wide-angle [13]. At the receiver end, the received power (P j t ) describes the product of transmission power efficiency, channel gain, propagation loss, and geometrical loss [79] of setup i.e.
where P j t , P j r , η i t , η j r , χ(ψ j i ) and ψ j i are denoted as the transmission and received signal powers, transceivers efficiencies, concentrator gain, and the incident angle concerning the receiver axis, respectively.
• UVLC Retro-reflector-assisted LOS Link: Optical retro-reflector type links are based on the back-scatter phenomenon principle consisting of a light source with an optical reflector. In this scenario, the capacity of the source is more powerful than the receiver node [125]. Although, the reflector is considered an interrogator with a basic anatomical design with minimal power availability. Consequently, the optical beam propagation source continuously emits a light beam during propagation of the light signal modulated and reflected by a local reflector then received by the destination. A UVLC retro-reflector LOS link is depicted by Fig.3. In more contrast, the retro-reflector scenario is sub-divided into two significant categories known as photon-limited and contrast-limited retro-reflector [5]. The photonlimited retro-reflector phenomena occurs in shallow waters such as lakes, deltas, and rivers either clear sea waters. In this scenario, the link range and the system capacity depends on the received falling photons at the detector end due to absorption phenomena. Although, the retro-reflector link is most widely used in turbid harbor water mediums, where signal scattering is considered an influential factor for system range and capacity. A contrast limitation for more specific uses underwater imaging applications is reducing the number of photons responsible for reducing the image contrast and quality. The reflector is capable of amplifying the modulated signal or light beam for the sake of better system performance in both scenarios as photon and contrast limited retro-reflectors [13]. Even, the retroreflectors reflect the modulated beam and signals to the interrogator when the modulated signals are received on it [125]. In this retro-reflector design, if η t , η r , and η Ret are the optical transmitter efficiency, receiver efficiency and retro-reflector efficiency, then the received power in retro-reflector link is given as follows [13], [125], where the aperture receiver area of reflector, the divergence angle of the reflector, and the angle between the receiver trajectory of the source are denoted by A r , θ r , and φ j i , respectively. • UVLC Non-LOS Configuration Link:-Non-line of sight (NLOS) optical signal propagation happens due to existing or suspended large scale particles or obstructions, even misalignment of the transceivers, and random orientation of the transceivers in underwater mediums. In this aspect, the reflected light from the sea surface or the bottom of water reservoirs (in shallow regions) could be beneficial to facilitate multi-point communication as depicted in Fig.4. In the NLOS scenario, the cone of light is emitted by the source with inner and outer angles θ min and θ max in an upward direction. This upward optical beam strikes with the water surface with an annular area A nn , either a reflection of the light beam strikes with the ocean surface and reached the desired nodes. In contrast, if the transceivers are designed vertically upward, then the propagated beam partially refracts and partially reflects with the water surface. The reflected light beam propagates from the transmitter, which locates x distance apart water surface as depicted in Fig.4, where A nn is defined as equal power density while apart at a distance x 0 . The annular surface area can be written as follows [125], where (x + x 0 ) is defined as the radius of the spherical geometry, which means the uniform power density in free space. The received power at the specified destination in the NLOS scenario is expressed as [126], .

B. THE NECESSITIES AND ADVANTAGES OF UVLC
UVLC has become a potential wireless candidate (transmission standard IEEE 802.15.7) for signal transmission in an underwater environment without any interference as in RF signaling. There are plenty of benefits to deploying VLC over traditional acoustic waves as very cost-effective, easy to install, high data transmission 3Gbit/s, real-time monitoring [105], and the collection of bathymetric data [127]. Under consideration of UVLC the blue-green colored optical wavelengths have been chosen for no-radiation and nonharmful for underwater species [128]. Numerous applications are deployed underwater where channel conditions emerge with a high-power LED-based VLC system. For real-time communication scenarios the underwater sensor network nodes (UWSNs) are connected with visible light sources and further communicate with the base station (BS) [129]. The sensors positioning system and localization in the UVLC system have been drawn attention in the literature in supporting and improving the system capacity and efficiency. For instance in the initial stage of VLC, it was deployed intensively in mining sectors and further used as a wireless candidate in underground areas [130]. The electromagnetic waves as VLC counterparts, are absorbed and effortlessly attenuated by underground channels. Therefore, the evolution of VLC technology to bridge the gap in such complex applications as mining sectors, tunnel operations, and underwater communication. Due to this, most organizations are seeking further developments in the VLC communication systems. It turns the clock back as Disney Research had developed an in-light lightning system for their theme parks [131]. Huawei just recently signed an agreement with a German company Schnell Li-Fi to build optical hospitals that make the hospitals smarter and remotely operation could be possible through the wireless connected medical equipment [132]. These applications will support safer operations because visible light is free of interference compared to electromagnetic carriers. Another key driver is that the VLC (enabled LEDs) can deploy and make the oceans smarter, supporting an urgent need for efficient and highly secure communication. The micro-LEDs are being used for high data transmission in VLC systems. However, LEDs are tiny in size, and transmission power is relatively low compared to the laser. At a glance, lasers are not associated with the efficiency Doppler effect, which means they can reach higher power and higher power density even a very tiny device. Also, the laser operation in the stimulated emission regime has a much higher modulation bandwidth compared with LEDs. The requirement is to move from LED to laser source because lasers have more advantages over LEDs. A recently developed narrow bandwidth blue lasers-based distributed feedback laser (DFB) wave-guide structure has been proposed [133]. The DFB laser structure with III-layer nitride is still under development. An experimented wave-guide structure builds a high-speed DFB blue laser with enormous potential for future speed Li-Fi applications.

C. THE CHARACTERIZATION OF OCEANIC LAYERS
Generally, the deep oceans are categorized into three distinct zones for optical signaling, i.e., Euphotic, Disphotic, and Aphotic zones widely known as sunlight zone, twilight zone, and midnight zone, respectively, depicted in Fig.5. The range of the visible light signal, living organisms, and the variation of pressure concerning the depth is also summarized in Fig.5.
• Euphotic zone: The sunlight zone is the top layer of the water surface; enough amount of sunlight penetrates this zone. The green algae can survive due to the primary source of sunlight, and photosynthesis happens [134]. Moreover, the living creatures can be found in this zone [135]. The typical depth of this zone depends on water quality, such as the clarity or murkiness of the water. The depth of this zone varies in different in water types of waters. Euphotic zone can be quite deep up to hundreds of meters while it can be 15m meters in coastal ocean water in vertical column [5], [136]. Generally, the depth of the sunlight zone is approximated up to 200m from the water level. In this zone, the temperature fluctuates approximately 28 0 C to freezing point −2 0 C [137].
• Twilight Zone: The twilight zone is formally known as the Disphotic zone, which is beneath the Euphotic zone. The expected depth of this zone varies from 200m to typically 1km in the ocean [138]. At this level, a small amount of light visualize and rarely the algae alive in this zone. The very dim light falls and the dim blue light could penetrate this zone. There is not sufficient light to sustain the photosynthesis process in the disphotic zone. Some adaptable creatures can be found in the Twilight zone. The pressure increases and the temperature falls due to increasing depth. • Mid-Night zone: A vast portion of the ocean belongs to this layer, known as the Aphotic zone. Approximately 90% of the ocean consists in the Midnight zone. It is entirely dark; there is no light extent this massive unguided water sector. The water pressure extremely low and below the freezing point. The depth of the midnight zone varies more than 1000m to 11km where is no light has taken place. The temperature fluctuates far from a chilling temperature of 4 0 C to −15 0 C or below. Rarely or more specific living organisms and creatures can be found in this zone [139].

D. DIFFERENT WATER TYPES, CHALLENGES & AFFECTING FACTORS IN UVLC SYSTEMS
Considering the channel conditions which affect optical signal propagation, the water has been categorized into different categories. According to [10], the three main types of waters exist and are classified for optical signaling as clearest water, intermediate water, and murkiest water. The geographical location of the clearest ocean water is located in the Mid-Pacific and Atlantic oceans, whereas intermediate water is possibly found in the Northern Pacific ocean. The North Sea and Eastern Atlantic oceans are the major VOLUME 10, 2022 water reservoirs of the murkiest water. Furthermore, another classification is commonly discussed in the open literature, which has been sub-divided into four distinctive categories: i) Pure seawater, ii) Clear ocean water, iii) Coastal ocean water, and finally, iv) Turbid harbor & estuary water. The light beam attenuates in these water differently. As in pure seawater the optical signals absorb because of the chemical composition, while the scattering loss occurs widely in clear ocean water due to its higher concentration of particles than pure seawater. A higher amount of particle concentration is founded in coastal ocean water rather than clear ocean water, which causes a higher rate of scattering and absorption of the light beam. Moreover, turbid harbor and estuary water has the highest concentration of particles. The deployment of UWOC is more challenging as per channel condition requirements. The channel properties vary with distinct water types and geographical conditions [10]. Additionally, another classification while the propagation of light beam based on the environmental, geographical conditions, and the chemical vitality of the water is categorized. The subcategories are called as IOPs, and AOPs [140]. The IOPs are only medium dependents on existing substances and the composition of the medium [140], while AOPs depend on both the mediums and nature of the light beam. The AOPs are used to evaluate the ambient light levels for communication near the water surface. AOPs contribute radiance, irradiance, and reluctance. In more contrast, the spectral absorptive coefficient and volume scattering function are the two main factors that are considered in IOPs [141]. The transmission of the light beam experiences the absorption of photons by water and further converts them into another form of energy to increase the temperature of water molecules. In absorption phenomena the signal pathloss occurs where the beam intensity does weaken and the propagated photons absorb by the channel. The absorption occurs by chlorophyll in phytoplankton at colored dissolved organic matter (CDOM), by water molecules and existing salt contents [140]. Moreover, the CDOM matters are the combinations of the two components known as fulvic and humic acid as mentioned in [142]. In more detail, the absorption and scattering phenomena in different waters have been discussed in further subsections. The absorption effect is calculated in terms of wavelength and represented by α(λ). Though, scattering is defined as the randomness of photons with the direction of beam propagation. The optical signal scattering phenomena occur by the vitality of salt ions in pure water and collimated with particle matters. The scattering coefficient measurement per meter is denoted by β(λ). Due to scattering and absorption, the energy of the optical beam falls and occurs the overall attenuation. The typical values of α(λ), β(λ), and extinction coefficients c(λ) in different water mediums are summarized in Table 5.

• Absorption & Scattering Phenomenon in UWC:-
Signal scattering in UVLC is the function that changes signal path, direction, and distance covered by photons in the underwater medium. The energy loss of The typical values of the contribution underwater absorption, scattering, and extinction coefficients. The data is applicable for different waters mediums [5], [82].
optical beam by water (interaction between photon and water molecules) cause absorption, while the collision happens with suspended particles in UWOC. The optical signal absorption is an irreversible process where the photons lose their energy and improve the water temperature of water molecules. In more detail, if the photons strike with suspended particles and change their direction of propagation, scattering happens. The total energy loss of propagated optical beam due to absorption and scattering can be defined as the energy loss per meter. In addition to that an another cause of energy loss is inter-symbol interference (ISI), where the receiver aperture captures fewer incoming photons on its crosssectional area. The extinction coefficient (summation absorption and scattering) is the signal loss phenomenon and directly affects the signal intensity and changes the direction of an optical beam. Concerning the physiochemical properties of the channel and the geometrical model of absorption and scattering coefficient are proposed in [5] and [13]. Due to the harsh water channel conditions, the best way to understand the signal attenuation mechanism through a geometrical model of a water element is depicted by Fig.6. The optical beam energy is either absorbed by dangle particles or is transformed into heat energy, or raises water molecules thermal energy. Therefore, signal absorption affects signal strength, while the scattering phenomenon occurs the spreading photons randomly and strikes with the suspended particle. As a result, signals are partially received by detecting aperture or delayed to reach the destination. Thus, multi-path fading, timejitter, and ISI occur in UWOC. To reduce ISI, the optimal multiple-symbol detection (MSD), as well as sub-optimal generalized MSD (GMSD) algorithms, are further investigated as in [105]. The recent surveys are detailed severe attenuation in optical signaling in harsh water cause absorption and scattering. Absorption and scattering are the two phenomenons responsible for signal fading caused by the suspended particles, the quality, and nature of water, dissolved organic matters affecting the signal strength, phase, and arrival angle of the received beam. The blue-green wavelength spectrum of visible light severs the minimum amount of absorption in pure seawater rather than coastal and turbid harbor water [143]. Moreover, light scattering is the cause of organic and inorganic matters due to the strong spatial and temporal dispersion of photons, which may affect photon strength and intensity [144].

E. AFFECTING FACTORS IN UVLC SIGNALING
In the UWOC system, the absorption and scattering effects light pulses by existing suspended particles leading to photons path loss and intensity deviation. Both of these signal fading factors are wavelength-dependent per meter of signal propagation distances. Generally, these factors occur due to the interaction of the light beam with micro and macromolecules of water, algal sediments, and color division organic materials. In general, the organic particles are responsible for absorbing photons while inorganic particles are creditworthy for scattering [143]. Further, the CDOM strongly absorbs the light photons in the ultra-violate (UV) spectrum [144]. The total signal fading contribution by absorption α(λ) and scattering β(λ) of optical beam energy loss describes in terms of extinction coefficient c(λ) known as signal attenuation as c(λ) = α(λ) + β(λ). The extinction coefficient depends on the different water types. To formulate the expression for absorption and scattering, an elemental unit volume of water δ(V ) with thickness δ(r) is considered and depicted by Fig.6. If the intensity of incident light-beam is denoted by P i (λ) with fixed wavelength λ collides with unitary elemental water volume. The incident beam is passing through the elemental volume and experiences absorption & scattering. The outcome beam is split into three fractions; as a certain amount of power, an incident beams P a (λ) absorbed and some extent of incident power P si (λ) will scatter at an angle ψ i . The remaining apparent beam P ri (λ) passed through the element and detected by receiver. Thus, the energy conservation law and total incident power is calculated as the addition of all above fractions, i.e., P i (λ) = P a (λ) + P si (λ) + P ri (λ). Furthermore, the absorbed signal power ratio A(λ) and B(λ) scattered signal power ratio are defined as, In (10), the absorption and scattering ratios with the radius δr of an elemental water volume are infinitesimally small. Then taking the limits by an elemental water volume, the absorption and scattering coefficients can be written as [117], VOLUME 10, 2022 Apart from the absorption and scattering, the seawater acts as a conductor medium for electromagnetic waves and offers dielectric properties for optical signaling [9]. In more precise terms, the seawater dramatically serves conductivity to dielectric at 250GHz frequency range [10]. However, underwater optical signals experience several extreme challenges and might be affected by fluctuation of scintillation index, scattering, absorption, dispersion, and temperature fluctuation due to the physio-chemical properties of the channel. Optical waves do not propagate simultaneously in water such as on a terrestrial basis, but accelerate exponentially in the water medium due to attenuation. The attenuation is the total contribution of signal fading, absorption, and scattering. In UWOC, scattering losses occur cause of discontinuity of the water molecules, while absorption occurs by the physical properties of channel conditions. In coastal ocean water, the optical signals are extremely corrupted by noise and path loss due to more scattering; the scattering event happens by the collision of the optical beam with the existing suspended particles and multiple reflections of angles [145]. The phenomena of absorption in UOWC have to be firmly taken into account cause several photons could reach the photo-detector or receiver end by passing harsh channel conditions.
Absorption is a high wavelength-dependent process where the transmitted photons are absorbed and converted into chemical or heat energy of the water. For instance, in [142] the authors have modeled absorption coefficient under consideration of absorptivity by pure seawater in terms of chlorophyll as a function of wavelength. The absorption by chlorophyll is defined as the change in light beam intensity per meter of path length. The total contribution of absorption is the chlorophyll and the color dissolved organic components (CDOMs) i.e., [146]- [148] In (13), the absorption coefficient of water, chlorophyll absorption coefficient are represented by α w (λ) and α cl (λ). Moreover, the two CDOM components fulvic acid absorption coefficient α f (λ), and humic acid absorption coefficient α h (λ) are defined, respectively. The individual absorption and scattering components for different water mediums are calculated in (14) and (15) to model. As a result, we consider transmitting the blue-colored spectrum wavelength (450nm) of visible light. The absorption coefficient by water types with respect to water concentration of ranges 0 ≤ w c ≤ 15 mg/m 3 , is calculated as follows, where w 0 c = 1 mg/m 3 is the reference water concentration. For the different types of water the a 0 w (λ) differentiate accordingly [142]. The water concentration for different water types such as for pure sea water, clear ocean water, coastal ocean water and turbid harbor water varies 0.0405λ, 0.114λ, 0.179λ and 0.266λ, respectively. According to [145], the absorption component due to chlorophyll a cl (λ) with respect to the chlorophyll concentration C 0 c varies 0 ≤ C c ≤ 12mg/m 3 , and can be written as, where the reference concentration of chlorophyll C 0 c = 1mg/m 3 , and α 0 c is equalized to 0.0602λ. Furthermore considering the CDOM components as, fulvic and humic acid absorption coefficients also formulated with respect to the chlorophyll concentration [146], where the specific absorption coefficient for fulvic and humic acid component are a 0 f and a 0 h respectively. The concentrations of fulvic acid C f and humic acid C h in mg/m 3 can be expressed as follows [147], Due to suspended large and small scale particles underwater, the scattering phenomena occurs. The scattering coefficient β(λ) defines the total signal loss due to the redirection of the photons. In contrast, the total scattering is the sum of scattering coefficient of water β w (λ), small particles β s (λ), and large particle β l (λ), respectively. The scattering function is wavelength and concentration-dependent. Therefore, total scattering can be calculated as [142], [146]- [148], where the small scale concentration C s and C l the concentration of large scale in g/m 3 can be calculated as [145] Another noticeable optical property concerning signal scattering is known as albedo scattering, which is the ratio of scattering signal loss to the total signal losses [144]. The albedo scattering number is formed as, ω 0 =(Scattering signals β(λ))/(Total signal scattering c(λ)). The albedo number shows a significant scale factor to measure absorptivity or scattering of water medium, which varies from 0.25 to 0.8 for clear ocean water to turbid coastal water. The scattering phenomenon is stated as the detected optical information in terms of angular, temporal, and polarization which causes the degradation in the number of photons at the receiver end. The cause of suspended CDOM and inorganic particles, the scattering coefficient may be termed as the function of the volumetric scattering function β v (VSF) over total angles (θ) as follows [149], Moreover, the VSF is prescribed the normalization of β(λ) by the scattering phase function. Therefore, the spectral VSF coefficientβ v (θ, λ) is defined as follows, Simultaneously, the optical beam propagated through the water channel and lost some fraction of its propagation energy in interaction with water molecules, number of biological substances, CDOM, etc. The are two signalaffecting mechanisms widely considered as absorption and scattering. The optical signals are severely affected by the channel turbidity due to the random fluctuation of the refraction index.

F. TURBULENCE
Turbulence is another cause of optical signal fading resulting from the random fluctuation in the refractive index. Plenty of valuable experimental results have recently been recorded in the open literature on characterizing underwater optical turbulence (UOT). In other words, turbidity is described as the varying changes in refractive index due to fluctuation of density, salinity, and pressure of underwater environment that affects optical signal propagation. It means the random variation in scintillation index induced turbidity in the channel environment. The consistency of oceanic turbulence occurs by wave current, physio-chemical properties of water, large, and small suspended particles. The wave currents are defined as weak to strong turbulence conditions based on Reynolds number. If Reynolds number ρ ≤ 1000, then water flow is considered laminar or steady flow, with the minimal fluctuation in scintillation index. Whether, if it is 1000 ≤ ρ ≤ 2000, then the flow has shown uncertainty, which means weak turbulence channel conditions. In the case where Reynolds number 2000 ≤ ρ exceeds 4000, then it is considered as heavy turbulence channel conditions.

G. AFFECT OF NOISE IN UWOC
Noise is the factor that corrupts the signals, which directly affects the signal efficiency during communication.
In UWOC, the noise occurs due to the reception of a random variation of photons propagation by transceivers. Different background noise sources are widely studied, such as quantum shot noise, optical-excess noise, optical background noise, photo-detector dark current noise, diffused extended background noise, background noise from sunlight, or other objects in the optical communication systems received at the receiver end. There are different types of noises in UWOC are recorded in existing literature [4], [5], [9], [150].
• Optical Excess Noise: The communication setup should be more accurate and reliable for information transmission purposes. However, some irregularities or faults in the experimental setup causes falling transmission accuracy. Due to inaccuracy of the transceivers noise occurrence, the glitches happen and corrupt the received signals defined as excess optical noise.
• Background Noise: In contrast to this noise, background noise is considered black-body radiation; the refracted sunlight is responsible as the main source of background noise. This type of noise majorly interfered by the sunlight, limiting the optical system performance for tens of meters (mostly in the Euphotic zone). The noise contribution of total background environment in the literature is recorded as in [4], [5], [9], and [150]. The total background noise power in optical signal propagation is defined as follows [150], For channel modeling, the background noise variance σ 2 BG = 2q P BG B, depends on electronic signal bandwidth B, power of solar background P BG sol , and electron charge q, respectively. Furthermore, P BG sol is expressed as, In (26), the receiver aperture area, the optical filter transmissivity is presented by A R , and T F , respectively. However, the solar radiance also depends on downwelling irradiance E, underwater reflectance of downwelling irradiance R, the directional dependence of underwater radiance L f , diffuse attenuation coefficient VOLUME 10, 2022 K , and the underwater depth d. Therefore, the solar radiance L sol is expressed by (27) as follows, Furthermore, the noise power of a black-body irradiance expressed as where c, h, k, and γ are described as the speed of light, the Plank constant, the Boltzmann constant and the radiant factor. Moreover, the signal transmission factor in underwater is defined as T A = exp(−τ 0 ). • Photo-Detector Dark Current Noise: The dark current noise is described and related to the photo-diode [35]. This type of noise occurs due to electrical current leakage from the photodetector. The variance of the dark current noise can be defined by (29), which is the function of electron charge q, the DC intensity I DC , and electronic bandwidth [150], • Current Shot Noise: This type of noise is generated while receiving the signals at the receiver end. If the signal power symbolized by P s , then the shot noise variance can be written as [10], • Thermal Noise: The random motion of electrons generates noise within the conductor medium due to a temperature rise. To model the thermal noise, if it considers The noise figure parameters F = 4, load resistance R l = 100 , and equivalent temperature T e = 290K are taken, then thermal noise variance given as [10], [150] In the UWOC system model, the sum of total noise variances is responsible for the system noise. Therefore, without any interference of optical signal, the noise variances of electrical current at the detector end is given as,

IV. TRANSCEIVERS FOR UVLC A. TRANSMITTER AND TRANSDUCER
The function of optical transducers is to transform the modulated information into an optical or electrical or vice-versa signal pattern. The transducers are mounted on transceivers and fixed on a well-defined path for smooth signal propagation in an underwater medium. In another words, the transducers are designed to generate an optical signal from an electrical signal made from optical transmitters during optical beam propagation. On the other hand, an actuator can be used in UWOC [9] to generate the pulse of optical signal in electrical format at the receiver end. With the rapidly increasing interest in exploring aquatic media, UWOC plays a vital role in realizing a wide range of scientific, commercial, and coastal security applications. VLC is an alternative communication media as a UWOC implementation in the range of LEDs illuminating light sources. In UVLC, the transmitter transforms digital information into analog format over channel conditions. Generally, LEDs and LDs are used for signal transmission over high data rates and bandwidth. Another adhesive advantage is LED structure, where the photons are emitted spontaneously at different phases. In coherent radiation, the photons are stimulate by another photon that is radiated with phase correlation [151]. More precisely, LEDs excel as incandescent reliable light sources, power consumption devices, and efficient luminous tools. The LEDs efficiency operate 210 m/W more than the incandescent lamps efficiency [45]. The multi-functional transmitter-enabled LEDs emit light and information simultaneously with the optical dimming power this is most widely used; for efficient signal propagation while the optical transmitters do not use flickering phenomena. In a general aspect, the LEDs are designed for white-light emission sources and divided into two categories: blue and red-green-blue (RGB) emitting types of LEDs. Both types of LEDs emit different colors of light, as yellow-white light causes phosphorus polishing. Blue LEDs are considered more energy-efficient and widely use for light illuminating systems rather than blue LEDs. The blue-LEDs could achieve reliability for optical signal transmission by combining yellow color (RGBY).
In optical beam propagation, the VLC link depends on the dimming or switching nature of LEDs. The optical sources are categorized as laser transmitter type or an LED type optical transmitter. Consequently, the laser type of technological sources quiet different and are used according to the application requirement. An argon-ion laser is a wellknown laser transmitter suitable for converting electrical signals into an optical format [10]. A transducer assists in converting and then transmitting all maximum quantities of photons toward the receiver or other nodes. For optical signal modulation, different types of laser modulators are being used. In general, the laser modulator is designed for low data rates over long-distance signal propagation, [9]. Compared to laser modulators, LEDs are cheaper though they cover very short ranges. The arrangement of an LED array is designed in a hexagonal type pyramidal shape for high-intensity and long-distance optical beam propagation, as proposed in [148]. In this type of pyramidal LED array structure, a single lens is used to illuminate the LED. This structure must be very compact for the output signals. For the sake of a signal, the direction can be created by switching an intense beam towards the destination.

B. OPTICAL RECEIVER
Optical receivers are the collection arrangements of alloptical signal transmission within a medium. In other words, receivers could be defined as the process of optical signal entities absorbed by the photodetector and the photons detection on its surface to generate an electrical signal called an optical receiver. The conversion process of photons energy into an electrical signal is performed through photoelectric effect. Otherwise stated, the received photons at the semiconductor junction diodes release either an electron or a hole. Subsequently, these released entities moved to the conductance and release their excessive energies [21]. The propagated optics are collected by a lens for the detection and demodulation process at the receiver end. A photo-sensor is used to convert optical signals for further analysis. Taking into account improving system performance, a photo-sensor is characterized as easily accessible, very cost-effective, robust, and small in size, along with the low current consumption [152]. To cope with these advantages, plenty of different types of photo-sensors are being used and summarized in recent surveys as [9], [152]. These photo-sensors are photo-resistors, phototransistors, photomultiplier tubes, avalanche photodiode (APD), p-n photodiodes, and a photon detector semiconductor, photosensors, and biologically-inspired quantum photo-sensors (BQP). PIN and avalanche photodiode (APD) are widely used as a photodetectors in optical communication [153].

V. CURRENT PROJECT SCHEMES ON UVLC
In this section, we provide the details of the most recent current project in underwater medium currently running VOLUME 10, 2022 worldwide. A wide detailed description are summarized in Table 6.
• Most recently, UNEXMIN project scheme [154]  • The Surf3DSLAM project [155] was coordinated by the University de Girona, Spain, and proposed an innovative  [156] was proposed and coordinated by the University Graz, Austria. This project was launched and aimed to achieve a long-lasting self-determination learning device, a self-configurable, self-maintainable underwater robotic tool in very high-impact-modified application areas in Italy. The main perspectives of this project are to develop a better quality of communication and energy harvesting among the underwater sensor nodes. The subCULTron is subdivided into few sections as the entitled project. The upper layer of sea surface in the proposed system, and the artificial objectives are introduced as the information collective robotic tools for long-run system exploration capabilities. The artificial robotic tool allows monitoring the marine environment, and the incredible natural oceanic environments, including biological materials such as algae, bacterium, and aquatic species. In contrast through subCULTron, a deployed artificial lily-pad on water surface communicates with human-culture and transmit energy along with the collected information from ship traffic. Apart from that, the artificial fish floats and exchanges information with mussels and lily-pad. The subCULTron is aimed to propose the real world for humans and animals in cohabitants of high impact such as venice canals and lagoon. Thus, the cultural evolution promotes a sub-culture development and an artificial society underwater along with the cultural services of human society over the water reservoir.
• The TRIDENT project scheme [157] formally known as Marine Robots, and Dexterous Manipulation for Enabling Autonomous Underwater Multipurpose Intervention Missions (MRDME-AUMIM) was proposed for multipurpose underwater challenges in highly turbid and complexity channel conditions. In other words, the development of novel methodology for marine robotics systems accommodates versatile dexterous adaption and the capabilities for operational activities in unguided underwater environments. This project scheme also has drawn attention to various specific skills in marine environments: navigation, bathymetric data, geographical mapping, vehicle-manipulator systems, dexterous manipulation, and the development of a wide range of intelligent control UWSNs architectural techniques. The project has been classified into two phases. In its first phase, the autonomous surface craft (ASC) is carried AUV towards the sector to be surveyed, while in the second phase, the assembly of ASC and AUV navigate the underwater targeted position.
• The latest project scheme [158]  • Oceans and seas are the source of life on earth and the most significant water source for humankind. Nevertheless, these water reservoirs are affected by human interactive activities. Therefore, exploring these massive water mediums are necessary by deploying artificial devices or robots, called marine robots. In general, the vitality of marine robotics technology is an enormous significance in an extensive range of functional underwater applications. These applications are mostly utilized to observe and monitor purpose and the life beneath the water surface, observation of water pollution, oil and gas rig observations, surveillance and coastal securities, fish farming, and early detection warning of natural disasters. Given this, an innovation of the project scheme Sibiu-HCEV [159], is introduced as the resolution of an intelligent hybrid high capacity electric underwater ROVs. The Sibiu-HCEV is an underwater floatable robot at different depths in an unguided water environment that is an innovative invention in underwater robotics technology to reduce risk to the capital expenditure (CAPEX) and operating expenditure (OPEX).
• Marine robotics industries are rapidly growing up to uncover the complexity of harsh oceanic environments. The marine robots have pulled back the curtains of the mysterious thesaurus of the deepest oceans. This underwater robotics industrial sector is a crucial, significantly cost-effective domain with considerable entry barriers to resources and developments. The project EUMarineRobots [160] is aimed to extend the underwater challenges into more depth, remotely operations, and in the very hostile marine environment. Additionally, the aim of EUMarineRobots are deploying for a full range of air and open international access to Europe. EUMarineRobots has an impressive research group from ten countries in marine robotics research infrastructure network. This network distinguishes the various technical infrastructures for diverse navigation beyond marine and in the marine robotic sectors.
• Since the early ages, underwater applications are based on acoustic links for long range communications. However, RF and optical waves are capable of supporting large bandwidth and high data rates but efficiently attenuated due to the physio-chemical vitality of the marine environment. The primary objectives of the project were recently proposed as UNDERWORLD [161] to re-evaluate the electromagnetic communication through UWSNs deployment for various underwater applications such as navigation, localization, natural disaster, detection of coral reefs, and coastal security purposes in real-time scenarios. This project aims to enhance industrialization in marine observatory systems, naval tactical operations, seaport management infrastructures, and underwater environmental monitoring. Moreover, underwater environmental monitoring in an aqueous medium is widely deployed with acoustic waves, but these applications apply in shallow waters such as rivers, dams, lakes, and remotely coastal areas. Throughout this project scheme. Another main contribution of this project is to improve the characteristics of antennas for two ways of communication among transceivers.
• Earthquakes, Tsunami, and floods are sudden natural events which occur due to an imbalance of water reservoirs and waves (water currents). These events are entirely impossible to control or prevent but can be detected earlier by deploying the sensors nodes or anchored architecture of UWSNs with the sea bottoms. The seismometers are used to sense the unacceptable seismic frequency at a standardized scale. These seismometers are highly detectable sensor nodes those capable for detecting and recording waves emitted by the most miniature earthquakes. It is far expensive to deploy seismometers at the ocean bottom to investigate water currents and seismic waves detection. Due to these issues, the novel project scheme funded by the EU entitled Ocean-DAS had recently proposed new tools for underwater seismology through oceanbottom distributed acoustic sensors [162]. The main aim of Ocean-DAS is to develop a low-cost deployable alternatives for monitoring seismicity in remote areas of the oceans. The Ocean-DAS are enabled with the fiber optical wires for telecommunication which transforms the data into powerful seismic sensing arrays. Through the support of this opto-electronic unit, more than a 50km span could be monitored.
• The most recent project scheme is proposed entitled as Cooperative Underwater Surveillance Networks (COUSIN) funded by Engineering and Physical Sciences Research Council (EPSRC) [163] during 2021s to 2023s. The project scheme focuses on the underwater surveillance system for a specific country or region.
The surveillance system is investigated for more precise benefits in various underwater applications in numerous fields such as pollution monitoring, coastal or harbor securities, import and export duties, human trafficking, smuggling, maintaining integrity, and detecting attacks on underwater infrastructure. The main expected contribution of this project scheme is to track, target and search for suspicious objects, and convey information to the base station for further investigation. The detectable target through this project scheme could be an artificial object such as UUV, surface vehicle, submarine, source of pollution, underwater species, active volcanos, oil & gas pipeline repaitment, ice objects, or divers, etc. Furthermore, surveillance and underwater monitoring are the main functions through the long-range acoustic link. For underwater geographical mapping and target detection, the SONAR waves are the key beneficial technologies while over the sea-surface detection, radio detection and ranging (RADAR) are the most widely used. The main objective of COUSIN is to design a low-cost underwater acoustic network to improve the underwater monitoring and surveillance system capabilities.
• The current ongoing project scheme entitled Technologies for Ocean Sensing (TechOceanS) [164] funded by the European Commission (EC) and coordinated by the National Oceanography Center, UK. TechOceanS is organized by the research group of ten universities in Europe. The oceans are affected by human beings by VOLUME 10, 2022 flushing and trashing unwanted decomposable materials such as plastics that are threatening to harm underwater species. Due to human activities direct and indirect involvement in the underwater environment, the ice caps melt, and the water level rises. Therefore, an urgent need to deploy an advanced ocean observational technology through resistance to extract the decompose plastic materials. At first glance, the project has developed the scheme to monitor and observe ocean biology, chemical composition changes, and plastic monitoring through underwater vehicles nine pioneering technologies. The proposed novel solution by TechnOceanS is comprised of a group of five sensors; the two sensors are used for imaging data collection of the specified seabed, marine biology, and plastics. A sampler is used to collect thousands of particle samples for further analysis and the rest of use for a new artificial intelligence (AI)based image processing method for data compression and information transmission.
• The most recent proceeding project scheme established for Long Distance Underwater Wireless Optical Communication System using Visible Light Laser Array with Directivity Control Function (LD-UWC-VLC-LADCF) [165], and funded by Japan Society for the Promotion Science (JSPS). The purpose of this scheme is to improve the quality of visible light signaling over long ranges in the underwater environment. In this research perspective, the main two challenges are extending the transmission distance in gigabit-class with high-speed UWOC. The first point is the verification of the effect of distance extension while multiple visible light laser are propagated propagating in a high turbid water medium. Secondly, the construction of a light beam in a particular direction through evaluating the experimental data available.

VI. COMPUTATIONAL METHODS
Nowadays, solving complex and huge data-based problems are typically possible without a real experimental setups. This is achieved through simulation-enabled computer science and engineering methodologies. Moreover, computational engineering methods support automaticity, design, and optimization of the complex problems. Therefore, the computational research methods provide the new advancements investigating the mythologies in computing such as algorithms, models, simulations, and system performances in order to understand the complexity of social, biological, technological, and everlasting patterns and their behaviors [166]. These computational research methodologies provide a solution for better understanding of random behavior of the system as follows: • Automatically categorization and information extraction of the selected data-sets; • Differentiate a bulk data into a specific group and particular level for the analysis; • The system design and model of the emergence of new behaviors from individual-level interactions; • A detailed explanation of emergence of a system network; • Recognizes and refrigerates the thresholds within a complex system; • Classifies and finds a the pattern within the massive data-sets for further analysis by using other techniques; • Identify meaningful structural patterns within the webs of relationships. In more detail, computational engineering methodologies are defined as the development and exploitation of more realistic computational models. As a result, the computational engineering methodology opens new perspectives and challenges of more sophisticated key problems of huge data structures. Additionally, computational methodologies are numerically mathematical and depends heavily on computer architectural system. It means that the numerical efficiency should be achieved by extracting the maximum amount of correct information from the minimum number of grid points, as well as establishing a linear proportionality between the incoming time and the unknown number. Due to this, several computational approaches are studied in this study as follows.

A. MONTE-CARLO COMPUTATIONAL METHOD
The probabilistic format of the computational method is widely known as the Monte-Carlo simulation. In UWOC, this computational method is used to track the number of photons emitted by an optical source within the underwater medium. A high bandwidth underwater optical communication channel by utilizing Monte-Carlo simulation is widely discussed in [167]. The simulation approach of photons propagation is rigorous within turbid medium, in a more detailed description, Monte-Carlo simulation computational methods are used to provide the specific details of the propagated photons geometry as mentioned in [168] and [169]. Generally, the optical signals propagation depends on the medium waves. This is an alternative computational approach to eliminate the unbounded error growth in co-variance equation computationally as described in [170].

B. NUMERICAL MODELING OF WAVE PROPAGATION
As aforementioned, that the optical beam spreads in an underwater medium and depends on the water flow (channel conditions). The different types of water flow have been categorized and summarized according to the flow pattern in [171]. Laminar flow is defined as where the fluid particles move along in a well-defined path and each water layer slides smoothly over the adjacent layer. While the turbulent flow is defined as the zig-zag flow pattern of water particles. The laminar flow changes into turbulent flow if the Reynold's number is more than that 4000. The Reynold's condition satisfies as laminar flow below 2000, otherwise turbulent flow [172]. Due to this, the optical beam propagation experiences a huge loss due to turbulence, signal absorption, and scattering by the suspended particles within the medium. Therefore, the necessity is to model the turbulence channel. Moreover, a numerical method for free surface flow that builds for nonlinear shallow water equation and utilizes a non-hydrostatic pressure term to describe short waves are presented in [173].

C. BEER LAMBERT's LAW
Computational methods are employed as the most significant solution for optical signaling in underwater mediums. An ordinary simulation method is widely employs for optical signaling in an aqueous medium known as Beer Lambert's law. This law is described the optical signal attenuation due to the existence of suspended particles underwater. It is mathematically defined as the function of optical received signal power, the Euclidean distance apart from the transceivers, and the extinction coefficient of the medium. This type of channel model is used for optical signaling for any type of water availability, especially, for path loss fading. The mathematical expression of Beer Lambert's law is shown below: where P r and P t are denoted as the received and transmitted optical signal strength at the receiver and transmitter ends. Also, the extinction coefficient c(λ) is the sum of total optical signal absorption and scattering which are the optical wavelength dependents. The experimental values of extinction coefficient, for the readers references are widely discussed in [5], [82], [84], [86], and [174]. Additionally, due to transceivers physical parameters, geometrical losses are also considered for diffused LDs and semi-collimated light sources LEDs. Therefore, the modified Beer-Lambert law is the function of path loss (h pl ) and geometrical losses (h gl ) are given as [101], The contributed channel condition coefficient h l in terms of path loss and geometrical losses are presented in (34). In (34), the receiver aperture diameter D r , full-width transmitter beam divergence angle θ F , and correction coefficient τ are defined. According to [101], the maximum achievable distance between communication nodes to obtain the BER performance of the vertically point-point system is given as follows, where W , Q, η, and T are defined as the Lambert-W function, error function, electrical to optical conversion efficiency, and correction coefficient.

D. RADIATIVE TRANSFER DIFFERENTIAL THEORY AND COMPUTATIONAL METHOD
Radiative transfer theory (RTE) deals with computing derivatives of radiometric measures with respect to desired visual of geometrical parametrization, and facilitates the gradient-based optimization in various applications [175]. Additionally, the RTE is deployable in neutron transportation, astrophysics, and biomedical image analysis [175], [176]. Furthermore, the energy conservation-based radio transfer theory is used to model the optical waves in participating media. For better RTE understanding the assumptions are made, if considers the test volume V ⊆ R 3 with the corresponding boundary ∂V , and the interior radiance field L(s, ω) is defined over the position s ∈ V /∂V , and lighttransport directions ω ∈ S 2 . Then, the RTE equation given as follows [175], where, K T , K C , and K S are denoted as the transport operator, collision operator, and interfacial scattering operator, respectively. Additionally, the Reynolds transport theorem is also an alternative method to solve computational problems. The Reynolds theorem for f scalar value function is defined for n-dimensional manifold ω parameterized with some π ∈ R as (37). If it is an assumption that (π ) ⊂ V (π) be an (n − 1) dimensional manifold given by the union of the external boundary ∂V (π) and the internal one containing the discontinuous locations of f . Then, Reynold's theorem could be specified as follows [177], < n,ẋ > (π). (37) Furthermore,ḟ := ∂ π ,ẋ := ∂ π x, dV and d denotes the standard measures associated with V and 0; Moreover < ·, · > indicated the dot (inner product between two vectors.) Furthermore, n is the normal direction at each x ∈ (π).

E. THE DATA ASSIMILATION ALGORITHMS FOR NUMERICAL MODELS
The data assimilation refers to the procedure of incorporating data into a model simulation to improve the upcoming predictions. The predictions of the system behavior can be defined by the measurement of the physically numerical based model. More specifically, the system should be more simplified for a better understanding of computational methods. In many applications, the data assimilation approaches are accurate to describe the dynamic behavior of the system. Utilizing the data assimilation techniques the measurement and model outcomes are used to obtain an optimal estimation of the state of the system. However, the main hurdles in advanced data assimilation schemes are the bulky computational load that is required for solving a real problem. On the other hand, the assimilation algorithm is an essential tool to obtain efficient variational data for further analysis. Due to this, More accurate data assimilation methods are variational data assimilation and Kalman filtering computational method is widely used. Data assimilation for minimization computational method widely discussed in [178] as follows, where X k is the system state, u k is the input of the system, f is a nonlinear function, p is a vector containing the uncertain parameters, and B(k) is an input matrix. Besides, the Kalman filter assimilation computational method could be written as, where the Gaussian noise process is denoted by W k with zero mean and covariance matrix Q(k) and G(k) is the noise input matrix.

VII. UVLC CHANNEL MODELLING
Modeling of UWOC channel leads to a better understanding of optical signal propagation and supports exploring aqueous mediums. UVLC is considered an alternative communication media to meet the requirements in high data rate, large bandwidth, and high communication speed to deploy in various underwater applications. Moreover, for long ranges UVLC communication, the necessity is to design a system for efficient signal transmission. Generally, the signal absorption reduces the signal strength that is attenuated in the transmitted signal. Despite that, the scattering phenomena is responsible for the optical beam deflection from its initial position, this contributes to signal fading. As a result, Monte-Carlo simulation is being used, which is an alternative approach to deal with the statistical methodology for the evaluation of channel characteristics by generating a number of random photons and further obtained the simulation performance interaction of each photon within the medium [179]. Optical signal propagation is categorized into two main streams based on the type of detection techniques. They are: non-coherent intensity modulation and direct detection (IM/DD) signal modulation technique. However, the IM/DD is generally used as an optical signal modulation scheme to decode information at the receiver. In non-coherent systems, the detector directly detects information through emitted light. Another aspect of the signal detection technique is the coherent detection, where a local oscillator (LO) is being used to mix optical signals before detection which improves the receiver sensitivity [180]. In more contrast, coherent and heterodyne detection techniques are widely discussed as in [29]. A local oscillator uses incoherent detection scheme which comprises of the received signal to the carrier beam detected by the photodetector. Furthermore, the coherent detection scheme is also sub-categorized into two main classes as heterodyne and homodyne detection [21]. The LO operates as the same frequency with the optical signal in the homodyne detection scheme, while the frequency could be different in the heterodyne detection scheme. In the VLC setup, the system consists of an optical transmitter for signal modulation by LED illumination phenomena. On the other hand, a receiver enables a photosensitive diode for the decoding process from the detected light power. The block diagram consists of a transmitter and a receiving system depicted by Fig. 8. The different approaches use to model the undersea VLC system.

A. OCEANIC TURBULENCE CHANNEL MODELING
The most influential optical signal phenomenon is channel turbidity. Turbulence occurs due to random fluctuation in the refractive index of the water medium caused by the variation in temperature and salinity. Therefore, turbulence phenomena affects the performance of optical signal propagation in an aqueous medium. The affecting performance and channel modeling of highly turbid water mediums are presented in existing literature [100], [102], [181]- [183]. The fluctuation in the refractive index is directly related to the nature of the water channel as a function of temperature, salinity, pressure, and density. Due to increasing underwater depth, the temperature decreases and pressure increases; nevertheless, the water is slightly compressible affecting highly pressurized and dense water environments. Taking into account underwater turbulence, the scintillation index (σ 2 I ) leads turbidity which is the main cause of optical signal fading. In more detail, the broad study of modified underwater optical turbulence (UOT) model is investigated mathematically by (40), as shown at the bottom of the next page, [184].
As stated UOT spectrum model in (40), where κ, and C 0 , C 1 is defined the magnitude of the spatial frequency, dissipa-tion rate of turbulent kinetic energy per unit mass of fluid and fluid constants, respectively. Moreover, Kolmogorov microscale length η is represented ratio of kinematic viscosity (ν) and . The dissipation rate of mean-squared refractive index χ n , the thermal expansion coefficient α, and saline contraction coefficients are given as, Considering plane and spherical wave models of scintillation index (σ 2 I ) for different Gaussian beams are widely discussed in [185]- [187]. However, the scintillation index is defined as the variance of the received signal normalized intensity I , which is expressed in [8], As varying scintillation index causes heavy turbulence, that can be highly affect of optical signal propagation. It means the more unsatisfactory performance of the system and degrades the system efficiency. Various existing works are recorded to channel models in weak and heavy turbidity channel conditions. In existing research, plenty of statistical channel models under weak turbulence intensity fluctuation and mathematically are defined PDFs for turbulence. The experimental model under lognormal distribution is mentioned as in [100], [188]- [190]. Therefore, the probability density function (PDF) of turbulence channel condition under lognormal distribution is given as follows, In spite of strong turbulence channel conditions, UVLC channel model into successive j th number of water layers, and following Gamma-Gamma (GG) distribution model is proposed as in [33] and [114], where the Gamma function and modified Bessel function of the second kind are denoted by (·) and K (α j −β j ) (·), respectively. The PDF in (44), for normalization conditions it satisfies the equality of turbulence channel coefficient as E{I J } 2 =1. Besides of it, the scintillation index for GG distribution is given as [191], Furthermore, under-consideration of j th vertical water layers, the large and small scale factors for the plane wave model are defined by α j and β j respectively. The α j and β j can be written as follows [33], [183], In (46)-(49), χ 2 I represents the scintillation index which is known as Rytov variance [102]. Consequently, underconsideration of plane wave model, the large scale factor α, and small scale factor β for multiple water layers can be written as [192], [193], In above expressions (48) and (49), the Rytove variance is calculated for k th environment layers as χ 2 I k =1.23C n 2 k 7/6 L 11/6 . Herewith, the optical wave number is denoted by k = 2π/λ. In (48) and (49), d = 10D 5π λz , with D which is the receiver aperture diameter and the optical wavelength (λ). It is noteworthy that the refraction index is denoted by C n 2 which varies from 10 −13 m −2/3 (strong turbulence regime) to 10 −17 m −2/3 (weak turbulence regime) with uncertainties values 10 −15 m −2/3 in terrestrial based communication [194]. Moreover, the Rytov variance χ 2 I k 1 depends for large scale cells for plane wave model while χ 2 I k 1 for small scale cells.

B. POINTING ERRORS AND NON-LOS COMMUNICATION LINK
Pointing error is another issue in optical signal propagation that contributes to the signal fading in the underwater environment. Due to the random water current flow and movement of floating nodes, a uniform constant tracking pointing system necessary to maintain for a reliable communication link. Generally, the pointing error is defined and considered with the two essential categories as bore-sight and jitter [195]. For more detail, in [195] the authors are defined the bore-sight determines on a stable displacement transceiver (between detector center and the beam center), whereas jitter is the random displacement process between of beam center at the detector plane [196].
Under consideration of Gaussian beam model for jitter phenomenon, if the optical beam propagates through the distance z between the source and destination along with the collected beam at the detector end with beam radius a. Then, the collected power at the receiver is given as [100], [195] [197], In (51), R denotes the random radial displacement at the receiver that could be further expressed as R = R 2 x + R 2 y with R 2 x and R 2 y are the horizontal and vertical components. The fraction of the collected power is denoted by A p at R = 0. The fraction of the collected power A p = [erf(v) 2 ] and v = π 2 D R 2w z are defined the ratio between aperture radius along with beam-width w z . The equivalent beam width defined as zeq = w z 2v exp −v 2 where erf denotes the error function. The beam-width at distance z given as w z ≈ w 0 1 + λz πw 2 0 . The spot size of the Gaussian beam wave and beam wavelength are denoted by w 0 and λ, respectively. The probability density function (PDF) of pointing error h p can be expressed as follows, where the ratio of equivalent beam radius with pointing error displacement standard deviation at the receiver end is presented by ζ = 2σ s . On the other hand, if neglecting the pointing error, then the observation the misalignment of the transceivers is modeled by utilizing the beam spread function (BSF) as [85].

VIII. MODULATION, DEMODULATION, AND CHANNEL CODING TECHNIQUES FOR UVLC
VLC signal modulation is totally different from terrestrialbased communication cause of non-encoding features in phase, and amplitude [198]. In general, the accomplishment of LEDs in the VLC system has triple features as illumination, high-speed data communication, and energy harvesting simultaneously [27]. The Maximum Flickering Time-Period (MFTP) of the light should be minimized for the minimal flickering phenomena [31]. On the other hand, the dimming process control of light supports the potential energy conservation purposes [31]. Generally, VLC modulation techniques deploy under IM/DD, and coherent modulation (CM) based scheme, which is further classified as a single carrier modulation (SCM), multi-carrier modulation (MCM), and color domain-based modulation (CDBM) schemes [3]. The various type of VLC modulation schemes are summarized by the Table 7. Moreover, the pictorial classification of VLC modulation techniques is depicted by Fig. 9. It is noteworthy that IM/DD modulation schemes are less suitable for high-data-rate applications [21]. For that reason, there is an alternative option Mach-Zehnder Modulator (MZM) which is used to modulate the intensity of the continuous optical waves and widely use for laser beams propagation [199]. Although the CM scheme offers more advantages over IM/DD enabled system in detection performance and minimize the background noise, the better sensitivity of the receiver, low cost, and complexity to install setup [200]. Modulation schemes are the most critical ways to expand bandwidth, such as OOK, pulse position modulation (PPM), and pulse width modulation (PMW) have advantages regarding system complexity when using to enhance the performance of the VLC system. However, the Discrete Multitone (DMT) modulation, OFDM modulation, and Carrier-less Amplitude Phase (CAP) modulation have better spectral efficiency [27].

A. SINGLE CARRIER MODULATION SCHEME
In SCM, the communication setup leads to improve channel capacity. Nevertheless, the system complexity is increased due to the cost factor. The IM/DD setups have several benefits as easy to install, very cost-effective, and less complex as compared with the coherent system. The photo-detectors are used to convert optical signals into electrical signals under IM/DD technique. As the dimming process is widely noticed in optical signaling consequently, several dimming processes have been proposed in [201]. Furthermore, the PPM, DMT, and OOK are also proposed to accommodate dimming phenomena [202]. Generally, for regular system model design, signal carrier modulation (SCM) schemes are widely used. In consideration of SCM-IM/DD, several modulation techniques are investigated such as OOK, PAM, variable pulse position modulation (VPPM) and differential pulse position modulation (DPPM) pulse position modulation (PPM), pulse width modulation (PWM) and color shift keying (CSK), etc., [21], [27] and are mentioned in Table 7.

B. OOK MODULATION SCHEME UNDER CONSIDERATION IM/DD
IM/DD scheme is well known and easy to install, and a cost-effective signal propagation scheme. In this scheme, the signals have to propagate through the source directly or by the external modulator. Despite the receiver side, the signal information is demodulated through the direct detection process; therefore, the whole system is named as intensity-modulation & direct-detection. An ordinary process to modulate optical signal information through light and the intensity regulated by the OOK scheme. The illumination of LEDs are turned ''off'' and ''on'' phenomena according to the bit-stream as the digital numbers ''0'' and ''1'' [27]. Such as the presence and absence pulse phenomena of light intensity depending on the bit duration. The LEDs are not completely turned off in the ''off states''; nevertheless, the VOLUME 10, 2022 signal intensity level decreases in this state. The OOK scheme uses and is divided into the pulse format with a return to zero (RZ) and without a return to zero (NRZ), [29]. In a more vast discussion, the format contains the digital bits ''1'' in NRZ while the RZ format occupies the remaining parts of the bits represented by ''0''. The performance of the OOK modulation technique severely degrades and can be improved by updating the higher modulation schemes as followed in Table 7.

C. PULSE MODULATION SCHEMES
Aforementioned, the OOK modulation technique is an essential signal modulation scheme that supports low data rates with low performances. Pulse amplitude modulation (PAM) is more tolerant of the effect of the multi-path channel as mentioned in [27]. PAM modulation scheme is the process where the message is encoded in terms of amplitude of the optical pulse. Despite the PAM scheme, OOK is recorded as widely used under consideration of IM/DD optical communication scheme because of its simplicity and resilience to the intrinsic nonlinearities of laser origin. Even, in pulse position modulation (PPM) scheme, the division of symbol k is transmitted into an equal time slot within the 2 M constellation symbol M . A single pulse in PPM scheme during each symbol period suffers from low datarate [45]. To cope up with this issue in PPM, a pulse width modulation (PWM) scheme is investigated as in [231], where it deals with dimming control cause of lower intensity control and varies with the pulse width proportion of the instantaneous values of the signals. Therefore, for multi-pulse in each symbol-time transmission the multi-pulse modulation (M-PPM) scheme was proposed [29]. On the other hand, in [3], authors comprised as PAM required more power for large bandwidth while PPM suffers low spectral efficiency. Continuously developing of optical pulse modulation schemes, a differential pulse position modulation (DPPM) scheme is introduced to improve the overall throughput spectral efficiency of PPM [232]. However, the DPPM is based on a very short pulse duration width for an alternative signal propagation. Due to this, the combination of DPPM and PWM in the form of DPWM has achieved a higher bandwidth efficiency than PPM and OOK. The most significant advantage of DPWM is that it solves the narrow pulse problem and has low power efficiency. To control the brightness of the LED in the PPM scheme, a Variable PPM (VPPM) is proposed as in [233]. However, the VPPM combines PWM and 2-PPM, which control the brightness and data transmission along with the controlling dimming level to flickering position. As a result, the multiple Pulse Position Modulation (MPPM) has gained a more power-efficient scheme than OOK, PAM, and PPM. Taken into an account of optimum power and bandwidth efficiency, a hybrid Multiple Pulse Amplitude and Position Modulation (MPAPM) signal propagation scheme is proposed in [234]. However, an Overlapping Pulse Position Modulation (OPPM) [235] is proposed for improving bandwidth efficiency and high data-rate in MPPM. Utilizing VPPM for data transmission, the system offers complexity; an alternative approach, a Variable Rate Multi-pulse Modulation (VR-MPPM) [233], is investigated to cope with the complex issues to achieve both data rate and brightness control simultaneously. Furthermore, an expurgated PPM (E-PPM) modulation scheme is proposed to improve the performance of a peak-power limited M-ary communication system mentioned in Table 7.

D. COLOR DOMAIN BASED SHIFT KEYING (CSK) MODULATION
Color Phase Shift Keying (CSK) is defined by VLC IEEE 802.15.7 standard to improve system performance compared with other modulation schemes [45]. The IEEE standards for the CSK scheme are categorized into three sub-categories based on data rates known as PHY-I operates 11.67 to 266.6 kb/s, PHY-II operates on 1.25 to 96 Mb/s and PHY-III that is operated on 12 to 96 Mb/s [31]. CSK modulation is the color space chromaticity dependent on utilizing the three colors intensities as Green, Red, and Blue LED of the source. The main principle of multi-color LED is the color shift keying where the VLC beam implemented as 4-CSK, 8-CSK, and 16-CSK modulation schemes light sources. The 3-Color CSK [210] and 4-CSK modulation schemes are proposed based on IEEE 802.15.7 standard [211]. The different CSK modulation schemes have been proposed in [236]- [238] and Table 7.

E. COHERENT & MULTI-CARRIER MODULATION TECHNIQUES
Despite IM/DD modulation scheme, the coherent scheme employs both phase and amplitude signals to modulate corresponding information. A local oscillator is used to convert the optical signals at the receiver end, which are referred to homodyne and hetrodyne detection mechanism. As a result, the coherent modulation scheme supports a high spectral efficiency and background noise resistivity but more channel complexity. However, multi-carrier modulation schemes are followed OFDM, where orthogonal sub-carriers are used as data transmission. A particular fading channel is converted into a flat fading channel as in the OFDM technique and using as a single tap equalizer. The different types of OFDM-based optical modulation schemes are proposed in open literature [239], [240]. The various multi-carrier modulation schemes are summarized in Table 7. The signals are converted from parallel format into serial ones for demodulation by using IFFT and then further transmitting to the LED transmitter. At the receiver end, the signal demodulation process happens by photo-diode (PD). Several multi-carrier modulation techniques are discussed in further Tables and subsections.
• DCO-OFDM:-Generally, OFDM uses RF communication which cannot be directly accessible to use in VLC because of the real and unipolar values of optical signals [241]. Therefore the conversion is needed; firstly, the bipolar signals are transformed into unipolar format and then the non-positive integers should be clipped. Generally, the RF-OFDM signals are converted into optical-OFDM by the Hermitian symmetry method. In contrast, the DC-bias converts into bipolar OFDM, which is known as DCO-OFDM. The mathematical expression of DC bias is given as [241], where DC bias is defined by B DC of 10 log 10 (µ 2 +1)dB. The proportionality constant and standard deviation of unclipped signals are represented by µ and E{x(t) 2 }, respectively. On the other hand, the expression for clipping operation can be written as follows [241], The continuous development of LED carries new trends in optical communication. It is obvious that in DCO-OFDM, the power efficiency decreases while the modulation sequence increases. This makes it less compatible with several VLC applications. Therefore, a substitutional modulation technique is required. The novel substitutional technique is investigated only heterogeneous subcarriers for operation and also removes subcarriers known as ACO-OFDM [213]. In contrast, the drawback of ACO-OFDM spectral efficiency is that half of DCO-OFDM causes half of the subcarriers rejection. PAM-DMT [221] has become an alternative methodology based on Fourier transform properties. In PAM-DMT, the real part of the signals is ignored while considering the imaginary values of the received signals. Moreover, for unipolar OFDM, the Flip-OFDM was also proposed [230]. The recently proposed Flip-OFDM technique is classified into two classes. In this classification, the second class is the replica of the original one. It means both of the classes contain the equivalency under DC biasbased modulation techniques. Therefore, in DCO-OFDM, the spectral efficiency falls up to half rather than ACO-OFDM.

F. HYBRID MODULATION TECHNIQUES
In optical signaling, long distances coverage is challenging. For covering, long-range communication hybrid links are the key solution approaches for further wireless communication and mentioned in Table 4 and Table 4. A single LED is used to transmit the signal in the further development of hybrid links instead of multiple arrays of LEDs. To deal with the issues in hybrid communication, spatial optical OFDM (SO-OFDM) is proposed. The SO-OFDM has received better BER performance than DCO-OFDM [241]. On the contrary, the reverse polarity optical OFDM (RPO-OFDM) is also introduced to control LED illumination in the VLC system [227]. The combined PWM technique deals with the real values in optical OFDM to control dimming phenomena in the VLC system.

IX. OPTICAL ENERGY HARVESTING (OEH)
The integration of IoTs, is a novel approach for wireless signaling in an underwater environment through IoUTs [70]. Optical waves can carry information and transfer energy simultaneously to power up the batteries of communication nodes. Even, to deploy an underwater communication setup is much more challenging than terrestrial-based communication. Therefore, the UVLC techniques are the potential wireless candidates for communication and energy transmitting media within the communication networks. Due to this, simultaneously, light-wave information and power transfer (SLIPT) has been proposed as a potential wireless candidate for communication and power transmission through optical link connectivity [242]. Compared with simultaneous wireless information and power transmission (SWIPT), SLIPT performs better in specific channel condition requirements. This section provides an overview of SLIPT methodology through time, power, and spatial space dependency. Moreover, the LEDs are capable of transmitting signals in underwater environments with conveniently lowcost rather than RF signaling [20]. Internet-connected devices are the power-hungry tools those need to harvest energy being in an active mode. WPT is quite challenging among wireless nodes, specifically underwater. Although this, WPT has gained research VOLUME 10, 2022 attention in the most recent years [41], [243]- [245]. Initially, the idea of WPT came up during the 1890s [246]. Later on, the integration of WPT was triggered by SWIPT, where only electromagnetic waves carry information and power in different required application scenarios. SWIPT has become a futuristic digital energy transmitting technology in the wireless communication domain. However, RF-energy harvesting (RF-EH) systems experienced major technical problems due to transceiver circuits and suffered from relatively low efficiency. It is noteworthy that EM waves propagate over few meters in the underwater environment. Consequently, to get rid of the radiation rate of EM waves, it is replaced by VLC enabled light sources such as laser diode and LEDs. A hybrid RF and light-wave power transfer scheme with the color diversity of optical beam has been recorded in [247]. The authors in [247], are proposed a novel RF-VLC ultra-small cell network and the proposed system model consists of multiple terminal devices along with an optical angle-diversity transmitter with a multi-antenna RF access point. For shallow water communication, the fading of the optical signal due to the limited sunlight or background light effect is the leading cause of EH harvesting efficiency.
SLIPT methodology can be a complementary energy harvesting (EH) approach to transfer energy via visible light in IoUT networking system. For instance, the RF/VCL hybrid communication and EH scheme is proposed in [248], where the authors designed a VLC link to harvest energy at the relay and furthered this harvested energy by transferring it with the base station through an RF link. However, a more sophisticated optical transceivers setup is required in the SLIPT scenario based on modulation and demodulation schemes, including the time, power, and space splitting format.

A. SWITCHING TECHNOLOGIES
As the main concern of the SLIPT scenario, the switching technique has played the most significant role in transmission power and simultaneously information transmission. Generally, time, power, and space switching schemes are proposed in the open literature [249]. In the time switching aspect, the receiver acts to decode information in the time slot t, whether the reaming time slot (1 − t) is assigned for the harvesting energy. Furthermore, the received signals split into photovoltaic and photo-conductive modes of receiving as well as decoding the signals. In addition, the low-power hybrid system can perform the function of switching between two solar cell modes in the cooperative communication aspect. While in power splitting mode, the receiving terminal self charges and detects the incident light beam information. The light beam splits into dual power factors as α and (1 − α), respectively. The power factor Pα is responsible for EH; else remaining power fraction P(1−α) is assigned for information decoding. In this power splitting scenario, the power splitting component could function passively; the reason is splitting the incident beam divides into two portions. Nevertheless, in this scenario, it is considering n th number of transceivers in a communication system and functioning for energy harvesting capabilities and information decoding. In this type of system architecture, the spatial splitting approach applicable. In this anticipated system design, each transmitter node is independent of each others and could transmit the signal and energy received by each receiving node. Furthermore, the time switching SPLIT approach could be utilized within this scenario for the same receiver, which acts as ''Energy Harvester'' and ''Signal Decoder'' over different periods. These two switching techniques would be beneficial to deploy in critical environments such as undersea and underground communication and wireless energy transfer.
For long-range communication, the UWSNs are deployed within a particular depth in an undersea environment. Being in an active mode of sensor networks to replace the batteries and transfer energy wirelessly is quite challenging. To cope up with this issue, a novel hybrid SWIPT and SLIPT wireless power transfer techniques are investigated [247]. RF and light-wave power transfers are complement to each other in their relative profits and losses during signal propagation. An alternatively switchable process towards the 5GB wireless network to transfer power in shallow waters over moderate depth in underwater mediums is summarized in this section.
SWIPT and SLIPT are the EH utility process where signal energy is converted into usable electric energy. An EHsystem harvests the energy from nature such as solar energy, wind, rising temperature, and vibrational EH approaches are recorded in [250]. As, we are surrounded by an electronic world, embedded devices, remote sensors, and low-power electronic devices are necessarily required for power transfer and batteries to store the electrical capacity. Due to the limitations of the battery lifespan, they should be replaced as per requirement. In underwater SLIPT, the signal recognition and simultaneously receiving energy is a novel approach amongst researchers. The most significant advantage of the EH scheme is the applicability for remote locations and harsh water channel conditions. This type of EH technology is costeffective, improves safety and reliability [251]. In the SLIPT scenario, it offers high bandwidth, extremely high speed, and energy efficiency. Furthermore, the movement of the renewal traditional EH technologies towards to SLIPT is the solution for environmentally friendly technology, which belongs to the class of green communication [252].

B. SLIPT IN UNDERWATER COMMUNICATION
An underwater SLIPT scenario is depicted by Fig. 10. Throughout the proposed system model, the received signal splits into two chunks simultaneously, which are responsible for energy harvesting and information decoding. At the receiver end the received signals are converted into AC and DC components. The AC component is responsible for the signal decoding while the DC component uses for the EH purposes [253].
According to Fig. 10, in the SLIPT system model, if m(t) represents the modulated electrical signal and A denotes the peak amplitude. If, +A I assigns for a bit ''1'' while −A I for ''0'' in modulation and demodulation of VLC signal. In consideration of modulation process to control the optical intensity of LED with the DC bias ψ, the peak amplitude for each bit is given as [41], where I L and I H are denoted the minimum and maximum input bias current. Furthermore, in the point-to-point communication scenario, the harvested energy is modified and given by [41], [254], where f , I DC , V t , T , r, P t , and I 0 are defined as the fill factor of solar panel, the DC component I DC = rIP t ψ, the thermal voltage, time duration during energy harvesting, the photo-detector resistivity, the power component, and the dark saturation current of solar panel, respectively.

• Time-Switching (TS) SLIPT Receiver Architecture
The TS receiver architecture is a disjoint scheme as well as the splitting factors t and (1 − t) denoted for the optimization in the SLIPT system. According to the splitting factor, the ability to gain the idea of exploit harvested energy with adequate BER performance. Moreover, the TS mechanism represents separate twotime frames for EH and information decoding (ID). Furthermore, TS can use for only one function at a time, energy transfer or receiving signal [253]. The TS-SLIPT block diagram architecture under the SPLIT system is depicted by Fig.11.

• Power-Switching (PS) SLIPT Receiver Architecture
The PS receiver architecture is another combined scheme as received current distributed into two phases.
The DC component of the current phase uses for EH, while the AC component of the current phase uses for information decoding purposes. Compared with the TS-SLIPT scheme, the PS design could reach high efficiency in EH at the receiver end. Additionally, the PS-SLIPT mechanism works at the same time with different power domains for EH and ID [42]. Fig.12 shows the block diagram of the PS-SLIPT receiver architecture.

C. SLIPT AND SWIPT SCHEMES
The authors in [42] widely discussed which comprised both of futuristic SLIPT and SWIPT schemes. In both schemes, the authors obtained the behavior of system BER & outage performances along with amount of harvested energy. Therefore, the BER and outage performances in the SLIPT scheme perform better than the SWIPT scheme at the consecutive photodetector sensitivity and power splitting factors. On the contrary case according to [42], the EH in SWIPT shows superior performance rather than indoor enabled SLIPT technology.

X. HYBRID UNDERWATER WIRELESS COMMUNICATION
Optical communication as VLC is a promising complementary approach instead of RF and acoustic waves signal transmission in the underwater medium over short distances. Enabling next generation of wireless network 5GB optical communication technique allows the massive number of connectivity of IoUTs. In recent years there is a growing interest in deploying VLC as a communication link in underwater integrated with existing RF and acoustic waves. Therefore, hybrid RF-VLC and FSO-VLC wireless communication links are proposed and showed impressive system performances expect to deploy individual point-to-point or point-to-multi-point communication links. A wide detailed summary of various hybrid communication techniques are depicted by Fig.1 and Fig.13. In RF-UWOC hybrid communication system [48] an electromagnetic link combines with an optical link, while in underwater FSO-VLC system [255] enables both of optical links for signal propagation. Additionally, a dual-hop acoustic-optical link is widely used for signal propagation in the underwater medium over longranges [256]. In hybrid underwater communication aspect various signal transmission techniques are developed and use to enable with FSO, VLC, sonar, and underwater acoustic waves. It is challenging to deploy an optical communication hybrid network system towards to 5GB and connecting IoUTs. Optical wireless communication link supports highly secure signal propagation media under visible light (VL), infra-red (IR), and ultraviolet (UV) frequency bands. Nowadays, numerous optical wireless communication systems are being developed under consideration above three optical frequency bands. In a hybrid link scenario, RF and acoustic waves are not required line-of-sight (LOS) nor short ranges but support high mobility and better performance in NLOS conditions. The hybrid approaches are integrated with two  multi-hops different wireless communication technologies such as RF-VLC [257], [258], RF-FSO [47], [48], and PLC-VLC [259], [260] are investigated in the open literature to provide various advantages as per requirement and deployment. Additionally, a hybrid wireless communication system facilitates the integration of multiple wireless carrier candidates to cover long-ranges communication along with better system performance that overcome the limitation of an individual technology.

A. RF/FSO DUAL-HOP HYBRID COMMUNICATION LINK
FSO has become an alternative solution for a wireless communication system with high bandwidth over long distances. Terrestrial FSO is most widely used as a LOS wireless communication approach that operates within 850−1550 nm of optical spectrum wavelength. Due to strong channel conditions, the FSO links are attenuated by atmospheric conditions, especially atmospheric turbulence, fog, snow, and dust limiting channels. Plenty of FSO based applications are recorded in open literature as in [171]. Mostly, in the FSO system the channel turbulence phenomenon is a highly noticeable event of the fluctuation of the scintillation index. The turbulence effect occurs due to the flexibility of refractive index variation inhomogeneities in an atmosphere where the temperature and pressure fluctuates. RF waves attenuate easily in an underwater environment and propagate over short distances. In hybrid communication over long ranges RF waves signify as a promising signal transmission technique. Therefore, RF-FSO underwater is a potential solution for acquiring high bandwidth over long distances. In this section, an RF-FSO hybrid communication system model is proposed to obtain the system performance. A source node S communicates with an underwater-based destination D through a floating buoy. The floating buoy works as a decodeand-forward (DF) relay protocol R and depicted by Fig.13. The source node transmits the RF signals with an average signal power P s . Further, the relay receives the transmitted signals re-generates then forwards to the destination D through FSO link. In another aspect, if we use the AF relay protocol, the relay received a signal copy from S and amplifies then forward to D. Additionally, we consider the RF-FSO combination follows Rayleigh flat fading and FSO link is experienced by strong turbulence channel conditions for communication between R and D. For strong turbulent channel conditions, the fading is modeled through Gamma-Gamma distribution.
In hybrid communication relaying technique has been widely promoted for signal transmission over long ranges. Due to the limitation of FSO signaling on terrestrial based communication, the channel impairments have to be taken into account. As turbulence and pointing errors are the main cause of optical signal fading, the hybrid communication link is widely studied in literature and the performance investigated through different modulation and demodulation techniques. In an underwater communication scenario, optical communication is a permissible communication media for signal transmission. Thus, a number of RF-Optical [46]- [48], [50], [51] and acoustic-optical [69], [256], [261], [262] hybrid communication systems are investigated.

FIGURE 13.
A hypothetical scenario of hybrid RF-optical, Acoustic-optical, and Acoustic-RF link in an underwater environment where each node communicates with the other. The monitoring and collect oceanographic data has to transfer an off-shore-based station for further analysis. The fixed and anchored nodes monitor the undersea environment while the floating nodes deployable monitoring for the water streams, water pollution, geo-chemical process, localization, etc. The architecture of underwater sensor network nodes is also depicted where 1D to 4D UWSNs architecture deployed and enabled with a unique wireless hybrid link towards the next generation 6G and beyond wireless networking system.

B. RF/VLC DUAL-HOP HYBRID COMMUNICATION LINK
In optical communication aspect VLC shows an impressive performance for signal transmission over short distances. VLC uses LED luminaries, which are very cost-effective, easy to install, and 100 Gbps data can be achieved. Similarly, VLC also affects by strong channel conditions, especially sunlight. VLC integrated with RF and a hybrid RF-VLC combination shows better performance in different waters. A dual-hop hybrid RF-VLC underwater communication link proposed in [114]. The system model consists of three or more nodes for signal transmission where a relay function re-generates or amplifies signal information and further communicates with the underwater-based destination. In [114], the system model is assumed in half-duplex mode with a single antenna at relay, while the nodes are fixed. Moreover, the relay should mount two directional antennas in full-duplex mode, where one antenna receives the transmitted signals from source and the other antenna towards to the destination for further communication with the destination. The BER performance of RF-VLC dualhop hybrid cooperative communication system [257] in different water types under strong channel conditions are depicted by Fig.14.

C. FSO/VLC DUAL-HOP HYBRID COMMUNICATION LINK
The traditional underwater communication media is modeled by acoustic waves that supports low data rate and bandwidth with high delayed signal propagation. FSO-VLC combination full-fill the acceptability of desired communication requirements with a highly achievable data rate. This hybrid communication could be operating in half and full-duplex mode as both are optical links and sever almost the same channel impairments. The high turbulence channel conditions, current water waves, misalignment, and physiochemical properties such as pressure, temperature, and salinity affects FSO and VLC links. A hybrid FSO-VLC  combination is proposed in the open literature, where an AUV communicates with terrestrial based station through a floating buoy that functions as a relay to assist received information from the source-based station. The BER performance of an investigate FSO/VLC dual-hop hybrid communication system [257] in different water types is depicted by Fig.15.

D. PLC/VLC DUAL-HOP HYBRID COMMUNICATION LINK
Another approach of hybrid underwater communication VLC link combined with power line communication (PLC) technology. A telecommunication technology PLC is based on the use of power grids for high-speed information transmission. The low transmission speed and poor noise immunity are the bottlenecks of this technology. Rather than this, the advent of PLC strengthens digital signal processors (DSP), making it possible to use more complex signal modulation methods such as OFDM modulation. PLC can be used to create a small base station where the main requirements for the network are ease of implementation, device mobility, and easy scalability. The small cell base station (CBS), e.g., femtocell access point (FAP) are used as a small cellular base station that serves 6-8 users. In hybrid PLC-VLC combined communication link a small power grid antenna connects with the small base station (SBS) through line cable. The signal is transmitted from SBS to the antenna via cable and further this antenna mounts by LEDs for further signal transmission to divers or UWSNs in shallow water.

E. OPTICAL-ACOUSTIC AND RF-ACOUSTIC DUAL-HOP HYBRID COMMUNICATION LINK
The study of acoustic waves involves the discovery and deployment of sound waves transmission in fluid media. Acoustic waves are generally defined as mechanical and longitudinal vibrational ripples that can travel through solid, liquid, or gas mediums in a wave patterns. Underwater acoustic is an alternative signal propagation method of sound in water, especially over long ranges undersea. Sound waves are the most effective energy transmission wireless carriers that propagate over long distances in the ocean. Moreover, acoustic waves can tolerate the pressure and the temperature of the deep oceans. Neither RF nor optical waves are suitable for long-range transmissions in an underwater environment as acoustic waves [263]. Usually, the speed of acoustic waves is comparably low (Approximately 1500 m/s) [74]. Although, according to the layer clarification the speed varies in cold water and lower than normal water. Acoustic waves are used for long-ranges transmissions approximately up to 15 − 20 km, but these waves cannot achieve high bandwidth signals undersea. Due to low frequency, bandwidth, speed, and additionally the received signals are delayed at the destination [264], [265].
However, the sound waves depend on the compressibility and density of the medium. In [266], an acoustic sound speed profile (SSP) system model has been proposed for 1 km as follows, (56) where the temperature in 0 C, salinity in PSU, depth in meters, and speed of sound in m/s are denoted by T , S, z, and c, respectively.
Sound waves propagate according to water current concerning every instant of time. An empirical model is described as a function of physical water properties such as temperature, salinity, and pressure varies with depth. Stratification of these parameters lead to stratification of wave motion, which entails the existence of specific profiles. Mackenzie has developed a mathematical formula in applied science during 1981s [267]. As already discussed, underwater optical communication is an alternative approach that refers to transmit signals in an aqueous medium. VLC communication has more superiority in comparison to acoustic waves but over short distances. To fulfill the gap of long-range communication, a hybrid optical-acoustic communication system is considerable. In this type of system model, the underwater-based mobile station transmits the acoustic signals and further convert into optical format for decoding purposes at the targeted node.

XI. UNDERWATER SENSOR NETWORKS ARCHITECTURE
Recently, there have been growing interests in underwater wireless sensor networks (UWSNs). The fundamental source of ocean exploration underwater sensor architecture is defined as a group of sensor nodes that are established undersea to exchange information wirelessly with the base station (BS) and floating vehicles (nodes). The growing interest of UWSNs for a wide variety of purposes about oceanography, monitoring marine life, seismic surveillance, oil-fields exploration, military applications, and deep-sea activities. In this regard, the necessity of UWSNs contributes a significant role in UWC. Generally, UWSN nodes are enabled with acoustic and optical links for data transmission within various applications. Energy conservation and self battery powered is a critical issue in UWSN technology. This survey estimates the target position and improves energy efficiency in hibernating and an active mode of underwater sensor nodes (USNs). The network of underwater sensors fixed with a certain depth or with the bottom of the deep ocean for early detection warning of an earthquake, tectonic plate movements, and active volcanos before striking the coastal and residential areas. The floating nodes are used to detect ice sheet melting and rising water level phenomenon, coral reef, water pollution, and physio-chemical properties of water. The development of floating nodes AUV supports to improve the quality of geographical mapping and navigation systems in underwater sectors. Localization is another significant application of UWSNs for tracking location or Global Positioning System (GPS) in harsh water mediums [268]. The integration of sensor nodes architecture has permitted to allow communication, localization, and GPS. In an underwater environment, navigation and localization are the two most significant challenges for mobile robots, floating nodes, and different underwater applications.
Aforementioned, acoustic and optical signal-based sensor networking systems are the most widely used for various underwater applications. Through the proposed underwater applications the acoustic and optical transponders are used to navigate an AUV. The sound navigation and ranging (SONAR) enabled AUV methodology has been proposed with the two basic configurations introduced as side-scan SONAR (SSS) and Forward-Looking SONAR (FLS) in [269] and [270]. The above SSS and FLS are used for monitoring marine life and detection for underwater environmental changes [271]. While an array of sensor nodes with a camera or without a camera can be implemented for long ranges navigation and imaging systems [272].

A. SIMULTANEOUS LOCATION AND MAPPING (SLAM)
SLAM is an AUV-enabled imaging technique and an essential solution for mapping unknown locations within the unknown environment to develop a consistent geographical mapping of an underwater environment [271]. In contrast, the SLAM imaging AUV-based technology that supports constructing a 3D image in different color frames is widely discussed in [273]. The authors in the above-proposed work defined a color-coding representation for the reconstruction of unknown mapping sectors, where each color code has its specified distance measurement between AUV and obstacles. The various methods are presented in the literature of SLAM to the reconstruction of the unknown sectors in underwater environment [271], [274]- [277]. Each method has its inadequacies and strengths according to the desired interaction, inspection, and navigation implementation. Furthermore, underwater SLAM is differentiated into acousticbased, and vision-based categories [278]. The acoustic-based underwater SLAM is used for intense unknown segments, while the vision-based SLAM is suffered from limited visibility and background noise by natural sources in shallow waters. Therefore, a high-definition FLS SLAM could be a promising alternative solution in geographical mapping to mitigate the problematic solution.

B. UNDERWATER ACOUSTIC SENSOR NETWORK
UASNs is an approach that supports the exploration of extremely dark and highly deep oceans. UASNs deploy for various underwater applications such as for detection submarines and unmanned operations for military purposes. The UWSNs architecture is categorized into fixed 2D and mobility 3D sensors. In the 3D type of architectural sensor system, the motion of nodes controls the internal navigation system. The floating nodes are free to move underwater with the drift and unpropelled type of portable devices such as unmanned underwater vehicles (UUVs), AUVs, ROVs, drifters, and gliders. The main function of these devices is to collect oceanographic data from an aqueous medium. The various types of UWSNs are summarized by Table 8. The drifter operates mostly through the ground, which drifts with wind and surface waves to acquire the possible measurement of marine sectors, further sharing the collected data with the base station through satellite. Moreover, the gliders are driven by the buoyant force and connected with the floating buoy or the cluster head for sharing collected data. In existing work, there are the two main groups of UASNs known as content-based and scheduled-based protocols proposed in [279]. UASNs are classified into two categories as content-based and scheduledbased nodes. The Content-based nodes exchange signals to each other, while scheduled-based nodes eliminate the exchange of aggregated data between transmission nodes. Alternatively stated content-based nodes do not meet the desired gap and are critically suitable for UWC. In contrast, due to high propagation latency and narrow bandwidth, the VOLUME 10, 2022 scheduled-based nodes such as TDMA and FDMA are less suitable. However, CDMA is a more suitable approach for UAN to be deployed in underwater medium [268].

C. ARCHITECTURE OF UNDERWATER SENSOR NETWORK NODES (UWSNs)
UASN is commonly used for underwater exploration but quite adequate to meet the demand for real-time streaming and high data rate scenario. RF wave-based underwater sensor network nodes (UWRFSNs) are the potential approachable technique TO full-fill the requirement of various underwater applications. In this section, the discussion is carried out of the different types UWSNs along with their deployment challenges.
• 1D-UWSNs Architecture: The single dimension (1D) UWSNs are the self-employed types of deployable networks underwater. In this type of sensor network, the individual node is self-configured by itself. In this type of architecture, each node has the capability of sensing and it is responsible for processing collection data within the remote area [281]. The sensor node could be fixed with a seabed or a floating buoy responsible for collecting the useful oceanographic data and further shared with the BS. In another scenario, the floating buoy could be the buoyant type of node that floats a certain depth underwater and then float towards the BS for information transmission. AUVs also function as self-configured individual nodes; they can floats at any horizontal and vertical depth to sense the data and share it with the onshore data center. The 1D-UWSNs type of architecture enables acoustic, EM, and optical communication-based wireless signaling technology.
The 1D-UWSNs are widely deployable for single-hop communication where the nodes directly communicate with the BS; in VLC scenario 1D type of UWSNs usable for plenty of underwater applications in shallow waters such as rivers, dams, lakes, etc.
• 2D-UWSNs Architecture: The 2D type of UWSN is referred as a group of sensor networks (cluster) where the nodes communicate with the cluster head. The fixed or anchored cluster head collects the sensed data from each node and further transmits it to the vertical buoyant transceiver of the BS. In 2D-UWSN architecture, it is considered that each node communicates with the cluster head horizontally and communicates with the vertical buoyant node as well. Thus, this type of node architecture is called 2D-UWSNs, based on acoustic, EM, and optical waves-based wireless signaling technology for long ranges in an underwater environment. In the UVLC scenario, all local and cluster head nodes link and transmit the information with the remote station. The 2D type of UWSNs architecture is proposed in [291] for the purpose of short-term aquatic exploration applications and extensively discussed. The 2D-type of UWSNs is depicted in Fig.1.
• 3D-UWSN Architecture: The 3D type of UWSN describes the network architecture where the sensor cluster nodes are deployed in different depths in underwater medium. Due to different depths, the clusters communication links established are variable in 3D space geometry in undersea. The communication link among the sensor nodes would be in three dimensions space for the mobility of data transmission. Additionally, in 3D-UWSNs, the cluster communicates among the neighboring clusters at varying depths, and the cluster network nodes communicate with their cluster head. All the cluster heads collect the sensed data by their local sensors and further transmit it to the buoyant node or with the BS. The 3D type of UWSNs can be deployed for underwater applications requirement based on acoustic, EM, and optical communication [281]. Moreover, the 3D underwater sensor network localization has been widely discussed in [282]. In UVLC enabled sensor network communication, the high probability of realtime monitoring and achieving high data rate. The 3D-type of UWSNs is depicted in Fig.1.
• 4D-UWSN Architecture: The 4D type of sensor network architectural system is designed by combining anchored sensor nodes with the floating or mobile UWSNs nodes at different underwater depths. The 4D-UWSN architecture is deployable for long ranges communication among the nodes. Mostly, the mobile UWSNs consist of ROVs for the data collection from an anchored or fixed nodes at the seabed in varying depth. The ROVs can be submersible robots, vehicles, ships, and even submarines. Each node can relay information with ROV and depending on the distance between the node and ROV. Furthermore, the signal transmission is the distance-dependent between underwater sensor nodes and ROV [292]. The 2D-type of UWSNs is depicted in Fig.16.

XII. RECENT DEVELOPMENT OF INTERNET OF UNDERWATER THINGS (IoUTs)
The network of the internet of underwater things (IoUTs) is a potential novel approach for data collection within the underwater environment that has to be transferred with the BS for further analysis. IoUTs is a group of wireless network-enabled smart devices and could be a wide range of underwater sensor nodes architecture. The different types of UWSNs architectures are discussed in the earlier sections for various underwater application purposes. The system architecture of IoUT has received attention for developing smart ocean and innovative business surrounded by smart interconnected underwater water objects [70]. The most widely deployable IoUTs are discussed as follow and depicted by Fig.16.

A. SEA GLIDERS AND AUTONOMOUS UNDERWATER VEHICLES
The Internet-connected devices structure provides a pretty reasonable communication bridge among the nodes. For underwater communication purposes, wirelessly connected devices are recognized as the internet of underwater things (IoUTs). These devices could be identified as network nodes, AUVs, moving gliders, ROVs and floating buoys, etc. Whereas sea gliders and AUVs provide the measurement of oceanic environment over long-ranges especially for bathymetric data collection. The thrust and movement of the sea-gliders are used as an electrically driven propellers and are operated by an installed battery that provides upthrust with buoyancy for forwarding or upward motion. The AUVs and sea-gliders are designed to cover thousands of miles for several months within underwater mediums. These devices sink up to 1km from the sea surface due to their energy capacity limits. Furthermore, these types of nodes are deployed for oceanographic data collection and GPS purposes. An architecture of IoUTs that involves the multiple wave gliders for acoustical observation is proposed [293]. The internal sensors of sea-gliders determine the direction of vehicle when it dives underwater. The external sensor scans the ocean to collect the data. The movement of sea-gliders, AUV, UUV, and ROVs, is shown in Fig.16.

B. UNDERWATER ANIMAL TELEMETRY
Animal telemetry is another effective approach for ocean exploration and uses to observe the marine environment. This tagging technology enables us to get to know about the marine life inhabitants and their interaction with coastal shelf ecosystem to polar seas and the open oceans [294]. Animal telemetry supports observing the exploration summary of the vast oceans, underwater environmental changes, and marine pollution. The air-sea interface is necessary to understand the observation and oceanic exploration of daily water conditions, driving global climate changes, biogeochemical cycle process, heavy wind and water currents to exchange the momentum and other micro-particles [294]. The various IoUTs such as HFR, satellites, drifters, and autonomous surface vehicles (ASVs) are widely used to observe the airsea interface. To mitigate such challenges, the development of a small communicating device that can be mounted on the floating seabird back that contained a sensing data unit with a battery is capable of measuring the water surface, water currents, and winds over the sea [294]- [297]. For more details about animal telemetry, a pictorial view is referred to for this approach in Fig.16.

C. A DEEP DETECTABLE TSUNAMI BUOY
Tsunamis and earthquakes are sudden events and highly impossible to control. Nevertheless, these events are predictabled by deploying an early detectable device called a tsunami buoy. The primary function of a tsunami buoy is the detection and monitoring of the high level ocean water waves (Tides). The tsunami events are generated by undersea earthquakes that bureau of meteorology confirms. The tsunami buoys are deployed for observation and recorded the changes in sea level and bottomless oceans. It enhances the capability for early detection warning of tsunamis and floods before hitting the land. The buoy splits into two major components: the pressure sensor anchored with the sea bead and a surface buoy that floats on the water surface.
The anchored sensors at the sea-bed are used to measure the changes in the height of the water column (the height h of the water column is the function of water pressure p as p = ρgh).
In more contrast, the measured water column height utilized for communication purposes with the floating buoy (works as a relay) through sound waves and further relayed via satellite to the tsunami warning center [298]. The function of the tsunami buoy is depicted in Fig.16.

D. UNDERWATER FLOATING DRIFTERS
Through the immense development of drifters, observing the global ocean currents and their effects are the main concerns. Recent advancements of drifters support the ocean circulation patterns in real-time monitoring the deployment of the drifters possibly in shallow water by a ship or another source. On the deployment of the drifter when it floats, the transmitter is activated and starts to share the collected oceanographic data with the satellite, this collected data is further to transmitted with the offshore BS for analysis. Other inbuilt sensors at drifters are responsible for collecting the various types of data such as water surface temperature, wind, ocean color, pressure, and salinity [299]. The data synchronization with the satellite has been depicted in Fig.16.

E. UNDERWATER HYDROPHONES
Generally, the signal propagation effects by channel impairments; noise is one of signal corrupting factors. The array of hydrophones are designed to be deployed for noise detection in an aqueous medium. Also, the hydrophones produce a limited voltage signal over high-frequency ranges while receiving the underwater sound from any direction. Deployment of an array of hydrophones amplifies and records the received noise more precisely. Omni-directional and Hemi-directional hydrophones support to collect the sound from a particular direction; it is also to track underwater the movement of underwater species. Therefore, hydrophones are the fundamental tools for plenty of undersea acoustic applications such as Sono-buoys, wired hydrophones, and autonomous hydrophones [287]. In addition, the NOAA's Pacific Marine Environmental Laboratory (PMEL) under NOAA's National Marine Sanctuaries also frequently uses hydrophones [300]. In addition to that NOAA's PMEL derives a long-term datasets for the global ocean acoustic environment. It has been utilized to identify and evaluate acoustic influences of human activities and natural disasters such as the eruption of an underwater volcano and glacial earthquakes in the marine environment [301]. The deployment of hydrophones in the underwater scenario has shown in Fig.16.

F. CLOD CARDS USE THE MEASUREMENT OF WATER FLOW
The clod cards are ice-shaped identical trapezoidal geometries and made by alabaster or plaster of Paris (gypsum blocks) and utilize to analyze underwater flow patterns. The ingredients of clod cards or made by the combination of the 100ml of freshwater with 80gm plaster of Paris powder manufactured by the Hobby Craft Trading limited, Dorest UK [302]. In [302], the clod cards were sanded at the bottom of the water bodies to accomplish a consistent weight of 12.5 ± 1.5 gm. Every individual plaster cube must be glued with the plastic plate size 3x8cm with silicone cement in this strategy. To measure the exact water flows analysis, the four clod cards established in four cardinal directions in front of their faces are fixed to a straight pole for at least 24 hours. The requirement of clod cards deployment in predetermined sites for various underwater applications such as divers, ROVs, and floating nodes. After a limited and fixed time duration (approximately 1 to 2 days), the clod cards have to be retrieved, dried at 40 0 C, and reweighed to calculate the decay and the strong water flow. Conversely, the exact weight loss of the plaster of Paris is determined from each card and sent to the laboratory for further analysis. As a result, it can be concluded that the clods who lost more weight are exposed to greater water flow than those who lost less weight. Accordingly, the cloud card can be used to measure several other essential parameters from the ocean. An implementation of a clod card has been depicted in Fig.16.

G. SATELLITE OCEANOGRAPHY
The weather observational satellite is the most significant tool to share constructive information with the base station.
Additionally, the satellite plays a vital role in ocean observation research and functions as an intermediate communication node for information transmission between the underwater and offshore base stations. For oceanographic data collection, environmental satellites are widely used to capture images and calculate the surface temperature of oceans. It is also used to know the water patterns and further shared the collected data with the terrestrial-based stations for analysis. For real-time video, steaming could be possible by utilizing satellites in different color coding techniques for water flow analysis. It supports determining the impact of floods and tsunami along the coastal area along with the detection of algal blooms. In the most recent years, remote sensing technology-enabled with the satellite that has boosted for data synchronization with altimetry (study to measures the height of sea surface), scatterometry (study to measures wind speed and direction), as well as the bathymetric (study of the ocean floor) approaches. These are the technologies that make the smart satellite for ocean observation, monitoring, and GPS. Additionally, to monitor early detection warnings to evacuate the coastal area and develop major storms, such as hurricanes and tornadoes, geosynchronous environmental satellites are being used. The most important information of coral reefs, coastal habitats, and similar environments can be analyzed by the satellite photography [303]. In Fig.16, an environmental satellite is depicted for ocean mapping.

H. REMOTELY OPERATED VEHICLES (ROVs)
ROVs are accessed remotely and functioned similarly as a robot. This type of node has been designed for underwater exploration and deployment for various underwater applications to find the lost ships, repair and monitor gas pipelines, habitat mapping, etc. Specifically, the ROVs are equipped with sensors and sampling tools for data collection at variable distances from the oceans. A network cable is used to detect and observing the objects by illuminating the light, establishes a communication link between the base station and floating node to controls commands of the proper node movement. The ROV is a well-equipped floating vehicle enabled with advanced features that contain a lighting system with a camera at the frontal portion for videography to record a better sub-aquatic panorama. It is especially utilized in the aquatic medium for the contribution of geology education and sea life learning [304]. The exploration and functioning of ROV in an underwater medium have shown in Fig.16. A. 5G NETWORKING SYSTEM IN UNDERWATER 5G wireless communication system has been designed to offer a significant improvement, especially for enhancing the system capacity, spectral efficiency, and energy efficiency of the communication system. For efficient and reliable wireless communication, 5G is widely accepted to combine the small cell, picocells, and macrocells [306]. The commercial use of 5G during 2019s established a collaborative communication system on a terrestrial basis, excluding the underwater environmental communication. The deployment of a 5G wireless network underwater will be an innovative step to improve data extraction from an aqueous medium for further analysis. As being improving the system capacity and essential assets of high data rate underwater wireless communication technology, optical communication towards 5G is the key solution that attracts the scientific community. Although, the most recent experimental study and the performance evaluation of 520nm wavelength laser-diode towards 5G is presented as [307]. In [307], the authors analyzed the performance of an underwater optical system under consideration of varying salinity conditions in pure water. Facing these issues, the Generalized Frequency Division Multiplexing (GFDM) is proposed as a novel underwater wireless network candidate towards the next generation standard based on multi-branch multi-carrier filter bank in [43]. It is presented and generalized the traditional OFDM technique with several advantages as minimal peak to average power ratio (PAPR). Minimizing PAPR is one of the key advantage features in GFDM over OFDM and a significant parameter to analyze the performance characteristics of the communication system [308]. As recorded the limited bandwidth in underwater acoustic, OFDM suits for communication purposes. In contrast, OFDM offers a profitable integration over single-carrier modulations against the frequency selective fading [309]. Therefore, to come up for all these issues, the requirement to deploy multi-carrier transmission schemes. GFDM [310] and filter bank multicarrier (FBMC) [311] are proposed the new promising emerging communication techniques towards 5G applications.

B. PREDICTABLE 5G AND BEYOND (5GB) NETWORKING SYSTEM IN UNDERWATER
The forthcoming next-generation 5G and beyond (5GB) communication will indeed offer numerous advanced services with ultra-high capabilities, an extensive number of device connectivity, very ultra-low energy consumption, and latency along with ultra-high security, extremely high quality of services (QoS), and high-resolution experience (HRE) [312]. The 6G networks are anticipated to accommodate and exceeded performance in comparison with 5G. Thus, the exceptionally high data will greatly impact networking users and the critical factor for more complex future challenges in various sectors such as in healthcare, long-range communication, vehicle-to-vehicle (V2V) communication, underwater communication, etc. Though, the prediction to launch the 6G communication system is anticipated till 2030s [313]. Indeed, the 6G wireless communication network will provide the next level of signal transmission qualitative performance compared with 5G communication.
The recent advances of and application of optical communication technologies towards 5G/6G are summarized in detail [314]. An extremely high data rate up to 100Gbps by deploying VLC has been confirmed in [315] and [316]. Internet of underwater things (IoUTs) is the essential part of an underwater communication system connected with a standardized communication network for data extraction in the real-time scenario from the underwater environment. The implementation of 6G integrating UWC will be a revolutionary step to explore aqueous mediums as IoUTs connected with VLC. The most significant aim is predicted to connect underwater-based nodes with satellites for global coverage and observation all around the globe.

C. INTERNET OF UNDERWATER THINGS (IoUTs)
IoUTs refer to internet-connected devices that can easily and legitimately retrieve information through cabled or wirelessenabled connectivity. UIoTs claims to be an extension and development phase of IoTs, which allows the extraction of anonymous information from smart ocean [317]. IoUTs are enabled by the most recent developments of AUVs, ROVs, smart underwater sensor networks, and routing protocols. The architecture of UIoTs depicted in Fig.1 and Fig.13, enable numerous applications in an underwater environment in terms of sensing, monitoring, and identify underwater objects. In underwater internet of things (UIoTs), the sensor network nodes collect data and communicate with the cluster head. Furthermore, the received data has to transmit with a base station for subsequent analysis. The IoUTs structure enables the UVLC link to be classified into two major categories: horizontal and slant communication systems. Described in more detail, the horizontal link has been an extensively focused study on increasing data rate and signal propagation length along with reducing underwater turbulence effects while slant links formed high depth considering wave height wind speed, sea surface slope, and heavy turbulence conditions. On the other hand, in the vertical link for high depth, pressure increases up to 10bar for each k th layer (each layer for k = 10m) in addition temperature decreases from 25 0 C to freezing point, the above matter facts due to varying salinity which affects the refractive index.

XIV. UNDERWATER OPTICAL WIRELESS NETWORK APPLICATIONS
Oceans are mysterious and challenging water bodies to explore. Plenty of natural disasters such as volcanic eruptions, earthquakes, and tsunamis occur by imbalanced water phenomena that could be observed and monitored by deploying communication links. To address the requirement of current emerging underwater applications which have been used in the context of rising water level, environmental monitoring, marine life, early detection tsunami, oil and gas rigs exploration, volcanic eruptions, coastal securities, and sliding tectonic plates, jointly with various hybrid wireless communication systems. Moreover, undersea wireless communication over long distances, UWSN plays an essential role in data transmission. The traditional acoustic modems are widely used in long-range UWC, whether low speed (approximately 1500 m/s) in such complexity of water channel conditions [318]. In more contrast, for high depth, the floating nodes AUVs, and UAVs are used to transfer information from source to destination. RF waves transmission show poor performance in an underwater environment cause the EM waves propagation depend on the intrinsic properties of the channel. Contrarily, the water medium has more density, permittivity, and higher electrical conductivity than air. Therefore, RF waves easily attenuate when propagating over successive distances [94]. For such transmission scenarios, a high data rate cooperative dual-hop RF-UVLC and FSO/UVLC systems are needed to be deployed [48], [180] [319]. As, VLC provides a high capacity of data rate over moderate distances, an acceptable wireless media for underwater mapping, and real-time video streaming with high resolution and various UVLC applications as follows.

A. OCEAN FLOOR BATHYMETRY
The study of exploring the ocean floor is referred to the depths and shape of the underwater terrain [320], as stated bathymetry is the most significant investigation that deploys in numerous underwater applications. Additionally, the collection of geographical and oceanographic data analysis updates of weather and natural hazards on our earth. The massive water reservoirs are still unmapped, unexplored, and unobserved sectors. Only a very few oceanic portions have been mapped systematically by direct measurement [127]. The rest of the unexplored fraction of oceans are predicted by the satellite altimeter data, which is the approximated estimation structure of the seabed. A newly sounding technique is proposed that supports measuring ocean depth over less than 18% of the seafloor resolution about 1km [321]. Furthermore, the two most significant seafloor mapping techniques are proposed as Single Beam Echo-Sounders (SBES) [127], [322] and Multi-beam Echo-Sounders (MBES) [127], [323], respectively. The use of the two-way travel time of sound waves methodologies are used for measurement sea depth directly, where the sound waves strike with the seabed. On the other hand, in the MBES system, the enabled fan generates sound waves for further data collection. These sound waves are responsible for re-echoing signals generated in areas of the strait perpendicular to that fan, leading to the ocean floor mapping. Therefore, the established system receives hundreds of beams capable of mapping the narrow sectors up to 150 degrees.

B. UNDERWATER ENVIRONMENTAL EXPLORATION AND MONITORING
Most of the underwater environmental monitoring applications are related explicitly to observing manual and habitable activities in the oceanic environment. By deploying the UWSNs, the water pollution, water flows, and lives could be monitored. Additionally, the tectonic plate movement, underwater thunderstorms, earthquakes, and new formation of lands due to the eruption of volcanoes (the lava plays a significant role in forming a chunk of the island, etc.) being monitored. The floating nodes at different depths can observe and collect data of the 3D geometry of open oceans and terrains as human beings are highly interested in exploring the underwater environment since their existence. The scientific community is a witness in the exploration of less than 5% of oceans. A vast number of underwater applications are immersed by floating nodes as AUVs, ROVs, and UUVs for monitoring and observing the marine lives along with the three-dimensional geographical data within real-time scenario [283]. The study of monitoring water quality, pollution, impurities underwater is more challenging. Various water samples are being used to monitor water quality regarding the pH level, oxygen (O 2 ), and ammonium nitrate (NH 3 ). Moreover, the underwater monitoring also includes water macrophytes or hydrophytes, marine species, coral reefs, and algae for further analysis. In more contrast, environmental monitoring is also a fundamental approach through IoUT enabled applications which include the physiochemical properties of water such as pressure, temperature, and variation in underwater are the main concerns [324].
The continuous monitoring of underwater helps to check remotely the pH water level. The water pollution monitoring is also substantial for the ecological system, especially for undersea species. IoUT provides an opportunity to explore the bottomless underwater reservoirs to discover lost human civilizations and lost treasure. Moreover, to identify the roughness of the seafloor, natural minerals, oil, and gas treasures. Many lost-treasure discoveries were made through the IoUTs.

C. NAVIGATION & SURVEILLANCE
Oceans are highly complex and unguided water bodies which are 95% unexplored in scientific research. As a result, underwater navigation is more complicated than terrestrial basis. UWSNs are appropriate and the most suitable technological approach that supports navigation in hulking aqueous mediums. Especially, for navigation the 3D underwater sensor network (USN) localization system is investigated in [282]. In [282], the authors propose the superiority of 3D Underwater Sensor Positioning (USP) systematic planning, which is beneficial for improving localization capabilities over existing 3D sensor architectures with predictable, balanced communication underwater. An AUVassisted localization scenario of USNs roaming across the underwater sensor fields is presented in [325]. A wide range of localization of acoustic network nodes in the underwater medium is summarized in open literature as in [326]- [332]. Since optical communication is a novel technique to implement the localization and navigation purposes. Therefore, the authors in [333] investigated the connectivity of underwater wireless optical sensor networks (UWOSNs) as the influential factor to the system performance. Most recently, an extensive study based on received signal strength to the centralized localization scheme is proposed in [13]. Counterpart surveillance is a more efficient tool for node detection for military purposes or coastal securities. AUVs are used as emerging communication nodes for surveillance and retrieving information for defense operations. A most recent survey summarized the AUV capabilities for underwater communication in [271]. The authors in [271] are widely discussed the possible underwater application through AUV such as signal transmission, localization, and navigation purposes where the RF waves are incapable of retrieving the digital information.

D. NAVAL-MINE RECONNAISSANCE
As we know that underwater environments are highly deepest and massive water reservoirs, countless mines in oceans could be found by deploying UWSNs connecting with ships safe voyage or floating and operational activities. A recent work is proposed to design an underwater mine detection system in [334]. The authors have suggested a model minefield that includes the type of mines to be detected and mine-clearing operations by deploying the sonar waves in the various depth of the oceanic environment. Another impressive work in the open literature, where the authors have proposed the detection of underwater mines in real-time scenarios [335]. In [335], an AUV floats at different depths with sonar waves transmission along with a camera for real-time observation. Naval mines are also a real threat to maritime and naval tactical operations. Underwater-based mines hide or buried by the high altitude of tides and water currents, even covered by muddy soil at the sea bottom [336]. In most of the conditions, the naval mines are undetectable by low-frequency ranges. To cope with this issue, the challenge is that a highfrequency range of sonar should be mounted through an AUV to detect or bathymetric data of naval mines. For instance, the ECA Group deals with the issues within an innovative technological solution to perform complex missions in hostile or harsh environments [336]. According to SOACYS, the first prototype in the form of a 5 to 40m longship is used to collect oceanic data 1 Teabyte within 2400km of survey lines [337]. This small ship prototype is used for various applications such as UneXploded Ordnance (UXO), debris, boulders, pipelines/cables, marine archeology, sedimentary studies, etc.

E. BORDER & COASTAL SECURITIES
The border and coastal securities applications are mainly used to observe unwanted activities in coastal areas such as entry port security, land border security, and maritime border security. Taking account of these issues, USWNs an emerging technique for various military operations and are used for military communication purposes. These integral networking signals have a key role in providing such facilities to the military as surveillance, intelligence, localization, targeting positions, and tracking nuclear, biological, and chemical weapons. A high capacity command control communication system of ASELSAN is used to enhance situational awareness, accurate decision making, and faster response qualities in coastal and border security system solutions [338].

F. SEISMIC MONITORING AND OCEAN SAMPLING
The oceans are extremely large water reservoirs that change over the time. The sensor nodes are the main advantageous communication networking tools for sampling and observing the underwater environment, collecting oceanographic data, and transmitting signals to the source (e.g., base stations, etc.). The floating sensor nodes could be deployed at high depth for ocean mapping and underwater sampling activities. Furthermore, earthquakes, tsunami, and floods are sudden and unexpected events that cause major catastrophy worldwide. Sudden slip of bedrock causes seismic waves that shake the ground. UWSNs are dedicated to monitoring seismic activities and recording data for early detection warning [339].

G. FUTURE DIRECTION
UVLC has drawn attention to the futuristic deployment of UWC, which opens the doors of advanced wireless communication systems towards 5GB networking system in the nearest future. Currently, it is possible to transmit the data at high speed as well as the wireless energy transfer through the illumination phenomena of LEDs. UVLC has a fruitful future perspectives due to the wide range of popularity of LEDs all around the world [42]. Therefore, UVLC has shown potential acceptability for next-generation 5GB networking systems for further developments in the digital wireless generation of communication phase. LEDs are very cost-effective optical signaling elements that have more superiority over traditional wireless communication media. The light spectrum of the optical beam visible light and LED illumination is a comfortable illusion for aquatic species. Therefore, the essentiality for promising extensive research is required for more advanced, very costeffective light-emitting and photon receiving detectors. The less expensive, tiny in size, less power consumption, and high compatibility with neighboring optical network UVLC claim the most significant wireless carriers. The traditional acoustic and RF wireless candidates are lagging behind for underwater signaling with low data-rate (few kbps); therefore UVLC fills the gap (Mbps to Gbps data transmission rates) of desired communication. In more contrast, the IoUT networks support the mobility of the devices in a very divisive medium within 3D channel coverage. Plenty of UWC applications enabled with ROVs, UUVs, and AUVs are widely employable with VLC format. The hybrid communication systems with 5GB networks are capable of connecting an enormous number of devices on terrestrial to underwater-based internet connecting things for further exploration of unguided oceanic sectors. The predictable scenario is establishing the underwater environment of the satellite becoming a universal digital communication sphere. As a substitute, UVLC supports high data rates (3Gb/s DMT VLC transmissions) [340] over short distances with low latency.

XV. CONCLUSION
UVLC approach builds up a wireless connection between terrestrial and floating nodes in an underwater environment. The possible solution to cope with the challenges and affecting factors of signal propagation are widely studied in this research. UVLC has become the most significant trade for signaling in aqueous mediums towards the futuristic 5GB wireless networking systems. The most recent advances through the current project schemes all around the globe to enhance the interest in marine industries along with various underwater applications are the main perspectives. The UVLC technological critics of signal propagation and channel impairments are the main issues due to the various types of underwater-based hybrid communication scenarios are summarized in this paper. Additionally, the architectural design of UWNs with higher modulation techniques is quite challenging to deploy over long-range communication in marine industries. Due to this, the different types of hybrid communication approaches are investigated according to requirements and geographical environments. The IoUTs play an ample role as a backbone in communication among transceivers. In the IoUT networks, the modems may contain some advanced signal tools for the purpose of achieving reliable high-data rates with diminishing influential factors such as noise and channel impairments in the correspondence of physical layers. This study contributes the solution of various range of channel impairment challenges and provides an overview of the latest projects schemes along with the future perspectives towards the 5GB networking system in this particular domain. undergraduate students. He held visiting and/or sabbatical positions at the Centre for Telecommunications Research, The University of Sydney, Australia, in 2015; and Texas A&M University, in 2018. He has published nearly 200 international peer-reviewed journal and conference papers and books. His research interests include PHY and NET layer prospective of 5G communications technologies, such as NOMA for 5G; cooperative wireless communications; device to device communications; LDPC codes; and unmanned aerial vehicle. In his career, so far, he has attracted nearly 6M research funding. He is a fellow of IET. He has received the Best Paper Award from the IEEE International Conference on Communication,