A Systematic Review on Practical Considerations, Recent Advances and Research Challenges in Underwater Optical Wireless Communication

Underwater Optical Wireless Communication (UOWC) has gained significant attraction in many underwater activities because of its high bandwidth as compared to radio frequency and acoustic technologies. Underwater Optical Wireless Communication (UOWC) has high stature in underwater observation, exploration and monitoring applications. However, due to complex nature of ocean water, several practical challenges exist in deployment of UOWC links. Qualitative and effective research has been carried out in UOWCs from last few decades. Ambition behind this research systematic study is to provide a comprehensive survey on latest research in UOWCs. Herein, we provide a brief discussion on major research challenges, limitations and development in UOWCs. We provide a periodical review on UOWC issues and potential challenges highlighted in previous studies. In this paper, we have also investigated research methods to gain attention of research fraternity towards future technologies and challenges on the basis of existing approaches. Thus, it is our foremost requirement to provide state-of-the-art analysis of existing UOWCs. Significant deliberation has been provided with recent bibliography. Keywords—Component; underwater optical wireless communication; underwater technologies; research questions; 5G/6G


I. INTRODUCTION
An extensive research has been carried out on free space optical wireless communications in different aspects and nowadays Underwater Optical Wireless Communication (UOWC) is generating considerable attention among recent researchers. UOWC is an emerging technology for premier research in underwater environment. It is a fusion of optical and wireless technologies having intelligent computing, smart sensing and communication abilities. UOWC is used in numerous applications such as mineral exploration, ocean observation, submarine communication, diver-to-diver communication, military applications, underwater navigation and surveillance. UOWC has successfully proven its stature in target detection, objection tracking, underwater robotics, monitoring, AUVs communication and oceanographic data transmission and collection. Though UOWC is providing cutting-edge solutions but many challenges exist at the same time. The researchers face challenges regarding link deployment, propagation delay, high bit error rates, Doppler spread, connectivity losses, attenuation, scattering, turbulence, low bandwidth, high latency, multipath propagation and salinity. The unpredictable condition of ocean environment generates these serious challenges and issues in designing and deployment of UOWC links. In this paper, we have investigated several challenges and presented our work summary on it. We have discussed several constraints and particularities in this systematic review. We have briefly discussed the feasible countermeasures to tackle these challenges in existing UOWC mechanism. UOWCs are vulnerable to numerous factors such as scattering, absorption and fading. In addition, UOWC performance is limited to short range [1] and extensive research is required for long range communications. Researchers are working to design systems and methodologies to transmit broadband optical signals at larger distances. In near future, submarine optical communication systems will be available commercially [2]. A general UOWC system is displayed in Fig. 1. It shows that different underwater platforms such as submarine, sensors, ships and divers are connected through light beams.
An extensive research is being carried out to develop a complementary technology which can allow broadband underwater communications such as real-time video transmissions, tele-operation of AUVs and remote monitoring of underwater stations [4]. In future, optical communication will be used for many underwater applications. However, UOWC cannot fully replace acoustic communications. For this purpose, scientists are focusing on hybrid optic/acoustic communications [5].
We have briefly discussed several potential features in our systematic review. We have included modulation schemes, channel coding, UOWC challenges and OWC revolution in 5G and 6G networks. We have organized this paper as shown in Fig. 2.

II. RELATED WORK
In this section, we have summarized practical considerations and issues in UOWCs in existing research literature. We provide an overview of UOWC technology, enhancing functions and issues highlighted in recent research publications. Previous researchers have provided scholarly discussions about communication technologies, technical aspects and challenges of UOWC. In a recent research study conducted in 2020 [6], researchers have provided a review on practical consideration and solution in UWOCs. They have addressed underwater challenges and possible solutions. They have discussed misalignment in underwater links and highlighted novel solutions to bring robustness in UWOC links. An exhaustive and inclusive survey of state-of-the-art UOWC is given in which authors have considered aspects of coding techniques, modulation and channel characterization [7] in UOWCs. A good scholarly approach based on overview of current advances in UOWC is given in an article [8]. In this study, authors have focused the reliability and feasibility of optical links with higher data rates. Emerging optical wireless communications, transcutaneous OWCs, optical scattering and free space optics are discussed in a previous study [9]. Khalighi et al. [10] have given a performance study of typical UOWC system and addressed open issues. Performance of FSO systems and various impairments which degrade FSO performance are highlighted in [11]. Authors have discussed FSO modulation, coding schemes using various detection techniques. In a Master's thesis, Z. Zeng has given an interesting survey about optical wireless communications [12]. Author has provided good remarks for UWOC channel modeling, channel modulation and coding techniques. Weihao Liu et al. provided a roadmap to characterize an UOWC channel with considering both underwater optical turbulence and absorption/scattering [13]. Pal A. van Walree carefully addressed scattering, propagation and fading effects [14]. Researchers have conducted a brief survey on recent advances, research activities and potential applications [15] of OWC. Aobo Lin et al. [16] successfully demonstrated UOWC by carrying out experiments with blue LEDs. A blue GaNbased micro-LED was used to achieve high speed in UOWC [17]. An efficient diver-to-diver communication system was demonstrated in [18], researchers claimed to transmit audio signal by using LEDs of different wavelengths. Peppas et al. [19] have discussed performance of UOWC systems and investigated effects of oceanic turbulence and inter-symbol interference. Rashed et al. [20] have constructed an underwater link budget for realistic ocean water by considering scattering and absorption. Depth dependent variation in attenuation and beam refraction are addressed by LJ Johnson [21][22]. He also outlined transmission characteristics for UOWC in [23]. A concise review on UOWCs and hidden abilities of optical spectrum is addressed [24]. Authors in [25] performed experimental trials to check optimum LED wavelengths for UOWCs. Opportunities and challenges in OWC technologies are highlighted in a study [26]. The effects of collimated laser beams in UOWC links are investigated [27]. D. Anguita et al. have proposed a UOWC model for AUVs [28]. Optical amplification and spatial diversity for UOWC is discussed in a previous study [29]. Johnson et al. have focused on the impact of link orientation in UOWC [30]. UOWC system for real time swimmers is designed by some researchers [31]. They have also addressed air bubbles effect in their study. Table I summarizes the research work and key aspects in different years.

III. UNDERWATER COMMUNICATION TECHNOLOGIES
Current underwater optical systems consider acoustic waves as EM waves cannot travel properly. However, acoustic waves have limitation in aspects of limited bandwidth and slow speed. Acoustic communication is preferred for long distance but optical communication gives better performance for short distance. Light signals lose energy and change direction due to water particles. These challenging factors limit the range of communication system. We have plotted a performance comparison of the four channels by considering their data rate and transmission range in MATLAB as shown in Fig. 3.

IV. UNDERWATER COMMUNICATION CHALLENGES
In this section, we have briefly discussed different potential challenges which degrade the performance of UOWC system.

A. Scattering and Absorption Issues
In optical region, waves are highly faded due to scattering and absorption. The extinction coefficient c of water medium is the attenuated beam over the complete path length. Extinction coefficient is the transmission wavelength which minimizes the propagation loss and it varies for different types 164 | P a g e www.ijacsa.thesai.org (IJACSA) International Journal of Advanced Computer Science and Applications, Vol. 11, No. 7, 2020 of water. It is obtained by adding both absorption and scattering coefficients [34] a and b.  Table V provides the scattering, absorption, and extinction coefficient for different types of types of waters. Thus, we conclude demonstrating a UOCS closer to shore is more challenging than wide open ocean.

B. Attenuation
In water, less attenuation appears in wavelengths corresponding to blue green spectrum. However, optical link is associated to optical properties of the water and it varies according to geographic location. Attenuation (dB/m) variation against different wavelength ranges is shown in Fig. 6.

C. Underwater Link Alignment Issue
Misalignment in optical transceivers can cause temporary disconnection in underwater optical links. Different UOWC systems make use of blue-green LEDs and lasers due to narrow divergence. However, it still requires a precise alignment [35]. A frequent misalignment is noticed at shallow depths; especially it occurs in applications based on vertical surface-to-bottom communication [21], [36]. Connection loss problem [37] occurs in case of random movements of sea surface.

D. UOWC Devices
Implementation and integration of UOWC systems needs appropriate and reliable underwater devices. Underwater characteristics such as pressure, salinity, flow and temperature highly influence the system performance and have strong impact on lifetime of UOWC devices. Energy efficient performance of whole UOWC system and reliability and sustainability of devices batteries is critical. Scholars should focus to design devices with low power consumption and higher lifetime.

V. MODULATION TECHNIQUES AND CHANNEL CODING
Modulation schemes and coding techniques are discussed in this section. UOWC is referred as FSO communication in water, conventional FSO modulation techniques can be used in UOWC systems. PWM, OOK, DPIM and PPM are the most common intensity modulation techniques. OOK is the simplest scheme having low bandwidth efficiency. The selection of modulation schemes needs inclusive knowledge and in depth understanding application framework. OOK is preferred for discrete underwater sensor nodes as it is cost effective and simple. However, more than one modulation schemes are 165 | P a g e www.ijacsa.thesai.org (IJACSA) International Journal of Advanced Computer Science and Applications, Vol. 11, No. 7, 2020 integrated to enhance the reliability and efficiency of UOWC system. Table VI gives performance comparison of different modulation schemes.
Different channel coding techniques are required to obtain low BER in UOWC system as it suffers from severe attenuation. Channel coding has benefits of low complexity, low implementation cost, error correction capability and low energy consumption. Thus, channel coding is appropriate for compact underwater sensors operating at high SNR in underwater environment. In order to enhance robustness, channel coding techniques such as Turbo and LDPC are integrated in main data processing node of UWSNs. We have summarized different coding techniques in Table VII. In past few years, considerable research interest has been given to UOWC but anomalies exist due to several challenges. In this section we have provided some future directions in UOWCs.

A. Channel Modeling
Researchers have designed simulation tools and close-toreality models for horizontal links but vertical links still need attention from research community. Another practical consideration is to resolve turbulence effect in UOWC. Though turbulence is not considerable in deep waters but it must be taken into account in shallow waters. A lack in channel modeling techniques exist which should deal accurately with turbulence. The difficulty in designing such models lies in the fact that turbulence is closely related to water conditions and operating scenarios.

B. Efficient Transmission Techniques
Another essential issue to be explored is to design an opposite signaling scheme for aquatic channel. Up to now, several coding and modulation techniques are involved in UOWC systems. An improved reliability and link performance can be obtained by considering underwater channel particularities in designing of these transmission schemes. Energy efficient modulation schemes are needed for data transmission in high turbid water and powerful channel coding is required at different layers including physical and data link.

C. Mitigating Link Misalignment
A major issue in underwater environment is power consumption. One solution is to reduce the intensity loss by minimizing the beam divergence. In such scenario, energy efficient techniques and solutions are crucially important for Tx/Rx beam alignment and localization. These issues can be mitigated through designing smart Tx/Rx and adding additional components like collimating lens in UOWCs. There are some challenging issues to implement end-to-end communication links, researchers need to investigate routing protocols and spatial diversity techniques. A common UOWC system with point-to-point link needs strict tracking and pointing mechanism especially in mobility scenario. Electronic beam steering and segmented FOV can solve the issue of tracking and pointing for narrow optical beams.

VII. RESEARCH METHODOLOGY
In this section, we discuss the subject's selection process of our findings and presented the framework and approaches we used to identify our results.

A. Approaches Selection
Our investigation starts with selecting the research articles providing a survey or review on challenges in UOWCs. In order to find initial list of target research articles, we used Google Scholar search. Google Scholar helped us to obtain relevant research articles, peer-reviewed publications, abstracts, preprints and research surveys and technical reports. Google Scholar gave us confidence to complete results based on articles collected from Google search, ResearchGate and academic publishers such as IEEE, Springer and ACM. It is worth noting that Google Scholar gives full-text search against our keywords and validates that obtained research articles are relevant to our performed queries. It gives convenience by giving direct access to download research papers from relevant databases. To achieve significant coverage of research work related to UOWCs, we performed several queries on Google Scholar. First we search challenges in UOWCs, later we checked while combining research survey, review study, new trends, recent advances and limitations. We collected research articles and considered only those published 166 | P a g e www.ijacsa.thesai.org (IJACSA) International Journal of Advanced Computer Science and Applications, Vol. 11, No. 7, 2020 in recent years in order to maintain good results and state-ofthe-art approach. Basically, we selected the recent research articles related to current challenges, new trends and issues in UOWCs. We made lists of relevant papers focusing same potential features and started reading abstracts and conclusions to achieve primary study related to this review. Our research methodology includes research questions, sources and keywords. A basic overview of our research procedure is displayed in Fig. 7.

B. Research Questions
We have composed a set of research questions (RQs) and motivation behind it. These questions will be helpful for researchers to recognize the research lack in this field. RQs and motivation is highlighted in Table  RQ6. Test each solution against possible challenges in real world environment to achieve required performance of designed UOWC system?

C. Search Strategy
We have defined a good research strategy which will be helpful for researchers to retrieve specific research literature. Our research strategy includes research method, various search terms and different data resources. Our search strategy starts with fundamental steps to identify search terms and data resources: 1) Prefatory search to retrieve previous relevant literature.

2) Check research papers published in good journals and leading conferences.
3) Consult with researchers to find relevant data from conferences and journals.
4) Trial search based on prior defined research questions. 5) Using own learning and experience related to UOWCs.

D. Search Method
In our search strategies, we used two search methods: initial manual search and later automatic search. For manual search, we selected research papers which were published in specific venues as listed in Table IX. For automatic data search, we used electronic data resources provided in Table X to obtain relevant papers.

E. Search Terms
We used search terms related to keywords, paper titles and abstracts found in differed electronic databases. Keywords are given in Table XI. We used below provided strategies to achieve the most relevant data by forming relevant search terms.

1)
Find key terms according to research questions and study topic.
2) Make list of keywords mentioned in most relevant articles.
3) Search synonyms and major term. 4) Use Boolean operator "AND" in main terms. 5) Use Boolean operator "OR" in synonyms and corresponding terms.

RQ1
It will help to find out current and future advancement in UOWC

RQ2
It will help to understand complete designing process of UOWCs

RQ3
It will help to investigate factors which degrade performance of UOWC system.

RQ4
It will help to recover communication and handle delay issue

RQ5
It will help to shape the model to meet required performance criteria.

RQ6
It will help to design system with real world applications. We defined inclusion and exclusion criteria to rectify this primary study to validate our research queries.
Inclusion Criteria: • Any paper which declares its key findings related to challenges in UOWCs.
• Any paper which is published in 2010 or after 2010.
• Any paper which is written in English language only.
Exclusion Criteria: • Article which do not meet specific required details against our approach.
• Articles which do not contain complete information about our desired research work.
• Articles which are extended by another article that we have selected already in our list. It gives us to find representative article towards our approach.
• Articles representing short paper, only abstract, an editorial, poster summary, panel summary or workshop summary. Such articles are missing with sufficient informative data.
• Articles such as white papers and technical reports as some research communities do not rectify such articles.
In short, our primary study must provide the right approach towards UOWCs with a strong focus on emerging trends, challenging issues, opportunities and limitations. After applying inclusion and exclusion criteria, we were able to find N=42 research papers published in recent years. Different stages of our research process are given in Fig. 8.

VIII. ANALYSIS STUDY
We have collected analytical findings from several research publications on UOWCs. Researchers have used different analytical approaches and designed communication systems, channel modeling, coding and modulation techniques. Researchers are working on innovative services, technical aspects, real world applications and emerging trend of UOWCs. The research challenges are found in proper exploitation of underwater environment before communication and clear understanding of propagation characteristics of channel. Possible solutions are found in simulations, laboratory artificial aquatic environment or testbeds. There is need of an effective approach for system design, testing and analysis before its final deployment in wide ocean environment. The analytical study must give confidence and significant results that designed system will meet the required performance and efficiency. Researchers should consider these analysis questions. 1) Designed system will meet real world application requirements?
2) Which modulation technique and signal processing tools will provide high performance?
3) What is the required communication range for data transmission between source and destination? 4) What is the best path for communication without delay in information transmission? 5) How to overcome temporary connectivity losses while communication?
6) How to control power utility in order to enhance system reliability? 7) How to cope with any possible threats and security attacks to achieve secure communication in network? 8) Which factors will mainly affect the UOWCs design, communication architecture and deployment? 9) Which factors will mainly affect the UOWCs design, communication architecture and deployment?
10) How to tackle transceiver position and orientation in LOS and NLOS conditions?
The researchers should consider these questions and find our factors to develop analysis techniques and solutions. Their solutions must validate the real systems. Research community should carefully address the reliability and feasibility of optical links in underwater scenario. We have found relevant research articles by using scientific keywords and references. We selected many papers from Google Scholar. In our exclusions, we selected relevant papers after reading title, abstract, conclusion and future aspects. A possibility of missing some recent papers or data still remains by using this keyword search, inclusion and exclusion methods. Here we have described the validity threats and limiting factors to this research study as follows: 1) Data sources: Our major sources for data collection were Google Scholar and ResearchGate. Though these sources are beneficial to search relevant data as they automatically fetch data from different databases, technical sources and academic publishers against single data query, but still we take it as a threat to validity as these sources suffer from certain limitations such as lack of search facility, partial control on content body and vulnerability to spam. To tackle this, we will include more sources, scientific search engines and publishers while extending our current work. Enabling this approach in extending our work, we are confident to get broader coverage of potential issues in UOWCs and will provide more statistical analysis.
2) Data collection process: While our data collection and analyzing process, we assigned a single researcher to read article and collect information from title, abstract and conclusion. It is a potential threat to validity of our findings and we tried to mitigate this concern through group discussion. We are aware that a single researcher can inject certain amount of inconsistencies in selected data. In future extended work, we will perform detailed statistical analysis through assigning this task to more than one researcher.
3) Selection of potential challenges: we tried to identify potential challenges of UOWCs. This investigation comes from preliminary analysis of different articles. Though we used an effective way to get appropriate analysis, still there can be more challenging factors. To alleviate this threat, one research member was assigned this task to keep a good record of possible challenges and issues discussed in each selected article. In the end, this approach provided confidence while validating our approach.

4) Time span:
In inclusion criteria, we considered articles published after 2010 till now. Although it gives us right approach to find latest articles and key topics but it can affect the completeness of our search results as we did not include papers published before 2010. In future, we expect to involve more research recourses and considering articles published at a wide range of years.

X. OPTICAL WIRELESS REVOLUTION: 5G AND 6G
Scientists have been focusing on 6G communication after the successful development of 5G networks. 6G is expected to be launched between 2027 and 2030. Although 5G communication has many advantages such as high-quality video streaming, internet TV and faster communication [40]; however, it does not support ground-breaking technologies as it mainly focuses on performance. While privacy, secrecy and security are main features of 6G communication [41]. In addition, research community should focus on security and privacy issues which exploit wireless communication such as channel fading and noise. In underwater environment, 6G network will assist to deploy UWSN nodes in the form of AUVs and sensors connected with underwater base stations. Recently, some countries have made strategic plan on revolutionary advances of 6G. German and U.K. governments have invested in quantum technology. China has also made official announcement that Chinese researchers are focusing on the development of 6G. Researchers from USA have started working on terahertz-based 6G mobile networks. Terahertz frequency communication offers 1000 times higher capacity than 5G networks. One goal of 6G is to achieve ubiquitous connectivity by integrating satellite communication networks and underwater communications to provide global coverage.
RF-based technologies are insufficient to meet the demand of 5G/6G and IoT networks. Therefore, OWC technologies are the best complementary solution of RF networks. A very large optical band is a good alternative as it offers prominent features of high security, low latency, high data rate, high QoE, massive connectivity, low cost and low energy consumption. The coexistence of RF and optical wireless systems can achieve the goals of such networks. OWC technologies, such as FSO, OCC, LiFi and VLC can be effective to deploy future 5G/6G and IoT networks. We have summarized data rates and latency in current backhaul technologies in Table XII. Optical fiber communication provides the highest throughput among all existing technologies. However, a similar throughput can be achieved in FSO as it has similar type of transceivers. In future, FSO network will emerge as promising solution to support higher data rates in 5G/6G networks. OWC technologies can provide enormous number of connections through low-power LEDs to meet green aspects of 5G/6G and IoT networks. In addition, researchers should proficiently address some challenging issues such as atmospheric loss, flickering, inter-cell interference and frequent handover. A few important 5G/6G and IoT platforms using the OWC technologies are presented in Fig. 9.

XI. CONCLUSION
There is a growing need to dig into elementary acuity to make UOWC a reality in future. For this, a detailed analysis of theoretical models is required. Herein, we have highlighted some key research areas with profound knowledge in our systematic review. We have carefully addressed UOWC strategies which influentially affect remote communication. However, research fraternity needs to focus to enhance data rate for video transmission at larger distances. UOWCs will offer potential features in real life applications and will put more impact in future. Our main objective was to identify key challenges for future research. It is concluded from our study that researchers are putting relevant efforts to handle challenges in system design, deployments, link configuration and analysis of UOWCs. For a reliable and efficient optical link, we have discussed system architecture, channel modeling, modulation techniques and operating wavelengths in this research study. We also conclude that rapid ongoing research in UOWC will be more conducive in better performance with game-changing features in future.

XII. FUTURE WORK
We have highlighted a limitation of system with real time operating conditions and researchers must give specific concern as it will be beneficial to control remotely operated vehicles. As a precondition, research community should focus on developing intelligent modulation techniques to improve system performance. Future research should contemplate to involve Internet of Underwater Things (IoUT) to properly achieve real world applications. Researchers should pay attention to future 6G green communications. The coexistence of optical wireless and RF systems can meet the goals of such revolutions. Merging some latest underwater technologies with UOWC can bring promising results in future as shown in Fig. 10.
Besides all this, there is still room to develop cheap, adaptive, robust, highly stable, low powered and real time underwater optical sensors for an efficient UOWC system. In order to enhance overall robustness, adaptive techniques must be observed to save energy and optimize communication efficiency.