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Underwater wireless communication (UWC) equipment facilitates information transmission and data exchange in underwater environments, playing vital roles in marine scientific research, underwater network monitoring, underwater collaborative operation, and marine safety maintenance. This study explores four primary UWC equipment categories: underwater acoustic communication, underwater optical communication, underwater electromagnetic communication, and underwater magnetic induction communication. It conducts in-depth analyses of the technical challenges associated with each category, comprehensively reviews their development status in China and abroad, and forecasts future trends. Focusing on the UWC industry of China, we summarize the development challenges in terms of overarching gaps, common issues, and top-level system, and propose the following development suggestions: (1) improving fundamental mechanisms and addressing common issues, (2) prioritizing breakthroughs in industry core areas, (3) elucidating the top-level system structure of the UWC equipment, and (4) enhancing safeguard measures and support policies. This study is expected to provide references for understanding the developmental trend and promoting the research and application of the UWC equipment.

We designed, fabricated, and evaluated a monopole water antenna (WA) filled with pure water. A 2.4 GHz patch antenna (PA) was used for measurement comparison, and the current density distribution and 3D field strength radiation distribution and reflection coefficient of the PA had a fundamental mode and a higher-order mode at 3.5 GHz, whose polarization was 90 degrees different. The 2.4 GHz monopole WA could receive only the fundamental mode of the PA. The 3.5 GHz WA could receive the higher-order mode of the PA by rotating the WA by 90 degrees. The transmission coefficient of the 2.4 GHz WA decreased with the square of the spacing, similar to the spatial propagation characteristics of electromagnetic waves. Almost the same results could be expected if planar or three-dimensional antennas were used instead of monopole electrodes.

Oceans cover more than 70% of the Earth's surface. For various reasons, almost 95% of these areas remain unexplored. Underwater wireless communication (UWC) has widespread applications, including real-time aquatic data collection, naval surveillance, natural disaster prevention, archaeological expeditions, oil and gas exploration, shipwreck exploration, maritime security, and the monitoring of aquatic species and water contamination. The promising concept of the Internet of Underwater Things (IoUT) is having a great influence in several areas, for example, in small research facilities and average-sized harbors, as well as in huge unexplored areas of ocean. The IoUT has emerged as an innovative technology with the potential to develop a smart ocean. The IoUT framework integrates different underwater communication techniques such as optical, magnetic induction, and acoustic signals. It is capable of revolutionizing industrial projects, scientific research, and business. The key enabler technology for the IoUT is the underwater wireless sensor network (UWSN); however, at present, this is characterized by limitations in reliability, long propagation delays, high energy consumption, a dynamic topology, and limited bandwidth. This study examines the literature to identify potential challenges and risks, as well as mitigating solutions, associated with the IoUT. Our findings reveal that the key contributing elements to the challenges facing the IoUT are underwater communications, energy storage, latency, mobility, a lack of standardization, transmission media, transmission range, and energy constraints. Furthermore, we discuss several IoUT applications while highlighting potential future research directions. [ABSTRACT FROM AUTHOR]

Summary: Underwater Wireless Communication is a rapidly growing technology in the research domain. The researchers have been working towards the developments in underwater communication by investigating climatic changes, prediction of natural disasters, marine environment, monitoring aquatic life, oceanographic transmission, and data collection. The main objective of underwater communication is to achieve lossless, high transmission with the least power consumption. The Unmanned Underwater Vehicles (UUVs) and Autonomous Underwater Vehicles (AUVs) are set up with remote instruments and sensors to enable the natural exploration of the resources in the undersea environment. This research article is based on the study of various modes of communication, architectures, and protocol layers involved in underwater communication. Furthermore, an underwater hybrid connection is established by opto‐acoustic signals, and the overview of various performance characteristics, channel behaviors, and issues in the underwater scenario are investigated. [ABSTRACT FROM AUTHOR]

Ocean observation system that involves multiple underwater vehicles and seafloor nodes plays an important role in better learning the ocean, where underwater wireless communication is mandatory for massive data interaction. Optical communication that has wide bandwidth and comprehensive working distance is the preferred method compared to acoustic and other methods. However, the presence of directionality makes the optical method difficult to use especially when the transceiver is equipped on a motive vehicle. In this study, an underwater free-space optical communication method of transmitting information is proposed. Characteristics of underwater optical transmission, as well as the photoelectric signal processing and modulation and demodulation algorithms, are studied and modeled. New approach for realizing underwater free-space optical communication is proposed and simulated. A prototype including a free-space optical transmitter and a receiver is developed; tests in different scenarios were carried out, and the results were observed: (1) by using the minimum number of LEDs, the effect of uniform lighting in space is achieved, and the transmitter coverage reaches 160°. (2) When the power of the transmitter is 10 W and the communication rate is 1 Mbps, the maximum communication distance reaches 13 m.

Underwater wireless communication is a rapidly growing field, especially with the recent emergence of technologies such as autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs). To support the high-bandwidth applications using these technologies, underwater optics has attracted significant attention, alongside its complementary technology – underwater acoustics. In this paper, we propose a hybrid opto-acoustic underwater wireless communication model that reduces network power consumption and supports high-data rate underwater applications by selecting appropriate communication links in response to varying traffic loads and dynamic weather conditions. Underwater optics offers high data rates and consumes less power. However, due to the severe absorption of light in the medium, the communication range is short in underwater optics. Conversely, acoustics suffers from low data rate and high power consumption, but provides longer communication ranges. Since most underwater equipment relies on battery power, energy-efficient communication is critical for reliable underwater communications. In this work, we derive analytical models for both underwater acoustics and optics, and calculate the required transmit power for reliable communications in various underwater communication environments. We then formulate an optimization problem that minimizes the network power consumption for carrying data from underwater nodes to surface sinks under varying traffic loads and weather conditions. The proposed optimization model can be solved offline periodically, hence the additional computational complexity to find the optimum solution for larger networks is not a limiting factor for practical applications. Our results indicate that the proposed technique yields up to 35% power savings compared to existing opto-acoustic solutions.

The visible light communication (VLC) technique using a rolling-shutter sensor-based commercial camera has an advantage in that the LED and camera mounted on an underwater device (e.g., drone, sensor) can be effectively used as a transmitter (Tx) and a receiver (Rx), respectively, under water. On the other hand, a suitable modulation technique has not been well considered to realize a VLC system using a rolling-shutter sensor in an underwater channel that includes high ambient light conditions. In this paper, we design VLC systems using phase-shift keying (PSK), non-return-to-zero on- off keying (NRZ-OOK), and orthogonal frequency division multiplexing (OFDM), and evaluate them through simulations and experiments. The results suggested that the VLC system using PSK or OFDM is more resilient than that using NRZ-OOK in an underwater channel with low-frequency ambient light, and that a VLC system using PSK or NRZ-OOK is more tolerant than that using OFDM in environments with high ambient light, resulting in pixel saturation. In conclusion, the present study has demonstrated that PSK is the most suitable modulation technique for a VLC system using a rolling-shutter sensor in an underwater channel that includes high ambient light conditions.

Oceans cover more than 70% of the Earth’s surface. For various reasons, almost 95% of these areas remain unexplored. Underwater wireless communication (UWC) has widespread applications, including real-time aquatic data collection, naval surveillance, natural disaster prevention, archaeological expeditions, oil and gas exploration, shipwreck exploration, maritime security, and the monitoring of aquatic species and water contamination. The promising concept of the Internet of Underwater Things (IoUT) is having a great influence in several areas, for example, in small research facilities and average-sized harbors, as well as in huge unexplored areas of ocean. The IoUT has emerged as an innovative technology with the potential to develop a smart ocean. The IoUT framework integrates different underwater communication techniques such as optical, magnetic induction, and acoustic signals. It is capable of revolutionizing industrial projects, scientific research, and business. The key enabler technology for the IoUT is the underwater wireless sensor network (UWSN); however, at present, this is characterized by limitations in reliability, long propagation delays, high energy consumption, a dynamic topology, and limited bandwidth. This study examines the literature to identify potential challenges and risks, as well as mitigating solutions, associated with the IoUT. Our findings reveal that the key contributing elements to the challenges facing the IoUT are underwater communications, energy storage, latency, mobility, a lack of standardization, transmission media, transmission range, and energy constraints. Furthermore, we discuss several IoUT applications while highlighting potential future research directions.

For application requirements of safety monitoring in complex underwater environment, wireless magnetic induction communication technology was adopted to be information transmission technology. Principle of underwater wireless magnetic induction communication was introduced. Design scheme of an underwater wireless magnetic induction communication platform based on FSK modulation was expounded and communication test was taken out by use of the platform in simulated underwater environment. The test results show that the platform can realize wireless information transmission in underwater environment. The platform using unidirectional antenna will have communication interruption under the condition of changing position or angle of receiving antenna. While the platform using omnidirectional antenna can communicate normally in process of changing position or angle of receiving antenna, which improves communication reliability.

Underwater communication is an emerging field of research currently, which is an important aspect in predicting the climatic conditions accurately. Underwater communication is disadvantageous when compared with aerial communication because water is basically non transparent and blurred to electromagnetic radiations excluding visible region. In the visible region, factors such as suspended sediments and the presence of aquatic life affects the penetration of light. It may penetrate only upto a few hundreds of metres in clear water and in turbid waters, penetration is even less. Underwater communication is also affected by Intersymbol Interference (ISI) due to multipath fading. Currently, a booming and mature technology called acoustic technology is employed for underwater communication systems. Acoustic systems are proficient for long distance communication. But acoustic communication has limitations. They have very low data rates for monitoring applications and due to velocity of sound in water, it has a huge latency which means a significant time delay. To cope up with the limitations of acoustic communication, we use optical communication whose wavelength lies in the visible region. Optical communication is known for high data rate and low latency. Many accidents take place under water such as the plane crash, ship sinking. It is a tedious job to find the lost debris. Hence an underwater optical link is developed which could identify the lost debris. In this project, a hybrid model comprising of acoustic as well as the optical link is proposed. The comparison between acoustic and optical technique is studied and simulated using MATLAB simulation and the responses were plotted. The simulated results can be used in various applications including climate monitoring, military services. [ABSTRACT FROM AUTHOR]

To improve the observation capability of the ocean, the combination of the static cabled ocean observatory network and the dynamic autonomous underwater vehicle has attracted more and more attention. In this paper, a non-contact docking system for the autonomous underwater vehicle is developed to combine the advantages of the cabled ocean observatory network and the autonomous underwater vehicle. The system includes both acoustic and optical navigation, underwater wireless communication, non-contact power transfer, and monitoring and controlling of the docking system. This docking system was verified by sea trials at depths of 50 and 105 m. The autonomous underwater vehicle successfully docked 11 times, during which non-contact power transfer and wireless communication were completed. The charging power reached 682 W, with a total efficiency of 78.5%, and the efficiency of the power transfer unit was 92%. The rate of wireless data transfer reached 3.1 MB/s. For the first time, this docking system has realized complete homing and docking navigation, wireless communication and charging in a water depth of more than 100 m, thus establishing a good foundation for the combination of the cabled ocean observatory network and autonomous underwater vehicles. [ABSTRACT FROM AUTHOR]

Autonomous Underwater Vehicles (AUVs) are robotic devices with a propulsion system for navigation and an onboard computer for decision making. AUV research is gaining popularity because of its extensive applications in fields from military to science. Robotic systems are need of the hour for exploration and environmental safety of the vast and deep oceans and water bodies. This paper presents current research trends in the field of AUVs and highlights future research directions. Here localization and navigation techniques such as inertial navigation to simultaneous localization and mapping being used in current AUVs are discussed in detail. Different optimal path planning and control methods are highlighted. Use of different sensor technology like sonar, laser, acoustic modems and stereo vision systems for localization, navigation and mapping is presented. Recent developments in underwater wireless communication along with the commercially available devices are discussed. • Review of AUVs based on structural design and propulsion technique are presented.. • Different adaptive control and path-planning techniques have been reviewed.. • Underwater localization techniques suitable for different applications are discussed.. • Developments of relatively new Intervention AUVs are presented.. • Underwater wireless communication methods and available systems are discussed.. [ABSTRACT FROM AUTHOR]

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The technologies used in underwater and air–water (A–W) wireless communication networks (WCN) are increasingly attracting attention due to numerous modern applications, e.g., internet of underwater things etc. However, for practical implementations of these modern applications, the security of the underwater and A–W WCNs needs to be ensured beforehand. Thus, the main focus of this survey is to systematically discuss the security needs of underwater and A–W WCNs, and solutions proposed to date. Before extending our discussion on security, we initially cover the fundamentals of underwater and A–W WCNs. First, we provide a comprehensive overview of different underwater communication technologies: radio frequency (RF), acoustic, optical, and magnetic induction (MI) in terms of channel characteristics, merits, and demerits. The discussion is further extended to A–W wireless communication by presenting direct and indirect (relay-aided) techniques. Then we present the primer on information security which highlights the four fundamental properties of security (i.e., confidentiality, integrity, authentication, and availability) and security solutions (i.e., cryptography and physical layer security) built for the considered underwater communication technologies. In addition, we discuss at length the security aspects of the underwater and A–W WCNs by reviewing and summarizing the existing work in the literature. We notice that the related work on security of RF-based, magnetic induction-based underwater WCNs, and A–W WCNs is practically non-existent. Thus, we highlight this research gap in the literature, and propose a few additional security-related open problems too, that we believe deserve to receive more attention from the underwater, marine, and oceanic research community. [ABSTRACT FROM AUTHOR]

The existing technologies using electromagnetic waves or lasers are not very efficient due to the large attenuation in the aquatic environment. Ultrasound reveals a lower attenuation, and thus has been used in underwater long-distance communications. For high data-rates and real-time applications it is necessary to use frequencies in the MHz range, allowing communication distances of hundreds of meters with a delay of milliseconds. To achieve this goal, it is necessary to develop ultrasound transducers able to work at high frequencies and wideband, with suitable responses to digital modulations. This work shows how the acoustic impedance influences the performance of an ultrasonic emitter transducer when digital modulations are used and operating at frequencies between 100 kHz and 1 MHz. The study includes a Finite Element Method and a MATLAB/Simulink simulation with an experimental validation to evaluate two types of piezoelectric materials: one based on ceramics (high acoustic impedance) with a resonance design and the other based in polymer (low acoustic impedance) designed to optimize the performance when digital modulations are used. The transducers performance for Binary Amplitude Shift Keying, On–Off Keying, Binary Phase Shift Keying and Binary Frequency Shift Keying modulations with a 1 MHz carrier at 125 kbps baud rate are compared. [ABSTRACT FROM AUTHOR]

In this paper, a new variable-focus LED light device is developed for underwater communication. Usually used as an underwater lighting fixture, the LED light device in this study is utilized as an underwater communication device (UCD) by controlling the distance between light source and lens when communication is needed. A transmission and a receiving part of LED light for communication using photoelectric sensor and Fresnel lens are also developed. The communication system was tested in fresh water and sea water to verify its communication performance; results of which are presented in this study. [ABSTRACT FROM AUTHOR]

This paper has presented our interesting in wireless underwater communications for different ocean water types. Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this paper is to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. As well as we have developed the underwater optical wireless communication systems to have shorter ranges, that can provide higher bandwidth (up to several hundred Mbit/s) communications by the assistant of exciting high brightness blue LED sources, and laser diodes suggest that high speed optical links can be viable for short range application. The received signal power, signal to noise ratio, bit error rate, transmitted signal bandwidth, and transmission bit rates are the major interesting parameters for different ocean water mediums as a criteria of the best signal transmission characteristics of short wireless optical communications over wide range of the affecting parameters. [ABSTRACT FROM AUTHOR]

In this paper, the problem of joint space-time parameter estimation for underwater wireless communication channels in a multipath environment is addressed. We consider the receive antenna array to be configured with multiple vector sensors, each of which consists of a pair of orthogonal velocity sensors. A quadrilinear model for the channel is formulated, and a quadrilinear decomposition method developed for joint angle and delay estimation (JADE). In addition, a computationally simple subspace-based algorithm is proposed for the problem under consideration. The basic idea behind this algorithm is to use the angle information embedded in the velocity vector sensors to start two polynomial rooting procedures for the angle and delay in succession. Simulation results are finally presented to verify the efficacy of the proposed algorithms. [ABSTRACT FROM PUBLISHER]

Abstract: Ultrasound transducers are typically based on piezoelectric materials, due to their good response at high frequencies. Depending on the application, ceramics, polymers and composite materials can be used. In this work, an optimization study of ultrasound transducers for underwater communications is addressed, focusing on a piston type emitter transducer operating in thickness mode (d 33). The piston is constituted by an active element disk with optimized dimensions. It is discussed how the acoustic impedance, thickness, resonance frequency and structure affect the transducer performance. This work allows a better understanding of the emitter transducer characteristics allowing reaching the optimum point of operation for specific applications. Focusing on underwater communication, the acoustic channel is defined and the transducer is optimized by finite element computer simulations. The results were compared with experimental tests, which show that four-layer structures increase up to 16dB in performance versus single-layer. [Copyright &y& Elsevier]