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Hydrophobic conductive gels have attracted much attention in underwater sensing as the soft robots and wearable sensors. Among the various methods for preparing conductive gels, UV‐initiated polymerization shows advantages of rapid curing and significant energy savings. However, traditional commonly used photoinitiators in UV‐initiated polymerization often migrate or diffuse within conductive gels, especially with prolonged use or underwater, posing potential environmental and health risks. Herein, a novel lignin‐based macromolecular photoinitiator (L‐BF) is first designed by incorporating benzoylformate into the lignin backbone, demonstrating excellent UV absorption, high radical initiation efficiency, and hydrophobicity. Subsequently, hydrophobic eutectogels (HEG) are prepared via UV‐initiated polymerization using L‐BF as photoinitiator. The HEG shows excellent mechanical properties (toughness of 5.67 MJ m−3 and strain of 941%) and scarcely L‐BF migration underwater. Moreover, the hydrophobic network of L‐BF endowed HEG with hydrophobicity (water contact angle exceeds 110°; swelling ratio is <5.8%), superior underwater adhesion (255 kPa), even maintained excellent sensing performance (gauge factor of 4.69), and long‐term reproducibility (20 000 cycles) in air and underwater. Validation confirms the potential of HEG as an effective wearable sensor for underwater posture monitoring and communication, advancing the use of lignin and low‐migration conductive gels in marine exploration. [ABSTRACT FROM AUTHOR]

Wireless sensor networks are used to monitor the environment, to detect anomalies or any other problems and risks in the system. If used in the transportation network, they can monitor traffic and detect traffic risks. In wireless sensor networks, energy constraints must be handled to enable continuous environmental monitoring and surveillance data gathering and communication. Energy-aware path planning of autonomous ground vehicle charging for sensor nodes can solve energy and battery replacement problems. This paper uses the Nearest Neighbour algorithm for the energy-aware path planning problem with an autonomous ground vehicle. Path planning simulations show that the Nearest Neighbour algorithm converges faster and produces a better solution than the genetic algorithm. We offer robust and energy-efficient path planning algorithms to swiftly collect sensor data with less energy, allowing the monitoring system to respond faster to anomalies. Positioning communicating sensors closer minimizes their energy usage and improves the network lifetime. This study also considers the scenario in which it is recommended to avoid taking direct travelling pathways between particular node pairs for a variety of different reasons. To address this more challenging scenario, we provide an Obstacle-Avoided Nearest Neighbour-based approach that has been adapted from the Nearest Neighbour approach. Within the context of this technique, the direct paths that connect the nodes are restricted. Even in this case, the Obstacle-Avoided Nearest Neighbour-based approach achieves almost the same performance as the the Neighbour-based approach. [ABSTRACT FROM AUTHOR]

The application of gel wearable sensors in aquatic environments is very promising, however, the preparation of underwater gel sensors with excellent anti‐swelling properties and sensing performance remains a great challenge. In this paper, a solvent exchange strategy is proposed to fabricate an anti‐swelling gel, in which the hydrophobic segments are induced to gather and microphase separation occurs inside the gel. The prepared gel possesses excellent anti‐swelling performance (swelling ratio of 4.79%), good mechanical properties (tensile strain of 712%), high sensitivity (GF = 1.07), and high linearity (R2 = 0.998, 0 to 70% strain range). Moreover, it can achieve high stability (more than 90 days) and fatigue resistance (1000 cycles at 50% strain) as an underwater sensor. Therefore, the designed underwater gel strain sensors can be utilized for monitoring human motion information, underwater distress and smart alarms, revealing the great potential application in the field of underwater strain sensors. [ABSTRACT FROM AUTHOR]

Reliable interfacial bonding is an essential guarantee that flexible electronics can output realistic signals, especially for underwater scenes. However, conventional self‐adhesive materials usually suffer from underwater adhesion failure, conflict between adhesion and cohesion, as well as adverse effects of isotropous adhesion and residue, greatly limiting their applications in flexible electronics. Herein, a Janus hydrophobic structural gel (HSG) with asymmetric adhesion is fabricated by a "grafting one twig on another" approach (in situ constructing a hydrophobic anti‐adhesive gel on the top of a hydrophobic self‐adhesive gel). The hydrophobic adhesion layer with long C18 aliphatic chains achieves a reliable underwater bonding (interfacial toughness exceeds 80 J m−2) with the assistance of high mobility of polymer chains, multiple interfacial interactions, and effective removal of interfacial liquid. The hydrophobic anti‐adhesion layer containing poly(ionic liquid) is more robust due to electrostatic and ion‐dipole interactions, ensuring the mechanical strength of the integral HSG. Such asymmetric heterostructure avoids common nonessential adhesion and residue, facilitating operation. The intrinsic hydrophobicity of HSG also prevents water erosion, achieving reliable underwater adhesion and sensing. As a result, the assembled sensor based on HSG can stably monitor human motions and wirelessly transmit underwater information, exhibiting an enormous potential in wearable electronics. [ABSTRACT FROM AUTHOR]

Summary: Underwater wireless sensor network (UWSN) connects the real world to the hidden resources available deep into the sea using sensor nodes deployed sparsely and are interconnected to gather information for any movement/change in the ocean. The computed information on nodes obtained from concurrently running UWSN's applications will be meaningful, only if the location of this change will be identified. Nevertheless, it is always difficult to obtain the exact location coordinates of any misadventure like tectonic plate's movement but using localization algorithms in UWSN helps to obtain the coordinates. Since localization algorithms for terrestrial networks are not feasible for UWSN because of environmental challenges. Moreover, GPS systems do not work after 15 m of depth and the radio frequency gets suppressed. So, the requirement arises to analyze existing localization approaches in the deep sea where acoustic signals are used for communication and data transfer. In this paper, along with describing the UWSN's applications and challenges, the underwater localization schemes are reviewed to present, summarize, and mention the scope of improvement. Further, classification into range‐based and range‐free categories of these schemes is depicted with implementation in the NS2.30 simulation environment of some of the recent techniques to showcase the reasons for better performance. [ABSTRACT FROM AUTHOR]

Wireless communication in underground environments presents formidable challenges due to the diverse and complex transmission mediums, including rocks, icebergs, soil, and water. These environments exhibit distinct properties such as varying densities, salinities, and numerous obstacles. Conventional wireless communication methods encounter difficulties in these scenarios, primarily stemming from issues like electrical conduction, line-of-sight obstacles, high path loss, dynamic channel conditions, and the requirement for large antennas, multiple scattering, and propagation delays constrained by the speed of light. However, electromagnetic induction (EMI) communication has emerged as a promising alternative solution, offering advantages such as reliable connectivity, consistent channel responses, rapid propagation, frequency offset capabilities, minimal propagation delays, and energy efficiency. Despite these merits in underwater environment ecosystems communications, EMI communication suffers from a limited communication range, typically spanning just a few meters. Recent research endeavors have sought to extend the effective range of EMI communication by exploring various strategies, including manipulations of coil parameters, structural enhancements, material advancements, and configuration improvements. This comprehensive study and evaluation delves into the diverse applications, notable advantages, techniques for extending communication range, and the pressing challenges and open issue facing EMI communication in underwater ecosystem environments, requiring further investigation and new research. [ABSTRACT FROM AUTHOR]

This study explores the application of single photon detection (SPD) technology in underwater wireless optical communication (UWOC) and analyzes the influence of different modulation modes and error correction coding types on communication performance. The study investigates the impact of on-off keying (OOK) and 2-pulse-position modulation (2-PPM) on the bit error rate (BER) in single-channel intensity and polarization multiplexing. Furthermore, it compares the error correction performance of low-density parity check (LDPC) and Reed-Solomon (RS) codes across different error correction coding types. The effects of unscattered photon ratio and depolarization ratio on BER are also verified. Finally, a UWOC system based on SPD is constructed, achieving 14.58 Mbps with polarization OOK multiplexing modulation and 4.37 Mbps with polarization 2-PPM multiplexing modulation using LDPC code error correction. [ABSTRACT FROM AUTHOR]

The exploration of underwater environments for applications like environmental monitoring, scientific research, and surveillance has propelled the significance of underwater wireless navigation. Light waves have emerged as a promising solution, offering the potential to achieve the required data rates and propagation speeds. However, underwater optical wireless navigation faces challenges, particularly limited range. This research investigates a novel FMCW (frequency-modulated continuous wave)-based photonic radar system's efficacy in detecting underwater vehicles across diverse salinity levels and distances. Numerical simulation evaluations reveal distinct signal-to-noise ratios (SNR) and detected power peaks corresponding to varying salinity levels, demonstrating the system's sensitivity. At 5 g/L salinity, the detected power peaked at −95 dBm, decreasing to −105 dBm at 15 g/L. SNR analysis indicates robust detection within a 4 m range, with challenges emerging at extended ranges and higher salinity. Despite these challenges, the system shows promise for near-range underwater navigation, contributing to sustainable marine exploration by enhancing the accuracy and efficiency of underwater monitoring systems. This advancement aligns with the goals of sustainable development by supporting the protection of marine ecosystems, promoting scientific understanding of underwater environments, and aiding in the sustainable management of marine resources. [ABSTRACT FROM AUTHOR]

A theory of surface electromagnetic waves in gradient media exhibiting arbitrary surface gradients of dielectric permittivity and magnetic permeability has been developed. Novel low-loss propagating surface wave solutions have been found in the gradient media in which both dielectric permittivity and magnetic permeability are dominated by their imaginary parts. Several examples of gradient geometries in which the surface wave problem may be solved analytically have been found. Examples of practically useful surface wave geometries spanning from radio communication underwater to UV nanophotonics have been demonstrated. [ABSTRACT FROM AUTHOR]

This paper proposes a whistle-generative adversarial network (WhistleGAN) that generates whistles for biomimetic underwater covert acoustic communication. The proposed method generates new whistles to maintain covertness by avoiding the repetitive use of the same whistles. Since the human ear perceives octave frequency such that low-frequency resolution is relatively larger than that of low frequencies, the proposed WhistleGAN uses mel filter banks to keep the fidelity in mimicking while reducing the complexity. The mean opinion score test verified that the whistles generated by the proposed method and the recorded real whistles have a similar score of 4.3, and the computer simulations proved that the bit error rate performance of the proposed method is the same as that of the real whistle. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) can be a promising alternative to radio-based communication for underwater applications. However, VLC suffers from limited range and coverage due to the absorption and scattering of light in water. The purpose of this research is to construct and assess a VLC's performance in turbid, turbulence, and typical ocean water. Utilizing Optisystem simulation software, we design and optimize a hybrid VLC underwater communication system, where a ship can communicate from water surface to inside and internal link between two submarines. The system's performance is measured in terms of bit error rate, data rate, and range under various water conditions, including turbid, normal ocean water, and turbulence water. Instead of implementing higher order modulation or achieving high data rates, the emphasis was on maximizing the simplicity of underwater light communication (ULC) in a variety of weather conditions. We observed BER 5.84x10-9 in the clear water within 500m and BER was 2.8x10-4 in the turbulence water by using OOK modulation scheme. Moreover, this system can transmit data maximum 500m in clear water 230m, and 210 m in turbid and turbulence water simultaneously. By systematically evaluating system performance under various types of water conditions, this model has the potential of effective communication solutions for underwater applications, with implications for various sectors including marine exploration, underwater surveillance, and environmental monitoring. [ABSTRACT FROM AUTHOR]

Underwater wireless communication remains a challenging topic, particularly for applications such as wreck penetration where multipath and Doppler effects are very intense. These effects are becoming even more difficult to mitigate for fast data transmission systems that utilize wideband signals. Due to the low propagation speed of acoustic wave in the water, there is a significant difference between the Doppler shift for lower and upper frequencies of the utilized spectrum. To address these challenges, this paper describes various methods for determining the Doppler frequency shift for MFSK signals, including cross-correlation, double FFT, pilots, and additional Up-Down chirp signals. The reception quality of the transmitted data in a real environment was used as an evaluation criterion for each method. The tests were carried out in motion within the towing tank for different movement speeds of the transmitter relative to the receiver. The tank’s limited dimensions created conditions for multipath signal propagation. Under very difficult multipath signal propagation conditions, the pilots method was found to be the most effective. It gave over two times lower BER than the well-known Up-Down chirp method.

This paper presents a simulation framework for modelling optical underwater turbulence in conjunction with absorption and scattering. Using this technique, the channel is evaluated in two ways: the turbulent statistics in different channel conditions; and the stationary channel characteristics. The turbulent statistics observed from the simulation show that the relative impact of turbulence on a received signal is lower in a highly scattering channel, showing an in-built resilience of these channels. Received intensity distributions are presented, showing that the commonly used Log-Normal fading model provides a good description of the fluctuations in received optical power due to the effect of turbulence. When considering stationary channel characteristics, the effect of turbulence induced scattering is shown to cause and increase both spatial and temporal spreading at the receiver plane. The impact of turbulence - as measured using this new modelling framework - on the channel capacity is equally investigated to provide context to the implications of the channel modelling findings on underwater optical wireless communications link performance.

A theory of surface electromagnetic waves in gradient media exhibiting arbitrary surface gradients of dielectric permittivity and magnetic permeability has been developed. Novel low-loss propagating surface wave solutions have been found in the gradient media in which both dielectric permittivity and magnetic permeability are dominated by their imaginary parts. Several examples of gradient geometries in which the surface wave problem may be solved analytically have been found. Examples of practically useful surface wave geometries spanning from radio communication underwater to UV nanophotonics have been demonstrated.

Wireless sensor networks are used to monitor the environment, to detect anomalies or any other problems and risks in the system. If used in the transportation network, they can monitor traffic and detect traffic risks. In wireless sensor networks, energy constraints must be handled to enable continuous environmental monitoring and surveillance data gathering and communication. Energy-aware path planning of autonomous ground vehicle charging for sensor nodes can solve energy and battery replacement problems. This paper uses the Nearest Neighbour algorithm for the energy-aware path planning problem with an autonomous ground vehicle. Path planning simulations show that the Nearest Neighbour algorithm converges faster and produces a better solution than the genetic algorithm. We offer robust and energy-efficient path planning algorithms to swiftly collect sensor data with less energy, allowing the monitoring system to respond faster to anomalies. Positioning communicating sensors closer minimizes their energy usage and improves the network lifetime. This study also considers the scenario in which it is recommended to avoid taking direct travelling pathways between particular node pairs for a variety of different reasons. To address this more challenging scenario, we provide an Obstacle-Avoided Nearest Neighbour-based approach that has been adapted from the Nearest Neighbour approach. Within the context of this technique, the direct paths that connect the nodes are restricted. Even in this case, the Obstacle-Avoided Nearest Neighbour-based approach achieves almost the same performance as the the Neighbour-based approach.

[Display omitted] • Hydrophobic eutectogel is facilely produced through the polymerization of hydrophobic monomers in a hydrophobic DES. • The hydrophobic eutectogel shows outstanding water resistance (water contact angle over 110°) and extraordinary durability (over 1000 testing cycles) underwater. • Hydrophobic eutectogel can be utilized as strain and pressure sensors underwater. • Hydrophobic eutectogel sensors can transmit information through Morse code, performing as a underwater communicator. Eutectogels derived from deep eutectic solvents (DESs) exhibit great potential for the fabrication of flexible sensors. However, the hygroscopicity of eutectogels hinders their applications in underwater sensing. In this work, a hydrophobic eutectogel with exceptional long-term underwater stability is produced through one-step polymerization of lauryl methacrylate and glycidyl methacrylate in a hydrophobic DES. The hydrophobic gel network and hydrophobic DES fulfill the eutectogel with outstanding water resistance (water contact angle > 110°) and excellent mechanical properties in an aqueous environment, thus leading to extraordinary durability (over 1000 testing cycles). Additionally, based on this eutectogel, underwater strain and pressure sensors with high sensitivity, rapid responsiveness, and superior durability were fabricated for accurate real-time monitoring of human activity. Furthermore, it has been demonstrated that the eutectogel sensor can transmit information through Morse code, performing as a wearable underwater communicator. This research provides an exemplary way for a demonstration method of hydrophobic eutectogel for durable underwater applications. [ABSTRACT FROM AUTHOR]

The development of compact hydroacoustic low-frequency emitters with a high power density is fraught with difficulties due to conflicting requirements on the dimensions, efficiency, radiated power, operating bandwidth, and manufacturability. For compact emitters, the body dimensions limit the possibility of combining the resonances of the active element and mechanical oscillatory system, which makes their development difficult. A compact hydroacoustic low-frequency longitudinal-bending type transducer with a complexly shaped radiating surface, 3D LFR, designed for large-scale modeling and verification of theoretical calculations of such design, has relatively small dimensions and demonstrates high efficiency. The paper presents the results, obtained with laser vibrometry, of in-air measurements of the vibrational characteristics of two titanium bodies with different sizes and corrugation patterns. The proposed designs for the transduce with maximum overall dimensions of less than 100 mm and a weight of approximately 1 kg provide a voltage sensitivity of about 1 Pa m/V in the operating frequency band and the fundamental resonance in the 1–2 kHz range. 3D LFR has a high mechanical conversion coefficient and uses the added mass; it also has a number of other advantages over similar designs. It is shown that the 10–12% differences in the sizes of the two emitters and the geometry of the radiating shells (12 and 16 corrugation waves) lead to a difference in the resonance frequencies measured in air (4.0 and 3.5 kHz, respectively). At the same time, the emitter is larger and has a smaller spread of the mechanical conversion coefficient along the ridges and troughs of the body, as well as a denser distribution of the spectral components outside the fundamental frequency band. [ABSTRACT FROM AUTHOR]

Conductive hydrogels have recently attracted extensive attention in the field of smart wearable electronics. Despite the current versatility of conductive hydrogels, the balance between mechanical properties (tensile properties, strength, and toughness) and electrical properties (electrical conductivity, sensitivity, and stability) still faces great challenges. Herein, a simplified method for constructing hydrophobic association hydrogels with excellent mechanical and electrical properties is proposed. The prepared conductive hydrogels exhibit high tensile properties (≈1224%), high linearity in the whole‐strain–range (R2 = 0.999), and a wide strain sensing range (2700%). The conductive hydrogel can realize more than 1000 cycles of sensing under 500% tensile strain. As an application demonstration, an underwater communication device is assembled in combination with polydimethylsiloxane/Triton X‐100 film coating, which successfully transmits underwater signals and provides warning of potential hazards. This study provides a new research method for developing underwater equipment with excellent mechanical properties and sensing properties. [ABSTRACT FROM AUTHOR]

Target signal acquisition and detection based on sonar images is a challenging task due to the complex underwater environment. In order to solve the problem that some semantic information in sonar images is lost and model detection performance is degraded due to the complex imaging environment, we proposed a more effective and robust target detection framework based on deep learning, which can make full use of the acoustic shadow information in the forward-looking sonar images to assist underwater target detection. Firstly, the weighted box fusion method is adopted to generate a fusion box by weighted fusion of prediction boxes with high confidence, so as to obtain accurate acoustic shadow boxes. Further, the acoustic shadow box is cut down to get the feature map containing the acoustic shadow information, and then the acoustic shadow feature map and the target information feature map are adaptively fused to make full use of the acoustic shadow feature information. In addition, we introduce a threshold processing module to improve the attention of the model to important feature information. Through the underwater sonar dataset provided by Pengcheng Laboratory, the proposed method improved the average accuracy by 3.14% at the IoU threshold of 0.7, which is better than the current traditional target detection model. [ABSTRACT FROM AUTHOR]

Summary: Wireless information routing beneath the ocean, or usually underwater, has supplied the most excellent technical methods of oceanic observations. Traditionally, ocean bottoms have been monitored by placing oceanographic sensors that collect data at specific and defined ocean zones. When the duties are performed, the oceanographic instruments are recovered. Because there is no collaborative exchange of collected data between the collecting point and the monitoring end, data cannot be observed remotely. Underwater sensor networks can be wirelessly connected with sensors and monitors without extensive cable. Underwater wireless sensor networks are what they are called (UWSNs). This article investigates the various aspects of UWSNs, such as their significance, applications, network design, demands, routing, deployments and challenges. [ABSTRACT FROM AUTHOR]

The smart management of water resources is an increasingly important topic in today's society. In this context, the paradigm of Smart Water Grids (SWGs) aims at a constant monitoring through a network of smart nodes deployed over the water distribution infrastructure. This facilitates a continuous assessment of water quality and the state of health of the pipeline infrastructure, enabling early detection of leaks and water contamination. Acoustic-wave-based technology has arisen as a viable communication technique among the nodes of the network. Such technology can be suitable for replacing traditional wireless networks in SWGs, as the acoustic channel is intrinsically embedded in the water supply network. However, the fluid-filled pipe is one of the most challenging media for data communication. Existing works proposing in-pipe acoustic communication systems are promising, but a comparison between the different implementations and their performance has not yet been reported. This paper reviews existing works dealing with acoustic-based communication networks in real large-scale urban water supply networks. For this purpose, an overview of the characteristics, trends and design challenges of existing works is provided in the present work as a guideline for future research. [ABSTRACT FROM AUTHOR]

This paper presents an underwater biomimetic covert acoustic communication system that achieves high covertness and a high data rate by mimicking dolphin group whistles. The proposed method uses combined time–frequency shift keying modulation with continuous varying carrier frequency modulation, which mitigates the interference between two overlapping multiple whistles while maintaining a high data rate. The data rate and bit error rate (BER) performance of the proposed method were compared with conventional underwater covert communication through an additive white Gaussian noise channel, a modeled underwater channel, and practical ocean experiments. For the covertness test, the similarity of the proposed multiple whistles was compared with the real dolphin group whistles using the mean opinion score test. As a result, the proposed method demonstrated a higher data rate, better BER performance, and large covertness to the real dolphin group whistles. [ABSTRACT FROM AUTHOR]

Recently, the realization of electromagnetic wave signal transmission and reception has been achieved through the utilization of the magnetoelectric effect, enabling the development of compact and portable low-frequency communication systems. In this paper, we present a miniaturized low-frequency communication system including a transmitter device and a receiver device, which operates at a frequency of 44.75 kHz, and the bandwidth is 1.1 kHz. The transmitter device employs a Terfenol-D (80 mm × 10 mm × 0.2 mm)/PZT (30 mm × 10 mm × 0.2 mm)/Terfenol-D glued composite heterojunction magnetoelectric antenna and the strongest radiation in the length direction, while the receiver device utilizes a manually crafted coil maximum size of 82 mm, yielding a minimum induced electromagnetic field of 1 pT at 44.75 kHz. With an input voltage of 150 V, the system effectively communicates over a distance of 16 m in air and achieves reception of electromagnetic wave signals within 1 m in simulated seawater with a salinity level of 35% at 25 °C. The miniaturized low-frequency communication system possesses wireless transmission capabilities, a compact size, and a rapid response, rendering it suitable for applications in mining communication, underwater communication, underwater wireless energy transmission, and underwater wireless sensor networks. [ABSTRACT FROM AUTHOR]

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This paper describes the hardware development of an underwater communication system using FPGA technology. The hardware device involves implementation of high-performance signal processing that also includes the platform's hardware, HDL, and software designs. A Microzed board is used to implement the underwater communication system which consists of Xlinix Zync-7000. The proposed work is broken down into two steps. The first stage involves creating OFDM Simulink blocks in Matlab, and the second involves transferring these Simulink blocks to SoC boards. 64-QAM modulation is used to map the OFDM symbol. Transmitter and receiver blocks are implemented on two SoC target devices. The data from both devices is transferred using the AXI-stream bus. The SoC devices employ a handshake logic as an acknowledgment mechanism. The findings of Matlab Simulink are compared to the decoded received bits from the SoC device. The performance of the system is presented in terms of BER and the dynamic power utilized by the device. The BER values are calculated theoretically and compared with the practical value of BER and from the result, it is observed that the BER value of the proposed method is 2% lower than the theoretical BER calculations. The Zynq SoC device uses 0.051 W of dynamic power with the logic loaded. The suggested hardware configuration is more useful and appropriate for real-time implementations. [ABSTRACT FROM AUTHOR]

The communication range of magnetic-induction (MI) technology in extreme environments such as underwater or underground is limited by the dipole-like attenuation behavior of the magnetic field as well as the eddy current induced loss in conductive media, and therefore a highly sensitive receiver is generally required. In this work, we propose the use of a highly sensitive superconducting quantum interference device (SQUID) in MI communication and try to provide a comprehensive investigation on developing a SQUID-based receiver for practical MI applications. A portable receiver scheme integrating a SQUID sensor and a coil-based flux transformer was proposed. The high sensitivity and long-range communication capability of the proposed receiver was experimentally demonstrated by spectroscopic measurements and reception experiments on a receiver prototype. Based on the experimental demonstrations, the sensitivity optimization of the proposed scheme was further investigated by simulation studies, which suggest that a communication distance exceeding 100 m and a channel capacity of ∼ 20 kb/s in underwater environment could be achieved based upon the optimization of the developed prototype. The results presented in this work have highlighted the potential of deploying SQUID sensors for long-range MI applications in extreme environments. [ABSTRACT FROM AUTHOR]

Frequency shift chirp modulation (FSCM) is the modulation technique utilized by the LoRa physical layer, a widely used and relatively new communication technique for radio low-power wide area networks (LPWANs). In this paper, we adapt and implement FSCM for low-power acoustic underwater communication. Opposed to over-water communication based on the electromagnetic wave, the acoustic wave in water travels much slower. Consequently, acoustic underwater communication is more sensitive to motion-based Doppler shifts. For resilient acoustic underwater communication, Doppler tracking and removal is mandatory. In this paper, we mathematically derive the effect of Doppler shifts to FSCM symbols. Afterwards, we describe a novel algorithm for a low-power receiver to track Doppler shifts without modification of the transmitted packet. Finally, we evaluate and compare our algorithm to another Doppler estimation algorithm from literature and study the effect on the bit error rate (BER). We show, that our algorithm can improve with little additional computational overhead the BER by a factor of more than 104 in some cases.

Underwater sensor and acoustic networks have several unique applications including water quality and ocean life monitoring, as well as ocean navigation and exploration. They also have peculiar physical layer characteristics with respect to operating frequency and attenuation which makes them different from terrestrial wireless sensor communication. Thus, coupled with their large cost of deployment and sensitivity, they are highly vulnerable to security attacks. For instance, a Sybil node could pretend to be at several other locations in the sparse network simultaneously, thereby deceiving legitimate nodes and infringing on the security of transmitted information. Over the last few years, researchers have studied means of preventing, detecting, and mitigating Sybil attacks for safe underwater communication under different assumptions and architectural setups. However, to our knowledge, these efforts have been scattered in the literature and concrete lessons have not been drawn from these efforts via a survey/review on this subject towards achieving safe underwater communication. This motivates the presentation of this paper that provides an exposition of the academic discussion on the solutions for addressing Sybil attacks in underwater wireless communication, with respect to attack prevention, detection and mitigation while identifying some of their limitations. Similarly, proposed methods and technical aspects peculiar to these works are identified, and a wide range of challenges, opportunities, and recommendations are provided.

As threats in the maritime domain diversify, securing data transmission becomes critical for underwater wireless networks designed for the surveillance of critical infrastructure and maritime border protection. This has sparked interest in underwater Quantum Key Distribution (QKD). In this paper, we present an FPGA-based real-time implementation of an underwater QKD system based on the BB84 protocol. The QKD unit is built on a hybrid computation system consisting of an FPGA and an on-board computer (OBC) interfaced with optical front-ends. A real-time photon counting module is implemented on FPGA. The transmitter and receiver units are powered with external UPS and all system parameters can be monitored from the connected computers. The system is equipped with a visible laser and an alignment indicator to validate successful manual alignment. Secure key distribution at a rate of 100 qubits per second was successfully tested over a link distance of 7 meters.

Abstract Real-time Doppler estimation of sonar signals is of great significance to underwater communication and detection. The Hyperbolic-Frequency-Modulated (HFM) waveform-based speed spectrum estimation is one of the modern methods for underwater Doppler estimation, and this paper presents a detailed analyses of the derivation and the performance of this method. The analyzing results show the robustness of the method on multipath channels and multi-highlight targets and provide the main lobe width of the speed spectrum. Moreover, the paper examines the spectral properties of the HFM signal from the frequency-domain Doppler invariance property, which could provide insights for better understanding and utilization of this Doppler invariant waveform.

With the increasing exploitation and use of marine resources, the limitations of acoustic, optical, and radio frequency technologies for underwater communications have become increasingly apparent. Magnetic induction (MI) is a new communication technology that enables wireless data transmission via magnetic field coupling between transmitting and receiving coils. MI offers advantages such as channel stability, small antenna size, and no multi-path loss. Multi-input–multi-output (MIMO) is a multi-antenna technology that significantly increases system capacity and spectrum utilization without increasing bandwidth. The whale optimization algorithm (WOA) is a well-known bio-inspired algorithm that mimics the hunting behavior of whales to optimize swarm intelligence. This paper proposes a model for an underwater MIMO communication system based on magnetic induction. We then construct a signal detection algorithm for MI-MIMO systems using the advanced whale optimization algorithm (AWOA) and conduct simulation experiments to compare the performance and complexity of three standard signal detection algorithms: zero-forcing (ZF), minimum mean square error (MMSE), and maximum likelihood (ML). The experimental results show that AWOA achieves suboptimal results, as its bit error rate (BER) is close to that of the ML algorithm. Furthermore, the complexity of AWOA is comparable to that of the MMSE strategy. This work supports the development of a high-performance MI-based underwater communication system. [ABSTRACT FROM AUTHOR]

Sensing and communication technology has been used successfully in various event monitoring applications over the last two decades, especially in places where long-term manual monitoring is infeasible. However, the major applicability of this technology was mostly limited to terrestrial environments. On the other hand, underwater wireless sensor networks (UWSNs) opens a new space for the remote monitoring of underwater species and faunas, along with communicating with underwater vehicles, submarines, and so on. However, as opposed to terrestrial radio communication, underwater environment brings new challenges for reliable communication due to the high conductivity of the aqueous medium which leads to major signal absorption. In this paper, we provide a detailed technical overview of different underwater communication technologies, namely acoustic, magnetic, and visual light, along with their potentials and challenges in submarine environments. Detailed comparison among these technologies have also been laid out along with their pros and cons using real experimental results. [ABSTRACT FROM AUTHOR]

Recently, there is an increasing demand for high-speed wireless communication network for short-range underwater communication. From previous research, most underwater antennas produced omnidirectional radiation pattern which has lower antenna gain. There are a few considerations that need to be taken if the antenna is designed to operate in water environment. This paper discusses the electromagnetic properties which affect the underwater antenna design. Physical properties such as electrical permittivity and conductivity of water contribute significant effect to the size of the antenna as it influences the behavior of electromagnetic signal that propagates in water. In this study, an axial mode helical antenna with waterproof container is presented which operates at 433 MHz. The axial mode helical antenna has circular polarization and is suitable to support wireless application which is surrounded by some obstruction. The proposed antenna produces a bidirectional radiation pattern by placing it into a waterproof casing. Good agreement between the simulation and measurement results validates the concept. However, a little discrepancy between the simulated and measured results may be attributed to the noise originated from the equipment and the environment. [ABSTRACT FROM AUTHOR]

In order to achieve underwater wireless dynamic optical communication, a laser communication system is proposed based on Pulse Position Modulation (PPM). In order to achieve underwater laser communication accurately, the mathematical model of underwater laser communication was constructed with small angle analysis. The pulse position modulation demodulation algorithm is designed, and the workflow of modulation and demodulation is given in the transmit module and the receive module. In the experiment, Lumileds-470 nm light source was selected for data communication for testing at a communication rate of 15 Mbp/s. In the servo control process, the square wave signal used for stepping motor drive had a stable amplitude output and a stable time width. It can well simulate the testing process of underwater dynamic scanning. In the experiment, laser light spots were obtained under different attenuation states, and the characteristics of the light spot distribution were analyzed. The numerical reconstruction of the light spot energy was completed in MATLAB. Three types of light attenuators, 1.0%, 0.1%, and 0.01%, were used to simulate different light attenuations underwater. The test results show that the system error rate is better than 10−6 when attenuation chip is 1.0%. When attenuation chip is 0.1%, the error rate of the system is reduced to 10−4. When attenuation chip is 0.01%, a valid signal cannot be obtained by the system. The feasibility of the system is verified.

Swelling is ubiquitous for conventional hydrogels but is not favorable for many situations, especially underwater applications. In this study, an anti‐swelling and mechanically robust polyacrylic acid (PAAc)/gelatin composite hydrogel is reported with a rapid gelation process (101 s) under mild conditions via the synergy of MXene‐activated initiation and zirconium ion (Zr4+)‐induced cross‐linking, without the requirement of external energy input. The MXene is found efficient to activate the chain initiation, while the Zr4+ is prone indispensable for facilitating the cross‐linking of formed polymer chains. The resulting hydrogel exhibits integration of exceptional anti‐swelling properties and high mechanical performance at room temperature, thanks to the dense hydrogen bonds between PAAc and gelatin chains that enable an upper critical solution temperature above room temperature. Also, desirable electrical conductivity emerges in the hydrogel due to the simultaneous contribution of MXene and Zr4+, allowing stable electrical signal output of the gel upon deformation underwater. As a demonstration, an underwater communicator by harnessing the gel as a sensing module is assembled, which is capable of wirelessly delivering messages to the decoder on the ground via Morse codes. This study provides an exemplary way for the rapid gelation of tough and anti‐swelling hydrogels for durable underwater applications. [ABSTRACT FROM AUTHOR]

Cetaceans show high cognitive abilities and strong social bonds. Their primary sensory modality to communicate and sense the environment is acoustics. Research on their echolocation and social vocalizations typically uses visual and tactile systems adapted from research on primates or birds. Such research would benefit from a purely acoustic communication system to better match their natural capabilities. We argue that a full duplex system, in which signals can flow in both directions simultaneously is essential for communication research. We designed and implemented a full duplex system to acoustically interact with cetaceans in the wild, featuring digital echo-suppression. We pilot tested the system in Arctic Norway and achieved an echo suppression of 18 dB. We discuss the limiting factors and how to improve the echo suppression further. The system enabled vocal interaction with the underwater acoustic scene by allowing experimenters to listen while producing sounds. We describe our motivations, then present our pilot deployment and give examples of initial explorative attempts to vocally interact with wild orcas and humpback whales. [ABSTRACT FROM AUTHOR]

Real-time Doppler estimation of sonar signals is of great significance to underwater communication and detection. The Hyperbolic-Frequency-Modulated (HFM) waveform-based speed spectrum estimation is one of the modern methods for underwater Doppler estimation, and this paper presents a detailed analyses of the derivation and the performance of this method. The analyzing results show the robustness of the method on multipath channels and multi-highlight targets and provide the main lobe width of the speed spectrum. Moreover, the paper examines the spectral properties of the HFM signal from the frequency-domain Doppler invariance property, which could provide insights for better understanding and utilization of this Doppler invariant waveform. [ABSTRACT FROM AUTHOR]

The growing interest in developing autonomous underwater vehicles (AUVs) and creating underwater sensor networks (USNs) has led to a need for communication tools in underwater environments. For obvious reasons, wireless means of communication are the most desirable. However, conducting research in real conditions is troublesome and costly. Moreover, as hydroacoustic propagation conditions change very significantly, even during the day, the assessment of proposed underwater wireless communication methods is very difficult. Therefore, in the literature, there are considered simulators based on real measurements of underwater acoustic (UWA) channels. However, these simulators make an assumption that, during the transmission of elementary signals, the impulse response does not change. In this article, the authors present the results of the measurements realized in a towing tank where the transmitter could move with a precisely set velocity and show that the analyzed channel was non-stationary, even during the time of the transmission of a single chirp signal. The article presents an evaluation method of channel stationarity at the time of the chirp transmission, which should be treated as novelty. There is also an analysis of the impulse responses measured in motion in a towing tank. [ABSTRACT FROM AUTHOR]

In this work, a wireless communication system based on magnetic coils for underwater vehicles is presented. Firstly, the mathematical model of magnetic field induction using magnetic coils is discussed. Then, a description of the proposed communication system is presented, including the main components of the transmitter and receiver module. The experimental results show that due to the properties of the magnetic field, the proposed communication system can work properly in different environments such as air or water with the same efficiency. Underwater tests were carried out in different water circumstances: varying the temperature in a range from 10 °C to 35 °C, varying concentrations of clay in a range from 0% to 10%, and varying the salinity concentration in a range from 1000 ppm (parts per million) to 35,000 ppm. It was observed that these conditions do not affect the information transfer. Finally, the advantages of using the proposed system compared to existing submarine communication systems are discussed. [ABSTRACT FROM AUTHOR]