Underwater Optical Wireless Communications: Overview

Two-way 5G NR FSO-HCF-UWOC converged systems with R/G/B 3-wavelength and SLM-based beam-tracking scheme

A two-way fifth-generation (5G) new radio (NR) free-space optical (FSO)-hollow-core fibre (HCF)-underwater wireless optical communication (UWOC) converged systems with a red/green/blue (R/G/B) 3-wavelengths and spatial light modulator (SLM)-based beam-tracking scheme is practically built. It is the first to practically build a two-way FSO-HCF-UWOC converged system with high-speed and long-distance optical wireless-wired-underwater wireless communication characteristics. It shows a 5G NR FSO-HCF-UWOC convergence from drone or buildings to undersea, using R/G/B 3-wavelengths and an SLM as a demonstration. The R/G/B 3-wavelengths are used to enhance the downstream and upstream aggregate transmission rates. An SLM with electrical comparator is used to adjust the laser beam and mitigate laser beam misalignment caused by drone movement or ocean flow. Over a hybrid of 1-km FSO, 10-m HCF, and 10.44-m ocean water-air-ocean water medium, downstream/upstream 5G-millimeter-wave (MMW) 9.1-Gb/s/24-GHz signals are transmitted with satisfactorily low bit error rates and error vector magnitudes, as well as distinct constellations. This demonstrated that the 5G NR FSO-HCF-UWOC converged system exhibits promising potential as it advances the scenario implemented by the 5G-MMW signals over FSO, HCF, and UWOC convergence, paving the way for high-speed and long-distance communications across diverse media.

Underwater Acoustic Orthogonal Frequency-Division Multiplexing Communication Using Deep Neural Network-Based Receiver: River Trial Results

In this article, a deep neural network (DNN)-based underwater acoustic (UA) communication receiver is proposed. Conventional orthogonal frequency-division multiplexing (OFDM) receivers perform channel estimation using linear interpolation. However, due to the significant delay spread in multipath UA channels, the frequency response often exhibits strong non-linearity between pilot subcarriers. Since the channel delay profile is generally unknown, this non-linearity cannot be modeled precisely. A neural network (NN)-based receiver effectively tackles this challenge by learning and compensating for the non-linearity through NN training. The performance of the DNN-based UA communication receiver was tested recently in river trials in Western Australia. The results obtained from the trials prove that the DNN-based receiver performs better than the conventional least-squares (LS) estimator-based receiver. This paper suggests that UA communication using DNN receivers holds great potential for revolutionizing underwater communication systems, enabling higher data rates, improved reliability, and enhanced adaptability to changing underwater conditions.

Sub-picometer level all-solid-state narrow linewidth single-frequency Pr3+:LiYF4 laser in the near-infrared spectral region

We report an all-solid-state near-infrared single-frequency (single longitudinal mode, SLM) Pr3+:LiYF4 (Pr:YLF) laser with the spectral linewidth at the sub-picometer level. The SLM lasers with center wavelengths of 868 and 907 nm are realized in Pr:YLF crystal for the first time to the best of our knowledge. The maximum output powers of SLM lasers at 868 and 907 nm are 102  and 213mW, corresponding to the narrowest spectral linewidths of 82 MHz (0.21 pm) and 94 MHz (0.26 pm), respectively. At the maximum output power, the beam quality factors in the x and y directions are measured as 1.25 and 1.16 at 868 nm and 1.21 and 1.13 at 907 nm, respectively. The output power stabilities of the 868 and 907 nm SLM lasers are calculated as 1.39% and 0.87%, respectively. The successful realization of 868 and 907 nm all-solid-state SLM lasers makes up for the gap that the Pr:YLF SLM lasers developed in the past are focused on the visible region, enriches the types of near-infrared (NIR) SLM lasers, and can provide practical applications in biomedicine, cold atom physics, and optical atom manipulation.

Analytical propagation model for underwater free-space optical communication through realistic levels of oceanic absorption and scattering

Optical communications (OC) through water bodies is an attractive technology for a variety of applications. Thanks to current single-photon detection capabilities, OC receiver systems can reliably decode very weak transmitted signals. This is the regime where pulse position modulation is an ideal scheme. However, there has to be at least one photon that goes through the pupil of the fore optics and lands in the assigned time bin. We estimate the detectable photon budget as a function of range for propagation through ocean water, both open and coastal. We make realistic assumptions about the water's inherent optical properties, specifically, absorption and scattering coefficients, as well as the strong directionality of the scattering phase function for typical hydrosol populations. We adopt an analytical (hence very fast) path-integral small-angle solution of the radiative transfer equation for multiple forward-peaked scattering across intermediate to large optical distances. Integrals are performed both along the directly transmitted beam (whether or not it is still populated) and radially away from it. We use this modeling framework to estimate transmission of a 1 J pulse of 532 nm light through open ocean and coastal waters. Thresholds for single-photon detection per time bin are a few km and a few 100 m. These are indicative estimates that will be reduced in practice due to sensor noise, background light, turbulence, bubbles, and so on, to be included in future work.

Mechanically controlled multifaceted dynamic transformations in twisted organic crystal waveguides

This study introduces mechanically induced phenomena such as standing, leaning, stacking, and interlocking behaviors in naturally twisted optical waveguiding microcrystals on a substrate. The microscale twisted crystal self-assembled from 2,4-dibromo-6-(((2-bromo-5-fluorophenyl)imino)methyl)phenol is flexible and emits orange fluorescence. Mechanistic analysis reveals the strain generated by the intergrowing orientationally mismatched nanocrystallites is responsible for the twisted crystal growth. The crystal's mechanical flexibility in the perpendicular direction to (001) and (010) planes can be attributed to intermolecular Br···Br, F···Br, and π···π stacking interactions. Through a systematic process involving step-by-step bending and subsequent optical waveguiding experiments at each bent position, a linear relationship between optical loss and mechanical strain is established. Additionally, the vertical standing and leaning of these crystals at different angles on a flat surface and the vertical stacking of multiple crystals reveal the three-dimensional aspects of organic crystal waveguides, introducing light trajectories in a 3D space. Furthermore, the integration of two axially interlocked twisted crystals enables the coupling of polarization rotation along their long axis. These crystal dynamics expand the horizons of crystal behavior and have the potential to revolutionize various applications, rendering these crystals invaluable in the realm of crystal-related science and technology.

A metasurface-based full-color circular auto-focusing Airy beam transmitter for stable high-speed underwater wireless optical communications

Due to its unique intensity distribution, self-acceleration, and beam self-healing properties, Airy beam holds great potential for optical wireless communications in challenging channels, such as underwater environments. As a vital part of 6G wireless network, the Internet of Underwater Things requires high-stability, low-latency, and high-capacity underwater wireless optical communication (UWOC). Currently, the primary challenge of UWOC lies in the prevalent time-varying and complex channel characteristics. Conventional blue Gaussian beam-based systems face difficulties in underwater randomly perturbed links. In this work, we report a full-color circular auto-focusing Airy beams metasurface transmitter for reliable, large-capacity and long-distance UWOC links. The metasurface is designed to exhibits high polarization conversion efficiency over a wide band (440-640 nm), enabling an increased data transmission rate of 91% and reliable 4 K video transmission in wavelength division multiplexing (WDM) based UWOC data link. The successful application of this metasurface in challenging UWOC links establishes a foundation for underwater interconnection scenarios in 6G communication.

Bubbles-induced turbulence channel prediction mechanism based on machine vision in underwater wireless optical communication

Bubbles-induced turbulence poses a significant challenge to the stability of underwater wireless optical communication (UWOC) system. Existing methods for understanding channel characteristics rely on the pilot information from the feed-back channel, which are ineffective and inaccurate due to the rapidly changing nature of the underwater channel. We propose a machine-vision-based channel prediction mechanism which contains three modules of motion judgment module, image processing module and scintillation index (SI) prediction module. The mechanism captures images of bubbles and calculates the bubble density. Subsequently, a relational function is applied to acquire the predicted SI which quantifies the impacts of bubbles on the channel. Experimental results validate the effectiveness of the proposed mechanism.

Liquid-crystal-based visible-light integrated optical phased arrays and application to underwater communications

In this Letter, we present the first, to the best of our knowledge, liquid-crystal-based integrated optical phased arrays (OPAs) that enable visible-light beam forming and steering. A cascaded OPA architecture is developed and experimentally shown to emit a beam in the far field at a 632.8-nm wavelength with a power full width at half maximum of 0.4°×1.6° and 7.2° beam-steering range within ±3.4 V. Furthermore, we show the first visible-light integrated-OPA-based free-space-optical-communications transmitter and use it to demonstrate the first integrated-OPA-based underwater-wireless-optical-communications link. We experimentally demonstrate a 1-Gbps on-off-keying link through water and an electronically-switchable point-to-multipoint link with channel selectivity greater than 19 dB through a water-filled tank.

Developing a comprehensive model for underwater MIMO OCC system

Due to its spatial modulation feature and potential applications, optical camera communication (OCC) has gained significant attention in recent years for a range of applications including underwater. Nonetheless, due to the low frame rates of the camera, the OCC data rate is rather low, which is why multiple-input-multiple-output (MIMO) has been adopted to compensate. In MIMO systems, however, the signal from one light emitting diode (LED) may result in interference on the image sensor (i.e., the camera) resulting in inter-pixel interference (IPI). This paper presents a comprehensive model of the underwater OCC (UOCC) and experimentally verifies its performance under IPI by comparing signal to interference and noise ratio (SINR). The effect of distance between LEDs according to LED diameter D on signal to interference ratio (SIR) is presented and results indicate that coastal water has the SIR gain ∼2.5 dB for the link span of 1 to 6 m, and for harbor water channel length from 0.4 to 1.4 m the gain increased from ∼2 to ∼5 dB for d of 2D compared with d of 0.5D.

High data-rate communication link supported through the exploitation of optical channels in a characterized turbulent underwater environment

Underwater turbulence presents a myriad of challenges for underwater optical systems through wavefront distortion and beam deflection. In this work, an underwater turbulence emulator is developed and thoroughly characterized to experimentally test the proposed underwater turbulence mitigation technique. This technique applies a modified HOBBIT system introduced in atmospheric turbulence to the relatively unknown underwater turbulence domain. By varying a beam's spatial position and relative phase gradient, a volume of turbulence is rapidly probed to determine the beam state for optimal propagation. This probe and control method is applied in multiple facets, including improved optical power transmission as well as supporting a 25-Gbps communication link through a dynamic environment.

Experimental demonstration of a real-time multi-user uplink UWOC system based on SIC-free NOMA

Non-orthogonal multiple access (NOMA) has been studied as a promising multiple access technology for optical communication systems due to its superior spectral efficiency. However, the multi-user communication systems that employ NOMA with successive interference cancellation (SIC) suffer from error propagation (EP). Besides, the issue of non-ideal rise and fall time of the received signal can result in severe bit error rate (BER) degradation while decoding by the SIC technique. In this paper, we propose a straightforward two-stage program judgment filter (PJF) for signal reshaping and a SIC-free decoding method for NOMA. Based on the amplitude threshold (AT) decoding method, we demonstrate a real-time, two-user uplink underwater wireless optical communication (UWOC) system via field programmable gate arrays (FPGAs). With a power allocation ratio (PAR) of 2:1 (user 1: user 2), the established real-time NOMA-based UWOC system utilizing commercial light emitting diodes (LEDs) achieves a data rate of 30 Mbps for each user with BERs of 7.8 × 10-6 and 3 × 10-4 for user 1 and user 2, respectively. The results show that the AT-based NOMA can obtain a lower BER compared to the SIC-based NOMA, especially for user 2.

Equalization equal gain combining for a single-input to multiple-output underwater wireless optical communication system under a Gaussian beam

In this study, we examined the performance of an underwater wireless optical communication (UWOC) system employing a single-input to multiple-output (SIMO) scheme and proposed an equalization equal gain combining (EEGC) algorithm for it under Gaussian beam conditions. Furthermore, based on a Yue spectrum with the instability of oceanic water stratification and a finite outer scale, we derived the closed analytical formulas for the scintillation index and spatial coherence radius in weak oceanic turbulence for a Gaussian beam, from which we could obtain the threshold of the detector spacing and the strength of oceanic turbulence. We then derived the closed-form formula for the upper bound average bit error rate of the EEGC SIMO system with ON-OFF keying modulation by using the hyperbolic tangent distribution function. Our simulations demonstrate two issues if oceanic water stratification is treated as a steady state: the performance of the diversity receiver system will be significantly underestimated in salinity-dominated weak oceanic turbulence channels and will be significantly overestimated in temperature-dominated weak oceanic turbulence channels. Additionally, the SIMO system performance improvement using the proposed EEGC algorithm was more evident with increasing detector spacing, and the EEGC algorithm reduced the impact of the layout of the avalanche photodiode arrays on the UWOC system performance, in contrast to the equal gain combining algorithm.

Chlorophyll-based model underpinned by measured inherent optical properties of Jerlov water types

An existing chlorophyll-based model has been updated and re-calibrated using measured data describing Jerlov water types, harvested from the World-wide Ocean Optics Database. This study has provided new chlorophyll concentration data, and used them in conjunction with recently published spectra of absorption and scattering coefficients to create an updated parameter set that describes eight of the 10 Jerlov water types. The updated model is consistent with other data, and it interprets the measured characteristics in terms of underlying properties. Techniques for inter-conversion between inherent and apparent optical properties have been further investigated, and the improved precision has uncovered new challenges that have been addressed using empirical techniques.

Enhanced blue-green response of nanoarray AlGaAs photocathodes for underwater low-light detection

Underwater optical communication and low-light detection are usually realized via blue-green laser sources and blue-green light-sensitive detectors. Negative-electron-affinity AlGaAs photocathode is an ideal photosensitive material for ocean exploration due to its adjustable spectrum range, long working lifetime, and easy epitaxy of materials. However, compared with other photocathodes, the main problem of AlGaAs photocathode is its low quantum efficiency. Based on Spicer's three-step photoemission model, nanoarray structures are designed on the surface of AlGaAs photocathode to improve its quantum efficiency from two aspects of optical absorption and photoelectron transport. Through simulation, it is concluded that the cylinder with diameter of 120 nm and height of 600 nm is the best nanoarray structure, and its absorptance is always greater than 90% in the 445∼532 nm range. Moreover, the absorptance and quantum efficiency of the cylinder nanoarray AlGaAs photocathode are less affected by the incident angle. When the angle of incident light reaches 70°, the minimum absorptance and quantum efficiency are still 64.6% and 24.9%. In addition, the square or hexagonal arrangement pattern of the nanoarray has little effect on the absorptance, however, a reduction in the overall emission layer thickness will decrease the absorptance near 532 nm.

Photon-counting-based underwater wireless optical communication employing orbital angular momentum multiplexing

Underwater wireless optical communication (UWOC) is a critical technology for underwater communication, providing high speed, low latency, and security advantages. However, the strong attenuation in the water channel still limits the UWOC systems and their performances require further improvement. In this study, an orbital angular momentum (OAM) multiplexing UWOC system that uses photon-counting detection is experimentally demonstrated. By employing a single-photon counting module to receive photon signals, we analyze the bit error rate (BER) and photon-counting statistics by building a theoretical model that fits the real system, and demodulate the OAM states in single photon level and implement signal processing using field programmable gate array (FPGA) programming. Based on these modules, a 2-OAM multiplexed UWOC link is established over a water channel of 9 m. By using on-off keying modulation and 2-pulse position modulation, we achieve a BER of 1.26×10^-3 with data rate of 20Mbps and 3.17×10^-4 with data rate of 10Mbps respectively, which below the forward error correction (FEC) threshold of 3.8×10^-3. The total transmission loss is 37 dB under an emission power of 0.5 mW, which is equivalent to the attenuation of 283 m Jerlov I type seawater from the perspective of energy loss. Our verified communication scheme will benefit the development of long-range and high-capacity UWOC.

Generation of 978 nm dispersion-managed solitons from a polarization-maintaining Yb-doped figure-of-9 fiber laser

Restricted by the narrow gain bandwidth of Yb³⁺ near 980 nm, it is challenging to generate dispersion-managed (DM) solitons at this wavelength. In this work, we demonstrate the generation of DM solitons at 978 nm in a polarization-maintaining (PM) figure-of-9 fiber laser. Highly coherent pulses with 14.4 nm spectral bandwidth and 175 fs pulse duration are experimentally obtained. To the best of our knowledge, this is the shortest ∼980 nm pulse ever reported in an Yb-doped mode-locked fiber laser. Numerical simulations are performed to reveal the DM solitons' temporal and spectral evolution inside the figure-of-9 cavity under the condition of a narrow gain bandwidth. This robust and cost-effective 978 nm femtosecond laser is a promising light source for applications such as underwater communication and biophotonics.

OCDMA transmission-based underwater wireless optical communication system: performance analysis

This paper introduces a novel underwater (UW) optical communication system that utilizes optical code division multiple access transmission technique using the permutation vector code. Three scenarios of water are considered, pure sea (PS), clear ocean (CL), and coastal ocean (CO). The performance is evaluated analytically in terms of bit error rate (BER), received power, signal to noise ratio for different UW links and data rates. The results show that the shortest UW range is achieved in the case of CO, achieving the highest extinction ratio compared to CL and PS. Considering a BER below the forward error correction (FEC) limit of 3 × $${10}^{-3}$$ 10 - 3 , the maximum UW ranges reached are 21 m for PS, 12 m for CL, and 8 m for CO at 3 Gbps with 20 dBm transmitted power. The propagation range could be increased when the transmitted power is increased to 25 dBm, achieving the ranges of 31 m, 18 m, and 9 m, for PS, CL, and CO, respectively.

Fractal superconducting nanowire single-photon detectors working in dual bands and their applications in free-space and underwater hybrid LIDAR

We demonstrate a fiber-coupled fractal superconducting nanowire single-photon detector (SNSPD) system with minimum polarization dependence of detection efficiency. Its system detection efficiency (SDE) was maximized at the wavelength of 1540 nm, which was measured to be 91 ± 4%; furthermore, we observed the second local maximum of SDE at the wavelength of 520 nm, which was measured to be 61 ± 2%. This dual-band feature of SDE was due to the enhancement of the optical absorptance by two longitudinal resonance modes of the micro-cavity. By using high SDE with minimum polarization dependence in these two bands, we implemented a hybrid LIDAR for imaging the remote objects in free space and under water.

Transfer Learning Strategy in Neural Network Application for Underwater Visible Light Communication System

Post-equalization using neural network (NN) is a promising technique that models and offsets the nonlinear distortion in visible light communication (VLC) channels, which is recognized as an essential component in the incoming 6G era. NN post-equalizer is good at modeling complex channel effects without previously knowing the law of physics during the transmission. However, the trained NN might be weak in generalization, and thus consumes considerable computation in retraining new models for different channel conditions. In this paper, we studied transfer learning strategy, growing DNN models from a well-trained 'stem model' instead of exhaustively training multiple models from randomly initialized states. It extracts the main feature of the channel first whose signal power balances the signal-to-noise ratio and the nonlinearity, and later focuses on the detailed difference in other channel conditions. Compared with the exhaustive training strategy, stem-originated DNN models achieve 64% of the working range with five times the training efficiency at most or more than 95% of the working range with 150% higher efficiency. This finding is beneficial to improving the feasibility of DNN application in real-world UVLC systems.

Measured IOPs of Jerlov water types

Inherent optical properties (IOPs) of typical ocean waters have been derived from a worldwide database of measured parameters. The optical quality of the world's oceans can be described in terms of their Jerlov water type, ranging from the clearest Jerlov I to the most turbid Jerlov 9C. These Jerlov classifications are defined in terms of an apparent optical property known as the downwelling diffuse attenuation coefficient (K_d). There is a need to relate these Jerlov water types to their IOPs, namely their absorption coefficient, a, and scattering coefficient, b. However, robust values of a and b for Jerlov water types have not previously existed. This study used the World-wide Ocean Optics Database to derive a series of experimentally measured a and b values for six Jerlov water types. Using data science techniques to group measurements in time and space, over 13.5 million data points were consolidated into 53 measured values for a and b. Established models were subsequently applied to generate a complete table of absorption and scattering coefficients from 300 to 800 nm for Jerlov IB to Jerlov 5C. The analysis includes the influence of changes in the solar zenith angle and the scattering phase function. These data are recommended for use in applications where IOPs are required to describe Jerlov water types.

Performance of hybrid LD/LED system for UWOC link in Baltic Sea

In this paper, the hybrid LD/LED link for the UWOC system is proposed. The Three different wavelengths, namely 715, 532, and 412 nm are adopted as a carrier signal at different water depth variations. A simulation was carried out to assess the quality of the received signal based on bit error rate (BER), quality factor (Q. factor), and data rate. The results indicate that the suggestion system can achieve a good link with different water depths. In addition, the APD PD is better than the PIN PD. The enhancement in the maximum distance link can be achieved with an LD link.

Investigation of hybrid LD/LED system for UWOC link with depth variations

In this paper, An UWOC system using a hybrid LD/LED link is proposed. A simulation was assumed to study the quality of the received optical signal with different modulation schemes such as 64-PPM, 4-QAM, NRZ-OOK, and RZ-OOK. A total of 64-PPM modulation schemes have enhanced the distance link compared with other modulation schemes. PIN and APD PDs are employed to validate the quality of the received optical signal under different water depths. Various metrics of BER, Q. factor, and data rate was introduced to assess the reliability of the hybrid LD/LED link. The simulation results show the proposed system can be achieved a good link with different water depths. The attenuation coefficient of water varies with depth and restricted the performance link distance by hybrid LD/LED link. The 64- PPM is a suitable modulation scheme for getting a long-distance link. The photodetector APD PD outperforms the PIN PD when 64-PPM and 4-QAM are applied.

Optimization and design of a diffuse optical wireless sensor network

Wireless sensor networks (WSNs) are currently being deployed in everyday objects to collect and transmit information related to humidity, temperature, heartbeat, motion, etc. Such networks are part of the massive machine-type communication (mMTC) scenario within the fifth/sixth generation of wireless networks. In this paper, we consider the optimization and design of an optical WSN composed of multiple battery-powered sensor nodes based on light-emitting diode transmitters. Extending our previous work, we take into account both line-of-sight and diffuse-light propagation, and show that in indoor scenarios, diffuse radiation can improve link availability under shadowing/blocking and extend battery life. In order to optimize the optical wireless link parameters, we use a machine-learning approach based on a genetic algorithm to ascertain the performance limits of the system. The presented results indicate that the proposed system is a viable wireless option for WSNs within the context of mMTC.

Anti error and erasure coding for water-to-air visible light communication through wavy water surface with wave height up to 0.6 meters

Considering large dynamic optical intensity range in a water-to-air (W2A) channel, we propose two promising channel coding schemes, namely the concatenated Reed Solomon-Low Density Parity Check (RS-LDPC) code and Raptor code, for W2A visible light communication (VLC). We establish a W2A-VLC link to verify the performance under different wavy water environments and different water depths with a green light emitting diode (LED). A wave generator is adopted to emulate the wavy water surface with wave height up to 0.6 m. The receiver is fixed 3.2 m above the water, and the transmitter varies from 2.5 m to 4.0 m under the water through a up-down-moveable platform. We test the coding schemes with different code lengths and code rates under 5 MSym/s air-interface symbol rate. Experimental results show that both schemes can reduce the bit error ratio (BER) and frame error rate (FER) of a W2A-VLC system, and thus can improve the reliability. Via comparing the two codes with the same overhead and approximately the same code length, it is demonstrated that Raptor code can generally outperform the concatenated RS-LDPC code. Our research provides promising channel coding methods without feedback for a W2A-VLC system.

Underwater blue-green LED communication using a double-layered, curved compound-eye optical system

Optical receiving systems with single-lens structures have problems such as low receiving efficiency and small field of view when applied to underwater optical wireless communication systems. In this study, a design scheme for a double-layered fly-eye-lens optical system with wide-angle focusing is proposed. Based on the analysis of the LED light source transmission model and seawater channel, the optical-power receiving equation of the fly-eye lens system is deduced. The fly-eye-lens receiving system was designed and simulated using Zemax according to the geometrical optics principle of the lens array. An experimental device for measuring the insertion loss and receiving efficiency of an underwater blue-green LED communication system was built, and the optical power of the receiving optical system was experimentally measured. For the link distances of 1, 3, and 5 m, the received optical power of the double-layered-compound eye system was higher than that of the single-layered system, with a power increase of 72%, 65%, and 60%, respectively. The results show that the double-layered fly-eye-lens receiving antenna can significantly improve the optical power received by the receiving end; therefore, this antenna structure has strong practicability and good development prospects in the field of underwater optical wireless communication.

GaN Laser Diode Technology for Visible-Light Communications

Gallium nitride (GaN) laser diodes (LDs) are considered for visible light communications (VLC) in free space, underwater, and in plastic optical fibers (POFs). A review of recent results is presented, showing high-frequency operation of AlGaInN laser diodes with data transmission rates up to 2.5 Gbit/s in free space and underwater and high bandwidths of up to 1.38 GHz through 10 m of plastic optical fiber. Distributed feedback (DFB) GaN LDs are fabricated to achieve single-frequency operation. We report on single-wavelength emissions of GaN DFB LDs with a side-mode suppression ratio (SMSR) in excess of 35 dB.

Underwater optical wireless communication performance enhancement using 4D 8PAM trellis-coded modulation OFDM with DFT precoding

A 4D 8-pulse amplitude modulation trellis-coded modulation-based orthogonal frequency division multiplexing (4D 8PAM TCM-OFDM) scheme combined with discrete Fourier transform (DFT) precoding is proposed and experimentally demonstrated in an underwater optical wireless communications system. It can resist power fading caused by the attenuation, scattering, and reflection in the water channel. The experimental results show that, after transmission over the water-air channel and at the bit error rate of 3.8×10^-3, the improvement in the receiver sensitivity is 0.88 dB using the 4D 8PAM TCM-OFDM scheme, compared to the conventional 64QAM-OFDM scheme. In addition, the proposed 4D 8PAM TCM-OFDM scheme combined with DFT precoding can compensate for unbalanced impairments, and it can obtain 1.28 dB receiver sensitivity improvement, compared to the conventional 64QAM-OFDM scheme with DFT precoding.

Deep Reinforcement Learning Based Optical and Acoustic Dual Channel Multiple Access in Heterogeneous Underwater Sensor Networks

In this paper, we investigate how to efficiently utilize channel bandwidth in heterogeneous hybrid optical and acoustic underwater sensor networks, where sensor nodes adopt different Media Access Control (MAC) protocols to transmit data packets to a common relay node on optical or acoustic channels. We propose a new MAC protocol based on deep reinforcement learning (DRL), referred to as optical and acoustic dual-channel deep-reinforcement learning multiple access (OA-DLMA), in which the sensor nodes utilizing the OA-DLMA protocol are called agents, and the remainder are non-agents. The agents can learn the transmission patterns of coexisting non-agents and find an optimal channel access strategy without any prior information. Moreover, in order to further enhance network performance, we develop a differentiated reward policy that rewards specific actions over optical and acoustic channels differently, with priority compensation being given to the optical channel to achieve greater data transmission. Furthermore, we have derived the optimal short-term sum throughput and channel utilization analytically and conducted extensive simulations to evaluate the OA-DLMA protocol. Simulation results show that our protocol performs with near-optimal performance and significantly outperforms other existing protocols in terms of short-term sum throughput and channel utilization.

A Systematic Review on Recent Trends, Challenges, Privacy and Security Issues of Underwater Internet of Things

Owing to the hasty growth of communication technologies in the Underwater Internet of Things (UIoT), many researchers and industries focus on enhancing the existing technologies of UIoT systems for developing numerous applications such as oceanography, diver networks monitoring, deep-sea exploration and early warning systems. In a constrained UIoT environment, communication media such as acoustic, infrared (IR), visible light, radiofrequency (RF) and magnet induction (MI) are generally used to transmit information via digitally linked underwater devices. However, each medium has its technical limitations: for example, the acoustic medium has challenges such as narrow-channel bandwidth, low data rate, high cost, etc., and optical medium has challenges such as high absorption, scattering, long-distance data transmission, etc. Moreover, the malicious node can steal the underwater data by employing blackhole attacks, routing attacks, Sybil attacks, etc. Furthermore, due to heavyweight, the existing privacy and security mechanism of the terrestrial internet of things (IoT) cannot be applied directly to UIoT environment. Hence, this paper aims to provide a systematic review of recent trends, applications, communication technologies, challenges, security threats and privacy issues of UIoT system. Additionally, this paper highlights the methods of preventing the technical challenges and security attacks of the UIoT environment. Finally, this systematic review contributes much to the profit of researchers to analyze and improve the performance of services in UIoT applications.

Review of Underwater Sensing Technologies and Applications

As the ocean development process speeds up, the technical means of ocean exploration are being upgraded. Due to the characteristics of seawater and the complex underwater environment, conventional measurement and sensing methods used for land are difficult to apply in the underwater environment directly. Especially for the seabed topography, it is impossible to carry out long-distance and accurate detection via electromagnetic waves. Therefore, various types of acoustic and even optical sensing devices for underwater applications have come into use. Equipped by submersibles, those underwater sensors can sense underwater wide-range and accurately. Moreover, the development of sensor technology will be modified and optimized according to the needs of ocean exploitation. This paper has made a summary of the ocean sensing technologies applied in some critical underwater scenarios, including geological surveys, navigation and communication, marine environmental parameters, and underwater inspections. In order to contain as many submersible-based sensors as possible, we have to make a trade-off on breadth and depth. In the end, the authors predict the development trend of underwater sensor technology based on the future ocean exploration requirements.

32 × 32 Pixelated High-Power Flip-Chip Blue Micro-LED-on-HFET Arrays for Submarine Optical Communication

This work proposes the use of integrated high-power InGaN/GaN multiple-quantum-well flip-chip blue micro light-emitting diode (μ-LED) arrays on an AlGaN/GaN-based heterojunction field-effect transistor (HFET), also known as a high electron mobility transistor (HEMT), for various applications: underwater wireless optical communication (UWOC) and smart lighting. Therefore, we demonstrate high-power μ-LED-on-HEMT arrays that consist of 32 × 32 pixelated μ-LED arrays and 32 × 32 pixelated HEMT arrays and that are interconnected by a solder bump bonding technique. Each pixel of the μ-LED arrays emits light in the HEMT on-state. The threshold voltage, the off-state leakage current, and the drain current of the HEMT arrays are -4.6 V, <~1.1 × 10^-9 A at gate-to-source voltage (V_GS) = -10 V, and 21 mA at V_GS = 4 V, respectively. At 12 mA, the forward voltage and the light output power (LOP) of μ-LED arrays are ~4.05 V and ~3.5 mW, respectively. The LOP of the integrated μ-LED-on-HEMT arrays increases from 0 to ~4 mW as the V_GS increases from -6 to 4 V at V_DD = 10 V. Each pixel of the integrated μ-LEDs exhibits a modulated high LOP at a peak wavelength of ~450 nm, showing their potential as candidates for use in UWOC.

Emerging IoT domains, current standings and open research challenges: a review

Over the last decade, the Internet of Things (IoT) domain has grown dramatically, from ultra-low-power hardware design to cloud-based solutions, and now, with the rise of 5G technology, a new horizon for edge computing on IoT devices will be introduced. A wide range of communication technologies has steadily evolved in recent years, representing a diverse range of domain areas and communication specifications. Because of the heterogeneity of technology and interconnectivity, the true realisation of the IoT ecosystem is currently hampered by multiple dynamic integration challenges. In this context, several emerging IoT domains necessitate a complete re-modeling, design, and standardisation from the ground up in order to achieve seamless IoT ecosystem integration. The Internet of Nano-Things (IoNT), Internet of Space-Things (IoST), Internet of Underwater-Things (IoUT) and Social Internet of Things (SIoT) are investigated in this paper with a broad future scope based on their integration and ability to source other IoT domains by highlighting their application domains, state-of-the-art research, and open challenges. To the best of our knowledge, there is little or no information on the current state of these ecosystems, which is the motivating factor behind this article. Finally, the paper summarises the integration of these ecosystems with current IoT domains and suggests future directions for overcoming the challenges.

On the Noise Effect of Fingerprinting-Based Positioning Error in Underwater Visible Light Networks

This paper assesses the performance of a localization technique for underwater visible light networks. The proposed approach is based on a fingerprinting technique, collecting the channel impulse responses from different wireless optical signals in the visible range. A local database related to the power level distribution within a maritime environment is built and exploited to estimate user position, e.g., a diver moving in a given space for underwater fish monitoring. In this paper, we investigate on the noise effect on the localization accuracy in underwater scenarios and for different water turbidity coefficient and we demonstrate that the estimation error suffers on variable channel impulse responses. Different configuration parameters and environmental scenarios have been considered, showing that the LED transmitter deployment can be effective in the localization estimation. A comparison of the proposed localization approach to the traditional triangulation method has been finally carried out, showing the effectiveness of the fingerprinting-based solution for a lower number of LED transmitters.

Investigation of the Scattering Noise in Underwater Optical Wireless Communications

In underwater optical wireless communications (UOWC), scattering of the propagating light beam results in both intensity and phase variations, which limit the transmission link range and channel bandwidth, respectively. Scattering of photons while propagating through the channel is a random process, which results in the channel-dependent scattering noise. In this work, we introduce for the first time an analytical model for this noise and investigate its effect on the bit error rate performance of the UOWC system for three types of waters and a range of transmission link spans. We show that, for a short range of un-clear water or a longer range of clear water, the number of photons experiencing scattering is high, thus leading to the increased scattering noise. The results demonstrate that the FEC limit of 3×10−3 and considering the scattering noise, the maximum link spans are 51.5, 20, and 4.6 m for the clear, coastal, and harbor waters, respectively.

Evaluation of Misalignment Effect in Vehicle-to-Vehicle Visible Light Communications: Experimental Demonstration of a 75 Meters Link

The use of visible light communications technology in communication-based vehicle applications is gaining more and more interest as the research community is constantly overcoming challenge after challenge. In this context, this article addresses the issues associated with the use of Visible Light Communications (VLC) technology in Vehicle-to-Vehicle (V2V) communications, while focusing on two crucial issues. On the one hand, it aims to investigate the achievable communication distance in V2V applications while addressing the least favorable case, namely the one when a standard vehicle rear lighting system is used as a VLC emitter. On the other hand, this article investigates another highly unfavorable use case scenario, i.e., the case when two vehicles are located on adjacent lanes, rather than on the same lane. In order to evaluate the compatibility of the VLC technology with the usage in inter-vehicle communication, a VLC prototype is intensively evaluated in outdoor conditions. The experimental results show a record V2V VLC distance of 75 m, while providing a Bit Error Ratio (BER) of 10⁻⁷–10⁻⁶. The results also show that the VLC technology is able to provide V2V connectivity even in a situation where the vehicles are located on adjacent lanes, without a major impact on the link performances. Nevertheless, this situation generates an initial no-coverage zone, which is determined by the VLC receiver reception angle, whereas in some cases, vehicle misalignment can generate a BER increase that can go up to two orders of magnitude.

Underwater localization system based on visible-light communications using neural networks

Underwater localization using visible-light communications is proposed based on neural networks (NNs) estimation of received signal strength (RSS). Our proposed work compromises two steps: data collection and NN training. First, data are collected with the aid of Zemax OpticStudio Monte Carlo ray tracing software, where we configure 40,000 receivers in a $100\;{\rm m} \times 100\;{\rm m}$ area in order to measure the channel gain for each detector in seawater. The channel gains represent the input data set to the NN, while the output of the NN is the coordinates of each detector based on the RSS intensity technique. Next, an NN system is built and trained with the aid of Orange data mining software. Several trials for NN implementations are performed, and the best training algorithms, activation functions, and number of neurons are determined. In addition, several performance measures are considered in order to evaluate the robustness of the proposed network. Specifically, we evaluate the following parameters: classification accuracy (CA), area under the curve (AUC), training time, testing time, F1, precision, recall, logloss, and specificity. The corresponding measures are as follows: 99.1% for AUC and 98.7% for CA, F1, precision, and recall. Further, the performance results of logloss and specificity are 7.3% and 99.3% respectively.

6G Enabled Smart Infrastructure for Sustainable Society: Opportunities, Challenges, and Research Roadmap

The 5G wireless communication network is currently faced with the challenge of limited data speed exacerbated by the proliferation of billions of data-intensive applications. To address this problem, researchers are developing cutting-edge technologies for the envisioned 6G wireless communication standards to satisfy the escalating wireless services demands. Though some of the candidate technologies in the 5G standards will apply to 6G wireless networks, key disruptive technologies that will guarantee the desired quality of physical experience to achieve ubiquitous wireless connectivity are expected in 6G. This article first provides a foundational background on the evolution of different wireless communication standards to have a proper insight into the vision and requirements of 6G. Second, we provide a panoramic view of the enabling technologies proposed to facilitate 6G and introduce emerging 6G applications such as multi-sensory–extended reality, digital replica, and more. Next, the technology-driven challenges, social, psychological, health and commercialization issues posed to actualizing 6G, and the probable solutions to tackle these challenges are discussed extensively. Additionally, we present new use cases of the 6G technology in agriculture, education, media and entertainment, logistics and transportation, and tourism. Furthermore, we discuss the multi-faceted communication capabilities of 6G that will contribute significantly to global sustainability and how 6G will bring about a dramatic change in the business arena. Finally, we highlight the research trends, open research issues, and key take-away lessons for future research exploration in 6G wireless communication.

Bessel Beam: Significance and Applications-A Progressive Review

Diffraction is a phenomenon related to the wave nature of light and arises when a propagating wave comes across an obstacle. Consequently, the wave can be transformed in amplitude or phase and diffraction occurs. Those parts of the wavefront avoiding an obstacle form a diffraction pattern after interfering with each other. In this review paper, we have discussed the topic of non-diffractive beams, explicitly Bessel beams. Such beams provide some resistance to diffraction and hence are hypothetically a phenomenal alternate to Gaussian beams in several circumstances. Several outstanding applications are coined to Bessel beams and have been employed in commercial applications. We have discussed several hot applications based on these magnificent beams such as optical trapping, material processing, free-space long-distance self-healing beams, optical coherence tomography, superresolution, sharp focusing, polarization transformation, increased depth of focus, birefringence detection based on astigmatic transformed BB and encryption in optical communication. According to our knowledge, each topic presented in this review is justifiably explained.

An AUV-Aided Cross-Layer Mobile Data Gathering Protocol for Underwater Sensor Networks

Underwater sensor networks (UWSNs) have recently attracted much attention due to their ability to discover and monitor the aquatic environment. However, acoustic communication has posed some significant challenges, such as high propagation delay, low available bandwidth, and high bit error rate. Therefore, proposing a cross-layer protocol is of high importance to the field to integrate different communication functionalities (i.e, an interaction between data link layer and network layer) to interact in a more reliable and flexible manner to overcome the consequences of applying acoustic signals. In this paper, a novel Cross-Layer Mobile Data gathering (CLMD) scheme for Underwater Sensor Networks (UWSNs) is presented to improve the performance by providing the interaction between the MAC and routing layers. In CLMD, an Autonomous Underwater Vehicle (AUV) is used to periodically visit a group of clusters which are responsible for data collection from members. The communications are managed by using a distributed cross-layer solution to enhance network performance in terms of packet delivery and energy saving. The cluster heads are replaced with other candidate members at the end of each operational phase to prolong the network lifetime. The effectiveness of CLMD is verified through an extensive simulation study which reveals the performance improvement in the energy-saving, network lifetime, and packet delivery ratio with varying number of nodes. The effects of MAC protocols are also studied by studying the network performance under various MAC protocols in terms of packet delivery ratio, goodput, and energy consumption with varying density of nodes.