Underwater Optical Communication System Research Articles

Experimental Demostration of a MIMO-OFDM Underwater Optical Communication System for Reducing Alignment Angle Requirements

Experimental Demostration of a MIMO-OFDM Underwater Optical Communication System for Reducing Alignment Angle Requirements

ОРГАНИЗАЦИЯ ПОДВОДНОЙ БЕСПРОВОДНОЙ СИСТЕМЫ СВЯЗИ

The article discusses underwater wireless communication systems. Research in the field of underwater optical wireless communication system will allow developing science, industry, to find solutions to defense tasks and emergencies, to carry out remote monitoring of environmental pollution of the underwater world and control underwater objects and equipment of oilrigs, to conduct underwater research and much more. The purpose of the work is to develop a classification of the principles of construction and organization of underwater wireless communication, taking into account modern achievements and development of technologies in the field of underwater wireless communication systems, to assess the strengths and weaknesses of underwater acoustic, radio frequency and optical communication systems. It is shown that the underwater optical wireless communication system has a great potential to enhance the traditional underwater acoustic communication system and underwater radio frequency communication system due to the high speed of information transmission, low latency, lower power consumption and compact size. To select the equipment and organize the communication channel, a generalized classification of an underwater optical wireless communication system has been developed, depending on the routing protocols used, on the configuration of the communication channel, on the optical properties of water, on the type of water in which the transmission channel is organized, on the mobility of underwater vehicles and the coverage area, on the type of modulation used, the method of underwater communication and from the factors affecting the organization of the communication channel.

Research on a Blue–Green LED Communication System Based on an Underwater Mobile Robot

Underwater robots have been widely used in ocean exploration, deep-sea observation, seabed operations, marine scientific research, and other fields. Underwater low-latency, efficient, and safe communication modes are key to realizing the application of an underwater robot data transmission system. This paper mainly studies the optical communication between underwater mobile robots, including the large-dispersion-angle light-emitting diode (LED) design, large field of view receiving technology, weak light detector technology, etc. By designing a 120° large divergence angle underwater optical communication system in this study, the receiving field-of-view angle of the receiving end can reach 60°, which is suitable for the optical communication system of an underwater mobile platform. The high-power LED driver circuit is designed to drive the high-power LED and adopt weak light detection technology to ensure its stability and reliability. The experimental results show that, in the case of incomplete alignment between the transmitter and receiver, stable communication of underwater robots in motion is achieved through the design of a large divergence angle and a receiving field-of-view angle and the use of an underwater weak light detection technology. The communication distance is 30 m, and the communication rate remains above 10 Mbps. The information transmission content can include network data transmission, real-time video, high-definition video, high-definition images, and other data types. This equipment provides a solution for cableless data transmission of remotely operated vehicles (ROVs) and substantially enhances the application field of ROVs.

Channel characteristics of underwater optical communication system

Underwater communication technology has seen significant development in recent years, with a focus on finding efficient and secure techniques for exploring the underwater world. While acoustic and electromagnetic waves have limitations in water, the use of optical communication has become a focus of research due to its high bandwidth and low delay. Research has shown that blue and green lights are absorbed less than other wavelengths in water, making them suitable for underwater optical communication. In recent years, people have been working with optical communication in harsh environments. The optical system and the acoustic system have been combined to achieve certain development. Several high-speed underwater laser transmission systems have also been developed, with varying transfer rates and distances. This paper focuses on the theory of underwater optical systems and their communication channel characteristics.

Achieving High-Performance Self-Powered Visible-Blind Ultraviolet Photodetection Using Alloy Engineering.

The exploration and development of self-powered visible-blind ultraviolet photodetectors (VBUV PDs) with high responsivity and wavelength selectivity have far-reaching significance for versatile applications. Although In2O3 shows potential for UV detection due to good UV absorption and electrical transport properties, the poor wavelength selectivity impedes further application in VBUV PDs. Here, a self-powered photoelectrochemical-type (PEC) VBUV PD is demonstrated by using gallium-indium oxide alloys (Ga-In OAs). The self-powered Ga-In OAs-based PEC VBUV PDs exhibit good VBUV photodetection performance, including a high responsivity of 50.04 mA/W and a high detectivity of 6.03 × 1010 Jones under 254 nm light irradiation, a good wavelength selectivity (UV/visible light rejection ratio of 262.45), and a fast response time (0.45/0.38 s). The good self-powered VBUV detection performance of Ga-In OAs is attributed to the larger band gap and smaller charge-transfer resistance induced by alloy engineering, which not only suppresses the absorption of visible light but also accelerates interfacial charge transfer. Moreover, an underwater optical communication system is demonstrated by using the self-powered Ga-In OAs PEC VBUV PDs. This study demonstrates that alloy engineering is a powerful tool to improve the performance of In2O3-based PEC PDs, and Ga-In OAs have great application potential for underwater optoelectronic devices.

90-m/660-Mbps Underwater Wireless Optical Communication Enabled by Interleaved Single-Carrier FDM Scheme Combined With Sparse Weight-Initiated DNN Equalizer

In this paper, we first utilize the interleaved single-carrier frequency division multiplexing (I-SC-FDM) scheme with a novel sparse weight-initiated deep neural network (SWI-DNN) equalizer for underwater optical communication (UWOC) systems. In addition to the superior characteristics of I-SC-FDM over the original OFDM and localized SC-FDM (L-SC-FDM), such as lower peak-to-average power ratio (PAPR) and lower computational complexity, the implementation of a special sparse weight-initiated (SWI) structure can significantly reduce the necessary training epochs up to 10.3%, which enables the proposed SWI-DNN equalizer to outperform the traditional random weight-initiated DNN equalizer. Besides, further pruning operation can dramatically enhance the final sparsity of the SWI-DNN equalizer to 96.88% at the expense of an inappreciable performance penalty, thereby effectively saving occupied computing resources. Following this, a data rate of up to 660 Mbps over a 90-m underwater transmission can be achieved in a standard 50-m swimming pool, with a relatively low received optical power of -31.12 dBm (at the BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ). Such a data rate is 17.9% higher than that obtained with a common hybrid time-frequency domain (TFD) equalizer. Moreover, the computational complexity of the SWI-DNN equalizer after pruning operation is only 11.83% of that of the hybrid TFD equalizer and can still support a 625-Mbps transmission. This is the first time to employ I-SC-FDM combined with an SWI-DNN equalizer for long-distance high-speed UWOC transmission, and it can be highly beneficial to the cost-sensitive and power-sensitive systems in future deployment.

Error Characterization of Differential Detection and Non-Differential Detection for MIMO UWOC Systems in Seawater Turbulent Channels

Ocean turbulence is an important factor affecting the development of underwater wireless optical communication (UWOC). To improve the error characteristics of the underwater optical communication system, we propose a differential detection-based multi-receiver-multi-transmitter (MIMO) underwater laser communication transmission method. Additionally, we derive the expressions for calculating the average BER of the MIMO underwater wireless optical communication system with differential detection and non-differential detection in the case that the two transmitted beams are completely uncorrelated. The error characteristics of the MIMO system are simulated and analyzed from the perspective of ocean turbulence intensity and link distance. The simulation results show that the differential detection method has a lower average BER compared to the non-differential detection method in the case of moderate-to-strong ocean turbulence. In addition, the differential detection methods do not have the error floor effect, and non-differential detection methods have the error floor effect. The more the turbulence intensity affects the average BER of the MIMO UWOC system with the increase of the communication link distance, the more obvious is the effect of the turbulence intensity on the average BER of the MIMO UWOC system. Accordingly, the simulation analysis shows that the differential detection method is more suitable for the construction of communication links under long-distance and medium-strong turbulence.

Empirical study of an underwater optical camera communication system under turbulent conditions.

This paper presents an experimental study of the turbulence impact caused by temperature inhomogeneity and air bubbles on a global shutter-based underwater optical camera communication (UOCC). The effects of these two phenomena on UOCC links are illustrated in terms of the intensity variations and an associated reduction in the average received intensity of the illuminated pixels corresponding to the optical source projection and the dispersion of the projection on the captured images. Additionally, it is shown that the area of illuminated pixels in the temperature-induced turbulence scenario is higher than in the bubbly water case. To analyze the effects of those two phenomena on the optical link performance, the signal-to-noise ratio (SNR) of the system is evaluated by considering different points as the regions of interest (ROI) from the light source projection of the captured images. The results indicate that the system performance is improved by averaging over the value of several pixels produced by the point spread function, compared to simply using the central and the maximum pixel value as the ROIs.

Research on underwater wireless dynamic optical communication system based on PPM modulation

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.

Simulation analysis of underwater wireless optical communication based on Monte Carlo method and experimental research in complex hydrological environment

This paper aims to address the challenges of short optical communication distances and high Bit Error Rates (BERs) caused by seawater absorption, scattering, and underwater turbulence. This paper establishes an optical communication channel model based on the Monte Carlo method and simulates and analyzes the process of received optical power attenuation when light is transmitted underwater due to absorption and scattering. The simulation model focuses on the influence of seawater type, transmission distance, receiver position, and angle on the received optical power attenuation. The results of this simulation will provide theoretical support for the development of prototype underwater optical communication systems and experiments in complex environments. To solve the long-range weak signal reception and processing issues in underwater optical wireless communication systems, this paper proposes the design and implementation of an underwater optical wireless communication system using optimized on-off keying high-power light-emitting diode/laser diode modulation technology and avalanche photodiode (APD)/photomultiplier (PMT) receiver modules. In addition, this paper describes the design of the underwater optical communication host computer software, which provides functions such as mode selection, BER monitoring, and file transfer. To test the performance of the proposed system, this paper conducts experiments in a 35 m long pool, fixed platform, and underwater mobile platform, as well as cross-air-water interface optical communication performance test experiments in a complex hydrological environment. The results show that the system achieved a 35 m underwater medium communication distance, transmission rate of 5–20 Mbps, and cross-domain wireless optical communication function under the condition of a BER of less than 10−6. The experiments also revealed that the communication distance decreases as the turbidity of the water body increases and the PMT receiver module selected at the receiver end has a longer communication distance than the APD receiver module. Overall, this paper demonstrates the feasibility of the proposed technology for underwater optical communication systems in complex waters, providing theoretical and data support for the next engineering practice.

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}, 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.

Adaptive Diversity Algorithm Based on Block STBC for Massive MIMO Link Misalignment in UWOC Systems

In the past few years, underwater wireless optical communication (UWOC) has become a promising wireless communication technology in the underwater environment. Aiming at the problem formulation of sub-channel correlation enhancement occurring due to the joint impact of underwater link misalignment and turbulence in the process of optical signal transmission in an underwater optical massive multiple-input multiple-output (MIMO) system, we propose an adaptive diversity approach depending on partition space time block code (STBC). STBC technology is used to reduce the random fading of optical signals caused by turbulence. At the same time, the channel correlation occurring due to channel misalignment is effectively alleviated by adaptive processing. The adaptive diversity algorithm based on segmented STBC effectively improves the reliability and decrease complexity of underwater optical Massive MIMO communication systems. It determines the particular link misalignment degree by the channel gain matrix obtained from the channel estimation and selects different combinations of detectors according to the degree of misalignment to obtain the maximum gain of the received signal combination. Compared with the chunking scheme, simulation and result shows that the adaptive diversity algorithm improves the tolerance of the system to the link misalignment error from 30 mm to 60 mm under the same condition number of channel gain matrix, and it can still demodulate the source signal directly without requiring detection algorithm in case of a large error in the link misalignment.

Propagation of hollow higher-order cosh-Gaussian beam in oceanic turbulence

The averaged received intensity of hollow higher-order cosh-Gaussian (HHOCG) beam propagating in oceanic turbulence is derived based on extended Huygens–Fresnel integral. In detail, the effect of beam parameters and oceanic turbulence parameters on the received intensity is analyzed. Interestingly, beam has a focusing nature along propagation. Our results indicate that received intensity distribution is not affected from the variation in source field parameters. Beam size at the receiver plane can vary according to the changes in turbulence nature. Accordingly, the provided results will contribute to the improvement of both underwater optical communication and imaging systems.

Achieving Record-High Photoelectrochemical Photoresponse Characteristics by Employing Co3O4 Nanoclusters as Hole Charging Layer for Underwater Optical Communication

The physicochemical properties of a semiconductor surface, especially in low-dimensional nanostructures, determine the electrical and optical behavior of the devices. Thereby, the precise control of surface properties is a prerequisite for not only preserving the intrinsic material quality but also manipulating carrier transport behavior for promoting device characteristics. Here, we report a facile approach to suppress the photocorrosion effect while boosting the photoresponse performance of n-GaN nanowires in a constructed photoelectrochemical-type photodetector by employing Co3O4 nanoclusters as a hole charging layer. Essentially, the Co3O4 nanoclusters not only alleviate nanowires from corrosion by optimizing the oxygen evolution reaction kinetics at the nanowire/electrolyte interface but also facilitate an efficient photogenerated carrier separation, migration, and collection process, leading to a significant ease of photocurrent attenuation (improved by nearly 867% after Co3O4 decoration). Strikingly, a record-high responsivity of 217.2 mA W-1 with an ultrafast response/recovery time of 0.03/0.02 ms can also be achieved, demonstrating one of the best performances among the reported photoelectrochemical-type photodetectors, that ultimately allowed us to build an underwater optical communication system based on the proposed nanowire array for practical applications. This work provides a perspective for the rational design of stable nanostructures for various applications in photo- and biosensing or energy-harvesting nanosystems.

Optical Equalization in SiPM-based Underwater Optical Communication System

Optical Equalization in SiPM-based Underwater Optical Communication System

Unified Performance Assessment of Optical Wireless Communication Over Multi-Layer Underwater Channels

In this paper, we model the multi-layer vertical underwater link as a cascaded channel and unify the performance analysis for the underwater optical communication (UWOC) system using generalized Gamma (GG), exponential GG (EGG), exponentiated Weibull (EW), and Gamma-Gamma ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Gamma \Gamma$</tex-math></inline-formula> ) oceanic turbulence models. We derive unified analytical expressions for probability density function (PDF) and cumulative distribution function (CDF) for the signal-to-noise ratios (SNR) considering independent and non-identical (i.ni.d.) turbulent models and zero bore-sight model for pointing errors. We develop performance metrics of the considered UWOC system using outage probability, average bit error rate (BER), and ergodic capacity with asymptotic expressions for outage probability and average BER. We develop the diversity order of the proposed system to provide a better insight into the system performance at a high SNR. We also integrate a terrestrial OWC (TOWC) subjected to the combined effect of generalized Malága atmospheric turbulence, fog-induced random path gain, and pointing errors to communicate with the UWOC link using the fixed-gain amplify-and-forward (AF) relaying. We analyze the performance of the mixed TWOC and multi-layer UWOC system by deriving PDF, CDF, outage probability, and average BER using the bivariate Fox H-function. We use Monte-Carlo simulation results to validate our exact and asymptotic expressions and demonstrate the performance of the considered underwater UWOC system using measurement-based parametric data available for turbulent oceanic channels.

New approach for designing an underwater free-space optical communication system

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

Parametric hologram optimization for enhanced underwater wireless optical communication.

The performance of the underwater optical communication (UWOC) systems was primarily limited by the low optical transmission efficiency due to the beam divergence and water interference. It has been proved in our previous works that holographic beam shaping can effectively increase the optical transmission efficiency and therefore the communication distances and speed. The conventional hologram optimisation method treated each pixel as an independent variable, leading to a large search space and a slow process. In this work, we proposed to use a small set of parameters to describe the beam shaping holograms that were able to limit the beam divergence and compensate for the wavefront distortion. This significantly reduced the number of variables to be optimised and enabled the optimisation to be more efficient and effective. In a proof-of-concept experiment based on the off-the-shelf components, the proposed method was able to generate the optimal hologram within 20 iterations while achieving a tenfold increase in the optical transmission efficiency for a 30 m link at 100 Mbps.

Modeling and performance analysis of oblique underwater optical communication links considering turbulence effects based on seawater depth layering.

Underwater wireless optical communication (UWOC) has been introduced to support emerging high-speed and low latency underwater communication applications. Most of the current studies on UWOC assume that the water temperature and salinity are constant, which can be justified only for horizontal links. In fact, as the temperature and salinity of seawater change with increasing depth, the seawater at different depths is bound to exhibit different optical properties. This implies that for the same link length, the communication system with the transmitter and receiver at different depths, will exhibit different performances. This paper first proposes an oblique optical link model considering turbulence effects, which is based on the layering of temperature and salinity with depth in realistic ocean water. Subsequently, the performance of the optical communication system with vertical and oblique links is analysed by adopting the oceanic power spectrum and seawater data from different ocean areas measured by the global ocean observation buoy, Argo. Our simulation shows that the performance of the underwater optical communication system is worse when the optical transmitter is located at the mixed layer than at the thermocline. When the transmitter is at the thermocline, the communication quality of the system will be worse at environments that temperature and salinity vary more slowly. When the tilt angle of the optical link in the vertical direction is less than 10°, the oblique link can be treated as a vertical link with the same link length.

Pulse Compressor based on Tunable Etched PM-MZI underwater Communication System

Underwater Optical Communication System (UOWC) has been used nowadays in modern optical communication systems. It has many militaries, industry, and scientific community applications. A compressed laser source was obtained in this experiment after using a pulse duration of 9 ns, peak power of 1.2297 mW, full width half maximum (FWHM) 286 pm, and a wavelength of 1546.7 nm. This work used three designs Single, cascaded, and nested, designed based on polarization-maintaining Mach-Zander interferometer (PM-MZI) this fiber etched. All three designs are tested in two different ways. The first structure consists of three variable lengths of (PMF) (8, 16, and 24) cm with etching 30 min. The Second structure has three constants of (PMF) 10 cm lengths with different etching times (10, 20, and 30) min correspond to (112.3, 86.7, 72.8) μm cladding diameter respectively. Four samples of water (seawater, shallow water, coastal water, distilled water) are regarded. This paper studies the effect of the four water samples placed on the fibers used in two structures within different three designs. The results obtained were single fiber, the compression factor FC = 1.1 when the PMF length is 24 cm and thickness cladding is 72.8 μm under shallow water. The peak power is (0.11) PW, the central wavelength is 1546.427 nm, and FWHM is 256 pm. while the cascaded shows FC = (1.5) in distilled water with variable fiber lengths. Finally, in design nested, FC is (1.2) in shallow water with constant fiber lengths.