Attenuation analysis of an underwater optical wireless communication system, based on red light-emitting diodes, in turbid water with preliminary uncertainty analysis

Underwater wireless optical communication (UWOC) has been widely studied as a key technology for ocean exploration and exploitation. However, current UWOC systems neglect semantic information of transmitted symbols, leading to unnecessary consumption of communication resources for transmitting non-essential data. In this paper, we propose and demonstrate a deep-learning-based underwater wireless optical semantic communication (UWOSC) system for image transmission. By utilizing a deep residual convolutional neural network, the semantic information can be extracted and mapped into the transmitted symbols. Moreover, we design a channel model based on long short-term memory network and employ a two-phase training strategy to ensure that the system matches the underwater channel. To evaluate the performance of the proposed UWOSC system, we conduct a series of experiments on an emulated UWOC experimental platform, in which the effects of different turbidity channel environments and bandwidth compression ratios are investigated. Experimental results show that the UWOSC system exhibits superior performance compared to the conventional communication schemes, particularly in challenging channel environments and low bandwidth compression ratios.

Underwater optical wireless communication system supplies extraordinary enthusiasm to the military, industry, and mainstream researchers, as it assumes a significant job in strategic observation, contamination checking, oil control and upkeep, seaward investigations, environmental change seeing, and oceanography research. To encourage every one of these exercises, there is an expansion in the quantity of unmanned vehicles or gadgets conveyed submerged, which require high data transmission and high limit with regards to data move submerged. Even if massive advancement has been made in the field of acoustic correspondence submerged, be that as it may, it is restricted by data transfer ability. This has prompted the expansion of underwater optical wireless communication system as it gives higher information rates than the customary acoustic correspondence frameworks with fundamentally lower power use and less difficult computational complexities for short-extend remote connections. Underwater optical wireless communication has numerous potential applications running from profound seas to beach front waters. Be that as it may, the greatest test for underwater optical wireless communication system from the principal attributes of sea or ocean water. Tending to these difficulties requires an exhaustive comprehension of complex physic-compound natural frameworks. This paper gives a comprehensive outline of ongoing advances in underwater optical wireless communication. Channel representation, parameter plans, suitable wavelength of underwater optical wireless communication, light used for optical communication, various noise sources, analysis of received and transmitted power are discussed for to underwater optical wireless communication. This paper not just gives comprehensive examination in underwater optical wireless communication system in addition intends to give the improvement of new thoughts that would help in the development of future underwater optical wireless communication.

In this paper, we investigate the photodetector responsivity, on the performance of underwater optical wireless communication (UOWC) systems. We examine its impact on the communication link performance in terms of bit-error-rate (BER) and Signal-to-noise ratio (SNR). This experiment is conducted for different types of photodiodes, to highlight the practical limitations of UWOC links in the presence of variable photodetector responsivity. We show how the photodiode responsivity can impact the performance of UWOC links for relatively less input power. The results provide valuable insight into the design of underwater optical wireless communication links, which are likely to be established at relatively less input power. They can be exploited not only for practical underwater optical wireless communication system deployment but also for the in-pool experimental set-ups since they elucidate the effect of photodiode responsivity on the measurements.

Electrowetting on dielectric (EWOD) is used for non-mechanical optical beam steering (OBS) in optical communication systems. High-capacitance ion gel is used to construct an efficient electrowetting interface that facilitates dynamic OBS. This integration facilitates precise control of beam steering and data transmission efficiency in optical wireless communication systems. An EWOD-based liquid prism (LP) manipulates beam direction via electrowetting. The theoretical framework is supported by the Young and Young-Lippmann equations for precise optical path control. We present a theoretical and experimental demonstration of a two-dimensional beam steering system using an EWOD-based LP, with beam steering up to 14.82° and 14.39° along the X and Y axes, respectively. The system achieves data rates of 1.9 Gbps in free-space optics (FSO) and 1.7 Gbps in underwater wireless optical communication (UWOC) systems, with a measured bit error rate that adheres to the standard threshold of the forward error correction limit. Our results suggest that the EWOD-based LP system offers a compact, efficient, and versatile design for optical devices in both FSO and UWOC systems.

Increasing interest in oceans increases the scientific, commercial and military studies in this field, in which underwater wireless communication systems are frequently used. In literature, underwater wireless, synchronous, high speed data transfer can only be done with optical systems. The distorting effects of underwater environment (UE) on optical waves significantly affect the communication distance and speed of underwater wireless optical communication (UWOC) systems. Therefore, in order to design reliable UWOC systems, it is important to examine effects of various UEs on UWOC systems on a realistic channel model. In this study, instead of the approximate channel models, Monte Carlo approach UWOC channel model, which is accepted in literature with its high accuracy and sensitivity, was used, and effects of the transmitter's beam divergence angle and aperture diameter of receiver parameters, which are important design parameters for UWOC systems, on a typical UWOC system were investigated. In this study, three various UEs, namely clean ocean, coastal ocean and harbor waters, which are frequently used as benchmarks in the literature, were taken into consideration, and channel impulse responses, signal to noise ratios and channel capacity distributions were presented and interpreted for each UE. With the results obtained, the communication speed and distance limits of a typical UWOC system were determined, and a prediction was provided for UWOC systems that can be used in various UEs and parameter values.

The present study proposed a high-data-rate underwater optical wireless communication (UOWC) system to propagate the laser blue–green waves through water. The presented study not only focuses on analysis of challenges in UOWC link including attenuation, absorption, scattering and turbulence model, but also investigates the performance of the proposed system using two different methods of balanced modulation schemes. Spectrum efficiency of the system can be improved by using appropriate modulation formats. Return-to-zero differential phase shift keying (RZ-DPSK) and non-return-to-zero differential phase shift keying (NRZ-DPSK) schemes are two modulation formats that we investigate them to improve the characteristics of the proposed UOWC system. The paper explains a real model and exhaustive analysis for advanced UOWC works by using channel model and modulation formats for presented underwater link. Performance of the proposed system under different modulation schemes and physical aspects of UOWC is studied with several parameters like max quality factor, min bit error rate (BER) and eye diagram. For clear ocean, the performance of the proposed system is good and min BER less than 10−90 for two modulation formats. Generally, results at different condition show that the operation of NRZ-DPSK modulation has better performance than RZ-DPSK scheme.

Various physical and chemical properties of seawater degrade the performance of the underwater optical wireless communication (UOWC) system. Implementing a forward error correction (FEC) coding scheme, advanced modulation, and signal processing techniques improves the UOWC system performance. So far, the implementation of FEC codes over the UOWC system has not been discussed with the system receiver (Rx) orientations. The undersea background noise is a challenge to the UOWC system, and the solar noise is robust in this category. The system Rx experiences unequal solar noise density when it faces different orientations. This paper evaluates the performance of the UOWC system, implementing the Reed-Solomon (RS) coding scheme when the Rx meets different orientations, and presents a computational model to find out energy-efficient RS code for a specific direction. The theoretical evaluation demonstrates that a fixed RS code is not always energy-efficient to the system when the Rx moves in various orientations.

Pulse position modulation (PPM) technology combined with the system of wireless optical communication received by the photon detector has the advantages of high energy efficiency and strong anti-interference capability. This technology has received extensive attention in the field of underwater wireless optical communication (UWOC) system. Affected by ocean turbulence, the UWOC system will produce the intensity fluctuations, leading the system performance to degrade. The Gamma-gamma intensity fluctuation probability model, which is a two-parameter model, possesses a wide range of applications. It can describe weak, medium and strong fluctuation in light intensity statistics. In this paper, firstly, based on the relationship between the weak atmospheric turbulent spherical wave scintillation index and the weak ocean anisotropic turbulent spherical wave scintillation index, the equivalent structural parameter expressed by both ocean turbulence parameters and anisotropy factor is derived. Then, using the structural parameter combined with the gamma-gamma turbulence channel and the asymptotic Rytov theory, the bit error rate (BER) under anisotropic ocean turbulence is calculated based on the BER formula of the PPM communication system. Finally, numerical simulations are carried out to analyze the ocean turbulence parameters, the average avalanche photodiode (APD) gain, the PPM modulation order, the data bit rate, and the influences of transmission distance on the BER under different anisotropic ocean turbulences. The results indicate that the negative effect of turbulence becomes stronger with increasing the ratio between the contributions of temperature and salinity to the refractive index spectrum, the dissipation rate of mean-squared temperature, data bit rate, and propagation distance. As the viscosity coefficient increases, the BER decreases. When the isotropic ocean turbulence and the anisotropy factors are very small, the increase of the rate of dissipation of kinetic energy per unit mass of fluid will result in a decrease in BER. When the turbulent environment anisotropy is further strengthened, the BER first increases and then decreases as the rate of dissipation of kinetic energy per unit mass of fluid increases. As the average APD gain increases, the BER first decreases and then increases. This trend is especially noticeable as the anisotropy factor increases. The choice of the average APD gain is important for finding the minimum value of the BER. In general, the system is more affected by salinity fluctuation than by temperature fluctuation. As the rate of dissipation of mean-squared temperature increases and the viscosity coefficient decreases, the negative effects of turbulence becomes more and more serious. When the system propagates longer distances or works at a higher data bit rate, the system is severely affected by turbulence, which limits the system operating distance and data transmission rate. However, using a smaller modulation order and choosing the right APD can conduce to improving the system performance. In addition, the PPM UWOC system can perform better when the system operates within acceptable bit error rate as the ocean turbulence environment becomes more anisotropic. This study will provide reference for the construction and performance estimation of UWOC system platform.

Coherent demodulated underwater wireless optical communication system based on convolutional neural network

We provide an experimental study of the channel characteristics in an underwater wireless optical communication (UWOC) system given by a 35-m transmission distance in various water types. A UWOC system using a low-power 488-nm laser diode is established to comprehensively evaluate the influence of the link distance, water turbidity, receiver parameters, and link misalignment on the communication link power. The results show that the link misalignment-induced power loss is significant and the effect of receiver parameters on power is limited. However, a higher turbidity or longer signal transmission distance can help to reduce the link misalignment effect and manifest the receiver parameters effect on the received power. In particular, in turbid waters with an attenuation coefficient of 0.471 m − 1, the change of the signal reception power curve is small over a certain degree of the link misalignment in the 35-m physical distance, and the reasonable configuration of receiver parameters effectively improves the signal reception quality in the UWOC system. These results can provide theoretical guidance for optimizing the UWOC system.

Probabilistic constellation shaping (PCS) is utilized to approach the channel capacity limit in discrete multitone (DMT) transmission for underwater optical wireless communication (UOWC) system. A fixed quadrature amplitude modulation (QAM) format with various probabilistic distributions is individually allocated for different subcarriers to obtain achievable maximum channel capacity in accordance with the pre-estimated signal-to-noise ratio. By using a 450-nm directly modulated laser diode (LD) with an available modulation bandwidth of ∼2.75 GHz, DMT with PCS technique is experimentally realized with a net data rate of 18.09Gbit/s over 5m, 17.21Gbit/s over 25m, and 12.62Gbit/s over 35m underwater transmission, giving substantial capacity improvement of 32.22%, 30.03%, and 27.55%, respectively, in comparison with the widely used regular QAM formats in DMT with bit-power loading scheme. The figure of merit of the UOWC system in terms of entropy, generalized mutual information (GMI), and normalized GMI are also presented. To the best of our knowledge, this is the first time to employ PCS-QAM-DMT in a UOWC system, and it is also the highest data rate ever reported for a single LD in UOWC.

In this paper, we first analyze the factors that affect the capacity of underwater optical wireless communication (UOWC) systems through deriving a new tight capacity upperbound. We find that the system capacity depends on the light wavelength in a complicated manner. Then we compare UOWC with the underwater acoustic communication (UAC) technology in terms of channel capacity, communication range, and energy efficiency. We show that UOWC requires a much lower energy-per-bit than UAC for short range communication. Finally, we study the multi-hop communication technique to extend the range of UOWC. The optimal number of hops is derived taking into account the cost of deploying relay nodes. Our study provides useful guidelines in designing a hybrid underwater acoustic/optical communication system which can achieve an increased range-rate product for underwater wireless communication.

The impact of temperature gradients on the average bit error rate (ABER) performance of low-density parity-check (LDPC)-coded underwater wireless optical communication (UWOC) systems is investigated over the generalized gamma fading channels. The effects of absorption and scattering are also taken into account in our computation on the basis of the Monte Carlo (MC) ray-tracing method. The decoded-and-forward multihop communication is applied to extend the viable communication range of UWOC systems. In particular, using the generalized Gauss–Laguerre quadrature rule, the ABER analytical expressions of the multihop UWOC system with binary phase shift keying and multiple phase shift keying modulation schemes are mathematically derived under weak temperature-induced oceanic turbulence condition, respectively, which are also confirmed by MC simulation. In addition, LDPC codes are adopted to improve the UWOC system performance. The results reveal that the ABER performance of this multihop UWOC system degrades with increasing temperature gradients, hop numbers, and modulation orders, while it can be significantly enhanced by LDPC codes and the coding gains increase with the increase of temperature gradients and hop numbers. Our work will benefit the design and development of UWOC system.

The performance of an underwater wireless optical communication (UWOC) system is affected by many factors, such as channel conditions, modulation and detection techniques, type of optical source, etc. The UWOC system performance was investigated in the presence of weak and strong turbulence. Pure sea was the water type used for the performance analysis. Binary phase shift keying, quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation, and non-return to zero-on–off keying were used for comparison. Further, a continuous wave laser was used as the optical source, and photodetectors are at the receiver. The performance of the UWOC system was analyzed in terms of the bit error rate (BER). The BER performance was evaluated using the log-normal probability density function (PDF) in weak turbulence and the gamma-gamma PDF in strong turbulence. In all modulation techniques, QPSK had the best BER performance. The BER was reduced further using avalanche photodiode (APD) as the photodetector instead of positive-intrinsic-negative diode for the same transmitter power and channel conditions. In weak turbulence, a BER of 10 − 8 was achieved at a transmitter power of 12.5 dBm using the QPSK modulation technique and APD as the photodetector. For the same conditions, a BER of 9 × 10 − 7 was achieved in strong turbulence. The multiplication gain, dark current, and carrier ionisation ratio of the photodetector were optimized for better BER performance in the UWOC system.

A 50 Gb/s four-level pulse amplitude modulation (PAM4) underwater wireless optical communication (UWOC) system across the water–air–water interface is demonstrated in practice. In practical scenarios, laser beam misalignment due to oceanic turbulence degrades performance in UWOC systems. With the adoption of a reflective spatial light modulator (SLM) with an electrical controller, not only can the laser be arbitrarily adjusted to attain a water–air–water scenario, but oceanic engineering problems can also be resolved to establish a reliable UWOC link. Brilliant bit error rate performance and clear PAM4 eye diagrams are attained by adopting a Keplerian beam expander and a reflective SLM with an electrical controller. This proposed PAM4 UWOC system presents a feasible state that outperforms existing UWOC systems due to its feature providing a high-speed water–air–water link.