Underwater Wireless Optical Communication System Research Articles

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.

Optimal Power Allocation for Multiuser Photon-Counting Underwater Optical Wireless Communications Under Poisson Shot Noise

Photon counting is an effective technique to detect low-power optical signals in underwater optical wireless communications (UOWC), but undergoes signal-dependent Poisson shot noises that lead to intractable data rate expressions and hinder effective power allocation of photon-counting systems. This paper presents a new approach to the optimal power allocation of a multiuser photon-counting UOWC system, where we first derive the asymptotic achievable rate as the background radiation is large under the signal-dependent Poisson shot noises. With the tractability of the asymptotic achievable rate, we formulate a new power allocation problem to maximize the weighted sum-rate of the multiuser photon-counting UOWC system. A new algorithm is developed to decompose the problem into subproblems with deterministic convexity or concavity and accordingly convexified and solved using successive convex approximation. We also propose to pre-select the subproblems, thereby reducing the complexity significantly with negligible loss of the weighted sum-rate. Simulations validate our asymptotic achievable rate, and show that the proposed algorithms can improve the weighted sum-rates of the UOWC systems by orders of magnitude, compared to the existing approaches.

Underwater Wireless Optical Communication System Channel Modelling With Oceanic Bubbles and Water Constituents Under Different Wind Conditions

Optical wireless data transmission from an autonomous underwater vehicle (AUV) to a sea-surface buoy has become a viable solution to support the increasing needs for many applications in the field of marine technology. Despite having unique characteristics and advantages, underwater wireless optical communication remains a challenging field due to the difficulties associated with the harsh marine environments and severe attenuation caused by the bubbles and particulates. This study aims to evaluate the uplink UWOC system in the presence of particles and bubbles under various wind speeds. To calculate the link distances and signal loss between a transmitter and a receiver of the UWOC system, simulations were performed with the inputs of the water optical properties obtained from the Bay of Bengal and Southern Ocean waters and bubble properties derived from the Hall-Novarini (HN) model. Consequently, the normalized received power were calculated for the different receiver configurations and oceanic conditions without and with bubbles. Our results showed that the received power decreased with the increasing wind speeds and smaller bubble populations. The angular and spatial distributions of the received beam increased for smaller bubble populations under high wind speeds. When compared to the clean bubbles, the received power slightly decreased for the non-absorbing coated bubbles with different film thicknesses (0.01 ∼ 1.0 μm) and increased for the absorbing coated bubbles with higher thicknesses (≥1 μm). These results will significantly help the system designer to develop and optimize an UWOC system under different bubble populations and wind speed conditions.

On the HARQ performance in underwater optical wireless communication systems with the presence of air bubbles

On the HARQ performance in underwater optical wireless communication systems with the presence of air bubbles

Power Optimization of UOWC-MIMO-OFDM System Based on PSO-WF Algorithm

Under the constraint that the total transmission power is constant, when multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM) technology is applied to the signal transmission design in the underwater optical wireless communication system (UOWC), the traditional water-filling algorithm (WF) uses mathematical derivation to solve the optimization problem, which requires multiple Lagrangian optimization solutions. In order to reduce the computational complexity and accelerate the convergence speed, a particle swarm optimization (PSO-WF) algorithm for water injection is proposed. It randomly selects particles in the target area, calculates the fitness, and gradually approximates the optimal solution to avoid negative power distribution. The simulation results show that, compared with the average power allocation scheme, the algorithm improves the system capacity and operation efficiency by about 80%; Compared with WF algorithm, it greatly reduces the complexity.

Underwater optical wireless communication system performance improvement using convolutional neural networks

Many applications that could benefit from the underwater optical wireless communication technique face challenges in using this technology due to the substantial, varying attenuation that affects optical signal transmission through waterbodies. This research demonstrated that convolutional neural networks (CNNs) could readily address these problems. A modified CNN model was proposed to recover the original data of a non-return to zero on–off keying modulated signal transmitted optically through a tank full of Gulf seawater. A comparison between the proposed CNN model and a conventional fixed-threshold decoder (FTD) demonstrates the excellent performance of the proposed CNN model, which improved the bit error ratio (BER), signal-to-noise ratio (SNR), and effective channel length. The BER of the optical signals that are transmitted at powers of 24, 26, and 27 dBm and a bit rate of 10 Mbit/s at a distance of 3 m from the transmitter when FTD is used is 7.826 × 10−7, 5.049 × 10−8, and 8.38 × 10−10, respectively. When the CNN decoder is used at the same distance and powers, the BER is 6.23 × 10−14, 1.44 × 10−16, and 2.69 × 10−18, respectively. In conclusion, the BER decreased by about seven orders of magnitude, the effective channel length increased by four times, and the SNR decreased by about 20 dB. The simplicity of the proposed CNN decoder is independent of the prior knowledge of the channel conditions. Furthermore, the magnificent obtained results make the proposed CNN decoder an ideal substitute for ordinary underwater optical wireless communication decoders.

Experiment-based channel characteristics for underwater wireless optical communication over medium-and-long distance

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.

Study on Temperature and Water Turbulence Impact on Saline Water-Based Wireless Optical Communication

Underwater wireless optical communication (UWOC) is a promising solution for Gb/s rate and long-distance underwater communication. However, random changes in the local temperature and salinity of seawater have caused different refractive indices of ocean water. This study investigated the UWOC system in different saline water while simultaneously changing the temperature and water flow. A maximum bit error rate (BER) of 4.851 × 10−6 was measured at −7.41 dBm in 3 m of 45.56 g/L saline water. By changing the temperature to 30 °C, the bit error rate (BER) value reached 5.12 × 10−6 in the saline water. On the other hand, water flow was generated in various types of water salinity to compare simultaneous environmental effects in the UWOC system. In 45.56 g/L of saline water with water flow, the UWOC system was still capable of reaching a BER value of 4 × 10−4.

Underwater optical wireless communication system: Deep learning CNN with NOMA-based performance analysis

This research is looking forward improving the performance for underwater optical wireless communication (UOWC) by applying a Non-orthogonal multiple access (NOMA) technique. We also get the benefit of the advantage the transmission based on convolutional neural network hybrid with a long short-term memory cell. The relays selection and power optimization are two main parameters to enhance the UOWC system performance. In this work, we suppose a pairing method for NOMA nodes. By replacing the inner dense connections with convolution layers, this model is proposed to overcome high complexity and over fitting to improve the model performance. The obtained performance for sum rates show that NOMA outperforms the orthogonal multiple access system by ~ 6%. Applying a step-by-step sub-optimization algorithm (SSOPA) yields better results than using fixed power allocation (FPA), while using a global optimal power allocation algorithm (GOPA) increases the sum rates over both FPA and SSOPA. It is found that the improvement when using GOPA combined with CNN approach enhances the performance of sum rates by ~ 2.5% than using the independent-relay-aided NOMA (ICNOMA) for UOWC. The GOPA improvement is 1.2%, 2.5%, 8.7% over FPA and is 0.12%, 0.34%, 2.09% over SSOPA, for clear, pure, and coastal water, respectively. The ICNOMA outperforms both ordinary NOMA (ONOMA) and cooperative NOMA (CNOMA) without independent relay nodes. The ICNOMA achieves an improvement over ONOMA and CNOMA by 20.4% and 3.2%, respectively.

Underwater VLC channel estimation based on Kalman filtering for direct current optical- and asymmetrically clipping optical- orthogonal frequency division multiplexing techniques

In this paper, we suppose an underwater optical wireless communication (UOWC) system, where the communication utilizes visible light communication for its advantages as wide spectrum, high data rate and high accuracy. The novelty of this paper is focused on improving the channel estimation between transmitter and receiver, where using Kalman Filter (KF) for channel estimation in UOWCs achieves the best results as compared to other traditional channel estimation methods. The scenario of this paper is summarized in transmitting data from transmitter to receiver via underwater harbor and coastal channels. Two channel models are utilized: weighted double gamma functions (WDGF) and a combination of exponential and arbitrary power function (CEAPF). The modulation technique used is optical orthogonal frequency division multiplexing with two kinds: direct current optical orthogonal frequency division multiplexing (DCO-OFDM) and asymmetrically clipping optical orthogonal frequency division multiplexing (ACO-OFDM). Three different techniques are used for channel estimation: Least Square (LS), minimum mean square error (MMSE), and KF. The simulation results reveal that the ACO-OFDM modulation technique with CEAPF channel modeling using KF achieves the lowest bit error rate (BER) compared to other channel estimation methods. The improvement percentage at BER = 10−1 is 13.3% for ACO-OFDM over DCO-OFDM with CEAPF in coastal water and is is 9.3% for WDGF. This indicates that CEAPF performs about 4% better than WDGF for ACO-OFDM than DCO-OFDM in terms of channel estimation.

Full-duplex underwater wireless blue light communication.

Owing to its relatively low absorption loss and high data transmission rate, wireless blue light communication is becoming an increasingly attractive technology for underwater applications. Here, we demonstrate an underwater optical wireless communication (UOWC) system that communicates using blue light-emitting diodes (LEDs) with a dominant wavelength of 455 nm. Under the on-off keying modulation scheme, the waterproof UOWC system achieves a bidirectional communication rate of 4 Mbps based on the transmission control protocol (TCP) and exhibits real-time full-duplex video communication with a transmission distance of 12 m in a swimming pool, offering great potential for practical use in real-world scenarios, such as carried around or attached to an autonomous vehicle.

Multilevel polarization shift keying modulation for turbulence-robust underwater optical wireless communication.

Turbulence is an intractable issue for underwater optical wireless communication (UOWC). Most literature has been mainly focused on the modeling of turbulence channels and performance analysis rather than mitigation of the turbulence effect, especially from the experimental aspects. In this paper, a multilevel polarization shift keying (PolSK) modulation based UOWC system is established utilizing a 15 m-long water tank, and the system performance is investigated under specific temperature gradient-induced turbulence and various transmitted optical powers. Experimental results show the feasibility of the PolSK in alleviating the effect of turbulence, and the bit error rate performance significantly outperforms traditional intensity-based modulation schemes which have difficulty in obtaining an optimal decision threshold in a turbulence channel.

Aperture-averaged scintillation for a weak underwater turbulence-affected Gaussian beam using the OTOPS model.

The calculation of optical system performance for laser beam propagation in optical turbulence, such as bit error rate (BER), signal-to-noise ratio, and probability of fade, requires the knowledge of scintillation. In this paper, we show the analytical expressions of the aperture-averaged scintillation using a new recently introduced power spectrum of the refractive index fluctuations for underwater turbulence, the oceanic turbulence optical power spectrum (OTOPS). In addition, we use this main result to investigate the impact of weak oceanic turbulence on free-space optical system performance for a propagating Gaussian beam wave. Similar to the atmospheric turbulence case, results show that aperture averaging can reduce the mean BER and the probability of fade several orders of magnitude if the receiver aperture is chosen with a diameter larger than the Fresnel zone, L/k. Being valid for weak turbulence regime in any natural waters, results present the variations of irradiance fluctuations and the performance of underwater optical wireless communication systems depending on the practical values of average temperature and average salinity concentration that can be encountered in any world's waters.

Outage performance analysis for relay-assisted UWOC systems over GGD weak turbulence with nonzero boresight pointing errors

In this paper, we mainly investigate the outage performance of serial and parallel relay-assisted underwater wireless optical communication (UWOC) systems based on a newly proposed aggregated underwater fading model. In order to overcome the deficiency of the traditional underwater weak turbulence models, that is, they could not accurately fit the measured data in the laboratory, the generalized gamma distribution (GGD) which has been verified by a series of experiments is chosen for the first time to characterize the weak oceanic turbulence. Then, we establish a new receiving signal model which has integrated the implicit path loss plus multipath propagation effect shown by fading free impulse response (FFIR), GGD weak oceanic turbulence, and nonzero boresight pointing errors. Next, we deduce the closed-form expression of the probability density function (PDF) of the hybrid fading considering GGD weak turbulence and nonzero boresight errors based on the new receiving signal model above through double-exponential Taylor expansion and higher transcendental Whittaker function. Finally, the analytical expressions of the outage probabilities for point-to-point (P2P) link, serial and parallel relay-assisted UWOC systems are further derived respectively under the proposed aggregated channel. Numerical simulations are also provided to validate the accuracy of the theoretical formulae derived above, and to show the effects of the key system parameters on the outage performance of relaying UWOC systems.

Channel Polarization Scheme for Ocean Turbulence Channels in Underwater Visible Light Communication

Underwater wireless communication technology plays an important role in marine environment monitoring and ecological protection. Underwater optical wireless communications (UWOCs) can currently achieve a transmission distance of hundreds of meters, and the rate can reach hundreds of Mbps or even Gbps, with low power consumption and high-speed features. In addition, UWOC also has the advantages of a small transceiver size and strong anti-electromagnetic interference ability, which is especially suitable for scenarios where underwater volume and power consumption are relatively limited. However, UWOC systems face problems such as unstable transceiver ends, ocean turbulence, and so on, resulting in reduced communication reliability and limited transmission distance. Establishing a stable and reliable communication link is critical to extending the communication distance of the UWOC system. In this paper, a model of ocean turbulence channels is established based on the power spectrum inversion method. The transmission characteristics of orbital angular momentum (OAM) light in an ocean turbulence channel are studied, then the mode selection of OAM light is determined. At the same time, the polarization coding technique is applied to the underwater OAM communication system for the first time. The simulation results show that this scheme can effectively extend the communication distance and reduce the system bit error rate.

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

The optical vortex beams carrying orbital angular momentum (OAM) is a beam with a special transverse spatial distribution, which has infinite orthogonal properties and can significantly improve the channel capacity of underwater wireless optical communication (UWOC) systems. However, for the underwater wireless optical communication system based on orbital angular momentum shift keying (UWOC-OAM-SK), turbulence in the seawater channel can distort the optical vortex beam and reduce the ability of the system to demodulate the OAM-SK signal, which can reduce the reliability of the system. To improve the performance of UWOC-OAM-SK systems in the channel with different turbulence, a coherent demodulation UWOC-OAM-SK system using an image recognizer based on convolutional neural network (CNN) as demodulator is proposed in this paper. Compared with the incoherent system, the proposed system can obtain a detection image with higher image contrast and more pattern features before the OAM mode is demodulated from the OAM-SK signal at the receiver, therefore has higher reliability. Moreover, the proposed system can recognize mutually conjugate OAM modes, which can greatly save the multiplexing resources in OAM communication. With the excellent image-data processing capability of CNN, the coherent demodulator based on CNN image recognizer has higher demodulation accuracy than the incoherent demodulator in both long distance channels or strongly turbulence channels underwater. The simulation results show that the demodulation accuracy of the proposed system is still higher than 90% in channels with strong turbulence intensity Cn2=1×10−14(K2m−2/3) or transmission distance z=80m. The work done provides some theoretical basis for the design of the UWOC-OAM-SK system.

Artificial Neural Network-Based Transmission Power Control for Underwater Wireless Optical Communication System

Several scientists have proposed using an underwater wireless optical communication system (UWOC) to deliver high-speed data services using the abundant optical spectrum. However, wireless optical signal propagation faces an antagonistic environment when using undersea channels due to various factors like scattering, absorption, turbulence, and optical link misalignment between the transmitter and the receiver. These factors will attenuate the optical signal and lead to degrading system performance. To reduce these factors impact on the communication system's performance, transmitted optical power (OTP) should be increased. Since the UWOC system is battery-powered, increasing OTP will consume more electrical power. Therefore, it is necessary to adjust OTP to a value commensurate with the underwater channel changes. So, an ANN model is proposed in this article for link adaptation, which can adjust the OTP level in tandem with the underwater channel conditions. Data for training, testing, and validation of the proposed system reliability was collected experimentally, and tap water was used as a transmission medium. Evaluation of the proposed model outcome demonstrates that reliable performance is achieved in predicting OTP needed in multiple scenarios. The MSE of the predicted OTP is(9.5×10-3,1.5×10-2, and 1.7×10-2) dBm in the training, testing, and validation stages, respectively. The regression values of the training, testing and validation sets are (0.9997,0.9990, and 0.9996). The results achieved by the proposed model prove it is reliable to be applied in UWOC systems.

Unified statistical thermocline channel model for underwater wireless optical communication.

Understanding the effect of ocean turbulence on optical beam propagation is critical to the design and performance evaluation of underwater wireless optical communication systems. In this Letter, we propose a unified Weibull-generalized gamma distribution to characterize the laser beam irradiance fluctuations of turbulent underwater thermocline wireless optical channels. The proposed model shows an excellent agreement with the measured data under various experimental emulated channel conditions that cover turbulences induced by temperature, salinity, and air bubbles. To the best of our knowledge, this is the first model that comprehensively describes the statistics of the laser beam irradiance fluctuations in underwater wireless optical channels due to both thermohaline gradient and air bubbles.

Optimization of LED Array Spatial Coverage Characteristics in Underwater Wireless Optical Communication

To achieve uniform spatial coverage characteristics in optical signals in an underwater wireless optical communication (UWOC) system, and therefore reduce the requirement of the alignment between the receiver and the transmitter, we propose an optimized scheme of optical signal coverage based on a light-emitting diode (LED) array in this paper. For high-efficiency coverage of the optical signals, the pitch angle of the LED light source is first optimized on the basis of the light beam geometry. Then, the layout of the LED array and the horizontal deflection angle of the light source are jointly optimized by an improved particle swarm optimization (PSO) algorithm. Taking a 16-LED array as an example, the performances of the spatial coverage characteristics with three different LED array layouts are analyzed in detail under four typical seawater environments. The results show that the LED array with the PSO-optimized layout can achieve better uniformity in the power distribution for the received optical signals, and enhance the robustness of the UWOC system in complex seawater environments.