Underwater Wireless Optical Communication Links Research Articles

Performance Improvement of Turbid Underwater Wireless Optical Communication Link Using Ratiometric Signal Processing

This article introduces a technique of improving the link performance of the underwater wireless optical communication (UWOC) system in terms of signal-to-noise ratio (SNR) by incorporating a novel ratiometric signal processing (RSP) scheme at the receiver node. In addition, the temporal fluctuations in the attenuation coefficient information (ACI) of the UWOC channel in the nonuniform turbulent regime are analyzed, and the corresponding distribution function is analytically derived. The consistency of the obtained statistical distribution of the ACI parameter is analyzed by simulating the experimental data in a UWOC prototype link within a laboratory-controlled turbulent regime. The experimental outcomes indicate a relative improvement in the SNR by 1.70 to 3.23 dB in the nonuniform turbulent conditions upon incorporating the proposed RSP technique at the receiver node. It is also observed that the mean square error and an average difference of the received images that are processed by incorporating the RSP technique are lower as compared to that of the images without processing at the receiver node in the UWOC prototype link. The approach of implementing the proposed RSP technique is observed to improve the performance of the UWOC link.

Absorptive Turbulent Seawater and Parameter Optimization of Perfect Optical Vortex for Optical Communication

In this paper, the optimization of perfect optical vortex (POV) parameter for underwater wireless optical communication link under M-QAM by average bit-error rate (ABER) and the effect of seawtaer turbulence on link information capacity are investigated. The link is absorbent, weakly turbulent, and bandwidth-limited. In investigating, we use the spectral absorption coefficient to describe the wavelength effect of seawater absorption. Specifically, under the paraxial approximation and Rytov approximation conditions, we define the average signal-to-noise-crosstalk ratio including the system bandwidth factor and derive the bandwidth-limited ABER of the OAM carrier link. Capitalizing on the defined average signal-to-noise crosstalk ratio and the derived bandwidth-limited ABER of link, the novel closed-form expression for the average information capacity of the perfect optical vortex link under M-QAM modulation is proposed. Through the numerical analysis of the ABER and the average information capacity, the POV optimization parameters in specific communication links are obtained and new conclusions are drawn that the average information capacity is restricted by both signal wavelength and the seawater absorption coefficient.

Performance analysis of hybrid free space optical and visible light communications for underwater applications using spatial diversity

In this paper, we analyze a combination based on a hybrid triple hop terrestrial free-space optical (FSO), a fiber optic cable (FOC), and an underwater optical wireless communication (UOWC) link for providing high-speed optical connectivity between onshore and submerge systems. UOWC, scattering, absorption, and turbulence severely degrade the reliability and transmission rate of the UOWC link. In our manuscript, a bend-insensitive intermediate FOC link and a spatial diversity (SD) are exploited to improve the error performance of UOWC links, which involves the deployment of multiple input single outputs (MISO). For this purpose, different transmission signaling schemes, such as pulse position modulation (PPM) with spatial modulation (SM) and on–off keying (OOK) with repetition coding (RC), are employed to evaluate the system performance in terms of average bit error rate (ABER). The behavior of the entire system is obtained by applying a decode-and-forward (DF), where we consider log-normal channels for FSO and UOWC. The path loss and turbulence are obtained using intensity modulation and direct detection schemes. To obtain performance results, Monte Carlo-based statistical simulation method is applied to validate the analytical ABER performance of the system. The achieved results reveal that MISO-OOK-RC has superior ABER performance as compared to MISO-PPM-SM in higher signal-to-noise ratio (SNR) conditions with different underwater turbulence strengths.

Comprehensive experimental and statistical analysis of the effects of salinity gradient, temperature gradient, air bubbles on the performance of underwater wireless optical communication link

Salinity gradients (SGs), temperature gradients (TGs), and different-sized air bubbles affect the optical signal propagating in the UWOC link. These effects contribute to beam wandering, spot dancing, and intensity fluctuations in the received optical signal. In this paper, experimental analysis has been carried out to estimate the effect of SG, TG and air bubbles on the underwater optical signal. The beam wandering, mean and variance deviation, displacement of weighted centroid, the statistical distribution of received intensities and link outage probability have been estimated in this work. It has been found that the beam wandering effects follow a Gaussian mixture model (GMM). It is also observed that in the presence of air bubbles in the SG, the chances of link outage increase from 48% to 80% and it increases from 7% to 28% in the TG conditions. Furthermore, the experimental and theoretical modelled outage probabilities are in close agreement.

The Investigation of Underwater Wireless Optical Communication Links Using the Total Reflection at the Air-Water Interface in the Presence of Waves

With the development of underwater exploration, underwater networking is in urgent demand. At present, underwater wireless optical communication (UWOC) is primarily based on line-of-sight (LOS) communication links. However, the underwater environment is so complicated that LOS communication links are easily affected by a couple of factors such as air bubbles, turbidity, oceanic turbulence, and so on. We put forward novel UWOC links using the total reflection at the air-water interface, which can mitigate those phenomena. This paper aims to investigate a UWOC link based on the total reflection at the air-water interface. In our work, we achieved the maximum data rate of 300 Mb/s and a bit error rate (BER) of 3.10 × 10−3 under the forward error correction (FEC) with the total reflection angle of 7°. Furthermore, we verified the performance of the total reflection-based UWOC links at the air-water interface in the presence of waves and evaluated the impact on the UWOC links when the frequency and amplitude of the waves varied.

Statistical channel model to characterize turbulence‐induced fluctuations in the underwater wireless optical communication links

SummaryUnderwater wireless optical communication (UWOC) turns out to be a primary sought alternative to wireless technology, which can provide high data rate, enormous bandwidth, and low deployment cost and resolve the crowded radio frequency band issues. The performance of the UWOC link is significantly affected by the turbulent ocean environment. In this paper, the different turbulence scenarios such as salinity gradients, temperature gradients, and air bubbles are considered to model the turbulent nature of the ocean environment experimentally. These turbulent conditions induce severe intensity fluctuations in the received optical signal. These intensity fluctuations are modeled stochastically, and a novel turbulence channel model is proposed, which excellently fitted with the experimental data. The conformity of the proposed model is validated by performing a goodness of fit test in terms of R2 and root mean square error (RMSE) coefficients. The results show that for all the considered turbulent scenarios, the Gaussian mixture model better approximates the experimental observations than the Weibull distribution. As the strength of the turbulence increases, the chances of link outage and the probability of bit errors increase. At a flow rate of 4 L/min, the bit error rate (BER) of 10−20, 10−14, and 10−10 is recorded under clear water, salinity, and temperature gradient channel conditions, respectively. It is observed that the air bubbles have a significant impact on the propagating optical beam in comparison to temperature and salinity‐induced turbulence. Moreover, a close agreement between the proposed model and the experimental data is also observed, which makes the proposed model more appropriate to describe the turbulent ocean environment.

High-speed long-range multihop underwater wireless optical communication convergent with free-space optical system for optical internet of underwater things and underwater optical wireless sensor network applications

We propose a multihop underwater wireless optical communication (UWOC) convergent with free-space optical (FSO) system for an optical internet of underwater things (O-IoUT) and underwater optical wireless sensor network (UOWSN) applications. A closed-form expression of outage probability was derived for the proposed system using the cumulative distribution functions gamma–gamma and hypertangent log-normal for FSO link and UWOC link, respectively. The outage performance of the proposed multihop UWOC convergent with FSO system was analyzed over various oceanic water types (clear ocean, coastal ocean, and turbid harbor) and different FSO weather conditions (clear air, haze, drizzle, and light fog). The results also depict the end-to-end system performance for different pointing errors (strong, moderate, and weak) and varying the number of hops. The maximum link range of ∼2.5 km, which includes 2 km of FSO link and 450 m of multihop UWOC link (nine hops with each of 50 m clear ocean), is considered.

Outage probability analysis of a vertical underwater wireless optical link subject to oceanic turbulence and pointing errors

The reliability of an underwater wireless optical communication (UWOC) network is seriously impacted by beam misalignment between transmitters (Txs) and receivers (Rxs). Also, the performance of UWOC systems can be affected by oceanic turbulence-induced fading due to fluctuations in the water refractive index as a result of variations in the pressure, water temperature, and salinity. In this work, we investigate the performance analysis of a vertical UWOC link subject to oceanic turbulence and pointing errors and further investigate the appropriate selection of link parameters to optimize link performance. This study is based on an accurate mathematical framework for link modeling while taking into account realistic Tx/Rx and channel parameters under different turbulence and beam misalignment conditions. We provide an analytical expression for calculating the link outage probability, whose accuracy is validated through numerical simulations. Last, the necessity of optimal Tx/Rx parameter selection to minimize the link outage is demonstrated. A laser beam is considered at the Tx, as well as an ultra-sensitive photodetector (silicon photo-multiplier) at the Rx to enable working at relatively long link ranges. The presented results give valuable insight into the practical aspects of deployment of UWOC networks.

Monocular vision aided optical tracking for underwater optical wireless communications.

Underwater optical wireless communication (UOWC) has received increasing attention due to its distinctive characteristics such as high bandwidth and low latency. However, the UOWC link is usually vulnerable to water flow, underwater turbulence and the terminals' own vibration. Thus, to maintain a stable transmission link, an accurate tracking method is essential for UOWC systems. In this paper, a monocular vision aided optical tracking method is proposed, where the deviation degree of the laser spot is employed to realize autonomous real-time alignment of the receiver with the transmitter. Experimental results verify that with the proposed tracking method, the bit error ratio can be ensured to be below the forward error correction (FEC) limit of 3.8×10-3 even under strong disturbance for the practical UOWC system.

Net 4 Gb/s underwater optical wireless communication system over 2 m using a single-pixel GaN-based blue mini-LED and linear equalization.

High-bandwidth GaN-based mini-LEDs on the c-sapphire substrate are promising candidates for underwater optical wireless communication (UOWC) systems due to their compatibility with the mature LED fabrication process. Here we fabricate and characterize mini-LEDs based on a single-layer InGaN active region with a peak emission wavelength around 484 nm for high-speed UOWC links. Since the LED diameter affects the trade-off between the modulation bandwidth and the optical modulation amplitude, mini-LEDs with varying mesa diameters from 100 µm to 175 µm are fabricated for the measurement. The 150 µm mini-LED with a 3-dB optical bandwidth of 906 MHz performs the best and enables the transmission of a net 4 Gb/s PAM-4 signal over 2 m of underwater distance using only linear equalization. This UOWC system has achieved, to the best of our knowledge, the highest net data rate and the highest data-rate-distance product based on a single-pixel mini-LED.

Performance enhancement of 112 Gbps UWOC link by mitigating the air bubbles induced turbulence with coherent detection MIMO DP-16QAM and advanced digital signal processing

Underwater applications like real-time marine observations, video streaming, communication between underwater sensor nodes require spectrally efficient, high-speed (>100 Gbps) and reliable wireless links. In this paper, a coherently detected dual-polarization (DP)-16 quadrature amplitude modulation (QAM) based system is proposed for turbulent underwater wireless optical communication (UWOC) which can transmit a data rate of 112 Gbps. The integration of digital signal processing (DSP) with a coherent system enables error-free transmission at longer distances. The proposed single-input single-output (SISO) system achieves reliable link ranges of 150 m, 55 m, 24 m, 10 m, and 5.5 m under the pure sea, clear ocean, coastal ocean, harbor I, and harbor II water conditions. Furthermore, spatial diversity with multiple-input multiple-output (MIMO) techniques is utilized to mitigate turbulence-induced losses and path losses. At the receiver, the multiple received MIMO branches are combined with the equal gain combining (EGC) method. The transmission range improvements have been observed with 2 × 2 MIMO and 3 × 3 MIMO techniques over the SISO UWOC system. The power budget enhancement of 6 dBm is observed by utilizing a 3 × 3 MIMO configuration over 1 × 1 SISO configuration, and 3 dBm power budget enhancement has been observed in the case of a 2 × 2 MIMO configuration over 1 × 1 SISO configuration. The proposed system can further be utilized in real ocean scenarios to transmit time-sensitive information among diverse underwater sensor nodes.

RIS Assisted Triple-Hop RF-FSO Convergent With UWOC System

The convergence of wireless optical communication (WOC) and radio-frequency (RF) systems is a promising technology that overcomes the shortcomings of standalone communication systems. By incorporating reconfigurable intelligent surfaces (RISs) on top of these WOC and RF communication systems, it is possible to circumvent the connection challenges associated with standard line of sight (LOS) communication links. Wireless communication systems with RIS assistance are a promising and evolving technology that enables more efficient and reliable link performance over long distances. The performance of the triple-hop RIS-assisted RF-FSO convergent with the underwater wireless optical communication (UWOC) system is investigated in this article. We considered the fading channel Nakagami-m over the RIS-RF connection and the fading channel Gamma-Gamma (GG) over the RIS-FSO and UWOC links. Then, the average bit error rate (ABER) and outage probability are determined using closed-form expressions. The ABER and outage probability performances of the triple-hop communication system is analysed by varying parameters such as turbulence, misalignment fading, and the number of RIS elements. The obtained results demonstrate an improvement in performance for low turbulence, low pointing error, and an increasing number of RIS elements. Additionally, the data demonstrate the accuracy of the analytical results.

Performance of Non-Line of Sight Underwater Optical Wireless Communication Links with Spatial Diversity

Line-of-sight (LOS) underwater optical wireless communication (UOWC) transmission may suffer blocking and are not always possible due to obstructions from sea creatures, bubbles, large suspended particles and features of the seabed, especially in coastal and turbid water environments. Thus, we present the performance of a spatially diverse non-line-of-sight (NLOS) UOWC system employing continuous phase modulation (CPM), which is shown to offer sensitivity benefits of several dBs over on–off keying (OOK) without coherent reception. We obtain the channel impulse response (CIR) by using Monte Carlo simulation, including absorption and multiple scattering. Turbulence is included by conditioning the CIR on log-normal statistics. To mitigate the resultant fading, we exploit spatial diversity with equal gain combining at the receiver side. Photon counting at the receiver is employed to accommodate shot noise. We compare the saddlepoint and Gaussian approximations for bit error rate (BER) calculations, using the latter for later calculations as it delivers excellent results and is simpler. Our results show that spatial diversity offers performance improvements, for example an 8 dB sensitivity gain at 10^-9 BER using 1 Gbps 3×1 multiple-input single-output (MISO) transmission over a 20 m link with 0.16 log-amplitude variance. We determine using an upper bound that Intersymbol Interference (ISI) has a significant impact at high bit rates, producing error floors for multiple-output arrangements.

Performance enhancement using multiple input multiple output in dual-hop convergent underwater wireless optical communication–free-space optical communication system under strong turbulence with pointing errors

For the first time, we propose a dual-hop multiple input multiple output (MIMO)-based convergent underwater wireless optical communication (UWOC)–free-space optical (FSO) system. The UWOC and FSO links are Gamma–Gamma (GG) distributed. Closed-form expression for the average bit error rate (ABER) is derived for the proposed MIMO-based dual-hop UWOC-FSO convergent system using the GG cumulative distribution function. The end-to-end system performance analysis is carried out by considering the turbulence, attenuation, and pointing error effects for UWOC and FSO links. For the UWOC link, different oceanic conditions, such as the clear ocean, coastal ocean, and turbid harbor, are considered. Various atmospheric effects, such as clear air, fog, rain, drizzle, and haze, are considered for the FSO link. The analytical results of the proposed MIMO-based convergent system are compared with single-input single-output (SISO) system. As a result, it is observed that the proposed MIMO 2 × 3 scheme offers an improvement of 35 dB in the average signal-to-noise ratio compared with the SISO system at ABER of 10 − 5 in the case of weak pointing error.

Wireless-optical-communication-based cooperative IoT and IoUT system for ocean monitoring applications.

This paper proposes the idea of a new cooperative communication between the Internet of Things (IoT) and the Internet of Underwater Things (IoUT) using wireless optical connectivity for ocean monitoring applications. We considered IoT communication using a hybrid radio frequency (RF)/free space optical (FSO) link and IoUT using a underwater wireless optical communication (UWOC) link. Channel models for RF, FSO, and UWOC links are considered to be Rayleigh, Malaga with pointing errors, and hyperbolic tangent log-normal distributions, respectively. The outage probability and the bit error rate (BER) expressions for the proposed system are derived over the combined channel model, which includes the effects of attenuation, turbulence, and pointing errors. The BER results are plotted for various binary digital modulation schemes such as on-off keying, binary phase-shift keying, binary frequency-shift keying, and differential phase-shift keying over UWOC, hybrid RF/FSO and RF-UWOC, FSO-UWOC with end-to-end systems. BER results are extended for various turbulence regions and pointing errors of the FSO link. Monte Carlo simulation results authenticate the correctness of the results.

Real‐time image transmission through underwater wireless optical communication link for Internet of Underwater Things

SummaryThe Internet of Underwater Things (IoUTs) can be considered as a potential candidate to interconnect physical devices (things) such as submarines, ships, divers, buoys, and unmanned underwater vehicles (AUVs) to collect and exchange information with each other in the ocean environment. Traditionally, sound waves have been used to transfer information between IoUTs, but due to the lack of limited bandwidth and data rate, the underwater wireless optical communication (UWOC)‐based IoUTs can be considered as a viable solution. In this work, a real‐time RGB image is transferred between two IoUTs using an intensity modulation direct detection (IM/DD) on–off keying (OOK) modulation scheme. To estimate the performance of the reconstructed image, structure similarity index (SSIM) and peak signal‐to‐noise ratio (PSNR) have been evaluated. During transmission of an image through the water, the quality of the received image degrades due to the absorption, scattering, and underwater turbulence which further limits the transmission range. Thus, to enhance the quality of the received images, two filters, that is, median and Wiener filters, have been proposed. The median filter improves the SSIM and PSNR of the reconstructed image more appropriately as compared to the Wiener filter. The results show that the proposed median filter can enhance the communication range of the UWOC point‐to‐point link by 100 m.

21.3: The Influence of NLOS on Underwater Wireless Optical Communication System Using an InGaN‐based Micro‐LED and NRZ‐OOK Modulation

In this paper, we have studied the influence of the total reflection angle on the underwater wireless optical communication (UWOC) link, and built a UWOC link with 7° total reflection angle. Based on this UWOC link, we achieved the maximum data rate of 300 Mbps and a bit error rate (BER) of 3.10×10‐3 under the forward error correction (FEC). In addition, we testified the performance of total reflection UWOC link at different wave frequencies and amplitudes.

Performance of an Underwater Optical Wireless Link With a Randomly Placed or Moving Receiver

The current work investigates the outage probability of a vertical diffused line-of-sight underwater optical wireless communication link where a fixed transmitter at the sea surface communicates with a receiver floated below sea level, which may be still but randomly placed at a given location or moving following the random waypoint model. In both considered scenarios, the channel gain follows a random distribution making the problem mathematically challenging. Novel mathematical expressions are derived for the channel gain statistics for either random placement and mobility effects. Afterward, the signal-to-noise ratio statistics are extracted, assuming indirect modulation/direct detection at the receiver. Hence, the outage probability and the average bit-error-rate for on-off keying are readily calculated. Finally, some relevant numerical results are depicted in a series of figures to highlight the overall performance.

Performance Analysis of UWOC using SISO and SIMO Techniques

Underwater communication technology is challenging and so far, there is no alternate to sonar technology for wireless communication. Recently, optical wireless communication is proposed to establish reliable communication link underwater. In this paper, underwater wireless optical communication link is simulated under SISO, SIMO communication model techniques using OptiSystem tool. Link distance is enhanced by increasing the number of receivers in the communication model technique for UWOC system. The performance parameters such as bit error rate, quality factor are analyzed under different UWOC model technique for different link lengths. It is observed that as the number of receivers is increased in the communication model technique, the UWOC efficiency improves in terms of Q factor and BER.

Investigation of Underwater Wireless Optical Communications Links With Surface Currents and Tides for Oceanic Signal Transmission

This report investigated underwater wireless optical communication (UWOC) links in the presence of surface currents and tides. The scenario of surface currents and tides was generated by a sea water tank, a water pipe, and a water pump of different flow rates. It built the oceanic channel of UWOC varied with the salinity and flow rate in the UWOC transmission. The data rate of UWOC links based on a 520-nm laser diode and none-return-to-zero on-off keying (NRZ-OOK) modulation can achieve 3.4 Gb/s at a bit error rate (BER) of 3.78 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10^{-4}$</tex-math></inline-formula> in 1.8 m long sea water tank when a flow rate reached 1.08 m/s. The BER, eye-diagrams, and received signal optical power measurements showed that both the increasing salinity and flow rate cause the UWOC channel attenuation, thus degrade the performance of UWOC links.