Underwater Wireless Optical Communication Links Research Articles

Hybrid FSO/RF and UWOC system for enabling terrestrial-underwater communication: Performance analysis

This work examines the performance of the terrestrial-underwater communication system utilizing hybrid free space optics (FSO)/radio-frequency (RF) and underwater wireless optical communication (UWOC) links. Here, the base station communicates with the underwater vehicle via a decode-and-forward (DF) based relay (buoy) in two phases. In the first phase, a hybrid FSO/RF link is used to transmit signal to the buoy, where the RF link acts as an alternative link to increase the reliability of the system, and in the next phase, the buoy forwards signal to the underwater vehicle through the UWOC link. To enhance the reliability of the RF link, the buoy is deployed with multiple antennas, and it exploits a maximal ratio combining scheme on the received RF signals. The analysis takes into consideration some primary variables that influence the system’s performance, such as atmospheric turbulence, attenuation, temperature gradient, air bubbles, water salinity variations, pointing errors, and detection techniques. Closed-form expressions for the outage probability, system throughput, and average channel capacity in terms of the Meijer-G and bivariate Fox-H functions are derived. Simulation results are presented to validate the analytical expressions and disclose valuable findings.

Demonstration of an 8-Gbit/s quadrature-phase-shift-keying coherent underwater wireless optical communication link using coherent heterodyne detection under scattering conditions.

In this paper, we experimentally demonstrate an 8-Gbit/s quadrature-phase-shift-keying (QPSK) coherent underwater wireless optical communication (UWOC) link under scattering conditions at 532 nm. At the transmitter, we generate the 532-nm QPSK signal using second-harmonic generation (SHG), where the 1064-nm signal modulated with four phase levels of an 8-phase-shift-keying (8-PSK) format is phase doubled to produce the 532-nm QPSK signal. To enhance the receiver sensitivity, we utilize a local oscillator (LO) at the receiver from an independent laser source. The received QPSK data beam is mixed with the independent LO for coherent heterodyne detection. Results show that the bit error rates (BERs) of the received QPSK signal can reach below the 7% forward error correction (FEC) limit under turbid water with attenuation lengths (γL) up to 7.4 and 6.1 for 2- and 8-Gbit/s QPSK, respectively. The corresponding receiver sensitivities are -34.0 and -28.4 dBm for 2- and 8-Gbit/s QPSK, respectively.

Performance analysis of the dual-hop mixed RF/UWOC system with fog channel and turbulence based on diversity

ABSTRACT In this paper, the performance of a dual-hop mixed radio frequency (RF)/underwater wireless optical communication (UWOC) system with amplifying and forwarding (AF) relaying is investigated. The diversity combining technology were combined with unifying Fisher-Snedecor ( F ), extended generalized- K (EGK), and α − μ distribution for atmospheric turbulence in foggy conditions to analyze the probability density function (PDF) of RF links. The UWOC link experiences mixed exponential generalized gamma (EGG) fading. By utilizing diversity combination methods and considering the statistical effects of atmospheric turbulence, random fog, and underwater turbulence, we derive the expression for the PDF of the RF/UWOC system. In addition, the tight asymptotic expression for the outage probability and average bit error rate under high signal-to-noise ratio (SNR) is provided. The analytical expressions are verified by Monte Carlo method, and the effects of channel number on RF/UWOC system performance under various atmospheric, underwater and weather conditions are demonstrated.

Weak signal detection based on pulse width counting method for underwater wireless optical communication with an analog mode PMT detector.

This work proposes a weak signal detection method based on pulse width counting (PWC) for the on-off keying (OOK) underwater wireless optical communication (UWOC) system with an analog mode photomultiplier tube (PMT) detector. The signal output model of the analog mode PMT in weak light communication and the influence of pulse overlap are investigated. We experimentally evaluate the proposed algorithm under different sampling rates, detection thresholds, data rates as well as received optical powers (ROPs), and compare the performance of the proposed approach with that of pulse amplitude detection and pulse peak counting. A 10-Mbps OOK UWOC link is realized with a sensitivity of -71.5 dBm in a 7-meter tank, which is 1.1-dB and 3.8-dB lower than that of pulse peak counting (PPC) and pulse amplitude detection (PAD) methods, respectively, and the total link attenuation is 94.8 dB. This system utilizes the analog mode PMT with larger dynamic range than photon-counting mode PMT to achieve weak light signal detection, which benefits design long-range UWOC systems.

Experimental demonstration of positioning the cooperation terminal for underwater wireless optical communication based on visual navigation

Underwater wireless optical communication (UWOC) is a useful way to transmit a large volume of data in ocean engineering. In application, one of the UWOC terminals is usually carried on an underwater robot to actively adjust its orientation and position to establish an optical link, which requires the mobile terminal to locate the cooperation partner. At a short communication distance, visual navigation is a preferred method to measure the position. In this method, a camera is installed on the mobile terminal while several light-emitting diodes (LEDs) are in the cooperation one. In the image of the camera, by capturing the pattern formed of LEDs light spots, the relative position and orientation between terminals could be calculated. However, in a UWOC system, visual navigation is interfered with by the UWOC transmitter since it forms an additional light spot in the camera’s image. To solve this issue, we present a UWOC-compatible visual navigation method, in which the UWOC light spot is distinguished with the hue, saturation, and value (HSV) color model. Then, we experimentally demonstrate the effectiveness of the method in a pool, applying signals with different bitrates and power on the transmitter. The results show that, while the distance between terminals is within 350 cm and the yaw angle is within 5°, the measured errors are within 1.77 cm and 0.92°, respectively, which is accurate enough for most UWOC applications. The method presented in this paper is simple, pragmatic, and useful for establishing a UWOC link.

Geometric Implications of Photodiode Arrays on Received Power Distribution in Mobile Underwater Optical Wireless Communication.

Underwater optical wireless communication (UOWC) has gained interest in recent years with the introduction of autonomous and remotely operated mobile systems in blue economic ventures such as offshore food production and energy generation. Here, we devised a model for estimating the received power distribution of diffused line-of-sight mobile optical links, accommodating irregular intensity distributions beyond the beam-spread angle of the emitter. We then used this model to conduct a spatial analysis investigating the parametric influence of the placement, orientation, and angular spread of photodiodes in array-based receivers on the mobile UOWC links in different Jerlov seawater types. It revealed that flat arrays were best for links where strict alignment could be maintained, whereas curved arrays performed better spatially but were not always optimal. Furthermore, utilizing two or more spectrally distinct wavelengths and more bandwidth-efficient modulation may be preferred for received-signal intensity-based localization and improving link range in clearer oceans, respectively. Considering the geometric implications of the array of receiver photodiodes for mobile UOWCs, we recommend the use of dynamically shape-shifting array geometries.

Performance analysis of an underwater wireless optical communication link with Lommel beam

In order to mitigate the stochastic interference of underwater channels and improve the quality of underwater communication systems, it is essential to study the performance of the underwater wireless optical communication (UWOC) links utilizing vortex beams with unique attributes. In this paper, the analytical formulae for the bit error rate (BER) and the average capacity of the UWOC link with diffraction-free Lommel beam are derived under the Rytov theory. Simulation results demonstrate that the system with a long wavelength, a high system signal-to-noise ratio(SNR), a small asymmetric parameter and receiving aperture diameter achieves a high average capacity and a low BER. Furthermore, in the underwater channel with a larger kinetic energy dissipation rate per unit mass of fluid and inner scale, a smaller mean-squared temperature dissipation rate, temperature salinity contribution ratio and transmission distance, the performance of the communication link can be improved. Meanwhile, it is found that the performance of the link with carrier Lommel beam are less sensitive to the topological charge, the scaling factor of the beam and the turbulent outer scale. These findings provide theoretical support for the design and implementation of an UWOC link utilizing the Lommel beam.

Propagation of auto-focusing hypergeometric Gaussian beams along a slant path in oceanic turbulence

Compared to horizontal transmission, the oceanic dissipation rate and temperature-salinity distribution ratio are no longer constant but vary with depth, imposing greater complexity on oceanic turbulence when beams propagate through a slant path and resulting in more limitations on the performance of underwater wireless optical communication (UWOC) links. This study focuses on investigating the performance, especially the auto-focusing characteristic, of auto-focusing hypergeometric Gaussian (AHGG) beams propagating along slant paths in oceanic turbulence. We theoretically derive the spatial coherence radius and the relative probability of the orbital angular momentum (OAM) mode for AHGG beams passing through such links. Numerical simulations reveal that AHGG beams exhibit superior propagation performance compared to hypergeometric Gaussian beams. Lower beam orders and OAM numbers contribute to improved performance, while careful selection of auto-focusing length can tangibly enhance detection performance as well. Additionally, tidal velocities and wind speeds have nonnegligible effects on OAM signal probability. Our results further demonstrate that surface buoyancy flux, temperature gradients, and waterside friction velocity significantly affect beam transmission under varying wind conditions. These findings, particularly controlling the auto-focusing length of AHGG beams to match the transmission distance, provide valuable insights for enhancing the quality of UWOC links.

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

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

Performance analysis of AUV-carried RISs-aided multihop UWOC convergent with RF MRC systems over WGG oceanic turbulence

To establish reliable connection between Internet of Underwater Things (IoUT) devices and terrestrial data centers, this work first proposes a reconfigurable intelligent surface (RIS) aided multihop underwater wireless optical communication (UWOC) convergent with radio frequency (RF) uplink system. Specifically, the RIS carried by an autonomous underwater vehicle (AUV) is introduced into UWOC link to relax the line-of-sight (LOS) requirement and the maximal-ratio combining (MRC) receiver is adopted at the terrestrial data center to mitigate the RF link fading. It is assumed that the underwater thermocline channel is subject to the newly proposed Weibull-generalized gamma (WGG) turbulence distribution and the RF link composite fading follows Fisher-Snedecor F distribution. Additionally, the optical link misalignment is characterized by the zero-boresight pointing errors model. With the decode-and-forward (DF) relaying scheme, the analytical closed-form expressions of the outage probability (OP) and average bit error rate (ABER) of this system are mathematically achieved, and then the impacts of air bubbles, thermohaline gradient, the number of RIS elements, pointing errors, system structure, and the number of receive antennas are further investigated. Meanwhile, the analytical results are verified by Monte Carlo (MC) simulations. Results reveal that this hybrid system performance would degrade with the increased air bubble levels and thermohaline gradients. Notably, RIS can effectively alleviate the impact of underwater turbulence and this effect would be more pronounced as the number of RIS elements increases. This work will benefit the design and research of hybrid UWOC-RF system.

Performance analysis of underwater vertical wireless optical communication links using selection combining.

In this paper, we have investigated the performance of underwater vertical wireless optical communication (UVWOC) links employing on-off key modulation for selection combining based receive diversity schemes in the presence of underwater turbulence, pointing errors, and attenuation losses. Due to variations in temperature and salinity along the ocean's depth, turbulence-induced fading affects the performance of the underwater vertical wireless optical link. The vertical link of the underwater medium influenced by a strong turbulence regime is modeled using cascaded gamma-gamma distribution considering multiple non-identical layers along with attenuation losses and pointing errors. We have derived closed-form expressions for the average bit error rate (BER) and outage probability (OP) for the link employing multiple detectors at the receiving end. The accuracy of all of the closed-form expressions derived in this paper has been validated using Monte Carlo simulations.

PLS performance analysis of the vertical UWOC system with perfect and imperfect CSI.

Although underwater wireless optical communication (UWOC) receives much interest lately, security issues associated with it get little attention. In this work, it is the first attempt to investigate the physical layer security (PLS) performance of the vertical UWOC system with perfect and imperfect channel state information (CSI). Specifically speaking, the communication between two legitimate peers in the presence of an external eavesdropper is studied from the information-theoretic security perspective. Assuming that turbulence-induced fading over the vertical UWOC links is respectively subject to cascaded lognormal (LN) and Gamma-Gamma (GG) distributions for weak and moderate/strong turbulence conditions, and the angular pointing error is randomized by the Beckmann distribution, the composite cascaded statistical fading models are derived with the comprehensive effects of path loss, underwater turbulence, angular pointing errors, and channel estimation error. On the basis of these models, analysis frameworks of the probability of strictly positive secrecy capacity (SPSC), secrecy outage probability (SOP), and average secrecy capacity (ASC) are further obtained for this UWOC system, which are confirmed by Monte Carlo (MC) simulations. Furthermore, the effects including the number of layers, the level of channel estimation error, the link distance, the location of the eavesdropper, the quality of the main and eavesdropping channels on this system are analyzed for different water conditions. The presented results give valuable insights into the practical aspects of deployment of UWOC networks.

A Long-Distance Underwater Wireless Optical Link Enabled by a Solar Array With a Baseline Compensator

In this paper, a self-designed solar cell-based optical detector is proposed and demonstrated in a long-distance underwater wireless optical communication (UWOC) link. Benefitting from a large detection area, the detector could successfully capture optical signals with a received power density of 5.62 μW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . To resist the baseline drift caused by the low-frequency cut-off of the system, an envelope detector-based baseline compensator is proposed. Comparing with digital solutions, the proposed compensator could be established in analog domain with common electronic components. With the compensator, a frequency domain equalizer (FDE) and a maximum ratio combing (MRC) algorithm, an 18 Mbps non-return-to-zero (NRZ) on-off keying (OOK) modulated link over an 80 m underwater channel is finally achieved. The proposed baseline compensator could compensate the low-frequency components effectively with low cost. Moreover, under a strong ambient light of approximately 1200 lux, the performance of the link shows little degradation. Our work shows a potential solution of long-distance UWOC system with ambient light tolerance, which is compatible with practical environment.

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

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

Analysis of Optical Wireless Communication Links in Turbulent Underwater Channels With Wide Range of Water Parameters

In this study, the performance of underwater optical wireless communication links is investigated by taking into account turbulence, absorption and scattering effects. Weak turbulent channel is modeled using log-normal distribution while moderate and strong turbulence channels are modeled using gamma-gamma distribution. Rytov variance of Gaussian beam is derived analytically for oceanic turbulence optical power spectrum. Subsequently, scintillation index is calculated using the computed Rytov variance. Moreover, the closed-form expression of bit-error-rate (BER) for underwater wireless optical communication (UWOC) systems using intensity-modulated/direct detection (IM/DD) implementation and on-off-keying (OOK) modulation scheme is obtained. Results show that the performance of wireless optical communication link between two platforms in underwater medium is degraded significantly due to turbulence, absorption and scattering. In fact, as the turbulence level increases, its effect becomes quantitatively comparable to those of absorption and scattering effects. The variation of both scintillation index and BER performance are presented for various underwater medium and communication system parameters, such as chlorophyll concentration, average temperature, average salinity concentration, temperature and dissipation rates, wavelength, link length and receiver aperture size. Optical network and internet of underwater things (IoUT) applications, which are growing day by day and requiring high data rates, will benefit from the results of this study.

Statistical channel modeling of intensity fluctuations in a turbulent underwater wireless optical communication system

Subject of study. The statistical modeling of received intensity fluctuations due to temperature-induced turbulence and the random presence of air bubbles in an underwater wireless optical communication channel is presented. Aim of study. Temperature-induced turbulence and air bubbles induce severe intensity fluctuations in the received optical signal, which degrade the underwater wireless optical communication link performance. Method. Therefore, to characterize the turbulent water channel, an experimental setup has been designed in the laboratory that considers uniform and gradient-based temperature variations along with variable-sized bubble populations. To estimate the effect of these random variations in the water channel, a statistical approach has been followed, which characterizes the nature of the recorded observations. Main results. It is observed that the behavior of random irradiance fluctuations due to uniform and gradient-based temperature variations in the underwater wireless optical communication link follows a single-lobe Gaussian distribution. Moreover, with the incorporation of air bubbles, the received irradiance fluctuations no longer follow a single-lobe Gaussian distribution but follow a multi-lobe Gaussian distribution, i.e., the Gaussian mixture model, which is the weighted sum of the Gaussian distribution. The accuracy of the proposed underwater wireless optical communication link models is verified by conducting the goodness of fit test; the observed confidence interval of 95% authenticates the proposed model. Also, the performance of the Gaussian-mixture-model-based underwater wireless optical communication link has been evaluated in terms of bit error rate, and the acceptable bit error rate levels between 1014 and 1010 have been observed. The proposed underwater wireless optical communication with Gaussian mixture model completely depicts thermally uniform, gradient-based nonuniform, and variable air bubble populations in the ocean. Practical significance. The Gaussian mixture model can further be utilized by the researchers to estimate numerous performance parameters of the turbulent underwater wireless optical communication links using advanced modulation schemes and diversity techniques and paves a valuable way to address the future aspects of research in this area.

Composite channel modeling for underwater optical wireless communication and analysis of multiple scattering characteristics.

In this study, we propose an innovative composite channel model that considers multi-size bubbles, absorption, and fading caused by scattering for investigating the effect of multiple scattering on the optical properties of a channel. The model is based on Mie theory, geometrical optics and the absorption-scattering model in the Monte-Carlo framework, and the performance of the optical communication system of the composite channel was analyzed for different positions, sizes, and number densities of bubbles. A comparison with the corresponding optical properties of conventional particle scattering indicated that a larger number of bubbles corresponded to greater attenuation of the composite channel, which was manifested by a low power at the receiver, an increased channel impulse response, and the observance of a prominent peak in the volume scattering function or critical scattering angles. Additionally, the effects of the position of large bubbles on the scattering property of the channel were investigated. The proposed composite channel model can provide reference data for designing a more reliable and comprehensive underwater optical wireless communication link.

Performance Analysis of MIMO-mQAM System with Pointing Errors and Beam Spreading in Underwater Málaga Turbulence Channel

Both the long-term beam spreading caused by ocean turbulence and the pointing errors induced by the jitter of transmitters and receivers degrade the performance of underwater wireless optical communication (UWOC) links. To effectively alleviate their effects, an in-depth study was carried out over the Málaga turbulence channel with pointing errors and beam spreading in multiple-input and multiple-output (MIMO) UWOC. First, we analyzed the long-term beam spreading and the received light power for the finite receiving aperture in the presence of pointing error displacements. Based on this, the relationship between beam spreading, pointing errors, and signal power was established. Second, the approximate expressions of the average bit error rate (BER) and the communication outage probability were derived theoretically for this MIMO system using maximal-ratio combining (MRC) diversity. Third, the effects of the pointing errors on the coding and the diversity gains were explored for the MIMO links. Finally, using the observed ocean data from the Global Ocean Argo gridded dataset, we numerically verified the combined effects of ocean turbulence strength, beam spreading, and pointing errors on the average BER and outage probability of this system. These results also proved that adjusting the size of the receiving aperture or the order of the multiple quadrature amplitude modulation (mQAM) could effectively mitigate their effects.

On the Physical Layer Security Performance Over RIS-Aided Dual-Hop RF-UOWC Mixed Network

Since security has been one of the crucial issues for high-yield communications such as 5G and 6G, researchers continuously come up with newer techniques to enhance the security and performance of these progressive wireless communications. Reconfigurable intelligent surface (RIS) is one of those techniques that artificially rearrange and optimize the propagation environment of electromagnetic waves to improve both spectrum and energy efficiency of wireless networks. Besides, in underwater communication, underwater optical wireless communication (UOWC) is a better alternative/replacement for conventional acoustic and radio frequency (RF) technologies. Hence, mixed RIS-aided RF-UOWC can be treated as a promising technology for future wireless networks. This work focuses on the secrecy performance of mixed dual-hop RIS-aided RF-UOWC networks under the intercepting effort of a probable eavesdropper. The RF link operates under generalized Gamma fading distribution; likewise, the UOWC link experiences the mixture exponential generalized Gamma distribution. The secrecy analysis subsumes the derivations of closed-form expressions for average secrecy capacity, exact and lower bound of secrecy outage probability, strictly positive secrecy capacity, and effective secrecy throughput, all in terms of Meijer's <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$G$</tex-math></inline-formula> functions. Capitalizing on these derivations, the effects of heterodyne and intensity modulation/direct detection systems, underwater turbulence resulting from air bubble levels, temperature gradients, and salinity gradients, are measured. Unlike conventional models that merely deal with thermally uniform scenarios, this proposed model is likely to be unique in terms of dealing with secrecy analysis of a temperature gradient RIS-aided RF-UOWC network. Lastly, the derivations are validated via Monte-Carlo simulations.

Improving the Performance of Underwater Wireless Optical Communications by Pointing Adjustable Beam Arrays

Considering the limited bandwidth of underwater wireless communications with radio frequency and acoustic waves and the constant increasing need for underwater high-speed data transmission, underwater wireless optical communication (UWOC) has been regarded as an attractive and viable alternative. However, UWOCs suffer from degradation attributed to underwater attenuation, oceanic turbulence, and pointing errors in oceanic channel. Therefore, performance enhancement technique plays a key role in the design and deployment of a UWOC system. In this article, we propose to improve the performance of a UWOC by transmitting with pointing adjustable beam arrays (PABA). In a PABA, we can adjust pointing directions of every single beam by reflectors. To decrease the path loss and turbulence-induced scintillations and weaken the influence of pointing errors in UWOCs, we make the one central beam and several surrounding beams in a PABA point toward the center of a receiver aperture, and make the remaining surrounding beams in a PABA point toward several target points located symmetrically on a ring with an adjustable radius around the center of a receiver aperture. This adjustable radius is set according to the average pointing errors in every specific UWOC link. To estimate the average pointing errors more accurate, we calculate pointing errors in view of both beam wander caused by oceanic turbulence and the misaligned displacements resulted by the changes in the position and attitude of an underwater platform. Finally, we validate the effect of PABA on improving the performance of UWOCs by simulations.