Underwater Wireless Optical Communication System Research Articles

Effect of Link Misalignment in the Optical-Internet of Underwater Things

Underwater wireless optical communication (UWOC) enables high-speed links in water for the optical Internet of Underwater Things (O-IoUT) networks. O-IoUT provides various marine applications, including ocean exploration, environmental monitoring, and underwater navigation. O-IoUT typically utilizes light-emitting diodes (LEDs) and different laser diodes (LDs) such as green/blue lasers to achieve efficient data communication in the underwater environment. The high-speed optical communication is limited up to a few tens of meters due to underwater channel impairments and misalignment between the transmitter (Tx) and the receiver (Rx). UWOC provides high-speed communications only in the line of sight conditions, and a small misalignment between the Tx and the Rx can degrade the system performance. In an attempt to understand and minimize this misalignment issue, we investigate how received power in a UWOC system depends on the transmitted beam’s divergence angle. Simulation results are provided to show the effectiveness of the study by comparing the plane, Gaussian, and spherical beams. Monte Carlo simulations are utilized to determine the maximum allowable lateral offset between Tx and Rx for a given Tx divergence angle. The results provide an overview and design-based trade-off between different parameters such as lateral offset, the power received, and bandwidth of the channel. The proposed method improves not only the maximum allowed link-span but also the bandwidth of the channel for a given transmission distance.

Performance studies of MIMO based DCO-OFDM in underwater wireless optical communication systems

The research in underwater communication system is focused towards wireless networks in view of recent developments in wireless technology and increasing need for deep sea data mining. The underwater wireless optical communication (UWOC) have more advantages for short-range wireless links due to very high bandwidth and data rate than acoustic communication. Although UWOC offers high capacity links at low latencies, they suffer from limited communication range due to various distinctive characteristics of underwater channel. A major degrading effect associated with the underwater channel is turbulence-induced fading. To mitigate this fading and to extend the viable communication range, spatial multipath diversity techniques can be adopted over UWOC links. The proposed work evaluates the performance of a multicarrier modulation scheme, i.e., MIMO based DC biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) for underwater wireless optical channel. The simulation of MIMO-DC biased optical OFDM technique employing spatial diversity in underwater optical communication is a novel approach towards the determination of an optimal modulation technique for underwater optical wireless channels. The performance is evaluated based on the various parameters such as Signal to Interference plus Noise Ratio (SINR), Throughput and Bit Error Rate (BER). A simple experiment is conducted to demonstrate the BER performance assessment for different link distance of 0.45 m and 1.5 m with different transmitter and receiver configurations (SIMO and MIMO) for both clear water and coastal water using MIMO DCO OFDM technique.

Performance Investigation of OAMSK Modulated Wireless Optical System Over Turbulent Ocean Using Convolutional Neural Networks

The adaptive demodulators based on convolutional neural networks (CNNs) are proposed for an M-ary orbital angular momentum-shift keying (OAMSK) modulated underwater wireless optical communication (UWOC) system. The impacts of different modulation orders, oceanic turbulence strengths, seawater types, signal to noise ratios (SNRs), transmission distance, and scales of CNNs are investigated. For the proposed system model, oceanic turbulence and pass loss caused by absorption and scattering are emulated by Monte Carlo (MC) methods. By comparing four CNNs, AlexNet and GoogLeNet inception v4 are adopted as the key part of M -ary OAMSK adaptive demodulating modules. After being trained by 100 000 OAM intensity specimens under unknown levels of oceanic turbulence, the suitable network parameters are achieved. Results show that the average bit error rate performances of both CNN-based demodulators outperform the traditional conjugate demodulator by several orders of magnitude. Moreover, deeper and more complex CNN would be more effective in enhancing the ABER performance under the strong oceanic turbulence condition. Furthermore, for both CNN-based and conjugate demodulators, the ABER of the system with noise will approach the saturation values when the instantaneous SNR is approximately 26 dB larger than the pass loss. This work benefits the design of OAMSK modulation-based UWOC system.

Performance Analysis of Underwater Optical Wireless Communication Systems Based on CD-Optical OFDM and Various Subcarriers Indexes

Performance Analysis of Underwater Optical Wireless Communication Systems Based on CD-Optical OFDM and Various Subcarriers Indexes

Experimental demonstration of an underwater wireless optical communication employing spread spectrum technology.

For some industrial underwater wireless optical communication (UWOC) applications, the transmission distance matters more than the communication rate. Attenuation length (AL) is an important distance indicator of UWOC system. In this paper, to the best of our knowledge, the spread spectrum (SS) technology is firstly applied in a UWOC system and the capability to extend transmission distance or AL is demonstrated. A 42-m UWOC is experimentally demonstrated with 6.68 ALs. Compared with the conventional not-return-to-zero on-off-keying (NRZ-OOK) modulation scheme, the proposed SS scheme with a spread spectrum gain (SSG) of 5 achieves an AL extension by 0.51 and 0.81, respectively, with the same data rate and bandwidth. And the minimum required signal-to-noise ratio (SNR) is reduced by 9 dB to as low as -0.8 dB. Besides, the feature of the SS scheme that could work in a bandwidth-limited long-reach underwater channel without the equalization process is experimentally demonstrated.

Co-operative RF-UWOC link performance over hyperbolic tangent log-normal distribution channel with pointing errors

In this paper, we have proposed an analytic model and determined the outage probability and average bit error rate (BER) performance of a co-operative radio frequency − underwater wireless optical communication (RF−UWOC) system. In recent years, UWOC has attracted attention as a useful enabler of underwater activities such as climate and ocean monitoring, surveillance, ocean exploration, underwater wireless optical sensor networks (UWOSN) and internet of underwater things (IoUT) because of its high speed, ease of deployability and wide bandwidth availability which is free of licensing fees. The proposed co-operative RF–UWOC system is designed to establish a connection between an underwater vehicle inside the ocean to a terrestrial ground station using decode−forward and amplify−forward relays. The RF link between the terrestrial ground station to relay is modeled as a Rayleigh distributed channel. The UWOC link between the relay to the underwater vehicle is modeled as being perturbed by the hyperbolic tangent log-normal (HTLN) distribution. To the best of our knowledge, it is for the first time that the perturbations due to weak oceanic turbulence have been modeled using HTLN distribution. This distribution is a member of the class of log-normal distributions derived from hyperbolic tangent distribution. Novel closed-form expressions have been derived for the outage probability and average BER for various modulation techniques that can be employed in this system. The analytical results are evaluated and validated with Monte-Carlo simulations in the presence and absence of pointing errors. The results show that the impact of pointing errors in the RF-UWOC system is to impose an additional SNR penalty of at-least 10 dB to obtain a BER of 10−6 when compared with the system operating without pointing errors.

Experimental demonstration and analysis of underwater wireless optical communication link: Design, BCH coded receiver diversity over the turbid and turbulent seawater channels

AbstractIn this article, we demonstrate an experimental underwater wireless optical communication (UWOC) system in the presence of air bubbles and weak turbulence for varying turbidity levels of the aquatic optical medium. The major limiting factors of the UWOC system are: absorption, scattering, and beam fluctuations; these effects can be mitigated by employing transmitter/receiver diversity schemes and channel codes. In this proposed system, we have employed receiver diversity (selection combining (SC), majority logic combining (MLC), and equal gain combining (EGC)) techniques augmented with Bose‐Chaudhuri‐Hocquenghem (BCH) codes to improve the performance of on‐off keying modulated UWOC system. The bit error rate (BER) expressions are derived for the proposed system and results are validated using analytic and experimental means. The results show that the proposed system, that is, the receiver employing SC, MLC, and EGC receiver combining techniques augmented with the BCH code provides a transmit power gain of 4, 6, and 8 dB respectively, when compared with the uncoded single‐input single‐output system, at a BER of 10−5.

ABER performance investigation of LDPC-coded multi-hop parallel underwater wireless optical communication system.

The average bit error rate (ABER) performance of a low-density parity-check (LDPC)-coded multi-hop parallel underwater wireless optical communication (UWOC) system is investigated with the combined effects of absorption, scattering, the misalignment characterized by the beam spread function, and the ocean turbulence-induced fading modeled by log-normal distribution. With the max-min criterion as the best path selection scheme, the cumulative distribution function for identically and independently distributed and non-identically and independently distributed links are derived, respectively. Then, the analytical ABER expressions of binary phase shift keying and multiple phase shift keying subcarrier intensity modulation schemes are deduced with the help of the Gauss-Laguerre quadrature rule, and they are also confirmed by Monte Carlo simulation. In addition, LDPC codes are applied in the simulation to improve the system performance. The results show that the combined degrading effects are mainly limited by the link length, especially under the coastal ocean condition. And the multi-hop parallel transmission demonstrates good ABER performance and can expand the communication range in ocean. Furthermore, LDPC codes can significantly improve the ABER performance of the UWOC system, and the coding gain is strongly affected by channel conditions and the corresponding parameters of LDPC codes. This work is beneficial for the UWOC system design.

Optimal Power Allocation for MIMO Underwater Wireless Optical Communication Systems Using Channel State Information at the Transmitter

For multiple-input-multiple-output underwater wireless optical communication (UWOC) systems with on-off keying modulation, we obtain the optimal power allocation policies that minimize the bit error rate (BER), subject to average power constraints. In this article, we consider a comprehensive channel model that takes into account absorption, scattering, and weak turbulence-indeed lognormal fading. The optimal power allocation policies are obtained using the perfect channel state information available at the receiver (CSIR) and transmitter (CSIT) for both MISO and MIMO systems. In the MISO UWOC link, the optimal policies result in allocating the whole available power to the optical source with highest instantaneous SNR, i.e., maximum fading coefficient. In the MIMO UWOC link, the equivalent fading coefficient of the ith optical source is defined as the sum of the fading coefficient associated with the ith optical source and other receiver apertures. In this case, the total available power should be allocated to the optical source with the highest equivalent fading coefficient to have the minimum BER. Furthermore, we analytically obtain the exact BER expressions for both MISO and MIMO systems assuming the optimal power allocation. Simulation results are further provided to verify our analytical results. The results demonstrate that MIMO techniques with optimal power allocation provide better performance compared to other alternatives such as equal power allocation.

Impact of temperature gradients on average bit error rate performance of low-density parity-check-coded multihop underwater wireless optical communication systems over the generalized gamma distribution

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.

High-speed and reliable Underwater Wireless Optical Communication system using Multiple-Input Multiple-Output and channel coding techniques for IoUT applications

In this paper, we investigate the performance of an Underwater Wireless Optical Communication (UWOC) system employing on–off keying modulation at a data-rate of 500 Mbps over a link-range of 30 m. Transmit/receive diversity schemes, namely Multiple-Input to Single-Output (MISO), Single-Input to Multiple-Output (SIMO) and Multiple-Input to Multiple-Output (MIMO) techniques with and without RS-coding have been employed to mitigate the effects of weak oceanic turbulence and beam attenuation. The novel closed-form analytical Bit Error Rate (BER) expressions of Single-Input to Single-Output (SISO), SIMO, MISO and MIMO links for un-coded and RS-coded cases have been computed using the hyperbolic tangent distribution and validated with Monte-Carlo simulation results. The obtained BER results show that the use of (63,51) RS-coded 4 × 5 MIMO UWOC system offers at-least 35 dB of transmit power gain compared with the un-coded SISO UWOC system at a BER of 10−5. Emerging technologies like the fifth-generation (5G) networks and the Internet of Underwater Things (IoUT) will have a high impact on UWOC as these systems require a high degree of information integrity, high data rates and energy efficiency when employed in conjunction with data transfer between underwater vehicles and objects. The proposed RS-coded MIMO UWOC system offers high reliability and power efficiency and it has the potential to be gainfully employed in IoUT applications.

Experimental study of the effects of thermally induced optical turbulence on underwater wireless optical communication link parameters

In this experimental research, the effects of refractive index fluctuations resulting from temperature fluctuations on some of the most important underwater communication link parameters such as the average received power, signal to noise ratio (SNR) and normalized power variance (aperture averaged scintillation) has been experimentally investigated. Moreover, by introducing a new probability density function which is fitted more appropriately to the experimental data obtained from our simulator tank, the bit error rate (BER) behavior of an underwater wireless optical communication (UWOC) link is also studied. These investigations are essential due to the optical turbulence limiting effects on the performance of UWOC systems. The experimental results indicate that reduction of the aperture averaged scintillation effects by using of optical components in the transmitter and receiver of UWOC links can enhance their performance. In general, increasing the strength of the optical turbulence (by increasing the temperature gradient induced by the rod heater) or the distance of the localized turbulence screen generated by temperature gradient from the transmitter plane, the aperture averaged scintillation decreases and consequently the SNR raises whilst the BER declines. Also, expanding the aperture diameter of the receiver optics reduces the normalized power variance which leads to decrease and increase in the BER and the SNR amounts, respectively. The results are presented as experimental simulation graphs.

Underwater Wireless Optical Communication Based on DPSK Modulation and Silicon Photomultiplier

In this paper, a promising phase-modulated underwater wireless optical communication (UWOC) system with silicon photomultiplier (SiPM) based receiver is proposed for the first time and its feasibility has been experimentally demonstrated in a laboratory environment. The phase modulation enables the additional degree of freedom available for encoding of information in UWOC, given that previous state-of-the-art systems mainly employ intensity modulation (IM) schemes with direct detection (DD). In the proposed UWOC system, the information is encoded in the phase of light wave carrier based on differential phase-shift keying (DPSK). A highly sensitive receiver is built from SiPM, which is able to compensate the transmission loss and dramatically enhances the performance of the DPSK UWOC system. We experimentally show the feasibility of the proposed system at a data rate of 200 Mbps. For comparison, the commonly used avalanche photodiode (APD) based receiver is also tested. Comparing with the APD, the use of SiPM reduces the BERs by about two orders of magnitudes. The minimum required optical power for achieving a BER below the FEC threshold is about −40.2 dBm, which is about 11.6 dB lower than the case of APD.

LED-Based Underwater Wireless Optical Communication for Small Mobile Platforms: Experimental Channel Study in Highly-Turbid Lake Water

Underwater collaborative work between small mobile platforms (SMP) such as divers, litter submarine or AUV, requires a high data rate communication system which is compact, reliable, affordable and eye-safe. The high bandwidth of LED-based underwater wireless optical communication (UWOC) is an advantage. Artificial, emulated as well as simulated, turbid underwater channel has been studied recently, but no practical nature water. To fully meet the requirements of challenging turbid underwater channel scenarios for SMP, we demonstrated a green LED-based UWOC system up to a bandwidth of 3.4 MHz in a highly-turbid nature lake water. Through a contrast wireless optical communication (WOC) experiment in air, turbid water channel is observed to compensates the receiving signal amplitude attenuates after propagating a longer attenuation length due to multiple-scattered light caused diffusion, and thus to be more like a single-input multi-output (SIMO) system. From our experimental observation, the SIMO channel model could be employed not only in turbulent UWOC, but also in highly turbid UWOC. Moreover, turbid water channel is observed a “frequency selection” effect, thus LED with lower threshold voltage at higher frequency would optimize bandwidth and link range, and a robust pre-code for interference cancellation would be a well-direct choice to constructing different UWOC system in highly turbid water for SMP. Our finding will provide a new reference complementing the current LED-based UWOC systems in realistic water environment.

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

Deep Learning Aided Signal Detection for SPAD-Based Underwater Optical Wireless Communications

In underwater optical wireless communication (UOWC) systems, using single photon avalanche photondiode (SPAD) as the detector can improve the transmission distance. However, the signal detection for SPAD-based systems is greatly challenged by the complex optical channel characteristics and SPAD nonlinear distortion. To address this issue, a novel deep learning aided signal detection scheme is proposed in this paper. By exploiting the physical mechanism and prior expert knowledge of the signal processing, a two-connected multilayer perception (MLP) network is integrated into the receiver. The first subnetwork is regarded as a channel compensation block while the second one works as a demodulator. With sophisticated numerical optical channel model and SPAD non-Poisson model, large amounts of training data are utilized to train the proposed model offline. Afterwards, the online data are recovered with the trained network. Simulation results verify that significant bit error ratio (BER) improvement can be achieved with the proposed scheme.

Statistical Modeling of the Impact of Underwater Bubbles on an Optical Wireless Channel

In underwater wireless optical communications (UWOC), the random obstruction of light propagation by air bubbles can cause fluctuations in the incoming light intensity of a receiver. In this paper, we propose a statistical model for determining the received power by a receiver in the presence of air bubbles. First, based on real experiments of the behavior of air bubbles underwater, we propose statistical models for the generation, size, and horizontal distribution of each air bubble. Second, we mathematically derive the obstruction caused by the shadow of each bubble as it passes over the beam area. We then compute the combined obstruction of all generated air bubbles to determine the total obstructed power, which is a random variable due to the randomness of bubble behavior. Next, we find the first and second moments of the total obstructed power to model the statistical distribution of the obstructed received power by using the method of moments, which shows that the Weibull distribution suitably matches the simulation data. We also estimate the shape and scale parameters by using two derived moments. Furthermore, we also construct a statistical model of the received power with complete blockage in the presence of air bubbles and we derive the distribution of the composite channel model combining the proposed bubble-obstruction model with a Gamma-Gamma turbulence model. Finally, we obtain and verify the analytic forms of the average bit error rate and the capacity of UWOC systems under this newly proposed composite channel model.

Time Jitter, Turbulence and Chromatic Dispersion in Underwater Optical Wireless Links

The performance of an underwater optical wireless communication link is investigated by taking into account—for the first time and to the best of our knowledge—the simultaneous influence of the chromatic dispersion, the time jitter and the turbulence effects, by assuming chirped longitudinal Gaussian pulse propagation as information carriers. The estimation procedure is presented and a novel probability density function is extracted in order to describe the irradiance fluctuations at the receiver side. Furthermore, the availability of the link is investigated by means of its probability of fade and various numerical results are presented using typical parameters for the underwater optical wireless communication systems.

A Review on Practical Considerations and Solutions in Underwater Wireless Optical Communication

Underwater wireless optical communication (UWOC) has attracted increasing interest in various underwater activities because of its order-of-magnitude higher bandwidth compared to acoustic and radio-frequency technologies. Testbeds and pre-aligned UWOC links were constructed for physical layer evaluation, which verified that UWOC systems can operate at tens of gigabits per second or close to a hundred meters of distance. This holds promise for realizing a globally connected Internet of Underwater Things (IoUT). However, due to the fundamental complexity of the ocean water environment, there are considerable practical challenges in establishing reliable UWOC links. Thus, in addition to providing an exhaustive overview of recent advances in UWOC, this article addresses various underwater challenges and offers insights into the solutions. In particular, oceanic turbulence, which induces scintillation and misalignment in underwater links, is one of the key factors in degrading UWOC performance. Novel solutions are proposed to ease the requirements on pointing, acquisition, and tracking (PAT) for establishing robustness in UWOC links. The solutions include light-scattering-based non-line-of-sight (NLOS) communication modality as well as PAT-relieving scintillating-fiber-based photoreceiver and large-photovoltaic cells as the optical signal detectors. Naturally, the dual-function photovoltaic–photodetector device readily offers a means of energy harvesting for powering up the future IoUT sensors.

Analysis of underwater wireless optical communication system performance.

Because the underwater channel environment is complicated, it is difficult to do an actual experiment in the ocean to analyze the performance of underwater wireless optical communication (UWOC) systems. In this study, the spot-expansion characteristics and time-domain broadening characteristics of underwater wireless optical signals are simulated and analyzed by a Monte Carlo statistical method. Thus, what we believe is an improved underwater channel transmission-attenuation model and time-domain broadening model based on UWOC are proposed, so the transmission distance characteristics of the UWOC system are obtained by combining the system parameters, and the transmission-rate characteristics can be analyzed by using the Shannon-Hartley theorem. The results show that the transmission distance is linear with the receiver sensitivity, and the transmission rate decays exponentially with the transmission distance and is limited by the receiver sensitivity in the UWOC system.