Underwater Communication System Research Articles

Impulse response of underwater optical wireless channel in the presence of turbulence, absorption, and scattering employing Monte Carlo simulation.

Recently, compared with acoustic and radio methods, underwater optical wireless communications has been considered as a high-speed and high-bandwidth transmitting method at a lower cost. Absorption, scattering, and optical turbulence are three destructive phenomena that affect the performance of underwater optical communication systems. In this work, we use computer simulations to mimic the statistical behavior of underwater media employing the Monte Carlo method. Our simulation results for optical turbulence are in good agreement with the lognormal probability density function, which describes weak turbulence well, and they deviate as the turbulence moves away from weak. By considering the combined effect of absorption, scattering, and turbulence (AST) phenomena, we obtain the underwater channel's impulse response (IR). We demonstrate that there is no noticeable difference between the mean of ensemble IRs of the AST channel and the IR of the channel when turbulence is not taken into account. Moreover, our results predict that tripling the coastal link length from 10 to 30 m increases the average variance of sample IRs of the AST channel from their ensemble average by more than five times.

Demonstration of a 2 × 2 MIMO-UWOC system with large spot against air bubbles

In order to reduce turbulence-induced scintillation and deal with alignment problems, a 2×2 multiple-input multiple-output (MIMO) underwater wireless optical communication (UWOC) system is proposed and experimentally demonstrated. With help of the large divergence angle of light beams and large field of view (FOV) of the detectors, the effect of high-density air bubbles is greatly eliminated. Simulation and experimental results confirm that, in most intensity-modulation/direct-detection (IM/DD) MIMO-UWOC systems, the repetition coding (RC) scheme performs better than the space-time block coding (STBC) scheme. In a 50 m swimming pool, the maximum horizontal offset can reach 97.9 cm, which is 421% and 192% higher than that of STBC multiple-input single-output (MISO) and RC-MISO/STBC-MIMO schemes, respectively. With a data rate of 233 Mbps and a transmission distance of 50 m, the large detection range can meet a variety of underwater wireless communication requirements. The experiment indicates that, when the difference in the transmission distance between the two optical signals is higher than 1 m, the bit error rate (BER) of the RC scheme increases sharply, while the BER of the STBC scheme is stable. The MIMO coding scheme needs to be selected according to the actual application environment.

Experimental demonstration of an OFDM-UWOC system using a direct decoding FC-DNN-based receiver

In this paper, a novel fully-connected deep neural network (FC-DNN)-based receiver is proposed and experimentally demonstrated in an underwater wireless optical communication (UWOC) system, with the objective of direct decoding orthogonal frequency division multiplexing (OFDM) signals via deep learning methods. Unlike existing schemes, which treat deep neural networks as a single equalizer or a symbol classifier that requires pre-processed labels, the receiver we proposed can achieve direct output from received OFDM signals to the desired bit stream without any redundant pre-processing and post-processing. The feasibility of the proposed receiver has been validated by changing the transmission data rate and the received optical power of experimental setups. Furthermore, our experiments indicate that the proposed direct decoding FC-DNN-based receiver can perform as well as the typical conventional bulky multi-structure receiver without using pilots and cyclic prefixes (CPs) in OFDM-UWOC systems.

Performance analysis of dual-mode adaptive switching blind equalization algorithm in an underwater wireless optical communication system

In a high-power underwater wireless optical communication (UWOC) system, the bandwidth limitations of high-power optical sources and high-sensitivity detectors and the multipath effect of seawater channels can cause intersymbol interference, which seriously affects the performance of the UWOC system. Based on the attenuation characteristics and time-domain broadening characteristics of underwater wireless optical signals, a dual-mode adaptive switching blind equalization algorithm is proposed; it combines the variable step size constant-to-mode fractional spaced equalizer (FSE) algorithm and the decision directed least-mean-square mode-FSE algorithm to improve the performance of long-distance UWOC systems. Simulations show that the proposed algorithm has antinoise performance under different seawater qualities. In particular, with a bit error rate performance of 10 − 4 in coastal seawater, the signal-to-noise ratio of the proposed algorithm is 5.2 dB lower than the traditional constant-to-mode decision directed least-mean-square algorithm and 9.2 dB lower than that when the algorithm is not equalized.

Experimental demonstration of underwater wireless optical OFDM communication system with a single SPAD receiver

Generally, only binary modulation formats, such as on–off keying (OOK) and pulse position modulation (PPM), can be detected by a single photon avalanche diode (SPAD). In this paper, we propose to use a single SPAD receiver to achieve underwater wireless optical communication (UWOC) with high-order modulation formats. The feasibility is theoretically analyzed, and firstly demonstrated in a proof-of-concept orthogonal frequency division multiplexing (OFDM) experiment. The proposed OFDM system using quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) can work with less than 68 and 390 photons per symbol, and with receiver sensitivities below the forward error correction (FEC) limit as high as −84.5 dBm and −77.8 dBm, respectively. These extraordinarily sensitive results indicate that the proposed scheme is a promising solution to long-distance UWOC systems or internet of underwater things (IoUT).

Performance Analysis and Altitude Optimization of UAV-Enabled Dual-Hop Mixed RF-UWOC System

This paper presents and investigates a dual-hop mixed radio frequency (RF) and underwater optical communication (UWOC) system. In the proposed system, an unmanned aerial vehicle (UAV) such as drone (S) located at low altitude is transmitting data signal towards the destination (D) which is located under water such as a submarine through a decode and forward (DF) relay (R) mounted on a ship over the sea surface. The data signal is transmitted from the UAV towards the relay located on the ship via RF channel modelled by the Nakagami- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ m$</tex-math></inline-formula> distributed fading statistics. The relay decodes the received RF data signal and re-transmits it towards the destination via visible light channel modelled by Exponential Generalized Gamma (EGG) distribution. Using the DF relaying protocol, we derive closed form analytical expressions for the outage probability and average bit error rate of the proposed mixed RF-UWOC system. Additionally, UAV optimal altitude analysis has been carried out to find the optimal elevation angle corresponding to the optimal altitude of the UAV where the system performance is maximized. The numerical simulation is carried out to support the performance analysis of the mixed RF-UWOC system and shows the influence of the various channel parameters such as air bubbles, UAV altitude, water salinity variations and scintillation on the end to end performance of the mixed RF-UWOC system.

Dual-Hop Underwater Optical Wireless Communication System With Simultaneous Lightwave Information and Power Transfer

The most significant challenge of underwater optical wireless communication (UOWC) system is to overcome its limited coverage. To expand the achievable communication range, we investigate the performance of the dual-hop UOWC system with simultaneous lightwave information and power transfer (SLIPT). The time splitting (TS) method is adopted for wireless power transfer in the proposed system, where the information and energy are transmitted in different phases. A suitable transmission strategy is designed for the model without additional power supply, which contains three phases, i.e. information transmission, energy transmission, and forwarding process. The expressions of the average bit error rate (BER) at the target node and the energy harvested by the relay node are derived over underwater attenuation channel. Then, the effects of the TS factor and the distances on the system performance are investigated in two sub-problems, which minimize the average BER while satisfying the energy harvesting and transmitting rate constraints. Numerical results indicate the performance improvement by adopting the relay node with SLIPT.

Channel Distribution and Noise Characteristics of Distributed Acoustic Sensing Underwater Communications

Due to the power and transmission rate limitations, it is challenging to achieve underwater acoustic (UWA) communications over long distances from the unmanned underwater vehicle (UUV) to the shore station. As an alternative solution, the distributed acoustic sensing (DAS) underwater communication uses submarine optical cable to acquire and demodulate acoustic communication signals. There is little research on the channel and noise of DAS underwater communication. This paper analyzes the channel characteristics and the background noise of a DAS underwater communication system with armored optical cable based on the experimental measurements. Channel analysis shows that the envelope amplitudes of impulse responses follow the Burr distribution. The amplitudes get increased concentration at lower values as the sound source goes closer to the position right above the optical cable. Moreover, we observe that the received signals are influenced by the adjacent segments, leading to multiple arrivals within each acoustic bounce. Noise analysis shows that for frequencies lower than 1 kHz, unlike the Wenz noise spectrum primarily caused by shipping and wind, the −18 dB/octave decrease in power spectral density (PSD) of DAS noise is mainly caused by the equipment and cable themselves. The noise also exhibits full-band fluctuations. In underwater communications, full-band noise poses significant challenges for signal design. As far as we know, this article is the first report on the field experiment using a DAS underwater communication system equipped with armored optical cable, providing a reference for future underwater applications.

Experimental Demonstration of High-Sensitivity Underwater Optical Wireless Communication Based on Photocounting Receiver

In this paper, we propose a high-sensitivity long-reach underwater optical wireless communication (UOWC) system with an Mbps-scale data rate. Using a commercial blue light-emitting diode (LED) source, a photon counting receiver, and return-to-zero on–off keying modulation, a receiver sensitivity of −70 dBm at 7% FEC limit is successfully achieved for a 5 Mbps intensity modulation direct detection UOWC system over 10 m underwater channel. For 1 Mbps and 2 Mbps data rates, the receiver sensitivity is enhanced to −76 dBm and −74 dBm, respectively. We further investigate the system performance under different water conditions: first type of seawater (c = 0.056 m−1), second type (c = 0.151 m−1), and third type (c = 0.398 m−1). The maximum distance of the 2 Mbps signal can be extended up to 100 m in the first type of seawater.

Underwater Ultrasonic Multipath Diffraction Model for Short Range Communication and Sensing Applications

In acoustic underwater communication and sonar applications, obstacles inside and outside the line-of-sight (LOS) affect signal propagation. Both reflection and diffraction occur in underwater communication and measurement systems due to these obstacles. To the best of our knowledge, the influence of diffraction and reflection is neither described nor modeled for finite pulses yet. We propose and develop a multipath propagation model for spectral diffraction components and phase information of the received signal based on knife-edge diffraction together with reflections, transmission effects, and backscatter. This paper designs a short range underwater ultrasonic experimental system composed of an ultrasonic transceiver with wideband pulses and advanced spectral signal processing. We evaluate our proposed model with measurements made in a water tank with an obstacle moved between the transmitter and receiver. When the model includes all major propagation components and effects, it achieves an accuracy for localization of 97% of the results in the range of twice the obstacle diameter in our test setup.

200-m/500-Mbps underwater wireless optical communication system utilizing a sparse nonlinear equalizer with a variable step size generalized orthogonal matching pursuit.

Linear and nonlinear impairments in underwater wireless optical communication (UWOC) systems caused by the limited bandwidth and nonlinearity of devices severely degrade the system performance. In this paper, we propose a sparse Volterra series model-based nonlinear post equalizer with greedy algorithms to mitigate the nonlinear impairments and the inter-symbol interference (ISI) in a UWOC system. A variable step size generalized orthogonal matching pursuit (VSgOMP) algorithm that combines generalized orthogonal matching pursuit (gOMP) and adaptive step size method is proposed and employed to compress the Volterra equalizer with low computational cost. A maximum data rate of 500 Mbps is realized with the received optical power of -32.5 dBm in a 7-m water tank. In a 50-m swimming pool, a data rate of 500 Mbps over 200-m underwater transmission is achieved with a BER lower than the forward error correction (FEC) threshold of 3.8 × 10-3. The number of kernels of the sparse Volterra equalizer is reduced to 70% of that of the traditional Volterra equalizer without significant BER performance degradation. Compared with orthogonal matching pursuit (OMP) scheme and regularized orthogonal match pursuit (ROMP) scheme, the VSgOMP scheme reduces the running time by 68.6% and 29.2%, respectively. To the best of our knowledge, this is the first time that a sparse Volterra equalizer combined with VSgOMP algorithm is employed for the nonlinear equalization in a long-distance high-speed UWOC system.

Effect of the APD Receiver’s Tilted Angle on Channel Capacity for Underwater Wireless Optical Communications

This paper focuses on investigating the effect of the receiver’s tilted angle on the channel capacity of an underwater wireless optical communication (UWOC) system, in which an avalanche photodiode (APD) detector is adopted as the receiver. Under the non-negativity, peak power, and average power constraints, the lower bounds on the capacity of UWOC are derived in detail according to different average-to-peak power ratios. With modeling achieving the maximum of the lower bounds of the capacity as an optimization object, we prove that the proposed optimization issue is in fact a simple convex optimization about the tilted angle of the APD receiver, and then present related theoretical solution for it. Both theoretical analysis and simulation results show that by appropriately tilting the receiver, we can significantly enhance the final capacity performance of the UWOC with APD receiver.

Scintillation and BER analysis of cosine and cosine-hyperbolic-Gaussian beams in turbulent ocean.

Effects of source beam, link, and oceanic turbulence parameters on the scintillation index and bit error rate (BER) performance of cosine (cos) and cosine-hyperbolic (cosh) Gaussian light beams have been investigated in order to improve wireless optical communication link performance in oceanic turbulence. The Nikishov and Nikishov power spectrum of oceanic water and extended Huygens Fresnel principle were used in our evaluations; the results were obtained via MATLAB. The scintillation index and BER were examined versus oceanic turbulence parameters, which are the rate of dissipation of mean-square temperature, the ratio of temperature and salinity contributions to the refractive index spectrum, and the dissipation rate of kinetic energy per unit fluid mass of fluid. Further, the scintillation index and BER are investigated against the source size, propagation distance, and complex displacement parameters of cos- and cosh-Gaussian beams. This study aimed to select the suitable sinusoidal beam to be employed in order to increase the performance of underwater wireless optical communication systems operating in oceanic turbulence.

Analytical Performance Evaluation of GFDM in Underwater Acoustic Communication Systems

The rapid growth of the Internet of Things (IoT) has extended its concept to underwater environments. However, the implementation of these systems via wired communication still represents a technological challenge, mainly due to the high cost of their deployment. Therefore, wireless communications are seen as an interesting solution for the deployment of underwater communications systems. Preliminary research indicated that underwater acoustic wireless communication could be used for some Internet of Underwater Things applications, mainly due to the wide range of communications involved. However, a significant disadvantage of acoustic systems is their low transmission data rate; thus, studies and analyses to improve this disadvantage must be carried out. Considering that new waveforms have been proposed to improve the performance of terrestrial wireless communications systems, this work presents the development of general analytical expressions that allow the performance evaluation of the Generalized Frequency Division Multiplexing (GFDM) waveform in underwater environments. These analytical expressions were obtained considering a continuous-time model for the GFDM signal and modeling the underwater acoustic communication channel as a time-varying multipath channel. Numerical results were obtained for many different systems and channel parameters, allowing a quantitative evaluation of the system performance degradation.

WIRELESS COMMUNICATION FOR UNDERWATER CRAFT USING LI-FI TECHNOLOGY

Data transmission is generally done with the help of a network. In many applications such as underwater communication, environmental monitoring etc., the wireless communication is necessary because of the problems associated with installation and maintenance of the traditional wired network. Authors have proposed the application of LI-FI technology for underwater communication. The proposed system design is presented in this paper. The present underwater communication systems, are based on the electromagnetic, acoustic and optical signals. To overcome the disadvantages of these systems, the LI-FI based communication system is proposed by the authors for underwater communication.

Average capacity of an absorptive turbulent ocean channel with diffraction-and attenuation-resistant beam carriers

The absorption and turbulent diffraction of seawater are important factors that restrict the channel capacity of an underwater optical communication system. Therefore, we derive the model for the capacity of the turbulent seawater communication channel with a diffraction-and attenuation-resistant random (DARR) vortex beam as a signal carrier, and study the influence of absorbing turbulent seawater on the capacity of optical communication channel. DARR beam is a large-scale Gaussian aperture truncated diffracting and absorption resistance waves. We find that with the increase of the number of channels, the total loss of channel capacity caused by absorptive turbulent ocean channel is basically unchanged, the optical communication channel with longer wavelength and smaller receiving diameter has higher channel capacity. Compared with the Laguerre–Gaussian beam and the Bessel–Gaussian beam, DARR beams are more resistant to seawater turbulence and seawater absorption.

Capacity of underwater optical wireless communication systems over salinity-induced oceanic turbulence channels with ISI.

Point-to-point underwater optical wireless communication (UOWC) links are mainly impaired by scattering due to impurities and turbidity in the open water, resulting in a significant inter-symbol interference (ISI) that limits seriously both channel capacity and the maximum practical information rate. This paper conducts, for the first time, the channel capacity analysis of UOWC systems in the presence of ISI and salinity-induced oceanic turbulence when the undersea optical channel is accurately modeled by linear discrete-time filtering of the input symbols. In this way, novel upper and lower bounds on channel capacity and mutual information are developed for non-uniform on-off keying (OOK) modulation when different constraints are imposed on the channel input. The results show that the capacity-achieving distribution, which is computed through numerical optimization, is discrete and depends on the optical signal-to-noise-ratio (SNR). Moreover, a non-uniform input distribution significantly improves the channel capacity of such systems affected by ISI and oceanic turbulence, especially at low optical SNR. Monte Carlo techniques are employed to test the developed bounds for different undersea optical channels with one, two and three casual ISI coefficients.

Efficient identification of orbital angular momentum modes carried by Bessel Gaussian beams in oceanic turbulence channels using convolutional neural network

As one of the spatial dimensions, orbital angular momentum (OAM) expands the channel capacity of underwater wireless optical communication (UWOC) system. However, the performance of the OAM-UWOC system is degraded due to the distortion of the OAM by oceanic turbulence. In this paper, we present a 7-layer convolutional neural network (CNN) and validate its ability to identify OAM patterns carried by Bessel Gaussian (BG) beams in oceanic turbulence channels. The trained CNN performs the task of recognizing OAM patterns with an accuracy of over 99% under weak-to-moderate oceanic turbulence at 100 m. Furthermore, in order to demonstrate the performance and robustness of the designed CNN, the effects of training sample number, epoch, turbulence intensity, balance parameter, transmission distance, type of training dataset, and coding method on the recognition ability of CNN are fully investigated. We hope that the method devised in our work will contribute to UWOC and facilitate its further development.

Quasi-omnidirectional transmitter for underwater wireless optical communication systems using a prismatic array of three high-power blue LED modules.

In this study, a quasi-omnidirectional underwater wireless optical communication (UWOC) system is implemented with a prismatic array consisting of three uniformly distributed high-power LED modules as the transmitter. Over a 10-m underwater channel in a 50-m standard swimming pool, a data rate of 22 Mbps is achieved without adopting any digital signal processing algorithm. With zero forcing (ZF) based frequency domain equalization (FDE) and a maximum ratio combining (MRC) algorithm, the maximum net data rates achieved are 69.65 Mbps, 39.8Mbps and 29.85 Mbps over 10-m, 30-m, and 40-m underwater channels, respectively. In the proposed UWOC system, the receiver could successfully capture optical signals at different directions from the transmitter and the bit error rates (BERs) measured in different directions show small fluctuations. The proposed system could meet the demands of high-speed data transmission among units in a swarm-robot system and last meter user access in an underwater optical cellular network system.

Improving the optical link for UVLC using MIMO technique

Abstract This paper proposed a theoretical treatment to study underwater wireless optical communications (UWOC) system with different modulation schemes by multiple input-multiple output (MIMO) technology in coastal water. MIMO technology provides high-speed data rates with longer distance link. This technique employed to assess the system by BER, Q. factor and data rate under coastal water types. The reliability of the system is examined by the techniques of 1Tx/1Rx, 2Tx/2Rx, 3Tx/3Rx and 4Tx/4Rx. The results shows the proposed technique by MIMO can get the better performance compared with the other techniques in terms of BER. Theoretical results were obtained to compare between PIN and APD photodetectors. Besides, 32-PPM is robust and considers a suitable modulation scheme for obtaining the low BER and high Q. factor in LoS scenarios.