Underwater Communication System Research Articles

Toward RIS-Enhanced Integrated Terrestrial/Non-Terrestrial Connectivity in 6G

The next generation of wireless systems will take the concept of communications and networking to another level through the seamless integration of terrestrial, aerial, satellite, maritime and underwater communication systems. Reconfigurable intelligent surface (RIS) is an innovative technology which, with its singular features and functionalities, can expedite the realization of this everywhere connectivity. Motivated by the unparalleled properties of this innovatory technology, this article provides a comprehensive discussion on how RIS can contribute to the actualization and proper functioning of future integrated terrestrial/non-terrestrial (INTENT) networks. As a case study, we explore the integration of RIS into non-orthogonal multiple access (NOMA)-based satellite communication networks and demonstrate the performance enhancement achieved by the inclusion of RIS via numerical simulations. Promising directions for future research in this area are set forth at the end of this article.

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.

Integrated Underwater Detection and Communication System Based on P4 Code-Modulated OFDM Signal

Because of the requirements of marine resources exploration and underwater cooperative operation, the underwater operation of common detection and communication equipment is difficult and unreliable. Therefore, it is urgent to establish an accurate underwater target detection and multi-node communication integrated system. An integrated method of underwater electric field detection and communication based on P4 code-modulated OFDM signal is proposed in response to the above requirements. The working principle, system structure, and signal processing of underwater electric field detection and electric field communication are similar. This article uses detection signal P4 code to modulate the phase of OFDM communication signal, thus realizing the integration of Underwater Detection and Communication System (IUDCS). The simulation results show that IUDCS can meet the detection range and velocity characteristics of underwater dynamic targets. The simulation results also meet the requirements for communication error rate and symbol error rate, thereby verifying the performance of underwater electric field IUDCS. This method is necessary for the realization of IUDCS in terms of reducing platform size, reducing power consumption, and enhancing concealment. Moreover, it shows good application prospects in underwater robot cluster operations.

A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems

In the past decades, researchers/scientists have paid attention to the physical layer of underwater communications (UWCs) due to a variety of scientific, military, and civil tasks completed beneath water. This includes numerous activities critical for communication, such as survey and monitoring of oceans, rescue, and response to disasters under the sea. Till the end of the last decade, many review articles addressing the history and survey of UWC have been published which were mostly focused on underwater sensor networks (UWSN), routing protocols, and underwater optical communication (UWOC). This paper provides an overview of underwater acoustic (UWA) physical layer techniques including cyclic prefix orthogonal frequency division multiplexing (CP-OFDM), zero padding orthogonal frequency division multiplexing (ZP-OFDM), time-domain synchronization orthogonal frequency division multiplexing (TDS-OFDM), multiple input multiple output orthogonal frequency division multiplexing (MIMO-OFDM), generalized frequency division multiplexing (GFDM), unfiltered orthogonal frequency division multiplexing (UF-OFDM), continuous phase modulation orthogonal frequency division multiplexing (CPM-OFDM), filter bank multicarrier (FBMC) modulation, MIMO, spatial modulation technologies (SMTs), and orthogonal frequency division multiplexing index modulation (OFDM-IM). Additionally, this paper provides a comprehensive review of UWA channel modeling problems and challenges, such as transmission loss, propagation delay, signal-to-noise ratio (SNR) and distance, multipath effect, ambient noise effect, delay spread, Doppler effect modeling, Doppler shift estimation. Further, modern technologies of the physical layer of UWC have been discussed. This study also discusses the different modulation technology in terms of spectral efficiency, computational complexity, date rate, bit error rate (BER), and energy efficiency along with their merits and demerits.

90-m/560-Mbps underwater wireless optical communication utilizing subband multiple-mode full permutation CAP combined with an SNR-weighted detector and multi-channel DFE.

In this paper, a joint signal processing scheme including a subband multiple-mode full permutation carrierless amplitude phase modulation (SMMP-CAP), signal-to-noise ratio weighted detector (SNR-WD), and multi-channel decision feedback equalizer (MC-DFE) is proposed to mitigate the bandwidth limitation of a high-speed long-reach underwater wireless optical communication (UWOC) system. Referring to the trellis coded modulation (TCM) subset division strategy, 16 quadrature amplitude modulation (QAM) mapping set is divided into four 4-QAM mapping subsets by SMMP-CAP scheme. An SNR-WD and an MC-DFE are employed to enhance the demodulation effect of this system in a fading channel. In a laboratory experiment, the minimal required received optical powers (ROPs) for data rates of 480 Mbps, 600 Mbps, and 720 Mbps, at hard-decision forward error correction (HD-FEC) threshold of 3.80 × 10-3, are -32.7 dBm, -31.3 dBm, and -25.5 dBm, respectively. Moreover, the proposed system successfully achieves a data rate of 560 Mbps in a swimming pool with a transmission distance up to 90 m and a total attenuation measured to be 54.64 dB. To the best of our knowledge, this is the first time to demonstrate a high-speed, long-distance UWOC system by employing an SMMP-CAP scheme.

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.

OCDMA transmission-based underwater wireless optical communication system: performance analysis

This paper introduces a novel underwater (UW) optical communication system that utilizes optical code division multiple access transmission technique using the permutation vector code. Three scenarios of water are considered, pure sea (PS), clear ocean (CL), and coastal ocean (CO). The performance is evaluated analytically in terms of bit error rate (BER), received power, signal to noise ratio for different UW links and data rates. The results show that the shortest UW range is achieved in the case of CO, achieving the highest extinction ratio compared to CL and PS. Considering a BER below the forward error correction (FEC) limit of 3 × {10}^{-3}, the maximum UW ranges reached are 21 m for PS, 12 m for CL, and 8 m for CO at 3 Gbps with 20 dBm transmitted power. The propagation range could be increased when the transmitted power is increased to 25 dBm, achieving the ranges of 31 m, 18 m, and 9 m, for PS, CL, and CO, respectively.

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

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

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

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

Underwater optical wireless communication system performance improvement using convolutional neural networks

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

Adaptive Diversity Algorithm Based on Block STBC for Massive MIMO Link Misalignment in UWOC Systems

In the past few years, underwater wireless optical communication (UWOC) has become a promising wireless communication technology in the underwater environment. Aiming at the problem formulation of sub-channel correlation enhancement occurring due to the joint impact of underwater link misalignment and turbulence in the process of optical signal transmission in an underwater optical massive multiple-input multiple-output (MIMO) system, we propose an adaptive diversity approach depending on partition space time block code (STBC). STBC technology is used to reduce the random fading of optical signals caused by turbulence. At the same time, the channel correlation occurring due to channel misalignment is effectively alleviated by adaptive processing. The adaptive diversity algorithm based on segmented STBC effectively improves the reliability and decrease complexity of underwater optical Massive MIMO communication systems. It determines the particular link misalignment degree by the channel gain matrix obtained from the channel estimation and selects different combinations of detectors according to the degree of misalignment to obtain the maximum gain of the received signal combination. Compared with the chunking scheme, simulation and result shows that the adaptive diversity algorithm improves the tolerance of the system to the link misalignment error from 30 mm to 60 mm under the same condition number of channel gain matrix, and it can still demodulate the source signal directly without requiring detection algorithm in case of a large error in the link misalignment.

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

We provide an experimental study of the channel characteristics in an underwater wireless optical communication (UWOC) system given by a 35-m transmission distance in various water types. A UWOC system using a low-power 488-nm laser diode is established to comprehensively evaluate the influence of the link distance, water turbidity, receiver parameters, and link misalignment on the communication link power. The results show that the link misalignment-induced power loss is significant and the effect of receiver parameters on power is limited. However, a higher turbidity or longer signal transmission distance can help to reduce the link misalignment effect and manifest the receiver parameters effect on the received power. In particular, in turbid waters with an attenuation coefficient of 0.471 m − 1, the change of the signal reception power curve is small over a certain degree of the link misalignment in the 35-m physical distance, and the reasonable configuration of receiver parameters effectively improves the signal reception quality in the UWOC system. These results can provide theoretical guidance for optimizing the UWOC system.

AWOA: An Advanced Whale Optimization Algorithm for Signal Detection in Underwater Magnetic Induction Multi-Input–Multi-Output Systems

With the increasing exploitation and use of marine resources, the limitations of acoustic, optical, and radio frequency technologies for underwater communications have become increasingly apparent. Magnetic induction (MI) is a new communication technology that enables wireless data transmission via magnetic field coupling between transmitting and receiving coils. MI offers advantages such as channel stability, small antenna size, and no multi-path loss. Multi-input–multi-output (MIMO) is a multi-antenna technology that significantly increases system capacity and spectrum utilization without increasing bandwidth. The whale optimization algorithm (WOA) is a well-known bio-inspired algorithm that mimics the hunting behavior of whales to optimize swarm intelligence. This paper proposes a model for an underwater MIMO communication system based on magnetic induction. We then construct a signal detection algorithm for MI-MIMO systems using the advanced whale optimization algorithm (AWOA) and conduct simulation experiments to compare the performance and complexity of three standard signal detection algorithms: zero-forcing (ZF), minimum mean square error (MMSE), and maximum likelihood (ML). The experimental results show that AWOA achieves suboptimal results, as its bit error rate (BER) is close to that of the ML algorithm. Furthermore, the complexity of AWOA is comparable to that of the MMSE strategy. This work supports the development of a high-performance MI-based underwater communication system.

Modeling and Simulation of SWARA Path Loss Model for Underwater Acoustic Communication in Multipath Environment

Effective communication in underwater environments is essential for numerous applications, including marine research, oil and gas exploration, and underwater surveillance. However, signal loss during propagation is a significant challenge for underwater communication systems. Accurate mathematical models are necessary to develop and optimize communication systems for underwater environments. This research proposes a mathematical model, SWARA, that incorporates crucial parameters such as transmitter and receiver responses, channel conditions, node placements, and signal frequency to accurately model short-range losses in underwater medium. We conducted tank trials at the UWAA Lab, CARE, IIT Delhi to validate our simulation results, using a chirp signal with a frequency ranging from 25–35 kHz. Our simulation results show that the SWARA model effectively predicts short-range propagation losses in underwater media. Compared to experimental losses, the maximum and minimum simulated losses deviated by only − 0.18–0.10 W/Hz and − 0.36–0.19 W/Hz, respectively, indicating the accuracy of the model. Our tank trials further revealed the significant impact of projector and hydrophone placements on incurred losses, highlighting the importance of careful placement in the design of underwater communication systems. In conclusion, this research proposes the SWARA model for studying short-range losses in underwater tank, which effectively incorporates crucial parameters for accurate modeling. The model has practical applications in the design and optimization of underwater communication systems for short-range applications. This research provides important insights for the development of effective underwater communication systems.

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

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

Self-interference cancellation in underwater acoustic communications systems using orthogonal pilots in IBFD

This paper proposes a Self-interference (SI) cancellation system model of Underwater acoustic (UWA) communication for in-band full-duplex (IBFD) technology. The SI channel is separated from the Far channel by exploiting a concurrently orthogonal pilot channel estimation technique using two orthogonal frequency-division multiplexing (OFDM) blocks to establish orthogonality between them based on a unitary matrix. Compared to the half-duplex channel estimator, the mean squared error (MSE) and the bit error rate (BER) provided strong evidence for the efficiency of the proposed SI cancellation. Since full-duplex systems are more efficient than half-duplex ones, the proposed approach might be seen as a viable option for them. The proposed method proved effective when used with a fixed full-duplex (FD) position and FD shifting of up to 4°. Different channel lengths and distances are adopted to evaluate the proposed method. Initial findings indicate that MSE for the SI channel minimum mean-square error (MMSE) estimator at 20 dB is 0.118 · 10−3, for fixed FD. In addition, this paper presents a geometry channel model for the Far channel in the IBFD underwater communication system that describes the propagation delay of the multipath reflection. The simulation results for the multipath propagation delay spread are similar to the traditional results, with the delay spread of the suggested model reaching (79 ms), which is close to the Bellhop simulator result (78 ms).

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

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

Ray tracing of long-range underwater acoustic vortex wave propagation

The underwater acoustic communications environment is severely band-limited, which leads to a bottleneck in data transfer. Existing methods of data transfer in underwater acoustic communications applications typically rely primarily on conventional temporal and frequency modulation techniques and achieve bit rates peaking at approximately 40 kb/s. One method of easing the bottleneck and increasing the data rate is to explore further potential degrees of freedom which may be utilized. Acoustic orbital angular momentum (OAM) is a physical quantity that characterizes the rotation in a propagating helical pressure wavefront. The unique phase patterns of OAM carrying vortex waves form an orthogonal basis which may be useful as an additional degree of freedom in acoustics communications applications; however, the long-distance propagation of these waves is largely unstudied. By employing BELLHOP’s ray tracing algorithm, the dominant features of a propagating OAM carrying vortex wave are tracked over long ranges (to and beyond 1 km) under various environmental conditions. This provides essential guidance in the design of the sending and receiving arrays of high-speed underwater communications systems, which rely on multiplexing acoustic OAMs.

Analysis of fluid excited noise characteristics of bulkhead opening of an Underwater Unmanned Vehicle

Acoustic stealth performance is an important performance index of Underwater Unmanned Aerial Vehicles. Due to equipment installation and other reasons, there will be a sea port on the bulkhead of the aircraft. Because the sea port destroys the flow field on the surface of the aircraft and produces noise, which affects the underwater communication of the aircraft. In this paper, the numerical simulation method is used to analyze the flow induced noise characteristics of the bulkhead of an underwater unmanned vehicle, and the main noise frequency characteristics at the bulkhead of the estuary are obtained, which provides data reference for the structural design of the estuary and the layout of the underwater communication system.

Very-Low-Frequency Magnetoelectric Antennas for Portable Underwater Communication: Theory and Experiment

Finding an efficient way for underwater communication with a portable antenna at very low frequency (VLF 3–30 kHz) is challenging since the conventional electrical antennas require the size to be larger than 1/10 of the wavelength. Recently, acoustically driven antennas were proposed to realize portable VLF communication in air but lack the demonstration in a lossy environment. Here, we reported the first VLF underwater communication system based on a pair of acoustically actuated magnetoelectric (ME) antennas with small size of 10 cm in length. A theoretical analysis of reflection and radiation performance of the ME antenna was conducted, where the electromechanical resonance (EMR) frequency and effective magnetic dipole moment were estimated, and a near-field coupling model of a pair of ME antennas was further established. The results of theoretical predictions and finite element model (FEM) simulations were then compared with experimental measurements and their differences were discussed. A prototype of underwater communication system based on the ME antenna pair was finally presented, where a binary digital modulation with a bit rate of 100 b/s has been demonstrated, confirming the feasibility of ME antennas for portable underwater communication.