Communication In Shallow Water Research Articles

Modeling and channel estimation for piezo-acoustic backscatter assisted underwater acoustic communications

Relying on direct and converse piezoelectric effects, piezo-acoustic backscatter (PAB) technology reflects ambient acoustic signals to enable underwater backscatter communications at near-zero power, which was first realized through a prototype. In this paper, we propose a mathematical model of the PAB assisted underwater acoustic (UWA) communication, and address the sparse channel estimation problem. First, we present a five-stage backscatter process to derive the backscatter coefficient, and propose the channel model for the shallow-water communications. Then, we formulate the shallow-water acoustic channel estimation problem as a sparse vector recovery one according to the compressed sensing theory, and leverage the orthogonal matching pursuit (OMP) algorithm to obtain the channel estimator. Finally, simulation results are provided to corroborate our proposed studies.

Background Noise Resistant Underwater Wireless Optical Communication Using Faraday Atomic Line Laser and Filter

High sensitive underwater wireless optical communication (UWOC) in shallow water or FSO-UWOC convergent applications suffers severely from solar noise and other background noise interference. This paper presents experimental demonstrations of broadband background noise resistant air-water and underwater wireless optical communication by using a pair of spectrum matched 852 nm Cs atomic Faraday line laser and filter, with a narrow passband of only 1 GHz. Experimental results show that by using the Faraday atomic line laser and filter, the Q factor can be improved by 3.37 dB compared to using the interference filter, and 14.67 dB compared to system without using noise resistant filter. A numerical model for UWOC system performance affected by broadband background noise with various filtering conditions is presented, where both the numerical and experimental results match each other quite well. Based on the experimental results substantiated model, performance of UWOC under solar noise interference is numerically studied. The numerical results show that under strong solar noise interference, the maximum depth of the receiver is extended from 0.77 m to 3.79 m for FSO-UWOC system and from 0.87 m to 3.54 m for UWOC system. It proves that FADOF can provide considerable performance improvement for both UWOC and FSO-UWOC system with broadband background noise interferences.

Influence of ice concentration and thickness on under-ice ambient noise levels in shallow coastal waters of Liaodong Bay, China

Underwater noise levels may be strongly influenced by sea ice. This study presents systematic measurements of under-ice ambient noise levels in shallow (<30 m) coastal waters using passive acoustic technology. Acoustic data were collected at 6 sites with different ice concentration (two levels: incomplete and complete ice cover) and thickness (three levels: <10 cm, 10–20 cm and >20 cm) in Liaodong Bay, China. Wind (max = 5.3 m/s) and current (max = 0.3 m/s) conditions were very weak, making minimal contributions to under-ice noise in this study. Root-mean-squared broadband (20–10,000 Hz) noise levels (BNLs) and one-third-octave-band noise levels (TOLs) at 63, 125 and 2000 Hz were measured. Median BNLs of the 6 recording sites ranged from 104.5 to 129.1 dB re 1 μPa, and median TOLs at 63, 125 and 2000 Hz were between 76.5 and 118.0 dB re 1 μPa at the 6 sites. Generalized Linear Models were used to analyze broadband and three one-third-octave-band noise levels in different ice concentration and thickness levels. The results showed that, regardless of the ice thickness, the sites with incomplete ice concentration had higher BNLs and 63, 125 and 2000 Hz TOLs than the sites with complete ice concentration, and regardless of the ice concentration, both BNLs and all three TOLs decreased with increased ice thickness. This study provides baseline characterization of under-ice noise in shallow coastal waters and indicating that both ice concentration and thickness have significant influence on the under-ice noise levels. These findings are important for future studies investigating acoustic communication and noise pollution in shallow coastal waters with sea ice.

A study on unpredictable shallow water channel behavior by various multipath patterns and experimental data analysis

In shallow water applications, multipath signal propagation is a major concern for robust communication. Multipath signal propagation is not explicitly seen in the channel, even though it is a main contributor for signal degradation. This fact motivated us to simulate multipath patterns to understand its influence in short-range communication. In this paper, a three-path signal propagation model is presented, where, besides the line of sight (LOS) signal, other two non-line of sight (NLOS) signals contact any point of channel boundary to reach the receiver. In simulation, the combination of eight possible multipath patterns is converged to estimate the received signal. A source fixed in water-tank periodically transmits low frequency acoustic signals 1 kHz and 1.5 kHz to the channel, and the receiver records them. The experiment was repeated for various input signal strengths. It has been observed that the simulation results coincide with the measured values. The good reception is noticed for signal frequency 1 kHz at 2.5m and 1.5 kHz at 1.2m. This study identifies the optimal signal strength for better signal reception in short range, which drives to the establishment of high-quality communication in shallow water.

Modification in the Range of Information Potentials for Blind Algorithms in Impulsive Noise Environments

In this paper a modification approach to the range of influence for information potentials (IP)of blind decision feedback equalizer (DFE) algorithms is presented to cope better with impulsive noise and severe channel distortion. Analysis of the IP range of influence reveals that the range should be applied separately based on the roles under impulsive noise. A new cost function that can manage ranges of information potentials is proposed, and a related DFE algorithm is derived. Simulation results from the environment of a severe multipath channel obtained from shallow-water communication experiments and impulsive noise show that the proposed DFE algorithm yields more than 10 dB in MSE gain by employing the proposed modification toranges of information potentials.

An Experimental Study of Passive Time Reversal Process for Underwater Acoustic Communication in Shallow Waters of the Bay of Bengal

An Experimental Study of Passive Time Reversal Process for Underwater Acoustic Communication in Shallow Waters of the Bay of Bengal

Performance analysis of an improved modal dispersion compensation receiver for low-frequency and long-range shallow water communication experiment

Low-frequency acoustic signal will suffer from dispersion effect when propagating in long-range shallow water waveguide, which leads to multi-mode and waveform expanding. Longer transmission range and shallower waveguide will result in more serious dispersion effect. In this paper, we propose an improved modal dispersion compensation receiver, and evaluate its performance for a shallow water acoustic communication experiment. The receiver not only exploits time-frequency analysis approach to compensate the waveform expanding effect of each mode simultaneously, but also combines the modified multi-mode energy to enhance symbol estimation accuracy. The experiment is conducted by a differential-binary-phase-shift-keying modulated direct-sequence-spread-spectrum communication system, which is suitable for long-range transmission. With this receiver, we obtain no error demodulation results without coding in the experiment. The experimental data analysis also shows that the proposed receiver can obtain about 3 dB gain than the conventional one, which is of significant importance for long-range communications.

Guest Editorial Underwater Acoustic Propagation Physics and Signal Processing Techniques for Shallow Water Acoustic Communications

The papers in this special issue focus on the rapidly evolving techniques on shallow water acoustic communications, and how underwater acoustic (UWA) propagation models based on shallow water physics may be harnessed with signal processing techniques for high-resolution channel estimation, equalization, and resolving cross-layer challenges to achieve high-data-rate communications in shallow water. The definition of shallow water is relatively broad in scope, and not limited to any particular frequency range. Any UWA channel that exhibits significant multipath and rapid fluctuations in delay spread can be included in the paradigm of interest.

Very-High-Frequency Single-Input–Multiple-Output Acoustic Communication in Shallow Water

Very-high-frequency (VHF) underwater acoustic communication is a relatively unexplored terrain, but its importance is expected to increase with the growing demands for larger bandwidths and higher data rates. Knowledge of VHF propagation channels is a prerequisite to achieve high data throughput. In this paper, we present time-variant angle-resolved impulse responses for a set of 250-kHz shallow-water channels. The measurements were performed at high resolution using 64 hydrophones arranged in a line array. The measured channels are characterized by delay spreads of several milliseconds, Doppler spreads of tens of hertz, and angular spreads up to 30 °. The angular spread is not separable from the delay-Doppler spread, and the impulse response depends heavily on the direction of arrival. Different beamforming and multichannel equalizer strategies are compared for a single-input-multiple-output configuration. A method combining subarray beamforming and multichannel equalization delivers in both noise- and interference-limited scenarios, transferring data at 78.125 kb/s over ranges up to 770 m.

MIMO Underwater Acoustic Communication in Shallow Water with Ice Cover

Although multiple-input multiple-output (MIMO) underwater acoustic (UWA) communication has been intensively investigated in the past years, existing works mainly focus on open-water environment. There is no work reporting MIMO acoustic communication in under-ice environment. This paper presents results from a recent MIMO acoustic communication experiment which was conducted in Bohai Gulf during winter. In this experiment, high frequency MIMO signals centered at 10 kHz were transmitted from a two-element source array to a four-element vertical receiving array at 1 km range. According to the received signal of different array elements, MIMO acoustic communication in under-ice environment suffers less effect from co-channel interference compared with that in open-water environment. In this paper, time reversal followed by a single channel decision feedback equalizer is used to process the experimental data. It is demonstrated that this simple receiver is capable of realizing robust performance using fewer hydrophones (i.e. 2) without the explicit use of complex co-channel interference cancelation algorithms, such as parallel interference cancelation or serial interference cancelation.

Time reversal communication using vertical particle velocity and pressure signals in shallow water

Acoustic communication in shallow water is characterized by multipath channels, which cause significant delay spreading, leading to inter-symbol interference. This inter-symbol interference necessarily causes a significant degradation in communication performance. Although a time reversal technique has been reported to produce satisfactory performance in multipath dominant environments, this technique requires a large receiver array covering the water column to achieve reliable communication performance. In this paper, a time reversal single-input multiple-output system using the pressure signal and the vertical component of the particle velocity is presented. The vertical component of the particle velocity was estimated using the finite difference in gradient between pressures measured by two vertically adjacent receivers. The experiment was conducted in shallow water off the south coast of Korea, where the water depth is 59 m and the bottom consists of silty clay. The results showed that the time reversal communication system based on vector quantities performed better than systems where only the pressure signals were used.

2P6-16 High Data Rate Scheduling for Underwater Acoustic Communication in Shallow Water(Poster Session)

2P6-16 High Data Rate Scheduling for Underwater Acoustic Communication in Shallow Water(Poster Session)

MTSW-SSTF: a wireless multimedia transmission scheme based on self-separation of time-frequency mode for shallow water

The random obstacles in the shallow sea environment, the irregular underwater pavement and the high quality requirements of multimedia transmission make the multimedia applications of shallow sea face the problems of high bit error rate, low transmission rate and low video quality. In order to solve these problems, this paper proposes a multimedia transmission mechanism and its architecture for wireless communication in shallow water based on time-frequency mode autonomous separation. Firstly, based on the complex and changeable seabed structure, the underwater biota movement track and the dynamic topology of end to end communication, a shallow sea wireless multimedia transmission system is constructed. Secondly, based on the performance of multimedia streaming in the time domain and frequency domain, a real-time multimedia transmission control mechanism for the time frequency separation of autonomous controlled multimedia signals from FS and TS is proposed. Finally, the simulation experiment and the field test results of shallow sea show that the proposed algorithm has superior performance in transmission rate, transmission efficiency, delivery rate and real-time performance.

Acoustic communication in marine shallow waters: testing the acoustic adaptive hypothesis in sand gobies.

Acoustic communication is an important part of social behaviour of fish species that live or breed in shallow noisy waters. Previous studies have shown that some fish species exploit a quiet window in the background noise for communication. However, it remains to be examined whether hearing abilities and sound production of fish are adapted to marine habitats presenting high hydrodynamism. Here, we investigated whether the communication system of the painted (Pomatoschistus pictus) and the marbled (Pomatoschistus marmoratus) gobies is adapted to enhance sound transmission and reception in Atlantic shallow water environments. We recorded and measured the sound pressure levels of social vocalisations of both species, as well as snapshots of ambient noise of habitats characterised by different hydrodynamics. Hearing thresholds (in terms of both sound pressure and particle acceleration) and responses to conspecific signals were determined using the auditory evoked potential recording technique. We found that the peak frequency range (100-300 Hz) of acoustic signals matched the best hearing sensitivity in both species and appeared well adapted for short-range communication in Atlantic habitats. Sandy/rocky exposed beaches presented a quiet window, observable even during the breaking of moderate waves, coincident with the main sound frequencies and best hearing sensitivities of both species. Our data demonstrate that the hearing abilities of these gobies are well suited to detect conspecific sounds within typical interacting distances (a few body lengths) in Atlantic shallow waters. These findings lend support to the acoustic adaptive hypothesis, under the sensory drive framework, proposing that signals and perception systems coevolve to be effective within local environment constraints.

Message Collision Avoidance Protocols for Detecting Stray Nodes in a Scuba Diving Group Using Ultrasonic Multi-Hop Message Communication

Recent years have seen a growing interest in underwater communication and some progress has been made in this area. However, underwater communication is still immature compared with terrestrial communication. A prime reason for this is that the underwater environment is intrinsically not suitable for propagation of electric waves. Instead, ultrasonic waves are mainly used for underwater communication. Since ultrasonic waves cannot provide sufficient communication speed or capacity, they cannot use existing network technologies, which assume use of radio waves. In particular, communication in shallow water is still an uncharted territory. Few communication technologies are employed in environments where people enjoy scuba diving. This paper addresses problems faced by recreational scuba divers. It proposes constructing an ad hoc mesh-shaped network between divers within a group and use ultrasonic waves as transmission media in order to enable the detection of a stray diver. It also proposes a communication protocol in which messages are relayed in multiple hops, and a message collision avoidance method, which is intended to reduce the rate of packet loss caused by message propagation delay. We have implemented the proposed methods in a network simulator, and compared them with an existing communication method that has no message collision avoidance function, in terms of the packet loss rate, the stray driver detection rate, and the rate of the ability to communicate in multiple hops.

Estimation and equalization for shallow water communication channel using geometric encoding of channel multipath

The shallow water acoustic channel is well-known to exhibit rapid fluctuations in its impulse response, which is challenging to estimate and compensate for in real time. In particular, the moving ocean surface and static sea bottom reflect the acoustic signal in unpredictable ways to create rapidly time-varying multipath arrivals. The talk will focus on the nexus of shallow water acoustic propagation paths and how multipath can be discovered and exploited to improve shallow water acoustic communications. In particular, the talk will review sampling strategies for the time-varying shallow water acoustic channel and propose novel geometric encoding techniques that exploit non-uniform channel sampling strategies. We will also demonstrate how geometric encoding can be harnessed for real-time channel estimation as well as semi-blind channel equalization. Results based on experimental field data in recent work as well simulation results will be presented.

Experimental Demonstration of Long-Range Underwater Acoustic Communication Using a Vertical Sensor Array.

This paper proposes a composite channel virtual time reversal mirror (CCVTRM) for vertical sensor array (VSA) processing and applies it to long-range underwater acoustic (UWA) communication in shallow water. Because of weak signal-to-noise ratio (SNR), it is unable to accurately estimate the channel impulse response of each sensor of the VSA, thus the traditional passive time reversal mirror (PTRM) cannot perform well in long-range UWA communication in shallow water. However, CCVTRM only needs to estimate the composite channel of the VSA to accomplish time reversal mirror (TRM), which can effectively mitigate the inter-symbol interference (ISI) and reduce the bit error rate (BER). In addition, the calculation of CCVTRM is simpler than traditional PTRM. An UWA communication experiment using a VSA of 12 sensors was conducted in the South China Sea. The experiment achieves a very low BER communication at communication rate of 66.7 bit/s over an 80 km range. The results of the sea trial demonstrate that CCVTRM is feasible and can be applied to long-range UWA communication in shallow water.

An Overview of Underwater Time-Reversal Communication

Time reversal (TR) exploits spatial diversity to achieve spatial and temporal focusing in complex environments. Over the last decade, the TR concept has been applied successfully to phase-coherent acoustic communications in time-varying multipath ocean environments, as an alternative to conventional adaptive multichannel equalization. Temporal focusing (pulse compression) mitigates the intersymbol interference (ISI) and subsequent single-channel equalization removes the residual ISI, thus providing nearly optimal performance in theory. The spatial focusing capability facilitates multiuser or multiple-input–multiple-output (MIMO) communications without explicit use of time, frequency, or code division multiplexing, while an adaptive TR approach can reduce further the crosstalk among users or multiple transmitters. TR communications can be extended easily to time-varying channels using a block-based approach with channel updates. This paper provides an overview of TR communications in both shallow and deep water and recent advances including bidirectional equalization, multiuser communications with mobile users, and communication with a glider serving as a mobile gateway.

비-가우시안 잡음하의 적응 시스템을 위한 바이어스된 영-오차확률의 반복적 추정법

바이어스된 영-오차확률 (biased zero-error probability)과 이에 관련된 알고리듬은 매 반복시간마다 합산과정을 지니고 있어 많은 계산상의 부담을 요구한다. 이 논문에서는 바이어스된 영-오차확률에 반복적 접근법을 적용한 알고리듬을 제안하였고 천해역 통신채널과 충격성 잡음 및 바이어스된 가우시안 잡음이 혼재한 실험 환경에서 성능을 비교하였다. 샘플 사이즈에 비례하는 계산 복잡도를 지닌 기존 알고리듬과 달리 제안한 반복적 방식은 샘플 사이즈와 무관하여 계산량의 부담을 크게 줄였다. 이러한 계산효율 특성을 지닌 제안한 알고리듬은 블록 처리방식의 기존 알고리듬과 비교하여 다중경로 페이딩, 바이어스된 잡음 및 충격성 잡음에 대한 강인성에서 동일한 성능을 나타냈다. The biased zero-error probability and its related algorithms require heavy computational burden related with some summation operations at each iteration time. In this paper, a recursive approach to the biased zero-error probability and related algorithms are proposed, and compared in the simulation environment of shallow water communication channels with ambient noise of biased Gaussian and impulsive noise. The proposed recursive method has significantly reduced computational burden regardless of sample size, contrast to the original MBZEP algorithm with computational complexity proportional to sample size. With this computational efficiency the proposed algorithm, compared with the block-processing method, shows the equivalent robustness to multipath fading, biased Gaussian and impulsive noise.

Underwater Sensor Network Applications: A Comprehensive Survey

There is no escaping fact that a huge amount of unexploited resources lies underwater which covers almost 70% of the Earth. Yet, the aquatic world has mainly been unaffected by the recent advances in the area of wireless sensor networks (WSNs) and their pervasive penetration in modern day research and industrial development. The current pace of research in the area of underwater sensor networks (UWSNs) is slow due to the difficulties arising in transferring the state-of-the-art WSNs to their underwater equivalent. Maximum underwater deployments rely on acoustics for enabling communication combined with special sensors having the capacity to take on harsh environment of the oceans. However, sensing and subsequent transmission tend to vary as per different subsea environments; for example, deep sea exploration requires altogether a different approach for communication as compared to shallow water communication. This paper particularly focuses on comprehensively gathering most recent developments in UWSN applications and their deployments. We have classified the underwater applications into five main classes, namely, monitoring, disaster, military, navigation, and sports, to cover the large spectrum of UWSN. The applications are further divided into relevant subclasses. We have also shown the challenges and opportunities faced by recent deployments of UWSN.