Shallow Water Channel Research Articles

Experimental investigation of acoustic channel reciprocity in shallow water

Channel reciprocity is recognized as one of the most fundamental properties of underwater acoustic propagation. The theoretical proof of channel reciprocity assumes that the ocean environment is static during the course of acoustic propagation. Since this assumption might not be valid for the case of shallow-water acoustic propagation, a question to be raised is: does channel reciprocity remain true in shallow water? A sea-going experiment was recently conducted to study the shallow-water channel reciprocity. In the experiment, two custom-built software-defined acoustic modems were used to probe their shared channel from two opposite directions. The channel reciprocity was examined by comparing the two probed channels directly. The probing signals and handshaking protocols were judiciously designed to minimize the effects of channel dynamics under the constraint of half-duplex acoustic modems. This paper will provide the details of this sea experiment (including its design, setups and execution) and report interesting findings. [Work supported by US Office of Naval Research.]

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.

Comparative Analysis of Horizontal Array Signal Processing in a Shallow-Water Channel with a Rough Windy Surface

The recent study [1] addressed how the characteristics of low-frequency acoustic signal propagation in a shallow-water channel under wind wave conditions affect the output performance of a horizontal phased array (PA) is further developed with the aim of comparative analysis of the efficiency of various array signal processing methods. The main focus is quantitative estimates of the additional array gain from optimal processing in comparison to the standard PA method compensated in the direction to the signal source, in dependence on the key parameters: direction and distance to the source, wind speed, and wave dimensions of the array. Numerical simulation is performed for a typical winter hydrological conditions of the Barents Sea, a signal frequency of 250 Hz, and a wide range of distances up to 500 km It is shown that the additional gain from optimizing the array processor can vary widely (from fractions of to a few dB), while the PA efficiency also varies widely and may have a nonmonotonic dependence on distance.

Shallow water channel estimation with energy efficient transmitted signal design.

This paper proposes channel estimation using energy efficient transmission of signal dictionaries for shallow water acoustic communications. Specifically, the multi-columned structure of the channel delay spread is exploited to design partially sampled dictionary in a two-dimensional (2-D) frequency representation of the channel. The key contribution of this work is to achieve considerable energy saving in the transmission of complex exponential signals, designed specifically for real-time shallow water channel estimation at the receiver. This is accomplished by harnessing 2-D frequency localization with compressive transmission and modified-compressive sensing with prior information to exploit the sparse structure of the rapidly fluctuating shallow water acoustic channel in real time. The proposed technique reduces demands on transmitted signal energy by harnessing the reconstruction ability of sparse sensing while retaining key non-sparse channel elements that represent important multipath phenomena. Numerical evidence based on experimental channel estimates demonstrates the efficacy of the proposed work.

Employing braid theory to disentangle overlapping oceanic events in the shallow water acoustic channel

The shallow water acoustic channel is well-known to exhibit rapid temporal fluctuations due to multipath effects, fluid motion as well as focusing events in the ocean. These events often overlap temporally thus contributing in unpredictable ways to the channel delay spread. We will present recent results from employing geometric signal processing techniques to shallow water channel estimation and will introduce new ways to identify and disambiguate overlapping oceanic phenomena within the channel delay spread. Specifically, we will explore how geometric braid theory and related operations can be employed to disentangle overlapping channel braids, and explore the efficacy of such signal processing techniques across diverse channel representations. Finally, we will discuss the importance of capturing oceanic phenomena at the appropriate resolution of time, frequency, and sparsity and the trade-offs in accuracy involved based on the chosen channel representation(s).

Acoustic radiation from a cylinder in shallow water by finite element-parabolic equation method

It can be a difficult problem to precisely predict the acoustic field radiated from a finite elastic structure in shallow water channel because of its strong coupling with up-down boundaries and the fluid medium, whose acoustic field cannot be calculated directly by existing methods, such as Ray theory, normal mode theory and other different methods, which are adaptable to sound fields from idealized point sources in waveguide. So, there is no reliable research method of predicting the acoustic radiation of elastic structure in shallow water at present. Based on the finite element method (FEM) coupled with the parabolic equation (PE), the theoretical model for structure acoustic radiation in shallow water at low frequency is established in this paper. This model mainly consists of three sections. First, obtaining the near-field vibro-acoustic characteristics of the elastic structure in shallow water by the multi-physics coupling model established by FEM, whose FEM model includes the up-down boundaries and the completely absorbent sound boundaries in the horizontal direction. Second, getting the acoustic information in the depth, which is set as the acoustic input condition i.e. starting field for the PE. Third, the acoustic information in the far-field quickly calculated by the PE and the finite difference method (FDM). The accuracy, efficiency and fast convergence of FEM-PE method are validated by numerical simulation and theoretical analysis through using a monopole source and structural source in the Pekeris waveguide, respectively. The vibro-acoustic characteristics of elastic cylinder influenced by upper and lower fluid boundaries of the Pekeris waveguide are calculated and analyzed. The cylindrical shell material is steel, and it is 1 m in radius and 10 m in length. The shallow water channel is a Pekeris waveguide with 30 m in depth, at the upper boundary, i.e., the free surface, the lower boundary is the semi-infinite liquid boundary. The analyzed frequencies range from 50 Hz to 200 Hz. The study shows that when the cylindrical shell approaches to the sea surface or bottom, the coupled frequency is higher or lower respectively than that of the shell immersed in the free field. When the diving depth reaches a certain distance range, the coupled frequency tends to be the same as that in free field. The acoustic field radiated from an elastic shell in Pekeris waveguide is similar to that from a point source at low frequency, but there exists a significant difference in high frequency between them, so the structural source can be equivalent to a point source conditionally. The sound radiation attenuation of the structure happens in sequence according to the near-field acoustic shadow zone, the spherical wave attenuation zone, the region between spherical wave and the cylindrical wave attenuation zone, and the cylindrical wave attenuation zone.

The frequency choice of continuous-wave signal in low frequency active detection

In active sonar, continuous-wave (CW) pulses are used to evaluate the velocity of a target because of its high-resolution of velocity. Unfortunately, due to multipath interface in shallow water, the received CW signals are fluctuating. This phenomenon degrades the performance of velocity measurement. The interference structure of low frequency sound field in shallow water is relatively stable and the intensity striations are regular in range-frequency graph, which makes it possible to reduce the influence of amplitude fluctuations by reasonable frequency design. In this paper, we obtain the separations of striations caused by different modes interfering. According to the separations, appropriate frequencies of multi-frequency CW pulses were selected to compensate the fluctuations. The simulation and experimental results show that this method can mitigate the influence of frequency selective fading in shallow water channel.

A numerical method for junctions in networks of shallow-water channels

There is growing interest in developing mathematical models and appropriate numerical methods for problems involving networks formed by, essentially, one-dimensional (1D) domains joined by junctions. Examples include hyperbolic equations in networks of gas tubes, water channels and vessel networks for blood and lymph transport in the human circulatory system. A key point in designing numerical methods for such applications is the treatment of junctions, i.e. points at which two or more 1D domains converge and where the flow exhibits multidimensional behaviour. This paper focuses on the design of methods for networks of water channels. Our methods adopt the finite volume approach to make full use of the two-dimensional shallow water equations on the true physical domain, locally at junctions, while solving the usual one-dimensional shallow water equations away from the junctions. In addition to mass conservation, our methods enforce conservation of momentum at junctions; the latter seems to be the missing element in methods currently available. Apart from simplicity and robustness, the salient feature of the proposed methods is their ability to successfully deal with transcritical and supercritical flows at junctions, a property not enjoyed by existing published methodologies. Systematic assessment of the proposed methods for a variety of flow configurations is carried out. The methods are directly applicable to other systems, provided the multidimensional versions of the 1D equations are available.

Matched-Filter Loss From Time-Varying Rough-Surface Reflection With a Small Effective Ensonified Area

Active sonar sensing often entails propagation paths that include a surface reflection, particularly in shallow-water scenarios. Surface reflection loss, which degrades sonar performance, depends on how rough the surface is with respect to the sensing wavelength and grazing angle. The Rayleigh roughness measure quantifies this relationship with small values representing an acoustically smooth surface and large values an acoustically rough surface. Models predicting surface reflection loss are generally derived assuming the surface shape is not varying over the time in which the pulse is reflecting from it and that the ensonified region of the surface is large in extent relative to the spatial correlation length of the surface. While these models are often appropriate for short-duration narrowband pulses, they are not necessarily applicable to long-duration broadband pulses, which are the focus of this paper. By assuming the effective ensonified area after matched filtering is smaller in extent than the spatial correlation length, a surface-reflection-loss model is derived as a function of pulse duration relative to surface wave period when the net surface displacement is Gaussian distributed. As might be expected the matched filter loss for the small-ensonified-area scenario increases with the Rayleigh roughness, the number of consecutive surface reflections, and the ratio between pulse duration and the surface wave period. With respect to the latter, the loss is predicted to saturate when the pulse duration exceeds one surface wave period. The model was compared with data measurements from the 2013 Target and Reverberation Experiment, as reported by Hines et al. (“The dependence of signal coherence on sea surface roughness for high and low duty cycle sonars in a shallow water channel,” IEEE J. Ocean. Eng. , vol. 42, no. 2, pp. 298–318, Apr. 2017). The data represent both short- and long-duration pulses with respect to the surface wave period. For the short-duration pulses, the model corroborates the data analysis by Hines et al. in predicting a very small matched filter loss. It also compared very favorably with the data for the long-duration pulses in both level and slope as a function of surface roughness. The models presented in this paper should be useful in sonar-equation analysis for predicting surface reflection loss with broadband waveforms when pulse duration is on par with or exceeds the surface wave period.

Time reversal MFSK acoustic communication in underwater channel with large multipath spread

It has been recognized that, compared with coherent acoustic communication, Multiple Frequency Shift Keying (MFSK) underwater acoustic communication offers the advantages of low complexity, easy-implementation and channel tolerance, but it is subject to significant performance degradation caused by inter-symbol interference (ISI) when the multipath spread is larger than symbol duration. The time reversal is capable of effectively suppressing the channel multipath by the means of temporal-spatial focusing, which has been widely examined and applied in coherent underwater acoustic communication systems. However, there is a lack of investigations to incorporate the time reversal with MFSK communication. In this paper, we report a multi-channel time reversal MFSK receiver, in which the channel estimate is initially obtained and periodically updated by matched filtering of the sync preamble, meanwhile, down-conversion is adopted to reduce computational complexity. The performance of the proposed receiver is evaluated in a shallow water channel with severe multipath spread, in terms of bit error rate (BER) and robustness upon time variations.

Sound radiation of cylinder in shallow water investigated by combined wave superposition method

It can be a difficult problem to precisely predict the sound field radiated from a finite elastic structure in shallow water channel because of its strong coupling with up-down boundaries and the fluid medium, whose sound field cannot be calculated directly by current methods, such as Ray theory, normal mode theory and other different methods, which are adaptable to sound fields from idealized point sources in waveguide. So far, there is no reliable prediction method to solve this kind of problem. A combined wave superposition method is proposed for such a problem, which combines the traditional wave superposition method with the transfer function in shallow water channel and the multi-physics field coupling numerical model. This method mainly consists of three sections:1) obtaining the normal velocity on the elastic structure surface in shallow water channel by the finite element method (FEM), whose FEM model includes the up-down boundaries and the completely absorbent sound boundaries in the horizontal direction; 2) getting the equivalent point source strength by traditional wave superposition method; 3) calculating the total sound field by adding up each point source field which is obtained by normal mode method. This method is verified by numerical simulation and theoretical analysis by using an imaginary and elastic spherical sound source respectively, and the results demonstrate that the method is valid and has high precision and calculating efficiency. The acoustic radiation characteristics from elastic cylindrical shells is investigated for different acoustic radiation sources, ocean environments and measurements. The cylindrical shell material is steel, whose radius and length are 3 m and 30 m respectively. The shallow water channel is an ideal waveguide with 50 m in depth, at the upper boundary, i.e., the free surface, the lower boundary is the Neumann boundary, i.e., the normal derivative of the acoustic pressure should be zero. The analysis frequency range is from 30 Hz to 200 Hz. The results show that due to a significant coupling effect of up-down direction boundaries on the sound field, the elastic structure can be equivalent to the point source only in low frequency and far field. The spatial field directivity distribution is more obvious at high frequency. The acoustic power measured by vertical line arrayis greatly influenced by ocean boundary and the depth of target.

Kalman Filter-Based Chip Differential Blind Adaptive Multiuser Detection for Variably Mobile Asynchronous Underwater Multiuser Communications

To mitigate the inter-symbol interference (ISI), multiple access interference (MAI), and the effect of different Doppler shifts within a data package incurred in variably mobile asynchronous underwater multiuser communications (VMAUMC), a Kalman filter-based chip differential blind adaptive multiuser detection (D-KF-BAMUD) algorithm is proposed in this paper. The Kalman filter-based blind adaptive multiuser detection (KF-BAMUD) algorithm has been adopted to combat ISI and MAI in the scenario of static users, and underwater and under-ice multiuser communication experiments have been carried out to verify the effectiveness of the KF-BAMUD algorithm. To tackle the more challenging VMAUMC, a differential modulation and demodulation technique operating at the chip level is proposed to reduce the impact of Doppler shift residues after the Doppler compensation with an average Doppler frequency offset within each data package. The technique is combined with the KF-BAMUD algorithm, leading to the D-KF-BAMUD algorithm, which is able to effectively handle ISI, MAI, and the effect of Doppler shift residues simultaneously in VMAUMC. This paper reports our experiments on VMAUMC carried out in Songhua River in April 2016 (seven users, data rate 7.87 bit/s, up to 2 m/s variably relative velocity in each data package, and horizontal distance 200-m shallow-water channel between transducer and hydrophone), and the encouraging experimental results demonstrate the effectiveness of the proposed system and the D-KF-BAMUD algorithm.

Effects of wind waves on horizontal array performance in shallow-water conditions

We analyze the influence of statistical effects of the propagation of an acoustic signal excited by a tone source in a shallow-water channel with a rough sea surface on the efficiency of a horizontal phased array. As the array characteristics, we consider the angular function of the array response for a given direction to the source and the coefficient of amplification of the signal-to-noise ratio (array gain). Numerical simulation was conducted in to the winter hydrological conditions of the Barents Sea in a wide range of parameters determining the spatial signal coherence. The results show the main physical effects of the influence of wind waves on the array characteristics and make it possible to quantitatively predict the efficiency of a large horizontal array in realistic shallow-water channels.

Studi Numerik Optimasi Propeller Kapal Selam 29 meter Dengan Menggunakan High Skew

Indonesia is an archipelagic nation and most shallow water channel in Western Indonesia, making the concept of mini submarine use by the Navy to be the right choice. One of the most important engineering is to design a mini submarine propeller with a high level of optimization. This research method is based on the result of numerical simulation using Computer Fluid Dynamic (CFD) which use B-series as base with skew variation 36o, 45o, 54o. It is expected that the results of this research can provide solutions in the selection of efficient design propeller for mini submarine 29m. With the open water diagrams generated then obtained the greatest efficiency value on B4-522 with skew obtained at 45 ° skew angle. The optimum velocity value is at J = 0.07 at 8.9 knots with an efficiency value of 0.856.

Numerical simulations of wake characteristics of a horizontal axis tidal stream turbine using actuator line model

The wake of a laboratory scale tidal stream turbine in a shallow water channel with a turbulent inflow is simulated using the hybrid LES/ALM technique, which combines large eddy simulation with the actuator line method. The turbulent inlet conditions are generated using the mapping method to avoid a precursor running and large space for saving data. The numerical results demonstrated the usefulness of the mapping technique as well as some shortcomings that still remain to be addressed. Good agreement between numerical predictions and experimental data is achieved for both the mean and turbulent characteristics of the flow behind the turbine. The examination of changes in turbulence intensity and turbulent kinetic energy in the streamwise direction confirms the existence of a peak and transition to a highly turbulent flow about three diameters downstream of the turbine, which means that the distinct characteristics of the streamwise changes of turbulence intensity or turbulent kinetic energy may serve as an effective indicator for the flow regime transition and wake behaviour.

Combining sparse recovery approaches with underwater acoustic channel models for robust communications in the shallow water paradigm

Shallow water acoustic communication techniques are fundamentally challenged by the rapidly fluctuating multipath effects due to oceanic phenomena such as surface wave focusing, specular reflections from the moving sea surface, Doppler effects due to fluid motion as well as sediment-dependent absorption from the sea bottom. Several signal processing techniques have been recently proposed that specialize in recovering the shallow water acoustic channel components using compressing sampling and mixed norm optimization theory. However, these novel techniques are typically agnostic of underlying underwater acoustic propagation phenomena. Furthermore, state-of-the-art in shallow water channel estimation typically does not account for the three-way uncertainty principles governing the localization of sparsity, time, and frequency for the time-varying shallow water acoustic channel. This talk will focus on recent techniques proposed in this domain, their relative benefits and shortcomings; as well as offer new insights into how we can combine knowledge of basic underwater acoustic propagation physics with state-of-the-art in sparse sensing and related techniques to achieve robust shallow water channel estimation. The talk will also provide an overview of equalization techniques that can be harnessed with channel estimation techniques proposed.

Effect of Pulse Duration on Echo Matched-Filter Statistics in a Shallow-Water Channel

Active sonar echo statistics (i.e., the probability density function of the match-filtered echo) are an important part of performance prediction. System design parameters such as pulse amplitude, duration and bandwidth are usually only assumed to alter the signal-to-noise-power ratio (SNR) at the detector and not the echo statistical model or its parameters. While this may be an accurate assumption for low bandwidth waveforms in a static propagation channel, it is not necessarily so when the echo is resolved or the channel is time varying. Echoes from a moored, vertical air hose were obtained during the 2013 Target and Reverberation Experiment (TREX-2013) and used to evaluate the impact of pulse duration on echo statistics. Using the scintillation index (SI) to represent the statistical character of the air-hose echo measurements, increasing pulse duration was seen to make the echoes more deterministic (i.e., to decrease SI). Assuming a time-varying Rician channel, a model was developed relating the pulse duration, channel correlation time, and ratio of coherent to incoherent power in the channel to the echo SI. Comparison with the TREX-13 air-hose measurements showed a good or excellent fit to five of the six 1-h measurement periods analyzed. Increasing pulse duration beyond the channel correlation time essentially increases the ratio of coherent to incoherent power in the echo, which improves detection performance when SNR is fixed (e.g., in a reverberation-limited scenario). If a single channel instantiation already produces a nearly deterministic echo, the performance improvement is minimal. However, when the channel is weakly Rician (e.g., a shallow-water channel at large ranges relative to water depth), the improvement can be significant.

The Dependence of Signal Coherence on Sea-Surface Roughness for High and Low Duty Cycle Sonars in a Shallow-Water Channel

It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time–bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product–moment correlation coefficient (Pearson's $R$ ) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.

Efficient cooperative multicarrier underwater acoustic communication over the Persian Gulf channel

Real-time and reliable transmission with high data rate is one of the most crucial design issues in underwater acoustic (UWA) communications. This paper is part of an operational project in the Persian Gulf which is known as a shallow water channel worldwide. Building on the promising combination of cooperative and multicarrier techniques, we first address the design of an efficient UWA communication system over the Persian Gulf channel. We assume sparse and frequency-selective Rician fading, non-white correlated Gaussian ambient noise, and non-uniform Doppler distortion among subcarriers. Based on the extensive field measurements, we adopt a comprehensive channel model including modified sound speed profile, modified absorption coefficient, reflections from the surface and bottom of the sea, and modified ambient noise model. In our work, carrier frequency is efficiently determined based on the system and environmental parameters. In addition, a simple criterion for Doppler scale calculation is proposed. Moreover, based on the approximate auto-correlation function of the ambient noise, whitening filter is utilized at the receiver. In many applications of underwater acoustic sensors network (UW-ASN), nodes may not be big enough to have more than one antenna. Therefore, to achieve spatial diversity, cooperation between nodes can be a proper alternate method. In another part of this research, toward a proper approach that each node operates under full-duplex mode, we address a cooperative virtually-aided transmission scenario which is called cooperative multiple input single output. Simulation results demonstrate that our proposed models can significantly improve the bit error rate performance of UW-ASN in the Persian Gulf channel.

Exploiting joint sparsity for underwater acoustic MIMO communications

MIMO communication has been recognized as a potential solution for high speed underwater acoustic communication, which unfortunately encounters significant difficulties posed by simultaneous presence of multipath and Co-channel interference (CoI). Sparsity contained in the multipath structure of underwater acoustic channels offers an effective way for improving channel estimation quality and thus enhancing the communication performance in the form of time reversal or channel estimation based equalization. However, for MIMO channels with extensive multipath and CoI, the performance gain achieved by classic sparsity exploitation channel estimation methods such as orthogonal matching pursuit (OMP) is still not enough to yield satisfactory performance. Under quasi-stationary assumption, underwater acoustic channels of adjacent data blocks exhibit correlated multipath structure, namely, multipath arrivals with similar time delay but different magnitude, which has not been exploited. In this paper, a joint sparse recovery approach is proposed to exploit the sparse correlation among adjacent data blocks to improve the performance of channel estimation. Under the framework of distributed compressed sensing (DCS), a joint sparse model which treats the multipath arrivals as sparse solutions with common time support is adopted to derive a joint sparse recovery algorithm for efficient channel estimation, the results of which are used to initialize and periodly update a channel estimation based time reversal receiver. Finally, underwater MIMO communication experimental results obtained in a shallow water channel are provided to demonstrate the effectiveness of the proposed method, compared to the same type of receiver that do not exploit the joint sparse.