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Since from last decade, there are significant research interests in improving the limit and information rate execution of development wireless communication frameworks. The millimetre wave (mmWave) communications introduced to achieve the explosive network capacity demands for future generation wireless communication systems. However, using mmWave with massive MIMO leads to significant challenges related to the spectrum and energy efficiency. The beam-space (BS) MIMO is recent invention to accomplish the exchange off between the spectrum and energy effectiveness of massive MIMO communications. The BS-MIMO significantly reduces the number radio frequency (RF) channels and increases the capacity. However, in one beam, only one user can be allocated using BS-MIMO. Thus in this paper, introduce the non-orthogonal multiple access (NOMA) to increase the multiple users allocations in one beam. The NOMA technique is combined with BS-MIMO and form the BS-MIMO-NOMA method in this paper. We presented the simple and normalized algorithm for iterative power allocations using the NOMA technique. The iterative power optimization method with less complexity is designed to realize the dynamic power allocation in order to achieve the energy efficiency performance. The simulation outcomes demonstrate the effectiveness of proposed technique in terms of spectrum and energy efficiency rates compared to underlying methods. [ABSTRACT FROM AUTHOR]

The global network of sensors commissioned to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty has proven capable of that task and more. [ABSTRACT FROM AUTHOR]

The medium with gradient change of acoustic impedance is a new acoustic structure which developed from multiple layer structures. In this paper, the inclusion is introduced and a new set of equations is developed. It can obtain better acoustic properties based on the medium with gradient change of acoustic impedance. Theoretical formulation has been systematically addressed which demonstrates how the idea of utilizing this method. The sound reflection and absorption coefficients were obtained. At last, the validity and the correctness of this method are assessed by simulations. The results show that appropriate design of parameters of the medium can improve underwater acoustic properties. [ABSTRACT FROM AUTHOR]

Vortex dynamics is one of problems arising in fluid dynamics. Vortices are a major characteristic of turbulent flow. We perform the Lattice Boltzmann Method (LBM) with Bhatnagar-Gross-Krook (BGK) approximation to analyze the underwater vortex dynamics close to the shoreline. Additionally, the Smagorinsky tubulence model is applied to treat turbulent flow and a special method for free surface treatment is applied to overcome free surface. Furthermore, we investigate the effect of the turbulence factor and the seabed profile to vortex dynamics. The results show a smaller turbulence factor affected to more turbulent flow and coral reefs reduced movement of vortex towards the shoreline. [ABSTRACT FROM AUTHOR]

In the underwater neutrino telescopes, the positions of the Cherenkov light sensors and their movements must be known with an accuracy of few tens of centimetres. In this work, the activities of the SMO and KM3NeT-Italia teams for the development of an acoustic positioning system for KM3NeT-Italia project are presented. The KM3NeT-Italia project foresees the construction, within two years, of 8 towers in the view of the several km³-scale neutrino telescope KM3NeT. [ABSTRACT FROM AUTHOR]

In situ acoustic calibration devices are necessary to assure the optimal performance of sensors and detectors for the acoustic detection of ultra-high energy neutrinos in underwater telescopes. Moreover, they provide evidences for the feasibility evaluation of the technique and for the determination of the entire detector efficiency. Following previous studies related to parametric acoustic sources, a first prototype of a compact acoustic array able to mimic the acoustic neutrino signal (a transient bipolar signal with 'pancake' directivity) is presented. The compact array developed has practical features such as easy handling and operation, and versatile functionality. In the latter sense, the transmitter is able to work in different frequency ranges for different application modes, and thus to carry out several tasks related to acoustics in underwater neutrino telescopes: emission of neutrino-like signals, calibration of sensor sensitivities and responses, emission of signals for positioning, etc. The design, construction and characterization of the prototype are described. A simulation study is also discussed, where experimental signals were propagated over distances in the kilometre range. [ABSTRACT FROM AUTHOR]

Chaotic Orthogonal Code-Division Multiplexing (OCDM) is a method of encoding digital data on multiple chaotic codes. More than ten years after its introduction, the Chaotic OCDM method is now a mature technology that has been adopted in long-range underwater acoustic communications systems. This paper presents an overview of this method and its applications. Firstly, we review the major evolutions of this method with an emphasis on some key technologies such as the paralleled M-ary mode and the constant envelope multi-signal synthesizing method. A sea test corresponding to these technologies is introduced thereafter. Finally, we make a conclusion and propose future research directions in this field. [ABSTRACT FROM AUTHOR]

The waveguide invariant is an important concept in ocean acoustics. It describes the interference striations of the spectral structure of acoustic signals at different ranges. This paper proposes a sound intensity based method for estimating the waveguide invariant and source range by using phase-shift-compensation of the underwater acoustic spectrum obtained by different receivers. When one of the waveguide invariant and the source range is known, this method can be used to estimate the other. With this method, waveguide invariants at different ranges are estimated experimentally using LFM signals radiated by one towed sound source and received by one sea-floor located horizontal-line-array. Data was collected in the North Yellow Sea in December 2011. It is shown that the waveguide invariant varies from 0.8 to 1.2, and that its mean value is near the theoretical value. On the other hand, ranges are estimated using the theoretical value of the waveguide invariant, and these are in good agreement with the ranges measured by GPS. [ABSTRACT FROM AUTHOR]

This paper presents a review of the underwater acoustics of the Arctic Ocean. It discusses the main features of the underwater acoustic environment and how they are so strongly affected by the presence of ice cover. The paper also discusses the history of Arctic Ocean acoustics research, how the motivation was originally military in character during the Cold War and how it changed to being driven by environmental considerations today. Originally, the physics of the Arctic Ocean was studied in order to predict its acoustic properties, and now acoustic techniques are used to help understand its physical environment. [ABSTRACT FROM AUTHOR]

The waveguide invariant concept describes the interference striations in the acoustic pressure spectrograms produced with an underwater broadband source. In this paper, the existence of interference structure in the vector acoustic field is examined using sea trial data, and the waveguide invariant is exploited to interpret fringes of the vector field. The experimental data, which recorded a merchant vessel passing on a straight path, were collected by a 2-dimensional vector sensor during an experiment in the South China Sea. The intensity and phase spectra of the energy flux density vector in the acoustic field radiated by the moving vessel are obtained from the magnitude and phase angle of the product of the pressure and the horizontal particle velocity's complex conjugate. Distinct interference patterns appear in the vector intensity and phase spectra. The characteristics of these have been analyzed by comparison with the scaled acoustic field. The equation describing the striations associated with the ship's trajectory is derived from waveguide invariant theory. The Hough transform method is used to extract the waveguide invariant from the data. To improve the quality of the patterns derived from the vector field, a better value of waveguide invariant can be estimated. Good agreement between reconstructed trajectories and real patterns suggests that it is feasible to use the interference structure in an acoustic vector field to determine the waveguide characteristics. [ABSTRACT FROM AUTHOR]

The 2001 Asian Sea International Experiment (ASIAEX2001) is a large-scale survey performed in the South China Sea. During May 2001 several minutes of M-sequence phase modulation signals were transmitted by a 400-Hz source repeatedly at intervals of half an hour, and received by an array 31 km away to conduct tomography of internal waves. The signal was treated as a binary-phase shift-keying (BPSK) communication signal with an information rate of 100 bps. The communication signals were demodulated by a decision-feedback equalizer. Since the intensity of the internal waves was not stable during the experiment period, data of two transmissions corresponding to a strong and a weak internal-wave activity were separately located and processed to investigate the impact of internal waves on the coherent underwater acoustic communication. The results show that internal waves cause a greatly fluctuating signal level and a rapidly varying multipath structure; consequently, these results show that the parameters of the equalizer need to be adjusted to mitigate the degradation of the communication performance. [ABSTRACT FROM AUTHOR]

Progress in the development of underwater sound projectors over the past thirty years in China is presented in this paper. This progress may be divided into four parts:(I) The development of new underwater sound projectors based on new transducer materials such as rare earth giant magnetostrictive materials, relaxation ferroelectric materials and piezoelectric composite materials; (II) The design of new structures and energy transfer mechanisms such as flex-tensional transducers, mosaic cylindrical transducers, longitudinal transducers, flexural disk and beam transducers; (III) The development of new configurations that broaden transducer bandwidth such as free-flooded ring transducers, broadband free-flooded flex-tensional transducers, doubly-excited longitudinal transducers, hybrid longitudinal transducers and matching layer technology; (IV) The development of new transducer design and analysis tools such as finite-element models for complex geometries, structure and fluid coupling mechanisms. These models have been used to optimize the design of a variety of projectors including low-frequency transducers, high power transducers and broadband wide-beam transducers. [ABSTRACT FROM AUTHOR]

Time-varying significant multipath interference is the major obstacle to reliable data communication in shallow water acoustic channels. In this paper, the performance of a low density parity check (LDPC) coded orthogonal frequency division multiplexing (OFDM) communication system is investigated for these channels. The initial message for LDPC, decoded by using the belief propagation (BP) algorithm, is deduced for OFDM underwater acoustic channels; based on this deduction, the noise thresholds of regular LDPC codes with different code rates are obtained by using the density evolution algorithm. Furthermore, a communication system model, developed with LDPC code, OFDM and channel interleaver for shallow water acoustic channels, is introduced. The effect of modulation and coding schemes on the LDPC codes performance is investigated by simulation. The results show that the system can achieve remarkable performance in shallow water acoustic channels, and the performance improves with increasing code length and decreasing code rate. The bit error rate (BER) of the system, under conditions with QPSK modulation, 1280-code length and 1/2-code rate, is less than 10-5 when the signal to noise ratio (SNR) is greater than 6.8dB. These values are obtained for a five-path shallow water acoustic channel of Xiamen harbor. [ABSTRACT FROM AUTHOR]

In any sonar system, suppression of the side lobes results in an increase in main beam to side lobe level (signal to noise) ratio, with a consequence in an increase in detection range and resolution. Traditionally, this undertaking is done as part of the signal processing operation of the array. The overall aim of this paper is to investigate the advantages and disadvantages of implementing various shading functions directly on the radiating surface of the individual elements (transducers) of a line array. In particular, this paper examines the performance of various shading (window) functions, such as rectangular, cosine, triangular, Hamming, and Blackman-Harris, by printing the actual shape of the function on a transmitting multi-element line array. The transducers used are of 1-3 piezocomposite material with acoustic window and operate at 300 kHz with over 100 kHz of bandwidth. The results of the acoustic tests and polar (directivity) responses are presented and compared to theoretical simulations. The results obtained show that significant improvement of the main beam to side lobe level ratio can be achieved with an unfortunate widening of the main beam. [ABSTRACT FROM AUTHOR]

The investigation of sound speed in seawater is one of the major interest to physical oceanographer and marine scientist. The application of sound speed has become important in hydrographic surveying as the acoustic method has been widely applied for remote observations of seas and oceans and fisheries purposes. The sound speed profile is an important source of information required to retrieve ocean properties. In liquid case, the speed of sound in water is approximately 1500 ms-1. In the context of shallow water environment, the speed of sound in seawater is variable and depends on its temperature and salinity. In this study, the sound speed of certain location in Sungai Merbok, Kedah was investigated. The sound speed calculation was based on field observation of temperature, salinity and depth of Sungai Merbok. The collected data showed a variation of sound speed between 1523.3-1537.8 ms-1. The sound speed variations followed more the variations in salinity compared to variations in temperature except for the evening data collection at the transverse direction of the river. However, the differences between both parameters that influence the variations were small. Therefore, in this study, the sound speed was more influenced by salinity rather than temperature. [ABSTRACT FROM AUTHOR]

This paper discusses what future research should be done in the area of nonlinear internal wave interactions with low frequency shallow water sound. [ABSTRACT FROM AUTHOR]

An acoustic inversion technique, known as Geo-Acoustic Doppler Spectroscopy, has recently been developed for estimating the geo-acoustic parameters of the seabed in shallow water. The technique is unusual in that it utilizes a low-flying, propeller-driven light aircraft as an acoustic source. Both the engine and propeller produce sound and, since they are rotating sources, the acoustic signature of each takes the form of a sequence of narrow-band harmonics. Although the coupling of the harmonics across the air-sea interface is inefficient, due to the large impedance mismatch between air and water, sufficient energy penetrates the sea surface to provide a useable underwater signal at sensors either in the water column or buried in the sediment. The received signals, which are significantly Doppler shifted due to the motion of the aircraft, will have experienced a number of reflections from the seabed and thus they contain information about the sediment. A geo-acoustic inversion of the Doppler-shifted modes associated with each harmonic yields an estimate of the sound speed in the sediment; and, once the sound speed has been determined, the known correlations between it and the remaining geo-acoustic parameters allow all of the latter to be computed. This inversion technique has been applied to aircraft data collected in the shallow water north of Scripps pier, returning values of the sound speed, shear speed, porosity, density and grain size that are consistent with the known properties of the sandy sediment in the channel. [ABSTRACT FROM AUTHOR]

This paper summarizes 10 years of NURC experience with the application of a state-of-the-art FE code [Zampolli et al., JASA 122, 1472-85 (2007)] to propagation and scattering problems in ocean acoustics. We show benchmark results for low-frequency propagation in a range-dependent waveguide with an elastic bottom, and object scattering results for spheres and cylinders placed near the seafloor. Despite the computational burden associated with the FE method, its generality in treating propagation in layered fluid-elastic media of complex geometry without theoretical approximations makes it very attractive for acoustic benchmarking. And, as experience shows, what is computationally impractical today, will be easily done on a desktop computer 10 years from now. [ABSTRACT FROM AUTHOR]

During ASIAEX, surface directional wave spectra were recorded for nine consecutive days. Data are analyzed in this paper to show the characteristics of the surface wave field in the East China Sea. Three surface spectrum models, Pierson-Moskowitz, JONSWAP, and Toba, are evaluated and compared with the ASIAEX RMS surface wave height data. The data-model comparisons show that the JONSWAP-Mitsuyasu model is the best fit to the experimental results. Model performance can be improved by a combined model, JONSWAP-Mitsuyasu for U/cp<0.8 and Toba-Donelan for U/cp>0.8. The preliminary results suggest that locally measured wave age can be used to find the most appropriate surface model for acoustic prediction purposes. [ABSTRACT FROM AUTHOR]

Low-frequency (LF) sound speed and attenuation in sandy/silty seabottoms have recently been analyzed and summarized from long-range field measurements in shallow water [Zhou, Zhang and Knobles, J. Acoust. Soc. Am., 125, 2847-2866, 2009]. Field measurements conducted at 20 locations in different coastal zones around the world were analyzed. The seabed attenuations, inverted from different acoustic field characteristics, exhibit similar magnitude and nonlinear frequency dependence below 2000 Hz. The present paper shows that both LF field-derived sound speed and attenuation in the bottom can be equally well described by the Biot-Stoll model, the Buckingham VGS model, the Chotiros-Isakson BICSQS model, and the Pierce-Carey model. Possible ranges of the physical parameters for each of these four seabed acoustic models are estimated based on the LF field-inverted sound speed and attenuation. These four models with different physical mechanisms in consideration may result in almost identical nonlinear frequency dependence of sound attenuation in sandy/silty seabottoms. [ABSTRACT FROM AUTHOR]

Over the last three decades several observational and theoretical studies have established the measureable effects of anisotropy in shallow water environment and its effects on the acoustic wave propagation. In this paper, the influence of shallow water internal waves on propagation of acoustic waves is revisited and verified by new experimental data. Results using a towed source show feasibility for remote mapping of the internal waves by underwater acoustics. [ABSTRACT FROM AUTHOR]

Internal waves are one of the most pronounced oceanic phenomenons to the oceanographer. During past decades much effort has been made to investigate the effect of internal waves on shallow water acoustic propagation. Even though many field observations, such as SWARM '95, have provided fruitful information about the relation between internal waves and acoustic propagation, it is necessary to conduct more numerical simulations due to their extensive feasibility. In this study, the shallow water internal wave environment is constructed by using a non-hydrostatic ocean model, the open boundary forcing is set by considering single or several internal wave modes at the M2 tidal frequency. In order to show the mode coupling caused by the internal wave field more clearly, the acoustic starting field with different single normal modes is adopted. The acoustic simulation can be used to check whether a specific combination of internal wave modes is related to the mode coupling, and which mode pair will be affected. The combination of internal wave modes can be separated into several groups. Even though the internal wave fields are different among every case in each group, the acoustic field structure and the mode coupling are similar. Different acoustic normal mode coupling occurs due to the different combinations of internal wave mode forcing. When the parameters of internal wave mode are modified gently, the acoustic mode coupling becomes quite different. It is interesting and important to investigate the sensitivity of acoustic fields to the variability of the internal mode combination. [ABSTRACT FROM AUTHOR]

This paper describes matched field inversion of data from the Shallow Water 06 (SW06) experiment that was carried out in a shallow water environment in which the water column sound speed profile was spatially and temporally variable. The profile was characterized by a variable thermocline between relatively stable upper and lower homogeneous layers. The inversion accounted for the variable water column sound speed profile by parameterizing the profile using empirical orthogonal function (EOFs), and including the EOFs as parameters in the inversion. Two approaches were used to obtain information about the sound speed variability. The first used only the variation in the thermocline, whereas the second used EOFs for the full water column. The inversions showed that consistent estimates of the geoacoustic model parameters could be obtained with either of the two approaches. (Work supported by the Ocean Acoustics Team at Office of Naval Research). [ABSTRACT FROM AUTHOR]

The Office of Naval Research and the Chinese Academy of Sciences established and continue to encourage an unprecedented collaboration between the United States and China in the basic research field of ocean acoustics. This paper describes the progression of events in the scientific relationship. This cooperative research effort, primarily focused on shallow-water acoustics, has varied over fifteen years. It has involved numerous researchers and institutions from the U.S., China, and other countries, and it has resulted in productive workshops, conferences, field experiments, research collaborations, and scientific publications. [ABSTRACT FROM AUTHOR]

Ambient noise in shallow water undergoes multiple reflections between the sea surface and seabed, thus, the spatial structure of ambient noise field depends strongly on the geo-acoustic characteristics of the seabed. A great number of measurements show that the noise vertical coherence is a stable feature, changing little with sea states, which reflects the invariance of seabed parameters. Therefore, the noise vertical coherence can be used to estimate geo-acoustic parameters. In this paper, geo-acoustic parameters are estimated by the vertical coherence of measured ambient noise. The differential evolution method is applied in the global optimization parameters search. However, geo-acoustic inversion is a strongly nonlinear process, so the inversion solution may be uncertain and non-unique. According to Bayesian theory, the solution is fully characterized by its posterior probability density (PPD), which combines prior information about the forward model with information from measured data. A fast Gibbs sampler approach was used to calculate the multi-dimensional integrals of the PPD in the paper, and the validity of the inversion method based on the vertical coherence of the ambient noise was checked. For these inversions two different seabed models were employed. First, the seabed was assumed to be a homogeneous fluid half-space. Another seabed model added a fluid sediment layer on top of a homogeneous fluid half-space. In the end, substituting these inversion results into the forward model, comparisons were made to estimate the uncertainties of inversion results by the broadband vertical coherence of shallow water noise data. [ABSTRACT FROM AUTHOR]

In the summer of 1996, a joint China-U.S. underwater acoustics experiment was conducted near the geographic center of the Yellow Sea with a water depth of 75 m. Hundreds of broadband explosive sources were deployed. The seabed sound speed in this experiment has been inverted using two methods based on data-derived mode depth functions and modal arrival time differences. The seabed attenuation has been estimated from three different approaches using modal attenuation coefficients, modal amplitude ratios, and transmission loss measurements [Wan, Zhou, and Rogers, J. Acoust. Soc. Am., 128(2), 2010]. The present paper analyzes the vertical coherence of the sound propagation as a function of range and frequency. The seabed sound attenuation is inferred by minimizing the difference between the theoretical and measured vertical coherence. The seabed attenuation, inverted from long-range broadband sound propagation with different methods over a frequency range of 80-1000 Hz, can be expressed as αb = (0.34±0.02)f187±004{dB/m·kHz). [ABSTRACT FROM AUTHOR]

This paper studied a method for extracting seabed reflection losses based on a towed line array for large scale sea areas with high resolution. The experimental system consisted of a towed tilted line array and three acoustic sources to cover a large scope of grazing angles (from 10° to 70°). The simulation results in shallow water show that different arrays have different experimental configurations in order to cover the same scope of grazing angles. The vertical line array must operate with several acoustic sources and requires a stationary platform of the array. The towed horizontal line array needs only one acoustic source but requires a large aperture line array. The towed tilted line array combines the merits of the vertical line array and the horizontal line array, which can invert the seabed acoustic parameters in a moving and high resolution way, and requires only a smaller aperture line array. The error resulting from the distortion of the array shape was analyzed for the inversion method. [ABSTRACT FROM AUTHOR]

The sound speed profile and bottom parameters play important roles in shallow water sound propagation. And much attention has been paid to how to obtain them. In concurrence with in-situ measurements, many inversion methods, such as the matched field inversion, have been put forward to invert the sound speed profile from acoustic signals. However, the matched field inversion may cost too much time in replica field calculations. The feasibility and robustness of an acoustic tomography scheme with matched field processing in shallow water is studied here. The sound speed profile is described by the empirical orthogonal functions (EOFs). The acoustic signals from a vertical linear array in ASIAEX2001 East China Sea are analyzed to invert sound speed profiles combined with the estimated EOFs. The parallel genetic algorithm is adopted as the optimization algorithm to increase the inversion speed. The results show that the inverted sound speed profiles are in good agreement with CTD measurements. Moreover, a posteriori probability analysis is used to verify the validity of the inverted results. [ABSTRACT FROM AUTHOR]

This overview of the mid-frequency (1-10 kHz) acoustics programs that were part of the Shallow Water 2006 Experiment includes descriptions of design goals, the experimental environment, and instrumentation deployed. Recent progress on the data analysis from the acoustics experiments is summarized. [ABSTRACT FROM AUTHOR]

Explosive sources are widely used as sound sources in underwater acoustics experiments. Unfortunately, the resulting direct pulse is often corrupted by bubble pulses, which has an undesired influence on the observation and its applications. Canceling these bubble pulses will greatly enhance the quality of the observation and will be helpful for its applications. To cancel the bubble pulse, an adaptive filter, updated from an adaptive least squares algorithm in order to model the physical transmission from the explosive source to the receiver, is proposed. The auto-correlation functions of both the original field trial data and the filtered results show the bubble pulses in the observation can be effectively cancelled. The normal modes of signal propagation can be reconstructed from the filtered results with an adaptive time-frequency representation. The estimated time delay of different normal modes is further applied in geo-acoustic inversion. [ABSTRACT FROM AUTHOR]

In the underwater sound channel we often use an array to receive signals from distant sources. The received signals are often mixed with environmental interference. In the complex acoustic environment, received signals are distorted greatly and elongated in time. In many practical applications such as sound communications, sound remote sensing and active sonar signals, we hope to obtain the original signal's waveform. In general theory, the received signals are the convolution of emission signals and Green's function of environment. In unknown Green's function of environment, simply relying on the array to record the information to determine the sound source signal wave propagation features and the environment is not enough. However, in certain circumstances, based on a physics method of underwater acoustics, the spread of recovery technology is successful. [ABSTRACT FROM AUTHOR]

The characteristics of a densely-coupled array of underwater acoustic transmitting transducers are studied. At first, the electro-acoustic characteristics such as the admittance, the resonant frequency and the transmitting voltage response, of a low frequency barrel-stave flextensional transducer and a densely-coupled compact array composed of three identical transducers uniformly distributed on a circle with spacing much less than half wavelength, are measured by experiments. Then, the radiation impedances of a single transducer and of transducers in the compact array are calculated by the boundary element model together with the finite element model. Based on the above results, the transducer's equivalent circuit model parameters are calculated in different cases, which include a single transducer in air and in water, and a densely-coupled array of three transducers parallel connected in water. The characteristics of the transducers and array are analyzed by the equivalent circuit model that was obtained. The research results show that when the transducers make up a densely-coupled compact array, the resonant frequency decreases and the transmitting bandwidth broadens. It is also shown that the mutual interactions among elements are significant for the compact array. The mutual radiation resistance between two transducers is close to the self-radiation resistance of the transducers. The vibration velocities of the transducers in the compact array are nearly 1/3 as those of a single transducer, and the radiation acoustic power and transmitting voltage response of the array are nearly the same as those of a single transducer, when the driving voltages of the array and single transducer are unchanged. Furthermore, the transmitting source level of the 3-element compact array is 8.9dB higher than that of the single transducer if the vibration velocities of the transducers in the array are the same as those of the single transducer. The proposed technique can be used to design densely-coupled compact arrays of transmitting transducers. [ABSTRACT FROM AUTHOR]

Time reversal can focus in both time and space. The temporal focusing properties have been used in underwater acoustics communications. Typical passive time reversal communication experiments use a vertical array of receivers and a sound source. The property of a horizontal time reversal array that it focuses well in the temporal domain indicates the utilization of synthetic aperture time reversal. Passive synthetic aperture processing repeats emitting same signals during the cruising and records the received signal using one or more receivers. Passive synthetic aperture time reversal communication techniques are presented and applied to results from a sea trail. The experiment consisted of a towed hydrophone and a vertical array. During the experiment, a probe signal is emitted to sample the transmit function then be used by time reversal processing. Due to the moving of the hydrophone, Doppler shift compensation and receive wave forms synchronization must be performed before the time reversal processing. The signal-to-noise ratio is increased and the temporal side lobes are decreased in the time reversal process. Additional communication algorithms can be appended after the time reversal process to increase performance. The result from the at-sea experiment confirms the ability of passive synthetic time reversal communications. [ABSTRACT FROM AUTHOR]

It has been proved theoretically that two incompletely correlated sources can be identified by linear signal processing methods. However, it is difficult in practice. A new method to separate two wideband sources with one vector sensor is presented in this paper. The method is the combination of subspace rotation and spatial matched filter. Simulations show that this method is insensitive to the initial azimuth error, independent of signal spectrum, and better man wideband focusing subspace methods at low SNR. The sea trial is performed and the experiment results show that the proposed method is effective to separate and track two wideband sources in the underwater environment. [ABSTRACT FROM AUTHOR]

Gaussian beam tracing methods have emerged as a standard approach for modeling sound propagation in the ocean. The first implementations were developed in the 1970's by Bucker and evolved significantly. Today there are actually some four different types of Gaussian beam algorithms. They are quite different in terms of both the beam characteristics and their performance. This paper will review the development of the methods and their application to typical ocean acoustic problems. [ABSTRACT FROM AUTHOR]

In 1926 Prufer introduced a method of transforming the second order Sturm-Liouville (SL) equation into two nonlinear first order differential equations for the phase o\ and 'magnitude', |o\2+o\2| for a Poincare phase space representation, (o\,o\). The useful property is the phase equation decouples from the magnitude one which leads to a nonlinear, two point boundary value problem for the eigenvalues, or SL numbers. The transformation has been used both theoretically, e.g. Atkinson, [1960] to prove certain properties of SL equations as well as numerically e.g Bailey [1978]. This paper examines the utility of the Prufer transformation in the context of numerical solutions for modes of the ocean acoustic wave equation. (Its use is certainly not well known in the ocean acoustics community.) Equations for the phase, o\, and natural logarithm of the 'magnitude', ln(|o\2+o\2|) lead to same decoupling and a fast and efficient numerical solution with the SL eigenvalues mapping to the horizontal wavenubers. The Prufer transformation has stabilty problems for low order modes at high frequecies, so a numerically stable method of integrating the phase equation is derived. This seems to be the first time the these stability issues have been highlighted to provide a robust algorthim for the modes. [ABSTRACT FROM AUTHOR]

The time-varying, dispersive, multipath underwater acoustic channel is a challenging environment for reliable coherent communications. A method proposed recently to cope with intersymbol interference (ISI) is Passive-Phase-Conjugation (PPC) cascaded with Decision-Feedback Equalization (DFE). Based on the theory of signal propagation in a waveguide, PPC can mitigate channel fading and improve the signal-to-noise ratio (SNR) by using a receiver array. At the same time the residual ISI will be removed by DFE. This method will lead to explosive divergence when the channel is changed by a large amount, because PPC estimates channels inaccurately. An improved algorithm is introduced in this paper to estimate the channel during all the communication process; as a result, the change of the channel can be found in time and the PPC could use more accurate channel estimated. Using simulated and at-sea data, we demonstrate that this algorithm can improve the stability of original algorithm in changed channels. [ABSTRACT FROM AUTHOR]

Due to the high resolution of time reversal processing in a shallow water waveguide, the target can not be treated as a point-like scatterer, but should be modeled as a distributed target composed of elemental scatterers. In this paper, the problem of using multiple focusing on the target to improve the detection performance is investigated. The signal processing method consists of both coherent time-reversal processing and incoherent accumulation of the output of time-reversal processing from each elemental scatterer. The influence of target scintillations on detection using only time reversal processing and the detection performance of joint processing are investigated by a waveguide experiment. The experimental results show that even though the target has a regular aspect, the detection performances differ significantly when different parts of the target are ensonified due to the wave propagation effects in a shallow water waveguide. The effectiveness of incoherent processing that takes advantage of spatial diversity to improve detection performance is also evaluated. [ABSTRACT FROM AUTHOR]

Time reversal (TR) processing is derived from the invariance of the wave equation for a lossless medium to change in the sign of the time variable. By retransmitting the TR version of the time-dispersed received signal propagated from a probe source (PS) to a source-receiver array, one can reacquire the transmitted pulse at the PS location (time compression and spatial focusing) when the waveguide environment is time invariant. However, if some environmental variations occur between the two transmissions, the retrofocusing signal will be defocused. This paper presents a novel method of environmental variation inversion by the focused/defocused signals measured at the PS location with environmental perturbations between the PS transmission and the SRA retransmission. Because the sound speed profile (SSP) plays a critical role in an uncertain ocean environment, inversion of the SSP represented in terms of the empirical orthogonal functions is developed and discussed in detail. Examples of the method applied to synthetic data are provided to demonstrate the method's feasibility. [ABSTRACT FROM AUTHOR]

Real time service and synchronization are very important to many underwater systems. But the time service and synchronization in existence cannot work well due to the multi-path propagation and random phase fluctuation of signals in the ocean channel. The time reversal mirror technique can realize energy concentration through self-matching of the ocean channel and has very good spatial and temporal focusing properties. Based on the TRM technique, we present the Time Reversal Mirror Real Time service and synchronization (TRMRT) method which can bypass the processing of multi-path on the server side and reduce multi-path contamination on the client side. So TRMRT can improve the accuracy of time service. Furthermore, as an efficient and precise method of time service, TRMRT could be widely used in underwater exploration activities and underwater navigation and positioning systems. [ABSTRACT FROM AUTHOR]

An algorithm for single-mode close-loop excitation in shallow water using two vertical hydrophone arrays is presented. Single mode excitation is a powerful tool for studying a great variety of oceanographic processes. The desired single-mode sound field is the only variable that needs to be given in single-mode close-loop excitation. In order to achieve an exact desired single-mode sound field, two vertical hydrophone arrays are used to search out the correct vertical wave number. By analyzing the covariance of estimation value of Green's function matrix, an optimal source array weight matrix is presented to estimate Green's function matrix. An algorithm based on the weight matrix makes single-mode excitation converge at maximum speed and be steady. Advantages of the algorithm are confirmed by numerical simulations. Results of an experiment performed in a coastal environment are presented. [ABSTRACT FROM AUTHOR]

Longitudinal correlation is a very important parameter in underwater acoustic signal processing. Normal mode interference is often the dominating effect on the decrease of the low frequency longitudinal correlation coefficient. Based on the waveguide invariance, a method was developed to enhance the correlation between the signals recorded by two horizontally separated hydrophones. In this research, the method is used to deal with other experimental data. Unlike the former research, the signal is generated by an explosive source, which is more complicated than a bulb source due to the strong bubble pulse following the shock wave. The results show that the spatial correlations of low frequency signals received by a horizontal array can be improved measurably. [ABSTRACT FROM AUTHOR]

The goal of this presentation is to provide first results concerning the optical noise of a fiber laser used as an underwater acoustic sensor: hydrophone. The main sensor characteristics are: -1): A sensitivity allowing to detect all noise levels above background sea noise (the so-called deep-sea state 0). Among other applications, one may mention: seismic risk prevention, oil prospection, ship detection, etc. -2) : An optical noise reduced to its minimal value: it is the lower bound below which no acoustic pressure variation is detectable. We therefore present here the first results for the expected sensitivity of the acousto-optic sensors, the frequency and amplitude of optical noises induced by the fiber laser and all the devices on the optical line. These results exemplify the possible detection of signal levels as low as the deep-sea state noise 0, especially for low frequency bandwidths, from several Hertz up to several kiloHertz. [ABSTRACT FROM AUTHOR]

Nonlinear ‘parametric’ sonar is distinguished by highly predictable in-water formations of identifiable von Helmholtz spectral energies produced directly as a result of two or more preselected primaries simultaneously contained in a transmit waveform. In the nearly half-century of scientific endeavors within the field of parametric sonar, the methodical investigation into formulation techniques and practical applications using higher-order combination tones has been noticeably lagging the attention received by their more commonly recognized kin of second-order sum and difference frequencies. Generalized mathematical and graphical viewing techniques are presented for elucidating the abundance of cross-band complexities and facilitating preliminary design efforts specifically employing any of these higher-order parametric frequency components on operational systems. Recent sonar experiments implementing pulsed parametric transmit waveforms intended to fully exploit their intrinsic broadband nonlinear energy have demonstrated the potential for improved underwater target detection and classification in acoustically harsh environments. However, research efforts could benefit from more efficient and universal tools for predetermining all of the desired in-water spectral-temporal characteristics. New developments utilizing this methodology have led to unique approaches for designing stepped CW, LFM and hyperbolic FM detection waveforms incorporating enhanced signal processing qualities and constructing coding schemes for reliable underwater acoustic digital communications. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Arrays of hydrophones have been part of Navy sonar systems for over fifty years now. They are used both passively and actively and now are quite sophisticated. These arrays were used in fixed configuration such as the SOSUS system and towed arrays deployed from surface ships and submarines. One can assert that these arrays were a key factor in winning the cold war. Similarly, medical ultrasonics have used many of the techniques initially developed for sonar systems save four orders of magnitude in frequency. The major area where modern sonars differ involves the use of adaptive array processing whereby one usually needs to localize a weak signal in the presence of strong interference in a nonstationary environment. Most of the recent literature concerns advancing these techniques especially making them more robust to environmental uncertainties including calibration and properties of the ocean. This presentation will give an overview of the state of the art in sonar and hopefully establish a few points of relevance with nondestructive testing. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Orthogonal frequency division multiplexing with on-off keying (OFDM-OOK) offers a means to near BPSK signaling rates with the simplicity of non-coherent processing. At high frequencies source receiver acceleration induces time varying signal dilations that adversely effect both frequency alignment and multicarrier orthogonality. Dilation process estimation is coupled with a decision directed frequency domain channel magnitude response estimator for non-coherent equalization of OFDM-OOK. Joint co-channel estimation is presented for near optimal decisions under loss of orthogonality due to Doppler spreading. Data from a moving source experiment at ranges from .8 km to 2.0 km, conducted in the shallow water off the coast of Elba Italy were used to test the feasibility of OFDM-OOK at 18kHz center frequency with 4 kHz bandwidth. The effects of source receiver relative motion, frequency selectivity, and Doppler spreading on bit error rates were assessed. Mollification of frequency selective fading by diversity combining is demonstrated. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The Rapid Geo-acoustic Characterization (RGC) algorithm was developed to perform geo-acoustic characterization using the interference patterns of surface ships of opportunity in shallow water. It has been applied successfully at low frequencies to synthetic inversion workshop data and at sea in near real-time. The RGC algorithm determines an effective sediment, which matches the slope (waveguide invariant) and spacing (reciprocal of the time -spread) of interference patterns as well as the fall off in range of the received level. In this paper, we evaluate the extension of the algorithm to mid and high-frequency acoustic signals in shallow water. The issues explored are the robustness of the waveguide invariant, time -spread and TL slope at frequencies above 2 kHz. The algorithm will be applied to a synthetic data example with a known bottom in a range-independent environment. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Channel estimate based equalizers are those for which observations of the received signal are used to estimate the channel impulse response and possibly the statistics of the interfering noise field, and these estimates are used to calculate the equalizer filter coefficients. Channel estimate based decision feedback equalizers (CE-DFE), linear MMSE equalizers (L-MMSE), and Passive Time-Reversal equalizers (P-TR) are all examples of coherent channel estimate based equalizers. Equations are derived for analyzing the performance of these channel estimated based equalizers. The performance is characterized in terms of the mean squared soft decision error (σs2) of each equalizer. This error is decomposed into two components. These are the minimum achievable error (σo2) and the excess error (σ[variant_greek_epsilon]2). The former is the soft decision error that would be realized by the equalizer if the filter coefficient calculation were based upon perfect knowledge of the channel impulse response and statistics of the interfering noise field. The latter is the additional soft decision error that is realized due to errors in the estimates of these channel parameters. Here, the impact of errors in the channel impulse estimation errors on σ[variant_greek_epsilon]2 is considered (i.e., the equalizer is assumed to have accurate knowledge of the statistics of the interfering noise field.). The error equations allow for a direct comparison of the performance characteristics of these equalizers as well as an evaluation of the impact of the characteristics of the acoustic channel on equalizer performance. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

High rate underwater communications have traditionally relied on equalization methods to overcome the intersymbol interference (ISI) caused by multipath propagation. An alternative technique has emerged in the form of time-reversal, which comes at virtually no cost in computational complexity, but sacrifices the data rate and relies on large arrays to reduce ISI. In this paperer, optimal multipath suppression using spatio-temporal processing is addressed analytically. A communication link between a single element and an array is considered in several scenarios: uplink and downlink transmission, with and without channel state information and varying implementation complexity. Transmit/receive techniques are designed which simultaneously maximize the data detection SNR and minimize the residual ISI, while maintaining maximal data rate in a given band-width and satisfying a constraint on transmitted energy. The performance of various techniques is compared on a shallow water channel operating in the 15 kHz band. Results demonstrate benefits of optimal focusing whose performance is not conditioned on the array size. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Underwater acoustic communications in waveguides is known to be prone to severe multipath, which strongly limits practical transmission rates with actual channel equalization techniques. The time reversal principle uses the ocean waveguide response to a basic pulse shape to matched filter the received data sequence. Assuming the ocean response to be a version of the actual pulse shape ocean response corrupted by additive noise, the matched filter output remains a sum of four terms from which only one has the required data sequence in usable form. This paper analysis the ability of such a peculiar matched filter to reject the unwanted terms both with fixed and time-varying source-receiver geometries. One particular parameter with practical interest is the sensitivity of the matched filter performance to a change of the source-receiver range during the processing that induces a mismatch limiting factor. Simulation examples in realistic situations and results obtained with real data, collected during the INTIFANTE’00 sea trial, illustrate the theoretical assertions. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]