Acoustic Channel Impulse Response Research Articles

Micro-multipath interference and its effect on sub-minute acoustic fluctuations: Observations of mid-frequency acoustic propagation in the upper ocean

An experiment was conducted to investigate how variability in the acoustic channel impulse response at signal processing timescales of seconds to minutes is related to internal-wave-driven changes in the ocean sound speed field on timescales of tens of minutes. We find that vertical fine-scale structures (≤10 m) embedded in the sound speed profile have strong second derivatives which allow for the formation of acoustic caustics as well as acoustic propagation pathways that interfere with each other (micro-multipaths). Both effects are sensitive to vertical displacement by internal waves, which can therefore produce sub-minute acoustic fluctuations from internal waves that have timescales longer than a minute.

Estimation of Acoustic Channel Impulse Response at Low Frequencies Using Sparse Bayesian Learning for Nonuniform Noise Power

Sparse Bayesian learning (SBL) has been extended to estimate acoustic channel impulse responses (CIRs) at low frequencies, where matched filter (MF)-based CIR estimation suffers from low resolution due to a limited frequency band. In this study, the extended SBL was developed to account for nonuniform noise power in a signal model for the CIR via a formulation that considers inconsistent noise and multiple measurements, which cannot be handled in the conventional SBL. The extended SBL is applied to simulated and measured acoustic data and then compared with the MF and existing SBLs. With the advancement in the schemes, the time resolution and denoising are enhanced; especially, the results of the extended SBL on the simulated data show that it clearly distinguishes the two adjacent arrivals with moderate errors in the estimated time delays. Additionally, as a result of applying it to the measured array data, the extended SBL can achieve a high resolution in the CIR estimation, which retains assured arrivals with a single measurement, while some arrivals are weakened by the insufficient measurement for the extended SBL.

Low resolution fourier synthesis modelling for underwater acoustic channel impulse response

Fourier synthesis is the most widely used method for underwater acoustic broadband modeling. Traditionally, high frequency resolution and large time window length are needed to generate broadband time series to avoid aliasing in time domain. An optimized Fourier synthesis method is presented in this paper to accelerate the impulse response function modelling. The method consists of two steps, the phase-frequency response reconstruction and the impulse response function time window determination. The impulse response function of a range-dependent ocean channel is modeled using the proposed method with different frequency resolutions and compared with the interpolation method and traditional Fourier synthesis method. Modeling results show that the proposed method requires only a tenth of the frequency resolution of the traditional Fourier synthesis method and the correlation coefficients of modelling output time series are preserved above 95%. The proposed method also exhibits better phase response reconstruction performances than the interpolation method when the frequency resolution equals to 1 Hz.

Modeling of the power delay profile for underwater acoustic channels—Effects of out-of-plane scattering and reverberation

Long reverberation tails seem to be a common feature of shallow water acoustic channel impulse responses. We show examples from two significantly different environments along the Norwegian coast, and demonstrate that 2-D propagation modeling fails to reproduce the long tails observed. The continuous nature of the impulse responses and the low correlation and slow decay rates of the tails point towards reverberation as an important mechanism behind the observations. The effects of reverberation and out-of-plane scattering are then included in the modeling by employing the 3-D ray-based model Rev3D. Scattering from the sea surface and sea bottom are described by scattering-strength functions including azimuthal variation. The modeling results indicate that the long reverberation tails can be explained by 3-D out-of-plane scattering. Although the measured levels are not perfectly matched, the importance of including reverberation in the modeling is clearly demonstrated.

Spectrum and Energy Efficient OFDM Multicarrier Modulation for an Underwater Acoustic Channel

Time domain synchronization orthogonal frequency division multiplexing technique (TDS-OFDM) can offer better efficiency in spectrum and energy use than standard cyclic prefix and zero padding schemes by replacing the guard interval with a pseudorandom noise sequence. In this paper, a type of frequency domain pseudorandom noise training sequence is proposed for underwater acoustic channel as a TDS-OFDM training sequence and used to update the Doppler scaling factor estimation obtained by using a preamble signal after the initial system is synchronized. A significant improvement can also be achieved in bit error rate performance by the use of the compressed sensing theory to estimate underwater acoustic channel impulse response based on the proposed TDS-OFDM training sequence. Furthermore, the guard interval amplitude in TDS-OFDM can be reduced to improve the resultant energy efficiency and signal-to-interfered-signal ratio. The proposed techniques are carefully developed based on intuitive analysis and combined in a receiver design. Such receiver is evaluated in a measured underwater acoustic channel and a simulated channel. Both technical analysis and experiment results show significant and consistent performance improvements in spectral and energy use.

Grid-free channel estimation of acoustic measurement data from SAVEX15

Acoustic channel impulse responses (CIRs) consisting of multiple arrivals are extracted from the measured data during SAVEX15 experiment (conducted in the northern East China Sea during May 2015) using compressive sensing, providing high-resolution results due to sparsity promoting objective function. Performance of grid-based compressive channel estimation degrades when basis mismatch occurs because delays of the arrivals do not correspond to delays on the grid. To overcome this, grid-free compressive channel estimation is derived from the formulation of grid-free compressive beamforming [J. Acoust. Soc. Am. 137, 1923-1935 (2015)]. The grid-free compressive channel estimation is applied to signals from SAVEX15; the signals were transmitted from a towed source and received at two separate vertical line arrays (16 elements each); they are used to measure the signals traveling through waveguides. Acoustic structures are observed clearly due to high-resolution CIRs along depth, which are varied by the moving source and internal wave activities.

Audio splicing detection and localization using environmental signature

Audio splicing is one of the most common manipulation techniques in the area of audio forensics. In this paper, the magnitudes of acoustic channel impulse response and ambient noise are proposed as the environmental signature. Specifically, the spliced audio segments are detected according to the magnitude correlation between the query frames and reference frames via a statically optimal threshold. The detection accuracy is further refined by comparing the adjacent frames. The effectiveness of the proposed method is tested on two data sets. One is generated from TIMIT database, and the other one is made in four acoustic environments using a commercial grade microphones. Experimental results show that the proposed method not only detects the presence of spliced frames, but also localizes the forgery segments with near perfect accuracy. Comparison results illustrate that the identification accuracy of the proposed scheme is higher than the previous schemes. In addition, experimental results also show that the proposed scheme is robust to MP3 compression attack, which is also superior to the previous works.

Low-Complexity Bayesian Estimation of Cluster-Sparse Channels

This paper addresses the problem of channel impulse response estimation for cluster-sparse channels under the Bayesian estimation framework. We develop a novel low-complexity minimum mean squared error (MMSE) estimator by exploiting the sparsity of the received signal profile and the structure of the measurement matrix. It is shown that, due to the banded Toeplitz/circulant structure of the measurement matrix, a channel impulse response, such as underwater acoustic channel impulse responses, can be partitioned into a number of orthogonal or approximately orthogonal clusters. The orthogonal clusters, the sparsity of the channel impulse response, and the structure of the measurement matrix, all combined, result in a computationally superior realization of the MMSE channel estimator. The MMSE estimator calculations boil down to simpler in-cluster calculations that can be reused in different clusters. The reduction in computational complexity allows for a more accurate implementation of the MMSE estimator. The proposed approach is tested using synthetic Gaussian channels, as well as simulated underwater acoustic channels. Symbol-error-rate performance and computation time confirm the superiority of the proposed method compared to selected benchmark methods in systems with preamble-based training signals transmitted over cluster-sparse channels.

Measurements and Modeling of Effects of Out-of-Plane Reverberation on the Power Delay Profile for Underwater Acoustic Channels

Long reverberation tails are often observed in shallow-water acoustic channel impulse responses (CIRs). Examples from three significantly different environments along the Norwegian coast are presented. It is shown that 2-D propagation modeling fails to reproduce the long tails. Nevertheless, inclusion of the impulse response tails is necessary in connection with model-based simulators of communication performance, to avoid too optimistic estimates of bit error rates, etc. This is demonstrated by simulation examples for one of the environments, comparing results for truncated and complete impulse responses. High wind speeds and Doppler spread signals indicate that surface reverberation must be taken into account for modeling the tail energy. In bistatic scenarios such as the ones considered here, backscattering computations in a single vertical 2-D plane are not sufficient. A 3-D ray-based model, Rev3D, is utilized to include effects of out-of-plane scattering from the sea bottom as well as the sea surface. The scattering-strength functions include azimuthal variation, with significantly enhanced scattering close to the forward direction. Rev3D modeling supports the hypothesis that out-of-plane scattering and reverberation are main mechanisms behind the observed continuous, exponentially decaying impulse responses. Modeling incorporates available environmental information concerning sound-speed profiles, bottom topography and type, surface wave spectra, etc. Good agreement of measured and modeled time series is obtained for two of the locations. For the remaining location, modeling correctly recovers a significantly lower decay rate of the reverberation tail in comparison to the other locations, but the modeled decay rate is somewhat too large.

A Study on the Characteristics of Underwater Sound Transmission by Short-term Variation of Sound Speed Profiles in Shallow-Water Channel with Thermocline

Underwater acoustic channel impulse responses (CIR) are influenced by sound speed profile (SSP), and the variation of CIR has significant effects on the performance of underwater acoustic communication systems. A significant change of SSP can occur within a short period, which must be considered during the design of underwater acoustic modems. This paper statistically analyzes the effect of the variation of SSP on the long-range acoustic signal propagation in shallow-water with thermocline using numerical modeling based on the data acquired from JACE13 experiment near Jeju island. The analysis result shows that CIR changes variously according to the SSP and the depth of the transmitter and receiver. We also found that when the transmitter and receiver are deeper, the variation of sound wave propagation pattern is smaller and signal level becomes higher. All CIR obtained in this study show that a series of bottom reflections due to downward refraction and small bottom loss in the shallow water with thermocline can be very important factor for long-range signal transmission and the performance of underwater acoustic communication system in time varying ocean environment can be very sensitive to the variation of SSP even for a short period of time.

Least Square and Trended Doppler Estimation in Fading Channel for High-Frequency Underwater Acoustic Communications

Hermes is an asymmetrical point-to-point underwater acoustic modem designed for short-range operations at very high bit rates in ports and shallow waters using broadband acoustic signaling (262-375 kHz). In exploring the possible conversion of Hermes into a multiple-input-multiple-output (MIMO) device, single-carrier phase-modulated spread-spectrum sequences were used for channel estimation and deconvolution purposes. It clearly appeared that the channel estimation and deconvolution routines were quite sensitive to rapid time changes in the acoustic channel impulse response (CIR), which usually reflects the presence of Doppler spread produced for the most part by moving boundaries and oscillating sensors. In this paper, the authors study the least square (LS) channel estimation routine ability to track the time-varying nature of the impulse response using broadband, single-carrier pseudonoise (PN) sequences transmitted by a single source and collected by a single receiver. In addition, the authors evaluate a trend estimation technique, based on the empirical modal decomposition (EMD) method applied to the LS estimate of the CIR. Simulated data produced with a Rayleigh channel model and experimental data collected in a marina are used. This paper shows that the channel estimation method can estimate the time-varying impulse response of the acoustic channel with a high resolution of both time and delay (down to 7 μs) at the expense of high computational requirements. In analyzing the time variation of the main and secondary echoes for a signal-to-noise ratio (SNR) of 32 dB, simulated results indicate that the root mean square error (RMSE) between theoretical and LS estimated response is 7.8% for the main path with an equivalent Doppler spread of 10 Hz and 15.7% for the second path with an equivalent Doppler spread of 15 Hz. Applying the trend estimation technique to the LS CIR greatly reduces this error, down to 2.9% for the main path and to 4.7% for the second path. The experimental data clearly show that the routine can closely track the time variations of the main echo and provide a meaningful estimate of the Doppler spread.

Region of Feasibility of Interference Alignment in Underwater Sensor Networks

To enable underwater applications such as coastal and tactical surveillance, undersea explorations, and real-time picture/video acquisition, there is a need to achieve high data-rate and reliable communications underwater, which translates into attaining high acoustic channel spectral efficiencies. Interference alignment (IA), which has been recently proposed for radio-frequency multiple-input-multiple-output (MIMO) terrestrial communication systems, aims at improving the spectral efficiency by enabling nodes to transmit data simultaneously at a rate equal to half of the interference-free channel capacity. The core of IA in the space domain lies in designing transmit precoding matrices for each transmitter such that all the interfering signals align at the receiver along a direction different from that of the desired signal. While promising, however, there are still challenges to solve for the practical use of IA underwater, i.e., imperfect acoustic channel knowledge, high computational complexity, and high communication delay. In this paper, a feasibility study on the employment of IA underwater is presented. A novel distributed computing framework for sharing processing resources in the network so to parallelize and speed IA algorithms up is proposed; also, such framework enables “ensemble learning” of various precoding matrices computed using different (competing) IA algorithms so to achieve efficient alignment of the interference at the receiver. The robustness of the IA technique against imperfect acoustic channel knowledge is also quantified by estimating precoding matrices based on predicted channel coefficients. Finally, the performance of an algorithm to predict the underwater acoustic channel impulse response is presented using real data sets.

Effect of wind-generated bubble plumes on shallow water acoustic channels

Wind blowing over the sea surface generates many dramatic effects. One of the most important effects is bubble plumes generated by breaking waves, which have major effects on high frequency acoustic propagation, sound speed profile and ambient noise due to the sound scattering and absorption properties of bubbles. On one hand, we will introduce several theoretical models of bubble plumes; on the other hand, we will propose how to calculate the acoustic signal attenuation and sound speed anomaly due to wind-generated bubble plumes. Simulation results will show the shallow water acoustic channel impulse response and sound speed anomaly with the effect of bubble plumes.

Application of time reverse mirror in underwater acoustic networks communication

Time reversal mirror (TRM) could match the acoustic channel impulse responses (CIR) automatically to achieve channel equalization without any transcendental knowledge. Active time reversal mirror (ATRM) needs monostatic sensor, and the signal propagates through the acoustic channel twice leading to inefficient. Further, the array processing enhances the complexity. In order to overcome disadvantages, the single sensor passive time reversal mirror (PTRM) and virtual time reversal mirror (VTRM) are presented. Based on the properties of underwater acoustic channel, using the uncorrelated character among different users' CIR, the single-element time reversal mirror is proposed to apply to underwater acoustic networks. The scheme could focus the desired user's information and suppress the undesired users' information. Furthermore, we have performed results of numerical simulations. Key words: time reversal mirror (TRM); channel equalization; underwater acoustic communication; networks

On Microphone-Array Beamforming From a MIMO Acoustic Signal Processing Perspective

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Although many microphone-array beamforming algorithms have been developed over the past few decades, most such algorithms so far can only offer limited performance in practical acoustic environments. The reason behind this has not been fully understood and further research on this matter is indispensable. In this paper, we treat a microphone array as a multiple-input multiple-output (MIMO) system and study its signal-enhancement performance. Our major contribution is fourfold. First, we develop a general framework for analyzing performance of beamforming algorithms based on the acoustic MIMO channel impulse responses. Second, we study the bounds for the length of the beamforming filter, which in turn shows the performance bounds of beamforming in terms of speech dereverberation and interference suppression. Third, we address the connection between beamforming and the multiple-input/output inverse theorem (MINT). Finally, we discuss the intrinsic relationships among different classical beamforming techniques and explain, from the channel condition perspective, what the prerequisites are for those techniques to work. </para>

Blind Estimation of the Ocean Acoustic Channel by Time&amp;ndash;Frequency Processing

A blind estimator of the ocean acoustic channel impulse response envelope is presented. The signal model is characterized by a deterministic multipath channel excited by a highly nonstationary deterministic source signal. The time-frequency (TF) representation of the received signal allows for the separation between the channel and the source signal. The proposed estimator proceeds in two steps: First, the unstable initial arrivals allow for the estimation of the source signal instantaneous frequency (IF) by maximization of the radially Gaussian kernel distribution; then, the Wigner-Ville distribution (WV) is sequentially windowed and integrated, where the window is defined by the previously estimated IF. The integral gives the channel impulse response envelope, which turns to be an approximation to the blind conventional matched filter (MF). The blind channel estimator (CE) is applicable upon the following conditions: that the multipath channel contains at least one dominant arrival well separated from the others, and that the IF of the source signal is a one-to-one function. Results obtained on real data from the INternal TIde Measurements with Acoustic Tomography Experiments (INTIMATE'96), where the acoustic channel was driven by an linear frequency modulation signal, show that the channel's envelope detailed structure could be accurately and consistently recovered, with the correlation of the estimates ranging from 0.796 to 0.973, as compared to the MF result

Coherent equalization for underwater acoustic communications: A performance evaluation of adaptive MMSE and time-reversal techniques

A simple analytical expression is presented for evaluating and comparing the performance of channel-estimate-based coherent equalizers. A channel-estimate-based equalizer is one whose filter coefficients are calculated as a function of an explicit estimate of the acoustic channel impulse response. This is in contrast to a direct adaptation equalizer whose coefficients are calculated directly as the solution to a data estimation error minimization problem. The equalizer performance expression is different from other commonly used expressions because it is formulated from the viewpoint of the estimate of the channel impulse response rather than the actual channel impulse response. The expression allows the dependence of the performance of coherent equalizers on channel complexity and the quality of the channel estimate to be separately evaluated. The expression is used to evaluate and compare the performance of adaptive MMSE and time-reversal equalizers. [Work supported by ONR Ocean Acoustics.]

Compound compensation strategies for wireless data communications over the multimodal acoustic ocean waveguide

Underwater acoustic communications (UAC) at the reverberation-limited range results in severely distorted information signals. Wide-band signals are subject to both intermodal and intramodal-type of dispersions. The underwater acoustic channel impulse response and the sidelobes strongly depend on the waveguide structure and the source and receiver positions. The motion and displacement from this position, as well as other environmental variabilities impose a real-time adaptivity for the receiver operation to keep track of the fluctuations. To increase the system's reliability and data rate, there is a need to employ adaptive equalizers and diversity techniques to improve the margins against noise, and intersymbol interference (ISI). Blind adaptive equalization (BAE) is the ideal adaptive compensation when operating point-to-multipoint networks, and centralized communication systems in general. Inherent optimum multiple resonant modes within the ocean acoustic waveguide can be exploited judiciously via a new proposed parallel data multicarrier modulation (MCM) scheme by sending data over the multiple subcarriers. MCM might eventually obviate equalization which introduces higher-order computational complexity to the receiver. The above modulation eliminates multipaths and allows operation at multiples of the single-carrier transmission rate. The system's immunity to distortions such as ISI, fast fades, and impulsive noises, is increased due to incorporation of symbol guard space. Direct comparisons with single carrier schemes (such as higher-order statistics (HOS)-based equalization) are of great interest, since the proposed new receiver configuration has low-complexity to provide a compact, portable and low-power practical acoustic modem. Finally, network topology issues are considered to determine optimum network architectures for underwater acoustic LANs. A central topology (CT) supported by BAE and MCM transmission is proposed.