Shallow Water Acoustic Communication Research Articles

Analysis of High-Frequency Communication Channel Characteristics in a Typical Deep-Sea Incomplete Sound Channel

Deep-sea acoustic communication has attracted widespread attention in recent years. Research on deep-sea acoustic communication channel characteristics is essential for the development and system design of deep-sea acoustic communication technologies. However, the structural and spatiotemporal characteristics of deep-sea high-frequency acoustic communication channels (unlike those of shallow-water acoustic communication channels) are poorly understood. Based on a channel measurement experiment in a typical deep-sea incomplete sound channel environment in the South China Sea, this paper analyzes the structural and spatiotemporal characteristics of high-frequency underwater acoustic channels in the direct-arrival and shadow zones. The channel multipath vertical structure, amplitudes of different path clusters, root mean square (RMS) delay spread, and channel temporal coherence are investigated at different depths. The randomness of different path clusters is quantified as the coefficient of variation. The channels in the shadow zone are characterized by a complex multi-path structure, high RMS delay spread, and low temporal coherence. On the contrary, the channels of the direct-arrival zone show multi-path convergence, low RMS delay spread, and high temporal coherence. Understanding the dynamic changes in these parameters can guide the statistical modeling of channels and the design of communication algorithms in different zones in deep-sea high-frequency acoustic communication scenarios.

Construction and application of polar codes in OFDM underwater acoustic communication

A polar-coded orthogonal frequency-division multiplexing (OFDM) underwater acoustic communication system was developed, and a Monte Carlo (MC) method for constructing optimal polar codes was proposed to improve the performance of the system in this study. Several optimal polar codes with different code lengths and rates were constructed for a shallow-water acoustic channel. Simulations showed that the polar codes constructed using the MC method rendered superior performance than those constructed using the Bhattacharyya parameter and the Gaussian approximation method. In addition, shallow-water acoustic communication experiments were conducted at Xiamen port. The results show that the developed polar-coded OFDM communication system has excellent performance with 196 out of 200 frames achieving error-free transmission, which is much greater than the performance (157 out of 200) of the LDPC-coded OFDM communication system.

Experimental evaluation of norm constraint sparsity exploitation for shallow water acoustic communication

It has been recognized that the highly time-frequency selective underwater acoustic (UWA) channels exhibit sparse multipath structure, thus provides the possibility for performance enhancement in channel estimation and equalization. In order to exploit the sparsity contained in underwater acoustic channels, there have been increasing interests in the investigation of sparse estimation algorithms. From the viewpoint of practical engineering implementation, imposing different norm constraint on the least means square (LMS) adaptive iteration offers a convenient way for derivation of low complexity sparse channel estimation algorithm. However, while the practical underwater acoustic channels exhibit random time variations, there is a lack of experimental evaluation and comparison in terms of the performance enhancement of different norm constraint sparse channel estimation algorithms under practical UWA communication scenario, in a relatively long time-scale such as hours. This paper presents the experimental evaluation and comparison of some typical norm constraint least mean square (LMS) algorithm, i.e., the classic l0-norm, l1-norm and the non-uniform norm (NN), in the context of a shallow water acoustic communication system. Experimental results obtained in a practical shallow water channel within a duration of 5 hours are provided and discussed for evaluating the sparsity exploitation behavior in terms of communication performance achieved via a channel estimation based decision feedback equalizer. The relationship between the performance curves and ocean environmental observations like sound velocity profile, temperature field and wind speed is investigated to explain the comparison results.

Estimation of Underwater Acoustic Channel With Hybrid Sparsity via Static-Dynamic Discriminative Compressed Sensing

The technology of underwater acoustic communication plays an important role in linking the underwater sensors for data collection and networking. Due to simultaneous presence of direct path as well as reflections upon static or dynamic boundaries, underwater acoustic (UWA) channel exhibits hybrid sparse, i.e., consists of relatively static multipath arrivals and time varying ones. While the classic sparse channel estimation algorithms generally do not take this type of hybrid sparsity into account, dynamic compressed sensing (DCS) technique enables the exploitation of time varying sparsity by incorporating CS with Kalman filtering, namely, Kalman Filtered Compressed Sensing (KF-CS). However, by assuming the whole UWA channel as sparse set with time varying support, static sparse components will unavoidably lead to model mismatch and performance degradation. In this paper, by formulating the hybrid multipath UWA channels as sparse set consisting of static and time varying supports, a static-dynamic discriminative compressed sensing (SDD-CS) approach is proposed to explore the hybrid sparsity. Hybrid multipath arrivals are firstly decomposed into static and time varying components by discriminative orthogonal matching pursuit (DOMP), and then the magnitude of time varying and static multipath is estimated by KF-CS and SOMP respectively, after that estimated results of static and time varying component are summed up to obtain the whole channel response. Numerical simulations verify the superiority of the proposed SDD-CS algorithm. Finally experimental results obtained from a shallow water acoustic communication scenario are provided to demonstrate the effectiveness of the proposed algorithm, compared to conventional algorithms.

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

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

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.

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

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

Adapting underwater acoustic communication networks to changing oceanic conditions Using opportunistic multipath signaling schemes

A fundamental bottleneck to high data-rate shallow water acoustic communications is rapidly fluctuating delay spread, which is caused by highly unpredictable multipath scattering by the moving sea surface, sea bottom as well as fluid motion. A rich literature exists on how to track and mitigate the mutipath interference and thus recover the direct arrival between the transmitter and receiver. This talk will review some of the recent methods proposed, in particular light of different representations of the channel multipath in delay, time, frequency domains and combinations thereof. The talk will also present how the different representations of channel multipath can be exploited to design opportunistic signaling schemes that exploit rather that mitigate multipath effects. We will also discuss the impact of such opportunistic signaling, particularly in the context of diversity strategies in communication networks.

Modal dispersion compensation receiver for the long range shallow water acoustic communications.

The received low frequency signals in the long range shallow water will suffer severe attenuation and modal dispersion effect, which will distort the signal and deteriorate the communication performance. In this letter, a modal dispersion compensation receiver is proposed, which exploits the de-dispersion transform to compensate intra-mode dispersion and the rake combination to combine the multi-mode components. With the proposed receiver, the waveform expansion of high order modes is compensated, and thus more modes can be distinguished from the noise and the rake combination gain is improved, which leads to high communication accuracy. Simulation results show that the proposed receiver can achieve about 3 dB gain for the 40 km range and signal with a center frequency 400 Hz.

Bit-error rate based Doppler estimation for shallow water acoustic OFDM communication

Orthogonal frequency division multiplexing (OFDM) has drawn extensive attention in the field of underwater acoustic (UWA) communication due to its advantages of high transmission rate and high spectrum efficiency. Unfortunately, the practical application of OFDM in UWA communication is subject to severe multi-path as well as Doppler sensitivity. To overcome the impact of Doppler, generally the value of Doppler is estimated by searching the best correlation with the standard copy in time or frequency domain, to enable Doppler compensation via the resampling. However, it has been recognized that, at the presence of diffuse or time varying multi-path, the Doppler estimated by searching signal-level correlation may cause significant mismatch and thus deteriorate the demodulation. The reason is that, while the Doppler is searched in terms of signal-level correlation function, the purpose of the demodulation is to achieve the minimum bit error rate (MBER) at bit-level. In this paper, a Doppler estimation method based on BER search is proposed to transform the signal-level Doppler estimation into a bit-level one. Finally, numerical simulation and sea trial experiments under different Doppler scenarios are performed to verify the effectiveness of the proposed Doppler estimation method.

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.

MIMO Underwater Acoustic Communication in Shallow Water with Ice Cover

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

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

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

Research and Development of a Highly Reconfigurable OFDM MODEM for Shallow Water Acoustic Communication

Orthogonal frequency division multiplexing (OFDM) provides an effective approach to combat multipath with high bandwidth efficiency for underwater acoustic communication, thus design and implementation of acoustic OFDM MODEMs draw more and more attention. Extensive work has been performed to mitigate multipath, Doppler shift, and peak average power ratio from the viewpoint of theoretical analysis. Meanwhile, design and implementation of a parameter-configurable OFDM to adapt to highly diverse underwater channels and various missions become more and more important. Traditional MODEM with fixed parameters cannot achieve the best tradeoff between data rate and bit error rate. In order to obtain the best communication performance under long time delay channels, Doppler channels, and frequency-selective channels, in this paper, we provide the design and implementation of a parameter-configurable underwater acoustic OFDM MODEM with the purpose to achieve flexibility. Specifically, the reconfigurable parameters of the OFDM MODEM include the number of receivers, the number of null subcarriers, the number of pilot subcarriers, as well as the type of channel estimation algorithms. The number of pilot subcarriers and type of channel estimation methods are determined based on channel characteristics, the number of null subcarriers is determined based on the Doppler shift, and the number of the received channels is based on fading channel. Moreover, in this paper, the Doppler estimation performance under different number of null subcarriers, and the channel estimation performance under different number of pilot subcarriers and under different channel estimation algorithms, are analyzed in terms of bit error rate. Finally, the effectiveness of the proposed highly configurable OFDM MODEM is demonstrated by two sea trials corresponding to different communication scenarios.

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

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

Exploiting time varying sparsity for underwater acoustic communication via dynamic compressed sensing

While it has been recognized that the multipath structure of the underwater acoustic (UWA) channel offers the potential for compressed sensing (CS) sparsity exploitation, the rapidly time varying arrivals induced by highly dynamic surfaces unfortunately pose significant difficulties to channel estimation. From the viewpoint of underwater acoustic propagation, with the exception of the highly time varying arrivals caused by dynamic surface, generally there exist relatively stationary or slowly changing arrivals caused by direct path or bottom reflection, which imply the adoption of a discriminate estimation method to handle sparse components with different time variation scale. By modeling the time varying UWA channels as a sparse set consisting of constant and time-varying supports, in this paper, estimation of time varying UWA channel is transformed into a problem of dynamic compressed sensing sparse recovery. The combination of a Kalman filter and compressed sensing is adopted to pursue the solution of it. Numerical simulations demonstrate the superiority of the proposed algorithm. In the form of a channel-estimation-based decision-feedback equalizer, the experimental results with the field data obtained in a shallow water acoustic communication experiment indicate that the proposed dynamic compressed sensing algorithm outperforms classic algorithms as well as CS algorithms.

Orthogonal Angle Domain Subspace Projection-Based Receiver Algorithm for Underwater Acoustic Communication

In this letter, the receiver design for shallow water acoustic communication with distinct Doppler is investigated. Conventional Doppler compensation technique may not distinguish different Doppler effects of multipath signals. Accordingly, we propose a receiver algorithm based on hydrophone array angle resolution, which utilizes orthogonal angle domain subspace projection methods to divide the received multipath signals into several subspaces. Moreover, the Dopplers in different subspaces are also estimated and compensated in each orthogonal subspace. Simulation results show that the proposed method can suppress the Doppler effect and achieve better performance in comparison to the conventional single and multiple resampling systems.

Delay-Doppler characteristics of surface interacting underwater sound

The interaction of underwater sound with the ocean surface is investigated. Laboratory measurements of the delay-Doppler structure of the surface multipath at a transmission frequency of 300 kHz are compared to physics based analytic predictions. Channel impulse responses and channel scattering functions are measured when the surface is characterized by wind generated roughness, swell-like roughness, and a mix of wind and swell. The distribution of surface scattered energy across delay time and Doppler shift is presented as a function of wind speed ranging from 0.5 m/s to 11.0 m/s. The effect of wave shape and nominal out-of-plane grazing angle on the delay time spread and Doppler spread of the surface multipath are also considered. Proper characterization of surface interacting acoustic energy has direct application to the improved performance of shallow water and near surface underwater acoustic communications systems.

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

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

Underwater Acoustic Propagation Physics and Signal Processing Techniques for Shallow-Water Acoustic Communications Special Issue

Underwater Acoustic Propagation Physics and Signal Processing Techniques for Shallow-Water Acoustic Communications Special Issue