Underwater Acoustic Communication Experiment Research Articles

Quantitative identification of causes of instrumental acoustic signal distortion in Global Navigation Satellite System-Acoustics Combination observations.

The Seafloor Geodetic Observation-Array (SGO-A), operated by the Japan Coast Guard, relies on the Global Navigation Satellite System-Acoustics combination (GNSS-A) technique, which integrates satellite positioning systems and undersea acoustic ranging to determine seafloor crustal deformation at the centimeter level for earthquake disaster prevention. Recently, we found distortion in the SGO-A 10-kHz carrier wave that degraded the accuracy. Carrier wave distortion can cause errors on the scale of several centimeters to twenty centimeters, which greatly impedes centimeter-level observations. This study investigated this carrier wave degradation by an underwater acoustic communication experiment conducted in 2022, using a transducer similar to that used by SGO-A. Also, we reproduced degraded waveforms through a grid search-like method for quantitatively evaluating the extent to which the interior of the equipment contributed to deterioration. Our results underscore the importance of careful consideration in signal processing, as the observed waveform degradation is not solely attributed to hardware structures but also to internal electrical circuits. The findings suggest that conventional signal identification methods may lead to errors, providing motivation for a shift towards experimental and experiential timing-based waveform identification approaches to enhance accuracy in GNSS-A systems.

Vector Sensor Steering-Dependent Performance in an Underwater Acoustic Communication Field Experiment.

This paper shows the performance resulting from combining vector sensor directional components in an underwater acoustic communication experiment. The objective is to relate performance with transmission direction and range. Receiver structures based on beamforming and passive time-reversal are tested in order to quantify and compare the steerability impact of vector sensor directional components. A shallow water experiment is carried out with a bottom fixed two-axis pressure-gradient vector sensor. A ship suspended acoustic source transmits coherent modulated communication signals at various ranges and from several directions. Results show that one vector sensor can provide an up to 10 times smaller error bit rate than a pressure sensor, favoring communication robustness without size penalty.

Real-Time Adaptive Modulation Schemes for Underwater Acoustic OFDM Communication.

Adaptive modulation received significant attention for underwater acoustic (UA) communication systems with the aim of increasing the system efficiency. It is challenging to attain a high data rate in UA communication, as UA channels vary fast, along with the environmental factors. For a time-varying UA channel, a self-adaptive system is an attractive option, which can choose the best method according to the channel condition to guarantee the continuous connectivity and high performance constantly. A real-time orthogonal frequency-division multiplexing (OFDM)-based adaptive UA communication system is presented in this paper, employing the National Instruments (NI) LabVIEW software and NI CompactDAQ device. In this paper, the received SNR is considered as a performance metric to select the transmission parameters, which are sent back to the transmitter for data transmission. In this research, a UA OFDM communication system is developed, employing adaptive modulation schemes for a nonstationary UA environment which allows to select subcarriers, modulation size, and allocate power adaptively to enhance the reliability of communication, guarantee continuous connectivity, and boost data rate. The recent UA communication experiments carried out in the Canning River, Western Australia, verify the performance of the proposed adaptive UA OFDM system, and the experimental results confirm the superiority of the proposed adaptive scheme.

Long-range underwater acoustic communication of M-eyry spread spectrum with cyclic prefix

The Spread Spectrum is an effective transmission method for complex channels in underwater acoustic (UWA) communication because of the high correlation of pseudo-random noise (PN) sequence. In particular, the spread spectrum method is effective in an environment with a low signal-to-noise ratio. The bandwidth of UWA communication channel is limited due to the fact that sound wave attenuation in water is positively associated with frequency. The limited bandwidth directly leads to the low data rate of conventional direct-sequence spread spectrum UWA communication. M-ary spread spectrum (MSS) schemes using multiple orthogonal PN sequences were proposed as a solution to this problem. However, even in the MSS method, the performance may be limited by the delay spread characteristics in the multipath propagation channels. We propose a more robust communication method in the UWA channel by including Cyclic Prefix (CP) to achieve a more robust performance in the multipath channel. In the proposed method, a CP is inserted for every symbol. In the 160 km long-range UWA communication experiment conducted in the East Sea, the proposed method showed more reliable results than the conventional MSS method. [Work supported by the Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Quadrature Acoustic Frequency Combs Multiplexing for Massive Parallel Underwater Acoustic Communications

AbstractUnderwater acoustic (UWA) communication has been developing rapidly over the past decades for its crucial position in resource exploration, environmental monitoring, and scientific research. However, the transmission data rate of UWA communication is limited by the narrow bandwidth of the underwater acoustic channel. Here, the generation of quadrature acoustic frequency combs (AFCs) is first reported. Massive parallel channels achieved with multiplexing of AFCs for UWA communication are demonstrated. The generated AFCs have unique characteristics which have a stable and precise division in frequency and time domain simultaneously with carrier spacing of 1 Hz with the stability of 10−8. The different frequency spacing in combs leads to different teeth spacings in time, which is orthogonal and separable in the time domain. The orthogonality among AFCs provides a new dimension for multiplexing and can drastically increase channel efficiency. The underwater acoustic communication experiments demonstrate that it is drastically increasing the transmission rate to 45.33 kb s−1 and high spectral efficiency of 1.51 (channel s−1) Hz−1 without utilizing other modulation techniques by a single transducer. Since AFCs multiplexing is a completely independent degree of freedom that can be readily integrated with other high order modulation techniques, such as quadrature amplitude modulation and phase‐shift keying, in a single channel, AFCs multiplexing opens a new dimension for acoustic communication.

Adaptive Turbo Equalization for Probabilistic Constellation Shaped Underwater Acoustic Communications

To increase the spectral efficiency of the underwater acoustic (UWA) communication system, the high order quadrature amplitude modulations (QAM) are deployed. Recently, the probabilistic constellation shaping (PCS) has been a novel technology to improve the spectral efficiency. The PCS with high-order QAM is introduced into the UWA communication system. A turbo equalization scheme with PCS was proposed to cancel the severe inter-symbol interference (ISI). The non-zero a priori information is available for the equalizer and decoder before turbo iteration. A priori hard decision approach is proposed to improve the detection performance and the equalizer convergence speed. At the initial turbo iteration, the relation between the a priori information and the probability of the amplitude of 16QAM symbols in one dimension is given. The simulation results verified the efficiency of the proposed method, and compared to the uniform distribution (UD), the PCS-16QAM had a significant improvement of the bit error rate (BER) performance with PCS-adaptive turbo equalization (PCS-ATEQ). The UWA communication experiments further verified the performance superiority of the proposed method.

Efficient interpolation based OMP for sparse channel estimation in underwater acoustic OFDM

To achieve higher accuracy of path delay estimation with lower computational complexity, the orthogonal matching pursuit (OMP) method with interpolation algorithm has been adopted for the sparse channel estimation, such as underwater acoustic (UWA) communication channels. For an orthogonal frequency division multiplexing (OFDM) system with uniform pilots, estimating the path delay by OMP in each its iteration is equivalent to the frequency estimation of single-tone signals with discrete Fourier transform (DFT). In this paper, based on the existing frequency estimation algorithms, we propose two novel interpolation based OMP methods for baseband sampling grid and over-sampling grid respectively, aiming at improving path delay estimation efficiency. Moreover, the closed-form of Cramer-Rao lower bound (CRLB) for single path delay estimation is derived to serve as a benchmark for simulations. The performances of the two proposed methods are verified by both simulations and UWA communication experiment in Yellow Sea, China, which show a higher estimation accuracy than that of the traditional OMP method, and the similar estimation performance but lower computational complexity than that of the existing high-accuracy interpolation based OMP method.

Integrated waveform for continuous active sonar detection and communication

A novel integrated waveform (IW) for continuous active sonar detection and communications is proposed in this study based on a generalised sinusoidal frequency-modulated signal. This study first introduces the mixed modulation of the IW and a model for the transmitting signal, and then it analyses the time and frequency resolution and anti-reverberation capability of the proposed IW. The performance of two underwater acoustic (UWA) communication receivers based on the proposed IW are compared and analysed under Gaussian noise and multipath conditions. A UWA communication experiment was carried out in Songhua Lake in December 2019. According to the numerical simulation and experimental results, it was concluded that the IW proposed in this study can meet the requirements of active sonar detection and UWA communications with satisfactory performance.

New Results on Joint Channel and Impulsive Noise Estimation and Tracking in Underwater Acoustic OFDM Systems

Impulsive noise can greatly affect the performance of underwater acoustic (UA) orthogonal frequency-division multiplexing (OFDM) systems. In this paper, by utilizing the sparsity of the UA channel impulse response and impulsive noise, we first propose a novel sparse Bayesian learning (SBL) based expectation maximization (EM) algorithm for joint channel estimation and impulsive noise mitigation in UA OFDM systems. Secondly, considering that the UA channel and impulsive noise are fast time-varying, we develop a new approach which combines the SBL with the forward-backward Kalman filtering to track the UA channel and impulsive noise. To further improve the system performance, we utilize the information available on data subcarriers for joint time-varying channel estimation and data detection, based on the SBL algorithm and the Kalman filter. The performance of our proposed algorithms is verified through both numerical simulations and by data collected during a UA communication experiment conducted in the estuary of the Swan River, Perth, Australia. The results demonstrate that compared with existing approaches, the proposed algorithms achieve a better system bit-error-rate and frame-error-rate performance.

An Underwater Acoustic OFDM System Based on NI CompactDAQ and LabVIEW

Recently the orthogonal frequency division multiplexing (OFDM) technique has attracted increasing interests in underwater acoustic (UA) communications. In this paper, we present an OFDM-based UA communication system, which is designed based on the National Instruments LabVIEW software and the CompactDAQ device. Details on both the transmitter and receiver system design are discussed. The performance of this UA OFDM system is verified through recent UA communication experiments performed in a tank, in the Canning River, and in the estuary of the Swan River, Western Australia. Experimental results show that the system achieves a reliable bit error rate performance even with high-order modulation schemes. Compared with conventional field-programmable gate array and digital signal processor based designs, the proposed implementation simplifies the prototype design process and reduces the software development time. The proposed system provides a flexible and reconfigurable prototype for researchers to test and validate the performance of UA communication algorithms in real UA channels.

An Underwater Time Reversal Communication Method Using Symbol-Based Doppler Compensation with a Single Sound Pressure Sensor

Due to the significant multipath and Doppler effects in the underwater acoustic (UWA) channel, the quality of the received signal is degraded, which seriously affects the performance of UWA communication. The paper proposes a time reversal UWA communication method combined with a symbol-based Doppler compensation (SBDC) technique to solve those problems. A single element time reversal mirror (TRM) is used to realize channel equalization and mitigate the inter-symbol interference (ISI) resulting from multipath propagation. The SBDC technique is subsequently used to compensate Doppler effects in the received signal, thereby reducing the bit error rate (BER) and improving the communication performance. In order to verify the performance of the proposed communication method, some simulations with real sounding channels were performed. Moreover, a field UWA communication experiment was conducted in the Songhua River (Harbin, China). The UWA communication experiment achieves nearly error-free performance with a communication rate of 100 bit/s in the bandwidth of 2 kHz. The results of the experiment demonstrate the feasibility and robustness of the proposed UWA communication method.

Efficient System Tracking With Decomposable Graph-Structured Inputs and Application to Adaptive Equalization With Cyclostationary Inputs

This paper introduces the graph-structured recursive least squares (GS-RLS) algorithm, which is a very efficient means to track a linear time-varying system when the inputs to the system have structure that can be modeled using a decomposable Gaussian graphical model. For graphs with small clique sizes, it is shown that GS-RLS can achieve tracking performance very close to that of the conventional RLS algorithm for a fraction of the computational cost. In particular, after proving that the outputs of wide-sense stationary time-varying communication channels have graphical model structure if the inputs are cyclostationary, significant computational gains are realized for adaptive equalization of the time-varying underwater acoustic communication channel using the GS-RLS algorithm. This is verified using field data from the SPACE08 underwater acoustic communication experiment.

Joint Channel Estimation and Impulsive Noise Mitigation in Underwater Acoustic OFDM Communication Systems

Impulsive noise occurs frequently in underwater acoustic (UA) channels and can significantly degrade the performance of UA orthogonal frequency-division multiplexing (OFDM) systems. In this paper, we propose two novel compressed sensing based algorithms for joint channel estimation and impulsive noise mitigation in UA OFDM systems. The first algorithm jointly estimates the channel impulse response and the impulsive noise by utilizing pilot subcarriers. The estimated impulsive noise is then converted to the time domain and removed from the received signals. We show that this algorithm reduces the system bit-error-rate through improved channel estimation and impulsive noise mitigation. In the second proposed algorithm, a joint estimation of the channel impulse response and the impulsive noise is performed by exploiting the initially detected data. Then, the estimated impulsive noise is removed from the received signals. The proposed algorithms are evaluated and compared with existing methods through numerical simulations and on real data collected during a UA communication experiment conducted in the estuary of the Swan River, WA, Australia, during December 2015. The results show that the proposed approaches consistently improve the accuracy of channel estimation and the performance of impulsive noise mitigation in UA OFDM communication systems.

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

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

Comparison of underwater communication performances in two shallow-water sites with different sediment types located off the coast of Korea

Shallow water waveguide characterized by multipath channel produces a significant delay spreading of transmitted signals, which is referred to as inter symbol interference (ISI). Since the ISI results in distortion of communication signals, many studies to reduce the effect of ISI have been conducted. For successful underwater communication, it is important to understand the correlation between the spatial and temporal properties of ISI and communication performance. Underwater acoustic communication experiments were conducted in two different seafloor environments with relatively fine-grained and coarse sediments. The experimental geometries were the same; water depths of both sites were about 40 m and source depths were about 32 m. Communication signals were measured by a four-channel receiving array, covering waters 5 to 35 m in depth. Sound speed profiles were measured by CTD casts and the surficial sediment samples were taken by a grab sampler. The received communication signals were demodulated using the time reversal, phase lock loop and decision feedback equalizer techniques. In this talk, the communication performances at two different sites are presented and compared with the channel impulse responses at two sites. [Work supported by ADD(UD140001DD) and KRISO(PMS3310).]

Underwater acoustic communication in time-varying channel environment based on passive time reversal

Channels estimated from probe signal or training symbols will mismatch with the true channels in passive time reversal (TR) underwater acoustic (UWA) communication when the ocean environment is time-varying, leading to poor channel equalization performance. In order to solve this problem, this paper studies Block-based TR processing structure. The received data are divided into small blocks and the channel estimated from previously decoded symbols is used to match filter the current block to suppress the channel mismatch. Decision feedback equalizer (DFE) is also combined with Block-based TR to remove the residual inter symbol interference (ISI) and Doppler effect (Block-based TR-DFE). Mobile UWA communication experiment was conducted near Xiao Changshan island, Dalian in July, 2015. Though the received data are preprocessed by resampling, residual Doppler effect still exists due to ununiform motion between the receiver and transmitter, making the channel time-varying fastly. Block-based TR realized effective tracking and compensation for time-varying channels. The output signal-to-noise ratio (SNR) after equalization is greatly improved and the bit error rate (BER) is significantly reduced.

Nonstationary System Analysis Methods for Underwater Acoustic Communications

The underwater environment can be considered a system with time-varying impulse response, causing time-dependent spectral changes to a transmitted acoustic signal. This is the result of the interaction of the signal with the water column and ocean boundaries or the presence of fast moving object scatterers in the ocean. In underwater acoustic communications using medium-to-high frequencies (0.3–20 kHz), the nonstationary transformation on the transmitted signals can be modeled as multiple time-delay and Doppler-scaling paths. When estimating the channel, a higher processing performance is thus expected if the techniques used employ a matched channel model compared to those that only compensate for wideband effects. Following a matched linear time-varying wideband system representation, we propose two different methods for estimating the underwater acoustic communication environment. The first method follows a canonical time-scale channel model and is based on estimating the coefficients of the discrete wideband spreading function. The second method follows a ray system model and is based on extracting time-scale features for different ray paths using the matching pursuit decomposition algorithm. Both methods are validated and compared using communication data from actual underwater acoustic communication experiments.

수중 센서네트워크 구현을 위한 단일 반송파 디지털 변조기법의 실험적 고찰

본 논문에서는 단일 반송파 기반의 디지털 변조 기법들의 성능을 수중 통신 실험을 통하여 분석하였다. 통신 실험은 실제 해상에서 수행되었으며 적용된 변조기법은 위상 비동기 방식인 ASK, FSK와 위상 동기 방식의 QPSK이다. 통신 성능을 평가하기 위해 이미지 파일을 600bps~3Kbps의 전송률을 가지는 신호로 변조하여 전송하였다. 수신된 신호를 복조하여 BER을 추정한 결과 ASK와 FSK의 경우 보상 알고리즘 없이 <TEX>$10^{-3}{\sim}10^{-4}$</TEX>의 오차율을 나타내었고, QPSK의 경우 선형 등화기를 이용하여 <TEX>$10^{-4}$</TEX>의 오차율을 가지는 통신이 가능함을 확인하였다. In this paper, underwater acoustic communication experiment was carried out to test a performance of single carrier digital modulation schemes. The communication experiment was performed at real sea and tested modulation schemes are ASK, FSK with non-coherent detection and QPSK with coherent detection. A modulated image data was transmitted with data rates of 600bps~3Kbps. From the results of BER of the demodulated signal, ASK and FSK show the achievable BER of <TEX>$10^{-3}{\sim}10^{-4}$</TEX> without compensation techniques and QPSK show that of <TEX>$10^{-4}$</TEX> with linear equalizer.

Experimental performance of concatenated coding in shallow-water channels

In a series of phase coherent underwater acoustic communication experiments near the New England continental shelfbreak, the coding gain realized by a concatenated outer block code and trellis coded modulation (TCM) inner code was measured and compared to ideal asymptotic limits. Viterbi decoding of the inner code was fully integrated into the adaptive decision feedback equalizer structure. Coding gain is typically not realized until after decision delays of several code constraint lengths. Delayed decisions for the tap update algorithm, however, degrades equalization performance. Two specific techniques were employed to mitigate this conflict, including linear extrapolation of the carrier phase correction and a unique evolution of the tap weights for each state of the Viterbi trellis. Coding gains were realized for both eight-level phase shift keyed (8 coding states) and 16 quadrature amplitude shift keyed (128 coding states) waveforms. Bit error rate as a function of output signal-to-noise ratio was computed to demonstrate practical, as opposed to asymptotic, coding gains. The extent to which coding increased the system performance envelope will then be discussed.

Model Studies of a Single Surface-Bounce, Underwater, Acoustic-Communication Channel

Underwater acoustic-communication experiments are being conducted in a 12×4×4-ft model tank equipped with a remote-controlled transducer-positioning system and a “sea-state” generator. The statistics of the various sea states are obtained by utilizing two waveheight indications. The output of these devices is instantaneous profiles of the surface at particular locations. The output of the waveheight indicators is digitized, recorded onto IBM magnetic tape, and analyzed on an IBM-1620 computer. Auto- and cross-correlation functions as a function of position are obtained for various modeled sea states. An advantage of the model tank is that it is possible to investigate any individual acoustic path desired without the confusion of multipaths that occur in typical real ocean channels. The path that we are investigating at this time is the single surface bounce. A train of constant-amplitude pulses, amplitude-modulating a 1-Mc/sec carrier, is reflected off the surface and the received wave is recorded. Using the same data-processing equipment described above, the correlation functions for the transmitted and received pulse trains are obtained for various “sea states.”