Long-range Underwater Acoustic Communication Research Articles

High-performance deep-sea long-range underwater acoustic communication: Deconvolved conventional beamforming based approach

To address the challenges posed by large propagation loss and severe multipath delay spread, while aiming to improve communication rates in long-range underwater acoustic (UWA) communication, this paper introduces a novel approach: multi-beam (MB) Orthogonal Frequency Division Multiplexing (OFDM) based on deconvolved conventional beamforming (dCv). The proposed method establishes a signal processing framework for UWA channel receivers, wherein received signals corresponding to each angle beam are isolated and concentrated using the dCv technique. Subsequently, the separated signals from each angle undergo maximal-ratio combining (MRC) channel equalization followed by demodulation. Compared to conventional single-beam (SB) processing methods, the proposed approach capitalizes on multipath diversity gain achieved by combining MB outputs originating from various arrival angles. Moreover, employing dCv processing demonstrates superior performance compared to popularly employed beamforming techniques, especially when dealing with paths arriving from closely aligned angles. Additionally, the array beamforming utilized significantly enhances the signal-to-noise ratio (SNR) of each beam output, mitigating the elevated error rates in channel estimation associated with combining low SNR signals received from individual elements. Simulation results utilizing BELLHOP and experimental data from the South China Sea showcase notably improved bit error rate (BER) performance for the proposed method compared to SB equalization, MB-MRC equalization based on conventional beamforming (CBF), minimum variance distortionless response (MVDR), and worst-case performance optimization (WCPO), as well as standard MRC equalization techniques. The proposed receiver achieves error-free decoding results at a communication distance of 80 km with a data rate of 247 bps.

Weighted Multiple-sensor Combining Algorithm for Long-range Underwater Acoustic Communications and Experimental Results

In very long-range underwater acoustic communications, enhancing communication performance often involves augmenting the received signal-to-noise ratio (SNR) through the integration of multiple receiver sensors. However, in cases where sensors are linearly combined, those with inferior performance can significantly impact overall communication efficacy. Consequently, when employing linear combinations, it becomes essential to assign lower weighting factors to sensors exhibiting higher error rates, thereby enhancing overall system performance. This paper introduces a novel approach: a weighted multiple-sensor combination algorithm that leverages the preamble error rate of individual sensors. Through the utilization of turbo coding with a rate of 1/3 and 4/8-ary frequency-shift keying method, extensive experiments spanning very long distances of over 160 km in the East Sea were conducted. The results of these experiments demonstrate that the proposed weighted multiple-sensor combination algorithm markedly improves BER performance compared to conventional linear combination algorithms.

Long-Range Underwater Acoustic Channel Estimation

Long-range underwater acoustic communication (LR-UWAC) is an essential technique for applications such as the control of unmanned underwater vehicles, gliders, or tactical submarines for long-term monitoring tasks. While techniques for estimating the channel impulse response (CIR) of short-range UWAC have been successfully applied, this is not the case for LR-UWAC. Here, the main challenge is to handle the diverse channel structure, which can be of a sparse or a non-sparse structure. In this paper, we propose a robust channel estimator for LR-UWAC. Our solution includes a switching mechanism to classify the channel as either sparse or non-sparse. In its sparse form, the LR-UWAC channel is characterized by a long time delay spread, time-invariant characteristics, and a block structure. Thus, we propose a non-trivial combination of the block subspace pursuit and the distributed subspace pursuit algorithms to exploit the channel’s block structure. For non-sparse LR-UWAC channel structures, we approximate the received signal as Gaussian, and derive a maximum-likelihood (ML) estimator to separate the non-resolvable multipath. Extensive numerical simulation and results from two long-range sea experiments demonstrate the efficiency of our approach in identifying the channel’s structure, and to accurately estimate the channel compared to state-of-the-art benchmarks.

Modeling the Geometry of an Underwater Channel for Acoustic Communication

The achievement of efficient data transmissions via underwater acoustic channels, while dealing with large data packets and real-time data fed by underwater sensors, requires a~high data rate. However, diffraction, refraction, and reflection phenomena, as well as phase and amplitude variations, are common problems experienced in underwater acoustic (UWA) channels. These factors make it difficult to achieve high-speed and long-range underwater acoustic communications. Due to multipath interference caused by surface and ocean floor reflections, the process of modeling acoustic channels under the water's surface is of key importance. This work proposes a~simple geometry-based channel model for underwater communication. The impact that varying numbers of reflections, low water depth values, and distances between the transmitter and the receiver exert on channel impulse response and transmission loss is examined. The high degree of similarity between numerical simulations and actual results demonstrates that the proposed model is suitable for describing shallow underwater acoustic communication environments.

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.]

Adaptive Modulation for Long-Range Underwater Acoustic Communication

Long-range underwater acoustic communication (LR-UWAC) refers to the peer-to-peer transmission of messages for distances of tens to hundreds of km. It is a key enabling technique for applications such as control over unmanned underwater vehicles for long-term surveying. While underwater acoustic communication over shorter ranges is an established technique, this is not the case for LR-UWAC. This gap is mostly due to channel uncertainties: in the absence of feedback from the receiver and due to the long transmission range, channel state information (CSI) at the transmitter may not reflect the actual channel. In this paper, we propose an adaptive approach to pre-set the modulation scheme for LR-UWAC. This is a channel classification approach which, based on environmental information and on prior training on various channel types, predicts the best modulation scheme for the expected channel. Our classification procedure is trained to identify the channel’s important features. Thus, compared to a direct decision approach, it becomes less sensitive to possible mismatches of environmental information. Our numerical simulation and sea experiment show that our approach successfully identifies the best modulation scheme based on the environmental information - even when the information is biased or only partially available.

Performance improvement of long-range underwater acoustic communication in deep water using spatiotemporal diversity

Performance improvement of long-range underwater acoustic communication in deep water using spatiotemporal diversity

Bidirectional equalization based on error propagation detection in long-range underwater acoustic communication

Bidirectional equalization is a typical equalization method that can compensate for the channel effectively using multichannel time-reversal combining followed by single-channel decision-feedback equalization. However, because conventional bidirectional equalization employs decision-feedback equalizers, performance can be significantly degraded if errors propagate. In this paper, we propose a method to improve the performance of bidirectional equalization by detecting error propagation and selecting the type of equalizer. The proposed method reduces the error rate by applying a decision-direct equalizer instead of a decision-feedback equalizer in the event of error propagation. Simulation results and analysis of experimental long-range communication data show that the proposed method produces superior performance in bidirectional equalization.

Bi-directional equalization for long-range underwater acoustic communication in East Sea of Korea

The long-range underwater acoustic communications using a sound channel in deep sea have been studied for many years. The acoustic waves transmitted through the sound channel can be propagated by long distance because there is no interaction with the sea surface and bottom interfaces. However, the transmitted signals are distorted by multipath propagation. Time reversal combining followed by a single channel equalizer can reduce efficiently the effects of multipath communication channel and thus increases signal-to-noise ratio by obtaining temporal and spatial diversity. However, the conventional time reversal tends to increase the entire error rate when burst errors occur. Long-range communication signals are particularly vulnerable to burst errors due to the weak signal strength. In this talk, the bi-directional equalization is applied to compensate for the distortion of long-range communication signals where the burst errors occur. Since the bi-directional equalization combines the soft outputs of forward and backward equalization, the effect of error propagation due to burst error can be reduced. The long-range communication experiment data acquired in the East Sea of Korea in October 1999 is used to verify the performance of the bi-directional equalization. The results show that the bi-directional equalization can be effectively applied when burst errors exist. [Work supported by the ADD(UD170022DD) and the National Research Foundation of Korea (NRF-2016R1D1A1B03930983).]

On the Relationship Between the Underwater Acoustic and Optical Channels

Wireless transmissions in water are mostly carried out via long-range (but low-rate) underwater acoustic communications, or short-range (but high-rate) underwater optical communications. In this paper, we are interested in finding out whether a statistical relationship exists between underwater acoustics and optics. Besides the theoretical interest of such a relationship, predicting the quality of the optical link through acoustics is also relevant in the context of a multimodal system with both acoustics and optics. Our study is based on a large data set acquired during the NATO ALOMEX 2015 expedition. During this experiment, we simultaneously measured several characteristics of the acoustic and optical links at multiple locations, reflecting a diversity of sea environments. Our results show a strong correlation between the properties of the acoustic link and the reliability of optical communications. This correlation makes it possible to predict the state of the underwater optical link at a certain depth and range. Due to the complexity of the acoustic and optical channels, we could not find the source of this correlation. This paper is, therefore, aimed to stimulate a theoretical study of the mutual properties of underwater acoustic and optical communication links. For reproducibility, we share the processed data from the experiment.

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.

Multi-channel time frequency shift keying in underwater acoustic communication

Current long-range underwater acoustic communication (UWAC) faces many difficulties such as great propagation loss, high ambient noise and long multi-path delay. In order to design an excellent long-range UWAC system, it is necessary to increase signal-to-noise ratio (SNR), suppress inter-symbol interference (ISI) and alleviate frequency selective fading. In this paper, a novel time frequency shift keying scheme named as Multi-channel Hex Four Time Four Frequency Shift Keying (McH-4T4FSK) is presented. Both theoretical analysis and simulation over Bellhop multi-path channel model indicate that McH-4T4FSK exhibits lower bit error rate (BER) at the same SNR compared with traditional Frequency Shift Keying (FSK) regardless of diversity or not. Moreover, shallow water sea trial experiments at Xiamen Port with 15km distance and at Taiwan Strait with 30km distance confirm that McH-4T4FSK possesses lower BER and better robustness.

Weakly dispersive modal pulses in long-range underwater acoustic communications

Weakly dispersive modal pulses are contributions to acoustic wave field from individual modes that are characterized by weak dispersion- and scattering-induced broadening. These pulses experience little propagation-induced distortions and are suitable for communications applications. This work investigates, using numerical simulations, how to design a communication system that takes advantage of the physics of weakly dispersive modal pulses. Two groups of weakly dispersive modal pulses are identified in typical mid-latitude ocean environments: the lowest order modes, and mode numbers with the waveguide invariant being near-zero (often around mode 20 at 75 Hz). This work analyzes the source and receiving array requirements for achieving low bit error rates (BERs) in a binary communication without performing channel equalization. It is shown that low BERs are achieved with only 3 hydrophones for mode 1 processing at 500 km and with 30 hydrophones for mode 20 at 400 km range with good signal-to-noise ratio (SNR). It is demonstrated that if depths of hydrophones are allowed to vary with the source-receiver distance, 2 hydrophones are often sufficient to achieve low BERs even with intermediate mode numbers. Thus, full modal resolution is often unnecessary to achieve low BERs. The effects of variable SNR are also studied. [Work supported by ONR.]

Research progress in chaotic orthogonal code-division multiplexing acoustic communications

Chaotic Orthogonal Code-Division Multiplexing (C-OCDM) is a method of encoding digital data on multiple chaotic codes. More than ten years after its introduction, the C-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. Several sea tests corresponding to these technologies are introduced thereafter. Finally, we make a conclusion and propose future research directions of this method.

Low-frequency source for very long-range underwater communication

Very long-range underwater acoustic communication (UAC) is crucial for long cruising (>1000 km) autonomous underwater vehicles (AUVs). Very long-range UAC source for AUV must exhibit high electro-acoustic efficiency (>60%) and compactness. This paper describes the Janus-Hammer Bell (JHB) transducer that has been designed for this purpose and meets those requirements. The transducer works on the 450-550 Hz bandwidth and reaches source level above 200 dB (ref. 1 μPa at 1 m) with 1 kW excitation and full immersion capability. JHB source has been used for communication experiments by the Japanese institute for marine technology (Japan Agency for Marine-Earth Science and Technology) achieving a baud rate of 100 bits/s at 1000 km.

M-ary chaotic spread spectrum Pattern time delay shift coding scheme for multichannel underwater acoustic communication

Pattern time delay shift coding scheme uses the time delay shift values of the Pattern to code the information whose anti-ISI (intersymbol interference) ability is related to the kind and duration of the Pattern. Spread spectrum communications is competent for long-range underwater acoustic communications for its spread spectrum gain, but the low communications rate seriously restricts its application. The proposed new scheme is adapted to the underwater acoustic environment, which integrates the spread spectrum with Pattern time delay shift coding and works at M-ary mode spread spectrum and several communications channels. The scheme could achieve good performance of the spread spectrum and also improve the data transfer rate. The results of lake trial show that the M-ary chaotic spread spectrum Pattern time delay shift coding scheme is robust and feasible.

Multiple focusing with adaptive time-reversal mirror.

Recently, adaptivity was introduced to time-reversal mirror to steer the nulls, and referred to as an adaptive time-reversal mirror (ATRM) [J. S. Kim, H. C. Song, and W. A. Kuperman, J. Acoust. Soc. Am. 109, 1817-1825 (2001)]. In this study, ATRM is extended to simultaneous multiple focusing in an ocean waveguide. The multiple focusing is achieved by imposing a set of constraints in the formulation to find the weight vectors. The algorithm is applied to the long-range underwater acoustic communication to show, via simulation, that the simultaneous pulse compression at multiple receiving locations is achieved.