Underwater Acoustic Modem Research Articles

Определение точности измерения дальности между подводными объектами при помощи гидроакустических модемов

В работе представлены результаты экспериментального исследования точности определения дальности между подводными объектами при помощи гидроакустических модемов uWave в условиях ледового эксперимента в мелком водоёме. Проведены две независимые проверки: определение фактической скорости звука на основе относительных измерений, и определение дальностей на основе скорости звука, вычисленной по измерению температуры встроенными в модемы датчиками. Общий размер выборки составляет 1260 измерений времени распространения сигнала, выполненных в 10 различных взаимных расположений двух объектов. Проведён анализ источников погрешностей, в ходе которого, в частности, выявлена линейная зависимость роста среднеквадратичного отклонения измерений времени распространения с дальностью. В качестве источника указан эффект удлинения траекторий лучей в виду нелинейности распространения звука в среде. Экспериментальная проверка точности определения времени распространения сигнала и наклонной дальности между модемами uWave показала, что среднеквадратичное отклонение имеет порядок 0.1% от дальности. The paper presents the results of an experimental study of the accuracy of measurment of the distance between underwater objects using uWave underwater acoustic modems under the conditions of an ice experiment in a shallow water body. Two independent tests were carried out: determining the actual speed of sound based on relative measurements, and determining ranges based on the speed of sound calculated from temperature measurements by sensors built into modems. The total sample size is 1260 signal propagation time measurements taken at 10 different relative positions of the two objects. An analysis of the sources of errors was carried out, during which, in particular, a linear dependence of the increase in the standard deviation of propagation time measurements with range was revealed. The source indicated is the effect of lengthening ray trajectories due to the nonlinearity of sound propagation in the medium. An experimental test of the accuracy of determining the signal propagation time and slant range between uWave modems showed that the standard deviation is on the order of 0.1% of the range.

Design and Development of an SVM-Powered Underwater Acoustic Modem

Design and Development of an SVM-Powered Underwater Acoustic Modem

System Architecture and Communication Infrastructure for the RoboVaaS Project

Current advancements in waterborne autonomous systems, together with the development of cloud-based service-oriented architectures and the recent availability of low-cost underwater acoustic modems and long-range above-water wireless devices, enable the development of new applications to support ships and port activities. Autonomous surface vehicles can, for instance, be used to perform bathymetry and environmental data collection tasks to ensure under keel clearance and to monitor the quality of the water. Similarly, remotely operated vehicles can be deployed to inspect ship hulls and typical port infrastructure elements, such as quay and sheet pilling walls. In this article, we present the complete system deployed for the small-scale demonstrations of the Robotic Vessels as-a-Service project, which introduces an on-demand service-based cloud system that dispatches unmanned vehicles capable of performing the required service either autonomously or piloted. These vessels are able to interact with sensors deployed in the port and with the shore station through an integrated underwater and above-water network. The developed system has been validated through sea trials and showcased through an underwater sensor data collection use case. The results of the test presented in this article provide a proof of concept of the system design and indicate its technical feasibility. It also shows the need for further developments for a mature technology allowing on-demand robotic maritime assistance services in real operational scenarios.

Adaptive switching for communication profiles in underwater acoustic modems based on reinforcement learning

The underwater acoustic (UWA) channel is a complex and stochastic process with large spatial and temporal dynamics. This work studies the adaptation of the communication strategy to channel dynamics. Specifically, a set of communication strategies are considered, including Frequency-Hopped Binary Frequency Shift Keying (FH-BFSK), Single-Carrier (SC) communication, and multicarrier communication. Based on the channel condition, a reinforcement learning (RL) algorithm, the Deep Deterministic Policy Gradient (DDPG) method along with a Gumbel-softmax scheme is employed for intelligent and adaptive switching among those communication strategies. The adaptive switching is performed on a transmission block-by-block basis, with the goal of maximizing long-term system performance. The reward function is defined based on the energy efficiency (EE) and the spectral efficiency (SE) of the communication strategies. Simulation results and experimental data processing results reveal that the proposed method outperforms a random selection method and a direct feedback method in time-varying channels.

A Software-Defined Underwater Acoustic Modem for Everyone: Design and Evaluation

The development of small low-cost autonomous underwater and surface vehicles has increased the need for underwater wireless communication and ranging to support swarm operations for collaborative data collection efforts. Specifically, newly available low-cost underwater and surface vehicles make the realization of swarm formation cost effective. However, their mission coordination involves the use of expensive acoustic modems whose price may be higher than that of the vehicle itself. In this paper, we describe and evaluate a low-cost software-defined acoustic modem developed with only off-the-shelf components, able to perform one-way travel-time ranging. The modem is specifically designed for dense mobile networks deployed in shallow water environments, such as rivers, lagoons and lakes. The modular software design of the modem allows us to easily configure parameters such as modulation and coding schemes, scheduling algorithm, source power, carrier frequency and bandwidth. In this paper we evaluate, in a shallow environment, the modem performance in terms of packet detection ratio, packet delivery ratio, and one-way travel-time ranging.

Survey on Low-Cost Underwater Sensor Networks: From Niche Applications to Everyday Use

Traditionally, underwater acoustic modems and positioning systems were developed for military and Oil & Gas industries, that require deep water deployments and extremely reliable systems, focusing on high power expensive systems and leaving the use of low-cost devices only attractive for academic studies. Conversely, recent developments of low-cost unmanned vehicles, such as remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), suitable for shallow water coastal missions, and the need of sensors network deployments for measuring water quality and studying the effect of climate change in coastal areas, called to the need of low-cost and low-power acoustic modems and positioning systems that are gaining more and more momentum to date. The use of these devices can enable a wide set of applications, often based on low-cost AUV swarm formations, where an acoustic link between the vehicles is required to coordinate the mission, perform the maneuvers, and maintain the formation along the time. Moreover, they can make environmental wireless sensor deployment cost effective by substituting wired systems. Underwater positioning systems, usually used in large-scale operations, can be finally applied to small-scale application thanks to the reduction in costs, at the price of a lower transmission and positioning range and precision. While in open-sea application this performance reduction is a huge limitation, in river, lagoon, port and lake deployments this is not an issue, given that the extremely shallow water and the presence of many obstacles would deteriorate the acoustic signal anyway, not allowing long range transmissions even with expensive and sophisticated acoustic devices. In this paper, we review the recent developments of low-cost and low-power acoustic communication and positioning systems, both analyzing University prototypes and new commercial devices available in the market, identifying advantages and limitations of these devices, and we describe potential new applications that can be enabled by these systems.

Research on Co-Channel Interference Cancellation for Underwater Acoustic MIMO Communications

Multiple-input–multiple-output (MIMO) communication systems utilize multiple transmitters to send different pieces of information in parallel. This offers a promising way to communicate at a high data rate over bandwidth-limited underwater acoustic channels. However, underwater acoustic MIMO communication not only suffers from serious inter-symbol interference, but also critical co-channel interference (CoI), both of which degrade the communication performance. In this paper, we propose a new framework for underwater acoustic MIMO communications. The proposed framework consists of a CoI-cancellation-based channel estimation method and channel-estimation-based decision feedback equalizer (CE-DFE) with CoI cancellation functionalities for underwater acoustic MIMO communication. We introduce a new channel estimation model that projects the received signal to a specific subspace where the interference is free; therefore, the CoI is cancelled. We also introduce a CE-DFE with CoI cancellation by appending some filters from traditional CE-DFE. In addition, the traditional direct adaptive decision feedback equalization (DA-DFE) method and the proposed method are compared in terms of communication performance and computational complexity. Finally, the sea trial experiment demonstrates the effectiveness and merits of the proposed method. The proposed method achieves a more than 1 dB of output SNR over traditional DA-DFE, and is less sensitive to parameters. The proposed method provides a new approach to the design of robust underwater acoustic MODEM.

An Unmanned Underwater Vehicle Defence System

Nowadays, military threats are very close to us and cover terrestrial, aerial, and maritime attacks. This research defines a submarine defence system capable of monitoring and controlling a certain perimeter, based on submersible drones known as Unmanned Underwater Vehicles (UUV) and a grid of stations fixed to the seabed, which provides positioning acoustic signals as well as full-duplex digital communication using underwater acoustic modems. The novelty is the proposed 3D grid system with each Positioning Grid Element (PGE) being an Ultra-Short Baseline (USBL) low or medium operating range device and the processing

UNDERWATER ACOUSTIC MODEM WITH HIGH PRECISION RANGING CAPABILITY BASED ON SECOND-LEVEL CESIUM ATOMIC CLOCK

Abstract. Equipped with a high-precision atomic clock can significantly improve the ranging accuracy of the underwater acoustic ranging system. However, the adverse characteristic of underwater acoustic channel (UAC) seriously deteriorates the ranging accuracy. In addition, as a timing device typically commercial cesium atomic clock is only designed to output second-level time reference, which cannot be applied to underwater ranging systems directly. To overcome this limitation, a new underwater acoustic modem with high precision ranging capability based on second-level cesium atomic clock is developed in this paper. The spread spectrum technology is chosen for highly reliable communication and the second-level commercial cesium atomic clock is used to ensure accurate clock synchronization. The proposed modem executes a predetermined ranging scheme based on spread spectrum technology. Specifically, at a certain specified second-level, the transmitter modem sends the ranging signal, and the receiver modem starts timing. Until the receiving modem detects the ranging signal, the propagation time is obtained and the single-item ranging capability is realized. The pool experiment shows the effectiveness of the modem, its ranging accuracy can reach cm level, and the average absolute error of ranging is 8 cm.

Performance Optimization of Underwater Communication Links at Different Ranges for AIS Relay to AUV

Autonomous Underwater Vehicles (AUVs) are becoming increasingly popular for large number of civil and military applications, such as environmental monitoring, oceanography, archaeology, or mine warfare. Operational safety issue still prevents the exploitation of the full potential of AUVs. Operations of AUVs are limited by constraints including the need to guarantee no collision with manned surface vehicles. To avoid collisions, a solution is to relay surface vessel positions to the AUVs by an underwater communication link. The communication must be optimized to establish a robust and reliable link at various range and depth. Commercial underwater acoustic modems are often dedicated for specific distances and bit rates with performances strongly dependent on the environmental conditions. For the considered application, the modem must be adaptive at the time and frequency selectivity of the channel which is varying according to the operation context. In this work, we propose an adaptive underwater acoustic waveform optimized to exploit at the best the time and frequency diversity the channel. To show the potential gain obtained by using diversity, the performance of the designed modem is evaluated using theoretical and realistic underwater channels. In addition, we propose a method for adapting the waveform based only on the knowledge of the transmission geometry. Finally, the proposed modem is tested to relay Automatic Identification Systems (AIS) picture of an area to AUVs.

Survey of Acoustic Frequency Use for Underwater Acoustic Cognitive Technology

The available underwater acoustic spectrum is limited. Therefore, it is imperative to avoid frequency interference from overlapping frequencies of underwater acoustic equipment (UAE) for the co-existence of the UAE. Cognitive technology that senses idle spectrum and actively avoids frequency interference is an efficient method to facilitate the collision-free operation of multiple UAE with overlapping frequencies. Cognitive technology is adopted to identify the frequency usage of UAE to apply cognitive technology. To this end, we investigated two principle underwater acoustic sources: UAE and marine animals. The UAE is classified into five types: underwater acoustic modem, acoustic positioning system, multi-beam echo-sounder, side-scan sonar, and sub-bottom profiler. We analyzed the parameters of the frequency band, directivity, range, and depth, which play a critical role in the design of underwater acoustic cognitive technology. Moreover, the frequency band of several marine species was also examined. The mid-frequency band from 10 - 40 kHz was found to be the busiest. Lastly, this study provides useful insights into the design of underwater acoustic cognitive technologies, where it is essential to avoid interference among the UAE in this mid-frequency band.

A step by step educational guide to designing and testing an underwater acoustic modem

We examine the design and systematic implementation of a receiver system for underwater acoustic communication. The system includes, but is not limited to, passband filter, down converter, matched filter, and detection logic. Our final implementation target is a frame-based system to minimize memory use and system latency. The matched filter uses the overlap-save method in frequency domain to reduce computational cost. Initially, the system is designed and tested for performance using the MATLAB platform in a non-real time vector-based implementation. The code is then restructured to a MATLAB frame-based model. The code contains several functions; each function performs a specific task and maintains its own histories as needed. Once the performance of the frame-based model is measured to be identical to the vector-based model, we systematically move each function to C + +. Each C + + function is individually substituted into the frame-based model using MEX files and verified for performance. Once all C + + functions are verified for performance, the entire C + + implementation is uploaded to the Popoto Modem™ real-time hardware and performs real-time transmission and reception in both air and underwater environments, with known and tested performance characteristics. [Work supported in part by NSF, Grant IIP-1500123, the INTERN program.]

A Novel Adaptive Filter for Cooperative Localization Under Time-Varying Delay and Non-Gaussian Noise

In this study, to cope with time-varying delay in acoustic communication under non-Gaussian noise caused by outliers, a novel robust delay filtering algorithm is proposed for cooperative localization of autonomous underwater vehicles (AUVs). First, the modified measurement equations of the nonlinear cooperative location system with time-varying delay are derived, and the delay caused by the information processing and propagation of the underwater acoustic modem is converted into measurement bias. Second, to improve the robustness of the system with outliers caused by abnormal measurement values of underwater acoustic communication and Doppler velocity log (DVL), the statistical similarity measure (SSM) is introduced to construct the cost function. This is accomplished by quantifying the similarity between the state vector and the predicted state vector, similarity between the delay measurement, and modified predicted delay measurement. Finally, the modified measurement noise variance is obtained by maximizing the cost function, and two algorithms for the approximate solution of the nonlinear cost function are presented. The proposed robust delayed algorithm alleviates the impacts of delayed measurement with outliers on localization accuracy. The effectiveness and potential of the proposed filter are verified by lake trials.

Self-interference channel modeling for in-band full-duplex underwater acoustic modem

In-band full-duplex (IBFD) underwater acoustic (UWA) communication can significantly improve the throughput of UWA communication networks. In this paper, we focus on the self-interference (SI) channel modeling which is essential for SI cancellation in an IBFD modem. The SI consists of direct self-loop interference (SLI) and self multi-path interference (SMI) due to reflections from water surface and bottom. Therefore, we first propose a simplified finite element model for SLI in an IBFD UWA modem. Then we model the underwater vertical channel to obtain the SMI path loss and propagation delays. Simulation results show that the SLI signal is composed of diffraction and scattering components, and it is greatly affected by the modem housing material and shape. The SLI channel has a long (several tens of ms) impulse response. To verify the proposed model, based on the IBFD UWA communication modem developed by our team, we conducted a lake experiment in December 2019 at Qiandao Lake in Hangzhou, China. The simulated results match well with the experimental results in time/frequency features and transmission loss. This study reveals the complexity of SI channel in IBFD UWA communication.

A Novel Robust Gaussian Approximate Smoother Based on EM for Cooperative Localization With Sensor Fault and Outliers

In this article, a novel robust Gaussian approximation smoother based on expectation–maximization (EM) algorithm is proposed for cooperative localization (CL) with faulty Doppler velocity log (DVL) and heavy-tailed measurement noise. In our model, the autonomous underwater vehicle (AUV) velocity information that is not available due to DVL failure and the bias in the underwater acoustic modem are considered as unknown inputs. Then, the Student’s t distribution is used to model the heavy-tailed measurement noise. An EM algorithm is also developed for the state-space model with heavy-tailed measurement noise. The state, noise covariance matrices, and auxiliary random variables are regarded as hidden variables to obtain the maximum likelihood estimation of unknown inputs. Gaussian smoother where modified process and measurement noise covariance are inferred by variational Bayesian (VB) approach is applied to estimate the state. The experimental results illustrate that given the heavy-tailed noise, the proposed method estimates the unknown input, and the state with a high level of accuracy. The proposed algorithm can be used as a backup algorithm for CL in cases where DVL is not available due to failure.

Underwater Data Transmission Using Frequency Shift Keying (FSK) Modulation with Bit Rate of 2400 bps

Underwater acoustic communication is a technology that uses sound or acoustic waves and water as its propagation medium. This technology has been used in various fields, such as underwater wireless sensor networks, underwater monitoring system, and surveillance systems. An acoustic modem is required to facilitate communication between nodes. In this paper, an underwater acoustic modem using Frequency Shift Keying (FSK) modulation has been designed. This modulation is widely used because of its reliability and simple design. FSK modem was designed using M=2 level or known as Binary FSK (BFSK) with 40 kHz mark frequency and 43 kHz space frequency. This study tested data packets sending and its error rate against the distance variation. Testing for 70-bit data resulted in 1% error at 100 cm distance and 37% error at 170 cm distance. When compared with the previous testing at 1200 bps which resulted in 0% and 35% error, it can be seen that error at 1200 bps is better than at 2400 bps, but the data transmission was better at 2400 bps. Addition to the number of bits sent and distance has an influence on the error value, i.e. the greater the distance and the amount of data sent, the greater the error value.

State-of-the-Art Underwater Acoustic Communication Modems: Classifications, Analyses and Design Challenges

Acoustic modem is one of the key elements of an underwater wireless sensor network (UWSN). Compared to a terrestrial wireless sensor network (WSN), designing a UWSN is more time consuming and expensive due to harsh conditions of the aquatic environment. Commercial modems provide better characteristics, but they consume more energy and are more expensive, while research modems have exploited diverse alternatives with varying success. The main contribution of the article is a comparative analyses of commercial and research modems based on their characteristics and design constraints, in order to describe the current trends and more promising techniques. This paper is focused on the state-of-the-art underwater acoustic modems designed, developed and implemented in the last few years. Various parameters of the modems are considered and analyzed: operating range, data-rate, modulation schemes, center frequency, bandwidth, power consumption, bit error rates, etc. Finally, design challenges, which need to be addressed, are identified. This study is useful for the engineering community to comprehend the characteristics, trends and design challenges of state-of-the-art underwater acoustic communication modems.

Event-Driven Data Gathering in Pure Asynchronous Multi-Hop Underwater Acoustic Sensor Networks.

In underwater acoustic modem design, pure asynchrony can contribute to improved wake-up coordination, thus avoiding energy-inefficient synchronization mechanisms. Nodes are designed with a pre-receptor and an acoustically adapted Radio Frequency Identification system, which wakes up the node when it receives an external tone. The facts that no synchronism protocol is necessary and that the time between waking up and packet reception is narrow make pure asynchronism highly efficient for energy saving. However, handshaking in the Medium Control Access layer must be adapted to maintain the premise of pure asynchronism. This paper explores different models to carry out this type of adaptation, comparing them via simulation in ns-3. Moreover, because energy saving is highly important to data gathering driven by underwater vehicles, where nodes can spend long periods without connection, this paper is focused on multi-hop topologies. When a vehicle appears in a 3D scenario, it is expected to gather as much information as possible in the minimum amount of time. Vehicle appearance is the event that triggers the gathering process, not only from the nearest nodes but from every node in the 3D volume. Therefore, this paper assumes, as a requirement, a topology of at least three hops. The results show that classic handshaking will perform better than tone reservation because hidden nodes annulate the positive effect of channel reservation. However, in highly dense networks, a combination model with polling will shorten the gathering time.

수중 음향 모뎀을 이용한 수중 이동체의 3차원 위치 추정 기술 개발

This paper presents the development of an underwater positioning system using underwater acoustic signals. The positioning system was developed on the basis of a conventional underwater acoustic modem. Additional hydrophones were installed to the conventional acoustic modem so that three-dimensional underwater positioning became available. A communication transducer was used for the acquisition of range information as well as underwater communication. The four additional hydrophones were used to estimate the three-dimensional angular information. The three-dimensional angular data were acquired by using the time difference of the arrivals of two pairs of acoustic signals. The time delay between two acoustic signals was acquired by using a probabilistic method based on the Bayesian update process. It could provide a reliable and accurate estimation. The acquired range and angular data were used to localize the underwater target equipped with a transmitting acoustic modem. The performance of the developed system was verified by experiments conducted in an inland water environment by using an unmanned surface vehicle.

Underwater Acoustic Modems with Synchronous Chip-Scale Atomic Clocks for Scalable Tasks of AUV Underwater Positioning

Accurate time synchronization of the nodes of digital underwater acoustic (UWA) networks is necessary for the effective use of information obtained from various kinds of underwater sensors over a vast water area. The sensors of autonomous underwater vehicles (AUV) are widely used to address new challenges. One of them is time synchronization of sensors on mobile carriers, as well as synchronization of the clocks on mobile nodes of UWA networks, for example, teams of AUVs that perform coordinated and/or cooperated operations. The paper presents theresults of the experiments on using UWA modems for AUV positioning, namely, UWA modems with chip-scale atomic clocks (CSAC) that allow accurate measurements of delays in propagation of UWA signals and, thus, accurate estimation of ranges to their georeferenced sources. In addition, the accuracy of CSAC operation in various situations as well as positioning accuracy of AUVs having UWA modems with integrated CSACs are analyzed based on the experimental results. Also given are practical recommendations on how to “discipline” (steer) CSACs and provide their phase synchronization with a source of timekeeping signals.