In a fading channel, bit error rate for frequency-shift- keying signals is determined predominantly by the envelope amplitude fading statistics of the signal. The narrowband envelope amplitude distributions are measured from the TREX04 data (as a function of frequency) using M-sequence signals centered at 17 kHz with a 5 kHz bandwidth. The results do not fit the Rayleigh, Rician, Nakagami m-distributions. In contrast, we find that the data are fitted well by a K-distribution. We also analyze the data in terms of long-term and short-term statistics. The long-term and short-term fading statistics are well fitted by the lognormal distribution and Rayleigh distribution respectively, choosing the average time scale to be ~0.2 sec. The joint probability distribution function of a lognormal and the Rayleigh distribution is approximately the K-distribution.
This study compares two sophisticated signaling schemes to compensate for distortions in underwater acoustic channels, where the channel rate of change approaches the reciprocal of the multipath time spread. The first scheme uses tonal pulses much longer than the channel multipath spread. The second (commonly called RAKE) uses spread spectrum pulses to resolve the multipath and an adaptive receiver to combine the multipath echoes coherently. Analysis shows that the RAKE scheme (1) performs considerably better for slowly fading channels and (2) can use a smaller decision interval and can therefore tolerate higher fade rates. (Author).
In recent years, interests in Underwater Acoustic (UWA) communications have exponentially grown due to many emerging commercial and military applications such as ocean pollution monitoring, off-shore oil exploration and data telemetry, oceanic environment sensing and surveillance, underwater wireless sensor networking, submarine communications, and so on. The UWA channels have specific properties which differ from radio channels, which make it a big challenge to apply current radio wireless systems to it directly. There are three main characters attached in UWA channels: low speed of sound that varies with medium conditions; attenuation that increases with both transmission range and frequency; time-varying multipath propagation that depends on boundary conditions. The wireless communication systems build on UWA channels would suffers from limited bandwidth, long multipath delays, large Doppler shift and spread which means low data rate, sever inter symbol interference (ISI) and complex equalization. The design and analysis of underwater acoustic communication systems rely on the fundamental characterization of underwater acoustic signal propagation. Several channel models have been developed to investigate channel properties in different environment set ups. Currently, there are no standardized models for acoustic channel fading. There are two kinds of models developed so far for different purposes: deterministic models and statistical models. While the former one focuses on the reflections when boundary conditions are fixed, the latter one concentrates on overall channel's statistical probability distributions with changing boundaries. Statistical models raise too much debate and the assumptions of the models with various statistical distributions remain to be further tested. On the other hand, many deterministic models have been tested and implemented as appropriate tools to investigate the reflection and refraction behavior of underwater acoustic signal propagation. In this thesis, we aim to study the power delay profile of underwater acoustic communication channels for given specific system configuration. Specifically, we investigated the multipath channel impulse responses of underwater acoustic channels by considering firstly a deterministic ray/beam tracing model and then a statistically random environment. We simulated an underwater acoustic channel model on MATLAB based on geometry of transceiver and surrounding environment and wave propagation equations. The amplitude and delays of the multipath channel impulse responses were compared and analyzed for underwater acoustic channels with various transceiver configurations such as range, depth, frequency, random water surface and bottom. Simulation results show that the depth location of underwater transceivers does not affect much the delay profile of the multipath received signals, however, it does change the power distribution of the multipath signals as the closer the transceiver to the boundaries, the less power received. Regarding various ranges between the transmitter and receiver, it is interesting to observe that the power delay profile of the multipath signals vary randomly, and the number of delay paths is uncorrelated to the ranges. Regarding underwater acoustic communication with different frequencies, while the delay profile of multipath signals remains stable with the same system geometry, the attenuation of multipath signals decreases as the frequency of acoustic signals increases. Moreover, the boundary conditions (water surface and/or bottom) of underwater acoustic channel affect the power delay profile of multipath signals significantly. In both fixed boundary and randomly varying boundary (e.g. water surface waves), the power delay profile of multipath signals exhibits a random pattern, which raises significant challenge in channel estimation in underwater acoustic communication. Our findings are helpful to the design of high-rate underwater acoustic communication systems. Our ultimate goal is to apply the MIMO-OFDM concept in underwater communication scenario in order to increase the date rate of underwater acoustic communication systems, in which channel estimation, channel multipath mitigation, signal processing, and data detection, need to be redesigned properly to take into account the unique characteristics of underwater acoustic channels.
Acoustic channel models provide a tool for predicting the performance of underwater communication systems prior to deployment, and are thus essential for system design. In this dissertation, we offer a statistical channel model, which incorporates physical laws of acoustic propagation (frequency-dependent attenuation, bottom-surface reflections) as well as the effects of inevitable random local displacements. We focus on random displacements on two scales: small-scale effects, that involve distances on the order of a few wavelengths, and large-scale effects, that involve many wavelengths. Small-scale effects include scattering and motion-induced Doppler shifting, and are responsible for fast variations of the instantaneous channel response; while large-scale effects describe the location uncertainty and changing environmental conditions, and affect the locally-averaged received power. We model each propagation path by a large-scale gain and micro-multipath components that cumulatively result in a complex Gaussian distortion. Random surface motion and transducer displacement introduce additional variation whose temporal correlation is described by Bessel-type functions. The total power, or the gain contained in the channel, averaged over small-scale, is modeled as log-normally distributed. The models are validated using real data obtained from four experiments. Specifically, experimental data are used to assess the distribution and the auto-correlation functions of the large-scale transmission loss and the short-term path gains. While the former indicates a log-normal distribution with an exponentially decaying auto-correlation, the latter indicates a conditional Ricean distribution with Bessel-type auto-correlation. Based on the proposed model, we design a channel simulator which we employ to generate a time-varying channel whose statistical characteristics match with those of a real underwater channel. The simulated channel is applied to convey an OFDM signal to coherent and differentially coherent detectors, and the MSE performance of the experimental and simulated systems are shown to be similar. Finally, we investigate the feasibility of adaptive power control using an experimental data set as well as theoretically. Based on the observed time-correlation properties of the large-scale channel gain, linear power prediction is employed and achievable power savings are obtained analytically (assuming a log-normal gain distribution) and experimentally. The results indicate that substantial power savings are possible over extended periods of time.
Digital Underwater Acoustic Communications focuses on describing the differences between underwater acoustic communication channels and radio channels, discusses loss of transmitted sound in underwater acoustic channels, describes digital underwater acoustic communication signal processing, and provides a comprehensive reference to digital underwater acoustic communication equipment. This book is designed to serve as a reference for postgraduate students and practicing engineers involved in the design and analysis of underwater acoustic communications systems as well as for engineers involved in underwater acoustic engineering. Introduces the basics of underwater acoustics, along with the advanced functionalities needed to achieve reliable communications in underwater environment Identifies challenges in underwater acoustic channels relative to radio channels, underwater acoustic propagation, and solutions Shows how multi-path structures can be thought of as time diversity signals Presents a new, robust signal processing system, and an advanced FH-SS system for multimedia underwater acoustic communications with moderate communication ranges (above 20km) and rates (above 600bps) Describes the APNFM system for underwater acoustic communication equipment (including both civil and military applications), to be employed in active sonar to improve its performance
Underwater acoustic digital signal processing and communications is an area of applied research that has witnessed major advances over the past decade. Rapid developments in this area were made possible by the use of powerful digital signal processors (DSPs) whose speed, computational power and portability allowed efficient implementation of complex signal processing algorithms and experimental demonstration of their performance in a variety of underwater environments. The early results served as a motivation for the development of new and improved signal processing methods for underwater applications, which today range from classical of autonomous underwater vehicles and sonar signal processing, to remote control underwater wireless communications. This book presents the diverse areas of underwater acoustic signal processing and communication systems through a collection of contributions from prominent researchers in these areas. Their results, both new and those published over the past few years, have been assembled to provide what we hope is a comprehensive overview of the recent developments in the field. The book is intended for a general audience of researchers, engineers and students working in the areas of underwater acoustic signal processing. It requires the reader to have a basic understanding of the digital signal processing concepts. Each topic is treated from a theoretical perspective, followed by practical implementation details. We hope that the book can serve both as a study text and an academic reference.
A blend of introductory material and advanced signal processing and communication techniques, of critical importance to underwater system and network development This book, which is the first to describe the processing techniques central to underwater OFDM, is arranged into four distinct sections: First, it describes the characteristics of underwater acoustic channels, and stresses the difference from wireless radio channels. Then it goes over the basics of OFDM and channel coding. The second part starts with an overview of the OFDM receiver, and develops various modules for the receiver design in systems with single or multiple transmitters. This is the main body of the book. Extensive experimental data sets are used to verify the receiver performance. In the third part, the authors discuss applications of the OFDM receiver in i) deep water channels, which may contain very long separated multipath clusters, ii) interference-rich environments, where an unintentional interference such as Sonar will be present, and iii) a network with multiple users where both non-cooperative and cooperative underwater communications are developed. Lastly, it describes the development of a positioning system with OFDM waveforms, and the progress on the OFDM modem development. Closely related industries include the development and manufacturing of autonomous underwater vehicles (AUVs) and scientific sensory equipment. AUVs and sensors in the future could integrate modems, based on the OFDM technology described in this book. Contents includes: Underwater acoustic channel characteristics/OFDM basics/Peak-to-average-ratio control/Detection and Doppler estimation (Doppler scale and CFO)/Channel estimation and noise estimation/A block-by-block progressive receiver and performance results/Extensions to multi-input multi-output OFDM/Receiver designs for multiple users/Cooperative underwater OFDM (Physical layer network coding and dynamic coded cooperation)/Localization with OFDM waveforms/Modem developments A valuable resource for Graduate and postgraduate students on electrical engineering or physics courses; electrical engineers, underwater acousticians, communications engineers
Modelling and simulation in acoustics is currently gaining importance. In fact, with the development and improvement of innovative computational techniques and with the growing need for predictive models, an impressive boost has been observed in several research and application areas, such as noise control, indoor acoustics, and industrial applications. This led us to the proposal of a special issue about “Modelling, Simulation and Data Analysis in Acoustical Problems”, as we believe in the importance of these topics in modern acoustics’ studies. In total, 81 papers were submitted and 33 of them were published, with an acceptance rate of 37.5%. According to the number of papers submitted, it can be affirmed that this is a trending topic in the scientific and academic community and this special issue will try to provide a future reference for the research that will be developed in coming years.
A channel characterization experiment for the underwater acoustic communication channel was carried out at Scripps Pier in May 1999. The experiment investigated acoustic transmission in very shallow water and breaking waves. In analyzing the data, several questions arose. The majority of the acoustic channel probe data was corrupted by crosstalk in the receiver array cable. This thesis investigates methods to correct for the effects of the crosstalk, to attempt to recover the channel probe data. In selected regions, the crosstalk could be removed quite effectively using a linear least-squares method to estimate the crosstalk coefficients. The bulk of the data could not be corrected, however, primarily due to crosstalk from a receiver channel which was not recorded, and hence could not be well estimated. A second question addressed by this thesis is concerned with acoustic propagation in shallow water under bubble clouds. The breaking waves injected air deep into the water column. The resulting bubble clouds heavily attenuated acoustic signals, effectively causing total dropouts of the acoustic communication channel. Due to buoyancy, the bubbles gradually rise, and the communication channel clears. The channel clearing was significantly slower than predicted by geometric ray acoustic propagation models, however. Proposed explanations included secondary, unobserved, breaking events causing additional bubble injection; delayed rising of bubbles due to turbulent currents; or failure of the geometric ray model due to suppression by bubble clouds of acoustic signals which are not along the geometric ray paths. This thesis investigated the final hypothesis, modeling the acoustic propagation in Scripps Pier environment, using the full wave equation modeling package OASES. It was determined that the attenuation of the propagating acoustic signal is not accurately predicted by the bubble-induced attenuation along the geometric ray path.
