Experimental studies of applications of time-reversal acoustics to noncoherent underwater communications

The most difficult problem in shallow underwater acoustic communications is considered to be the time-varying multipath propagation because it impacts negatively on data rates. At high data rates the intersymbol interference requires adaptive algorithms on the receiver side that lead to computationally intensive and complex signal processing. A novel technique called time-reversal acoustics (TRA) can environmentally adapt the acoustic propagation effects of a complex medium in order to focus energy at a particular target range and depth. Using TRA, the multipath structure is reduced because all the propagation paths add coherently at the intended target location. This property of time-reversal acoustics suggests a potential application in the field of noncoherent acoustic communications. This work presents results of a tank scale experiment using an algorithm for rapid transmission of binary data in a complex underwater environment with the TRA approach. A simple 15-symbol code provides an example of the simplicity and feasibility of the approach. Covert coding due to the inherent scrambling induced by the environment at points other than the intended receiver is also investigated. The experiments described suggest a high potential in data rate for the time-reversal approach in underwater acoustic communications while keeping the computational complexity low.     

浅海环境中的时间反转多用户水声通信

在无线电通信中,多用户通信可以采用时分多址(TDMA)、频分多址(FDMA)或者码分多址(CDMA)技术来实现,在水声通信中,信道的多途传播特性带来的空间差异,提供了另外的多用户通信手段。时间反转(或相位共轭)技术,能够实现对空间中指定点的聚焦接收和多途压缩,它为空间位置不同的多个用户提供了相互独立的通信通道,能够很好地克服用户之间的同道干扰(CI)。本文在垂直阵接收的基础上,利用时间反转技术来实现不同用户在同一信道中的同时通信,结合带锁相环的自适应判决反馈均衡技术来消除残余的多途码间干扰,并进行了初步的海上试验,实现了两个不同深度上用户的同时通信。     

复射线法及其在水声技术中的应用

本文介绍复源点射线技术原理,概述各种复射线分析方法,并举例说明这种方法在水声技术中一些可能的应用,包括指向性声场的复射线分析,声辐射器阵列方向性的复射线模拟,近区和远区声场的复射线变换,以及声呐目标强度的复射线理论预估等。     

Intersymbol interference in underwater acoustic communications using time-reversal signal processing

Coherent underwater communication is hampered by the time spread inherent to acoustic propagation in the ocean. Because time-reversal signal processing produces pulse compression, communications has been suggested as a natural application of the technique. Passive versions of time-reversal processing use a receive-only array to do combined temporal and spatial matched filtering. It can be shown, however, that the pulse compression it achieves is not perfect and that an equalizer that relies solely on time-reversal processing will have an error floor caused by uncompensated intersymbol interference (ISI). In the present paper, a physics-based model is developed for the uncompensated ISI in a passive time-reversal equalizer. The model makes use of a normal-mode expansion for the acoustic field. The matched-filtering integral is approximated and the intermediate result interpreted using the waveguide invariant. After combining across the array and sampling, formal statistical averages of the soft demodulation output are calculated. The results show how performance scales with bandwidth, with the number and position of array elements, and with the length of the finite impulse response matched filters. Good agreement is obtained between the predicted scaling and that observed in field experiments.     

Complex ray method and its applications to underwater acoustics

In this paper,the principle of complex source point and various analyt-ical methods of complex ray are summarized.Some possible applications ofcomplex ray method to underwater acoustics are proposed,including complexray analysis of directional acoustic fields,complex ray simulation of directivityof acoustic radiator arrays,complex ray transformation of acoustic fields fromnear zone to far zone,as wel1 as theoretical prediction of sonar target strengthby complex ray method.     

Effects of ocean thermocline variability on noncoherent underwater acoustic communications

The performance of acoustic modems in the ocean is strongly affected by the ocean environment. A storm can drive up the ambient noise levels, eliminate a thermocline by wind mixing, and whip up violent waves and thereby break up the acoustic mirror formed by the ocean surface. The combined effects of these and other processes on modem performance are not well understood. The authors have been conducting experiments to study these environmental effects on various modulation schemes. Here the focus is on the role of the thermocline on a widely used modulation scheme (frequency-shift keying). Using data from a recent experiment conducted in 100-m-deep water off the coast of Kauai, HI, frequency-shift-key modulation performance is shown to be strongly affected by diurnal cycles in the thermocline. There is dramatic variation in performance (measured by bit error rates) between receivers in the surface duct and receivers in the thermocline. To interpret the performance variations in a quantitative way, a precise metric is introduced based on a signal-to-interference-noise ratio that encompasses both the ambient noise and intersymbol interference. Further, it will be shown that differences in the fading statistics for receivers in and out of the thermocline explain the differences in modem performance.     

Relating the performance of time-reversal-based underwater acoustic communications in different shallow water environments

The performance of underwater acoustic communications, such as the output signal-to-noise ratio (OSNR), is generally dependent on the channel specifics, hence a channel model is normally required as the performance of the channel equalizer depends on the number of tap coefficients used (e.g., a sparse equalizer) which are different for different oceans having different multipath arrivals. This letter presents theoretical arguments, and experimental data from different oceans that suggest that the increase of OSNR with the number of diverse receivers (in terms of the effective number of receivers) and the decrease of OSNR with the channel-estimation error follow a universal relationship using the time-reversal or correlation-based equalizer, despite the fact that the channels have very different properties. The reason is due to the fact that the OSNR is a function of the q function, the auto-correlation of the received impulse responses summed over all receiver channels, and the q function is approximately the same for all shallow waters given a sufficient (≥4-6) number of receivers.     

Pressure sensitivity kernels applied to time-reversal acoustics

Sensitivity kernels for receptions of broadband sound transmissions are used to study the effect of the transmitted signal on the sensitivity of the reception to environmental perturbations. A first-order Born approximation is used to obtain the pressure sensitivity of the received signal to small changes in medium sound speed. The pressure perturbation to the received signal caused by medium sound speed changes is expressed as a linear combination of single-frequency sensitivity kernels weighted by the signal in the frequency domain. This formulation can be used to predict the response of a source transmission to sound speed perturbations. The stability of time-reversal is studied and compared to that of a one-way transmission using sensitivity kernels. In the absence of multipath, a reduction in pressure sensitivity using time reversal is only obtained with multiple sources. This can be attributed both to the presence of independent paths and to cancellations that occur due to the overlap of sensitivity kernels for different source-receiver paths. The sensitivity kernel is then optimized to give a new source transmission scheme that takes into account knowledge of the medium statistics and is related to the regularized inverse filter.     

High-rate multiuser communications in shallow water

Passive multiuser communications in shallow water previously was demonstrated in the 3-4 kHz band using a time reversal approach. This paper extends those experimental results in three respects. First, a larger bandwidth at higher frequency (11-19 kHz) is employed allowing for the use of various symbol rates (or bandwidths). Second, two different shaping pulses are examined: a raised cosine filter and LFM (linear frequency modulation) chirp. Third, the adaptive time reversal approach with spatial nulling is applied to suppress the crosstalk among users. It is shown that the use of a larger bandwidth is beneficial along with the time reversal receiver which can handle significant intersymbol interference with minimal computational complexity. In addition, adding each user degrades the performance by about 4 dB for the benefit of linear increase in data rate. It is demonstrated that an aggregate data rate of 60 kbits/s can be achieved with a 7.5 kHz bandwidth (a spectral efficiency of 8 bits/s?Hz) by three users distributed over 4.2-m depth at a 2.2 km range in shallow water using 16-QAM (quadrature amplitude modulation).     

Properties of underwater acoustic communication channels in shallow water

Underwater acoustic channels are band-limited and reverberant, posing many obstacles to reliable, phase-coherent acoustic communications. While many high frequency communication experiments have been conducted in shallow water, few have carried out systematic studies on the channel properties at a time scale relevant for communications. To aid communication system design, this paper analyzes at-sea data collected in shallow water under various conditions to illustrate how the ocean environments (sea surface waves and random ocean medium) can affect the signal properties. Channel properties studied include amplitude and phase variations, and temporal coherence of individual paths as well as the temporal and spatial coherence of multipaths at different time scales. Reasons for the coherence loss are hypothesized.     

水声学研究进展

声学是一门古老而又年轻的科学,水声学是声学的一个分支。本文介绍近年来水声学研究的主要进展。包括新型水声换能器材料、水中非线性声学、匹配场过滤、宽容性信号处理、水声通信、合成孔径成象技术、数据融合、声层析、水下GPS技术等。文中还介绍了军事海洋学和基于模型的声呐设计等新概念。     

Features of underwater acoustics from Aristotle to our time

Underwater acoustics has been one of the fastest growing fields of research in acoustics. In particular, the 20th Century has taken our understanding of underwater acoustics phenomena a great step forward. The two World Wars contributed to the recognition of the importance of research in underwater acoustics, and the momentum in research and development gained during World War II did not reduce in the years after the war. The so-called cold war and the development in computer technology both contributed substantially to the development in underwater acoustics over the second half of the 20th Century. However, the very widespread field of underwater acoustic activities started nearly 2300 years ago with human curiosity about the fundamental nature of sound in the sea. From primitive philosophical and experimental studies of the velocity of sound in the sea and through centuries of successes and failures, the knowledge about underwater acoustics has developed into its high-technological status of today. In particular the development through the period from Aristotle (384–322 BC) to 1960 formed the basis for the tremendous research and development efforts we have witnessed in our time. In this paper most emphasis will be put on the development in underwater acoustics through this period of nearly 2300 years duration, and only the main trends in later research will be mentioned.     

New approach to the normal mode method in underwater acoustics

A new approach to the numerical solution of normal model problems in underwater acoustics is presented,in which the corresponding normal mode problem is transformed to the problem of solving a dynamic system.Three applications are considered:(1)the broad band normal mode problem;(2) the range-dependent problem with perturbation proportional to the range parameter;and (3) the evolution of the normal mode with environmental parameters.A numerical simulation for a broad band problem is performed,and the calculated eigenvalues have good agreement with those obtained by the standard normal mode code KRAKAN.     

Progresses and advances in Arctic underwater acoustics study

As a part of underwater acoustics, the study and development of Arctic Acoustics is later than underwater acoustics about several hundred years. After World War II, the study of Arctic Acoustics attracts many interests by some developed countries, especially U.S. and pre-Soviet Union. The research works obviously have some kinds of cold war brand. After the Cold War, with the gradual warming trend of Arctic area, the ocean acoustic environment in Arctic and its adjacent area have been considerably concerned. The 8 countries of Arctic rim organized exclusive "Arctic Council" in 1996. The white paper of "China Arctic Policy"was published in January 26, 2018, the Chinese government declared that China is a close Arctic country, and is the responsible stakeholder of Arctic interests. The new advances in Arctic research works axe introduced in this paper, including the results of scientific survey and studies about Arctic underwater acoustics of Chinese researchers. It is showed that the Arctic underwater acoustical research area is not only parallely copy the topics what traditional underwater acoustics covered, e.g. environment noise,reverberation,and propagation, but also the topics which is specifically based on the Arctic environment. Some of these research fields cannot be included in the traditional shallow water, deep water acoustics, e.g. the ice-water interface feature, ice covered semi-acoustic channel effect, the communication and navigation of UUV in the condition of under ice, and the adaptation of sonar technique, equipment in the ice-covered environment, etc.     

Characterization of an elastic target in a shallow water waveguide by decomposition of the time-reversal operator

This paper reports the results of an investigation into extracting of the backscattered frequency signature of a target in a waveguide. Retrieving the target signature is difficult because it is blurred by waveguide reflections and modal interference. It is shown that the decomposition of the time-reversal operator method provides a solution to this problem. Using a modal theory, this paper shows that the first singular value associated with a target is proportional to the backscattering form function. It is linked to the waveguide geometry through a factor that weakly depends on frequency as long as the target is far from the boundaries. Using the same approach, the second singular value is shown to be proportional to the second derivative of the angular form function which is a relevant parameter for target identification. Within this framework the coupling between two targets is considered. Small scale experimental studies are performed in the 3.5 MHz frequency range for 3 mm spheres in a 28 mm deep and 570 mm long waveguide and confirm the theoretical results.     

Wave analysis of ray chaos in underwater acoustics

The dispersion of a wave packet in an acoustic medium is considered in the paraxial wave approximation, where the effective potential, due to variation of the speed of propagation, varies both with depth and propagation distance. The analysis of the resulting parabolic equation, similar to the Schrodinger equation, clearly demonstrates the role of ray chaos in enhancing the dispersion of the initial packet. However, wave coherence effects are also seen that suppress the effects of the ray chaos in a manner analogous to the effects of quantum chaos. (c) 1999 American Institute of Physics.     

A high-frequency warm shallow water acoustic communications channel model and measurements

Underwater acoustic communication is a core enabling technology with applications in ocean monitoring using remote sensors and autonomous underwater vehicles. One of the more challenging underwater acoustic communication channels is the medium-range very shallow warm-water channel, common in tropical coastal regions. This channel exhibits two key features-extensive time-varying multipath and high levels of non-Gaussian ambient noise due to snapping shrimp-both of which limit the performance of traditional communication techniques. A good understanding of the communications channel is key to the design of communication systems. It aids in the development of signal processing techniques as well as in the testing of the techniques via simulation. In this article, a physics-based channel model for the very shallow warm-water acoustic channel at high frequencies is developed, which are of interest to medium-range communication system developers. The model is based on ray acoustics and includes time-varying statistical effects as well as non-Gaussian ambient noise statistics observed during channel studies. The model is calibrated and its accuracy validated using measurements made at sea.