A computationally efficient Doppler compensation system forunderwater acoustic communications

A Doppler compensation system is presented which is suitable for high-data-rate acoustic communication between rapidly moving platforms such as autonomous underwater vehicles. The proposed approach provides a generic preprocessor to conventional adaptive receiver structures with only a marginal increase in computational load and hardware cost. The preprocessor employs a novel Doppler estimation technique and efficient sample rate conversion to remove Doppler shift induced by platform velocity and acceleration. Performance predicted by simulation is compared to that of sea trials of a prototype communication system in the North Sea. Successful communication is demonstrated at 16 kbit/s with a transmitting platform moving at up to ±2.6 m/s     

Digital underwater acoustic voice communications

This paper describes the design of an underwater acoustic diver communication system controlled by a digital signal processor. The speech signal transmission rate is compressed by using linear predictive coding (LPC) and the extracted parameters are transmitted through the water to a synchronized receiver by employing digital pulse position modulation (DPPM). The pulse position in each time frame is estimated by an energy detection and decision algorithm which enables the received LPC parameters to be recovered and used to synthesize the speech signal     

Recent advances in high-speed underwater acoustic communications

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel     

Correlation-based decision-feedback equalizer for underwater acoustic communications

The purpose of this paper is to develop a decision-feedback equalizer (DFE) using a fixed set of parameters applicable to most shallow oceans with minimal user supervision (i.e., a turn key system). This work is motivated by the superior performance [bit error rate (BER)] of the multichannel DFE compared with other methods, such as passive-phase conjugation (PPC), at the same time noting its sensitivity to different acoustic environments. The approach is to couple PPC, utilizing its adaptability to different environments, with a single-channel DFE. This coupling forms an optimal processor for acoustic communications in theory, but it has never been implemented in practice. By coupling with DFE, the method achieves the same spatial diversity as conventional multichannel DFE, without requiring a large number of receivers as does PPC. The correlation-based DFE in terms of the autocorrelation functions of the channel impulse responses summed over the receiver channels (the Q function) is derived. This paper shows in terms of waveguide physics, further supported by real data, the many desirable features of the Q function that suggest, given adequate sampling of the water column, a general applicability of the correlation-based equalizer to different environments, irrespective of the sound speed profiles, bottom properties, and source-receiver ranges/depths. This property can be expected to hold approximately for a small number of receivers with spatial diversity. This paper demonstrates the robustness of the new equalizer with moving source data despite the range change (which modifies the impulse response) and symbol phase change due to time-varying Doppler.     

Low complexity adaptive equalizers for underwater acoustic communications

Interference signals due to scattering from surface and reflecting from bottom is one of the most important problems of reliable communications in shallow water channels. To solve this problem, one of the best suggested ways is to use adaptive equalizers. Convergence rate and misadjustment error in adaptive algorithms play important roles in adaptive equalizer performance. In this paper, affine projection algorithm （APA）, selective regressor APA（SR-APA）, family of selective partial update （SPU） algorithms, family of set-membership （SM） algorithms and selective partial update selective regressor APA （SPU-SR-APA） are compared with conventional algorithms such as the least mean square （LMS） in underwater acoustic communications. We apply experimental data from the Strait of Hormuz for demonstrating the efficiency of the proposed methods over shallow water channel. We observe that the values of the steady-state mean square error （MSE） of SR-APA, SPU-APA0 SPU-normalized least mean square （SPU-NLMS）, SPU-SR-APA0 SM-APA and SM-NLMS algorithms decrease in comparison with the LMS algorithm. Also these algorithms have better convergence rates than LMS type algorithm.     

Multiple-input-multiple-output coherent time reversal communications in a shallow-water acoustic channel

A recent time reversal (TR) experiment demonstrated that multiple foci can be projected from an array of sources to the same range but at different depths. This multiple input/multiple output process can potentially increase the information data rate. This paper presents experimental results of coherent TR communications (binary phase-shift keying, quaternary phase-shift keying, and 8-quadratic-amplitude modulation) at 3.5 kHz with a 1-kHz bandwidth where different messages were sent simultaneously to either two or three different depths at an 8.6-km range in a 105-m-deep water.     

A queing system approach for the design of coast guard vessel traffic services communications

In order to provide for the safe and expeditious passage of maritime traffic in congested waters, the U.S. Coast Guard is authorized by the Ports and Waterways Safety Act of 1972 to establish, operate, and maintain Vessel Traffic Services (VTS) where needed. In larger areas, a VTS will generally require a communications system to enable the vessel traffic center and the participating vessels to exchange information. In designing such a system, it is necessary to assess the expected communications loading in order to determine frequency requirements and evaluate alternative configurations for the system. Here, VTS communications are viewed as a queuing system. The customers (messages) arrive at the service facility (communications channel) according to some probabilistic process, and are then serviced (transmitted) according to some other probabilistic process. Queues or waiting lines form as arriving messages wait to be transmitted, because the communications channels are busy. Three classes of messages are considered in the arrival process: check in/check out (basic VTS) messages; Vessel Movement Reporting System (VMRS) messages; and bridge-to-bridge messages. Each class is characterized by an independent Poisson distribution, and the resultant arrival process is a well-defined nonhomogeneous Poisson process. The service time is characterized by a general distribution with a known mean and variance. The queuing results, which are developed, include the utilization factor, the expected waiting time, and the expected number of messages waiting to be transmitted. The arrival process and the queuing results vary according to the time of the day, reflecting the varying traffic load throughout the day. A detailed example is given for a preliminary analysis of New York Harbor VTS communications.     

Temporal resolutions of time-reversal and passive-phase conjugation for underwater acoustic communications

In this paper, we study the temporal resolution of a time-reversal or passive-phase conjugation process as applied to underwater acoustic communications. Specifically, we address 1) the time resolution or the pulse width of a back-propagated time-compressed pulse as compared with the original transmitted pulse; 2) the effectiveness of temporal focusing as measured by the peak-to-sidelobe ratio of the back-propagated or phase-conjugated pulse (both pulse elongation and sidelobe leakages are causes of intersymbol interference and bit errors for communications); 3) the duration of temporal focusing or the temporal coherence time of the underwater acoustic channel; and 4) the stability of temporal focusing as measured by the phase fluctuations of successive pulses (symbols). Binary phase-shift keying signals collected at sea from a fixed source to a fixed receiver are used to extract the above four parameters and are compared with simulated results. Mid-frequency (3-4-kHz) data were collected in a dynamic shallow-water environment, exhibiting high temporal fluctuations over a scale of minutes. Despite this, the channel is found to be highly coherent over a length of 17 s. As a result, only one probe signal is used for 17 s of data. The bit error rate and variance of the symbol phase fluctuations are measured as a function of the number of receivers. They are of the same order as that calculated from the simulated data. The agreement suggests that these two quantities could be modeled for a communication channel with high coherence time. The phase variance can be used to determine the maximum data rate for a phase-shift keying signal for a given signal bandwidth and a given number of receivers.     

Differences between passive-phase conjugation and decision-feedback equalizer for underwater acoustic communications

Passive-phase conjugation (PPC) uses passive time reversal to remove intersymbol interferences (ISIs) for acoustic communications in a multipath environment. It is based on the theory of signal propagation in a waveguide, which says that the Green's function (or the impulse-response function) convolved with its time-reversed conjugate, summed over a (large-aperture) vertical array of receivers (denoted as the Q function) is approximately a delta function in space and time. A decision feedback equalizer (DFE) uses a nonlinear filter to remove ISI based on the minimum mean-square errors (mmse) between the estimated symbols and the true (or decision) symbols. These two approaches are motivated by different principles. In this paper, we analyze both using a common framework. We note the commonality and differences, and pros and cons, between the two methods and compare their performance in realistic ocean environments, using simulated and at-sea data. The performance measures are mean-square error (mse), output signal-to-noise ratio (SNR), and bit-error rate (BER) as a function of the number of receivers. For a small number of receivers, the DFE outperforms PPC in all measures. The reason for poor PPC performance is that, for a small number of receivers, the Q function has nonnegligible sidelobes, resulting in nonzero ISI. As the number of receivers increases, the BER for both processors approaches zero, but at a different rate. The modeled performance differences (in mse and SNR) between PPC and DFE are in general agreement with the measured values from at-sea data, providing a basis for performance prediction.     

Optimum design of the ocean acoustic tomography system for the Sea of Japan

An ocean acoustic tomography system covering the region of 800×1000 km with the spatial resolution of eddy-resolving scales has been designed on the basis of computer experiments using the hydrographic data collected in the Sea of Japan. The optimum number of acoustic sources required for 20 acoustic receivers was estimated as 13 by changing the source number. The spatial resolution for the optimum system was 41 km smaller than the dominant size of meso-scale eddies in the Sea of Japan. The effect of travel-time errors on tomographic maps is also quantified.     

Marine sediment acoustic measurement system

This paper describes a system that has been developed to measure compressional wave speed in cored marine sediments onboard ship. The structure enables one to secure an extruded core sample to its base and to move acoustic probes to a desired location, implant them to a specified depth into the sample and perform the measurement. The acoustic measurement system is a pulse-time delay system measuring time difference over a fixed path length and the temperature of the sediment. The time difference and temperature measurement systems are comprised of task oriented components and are housed in a single portable box. The system is adaptable to the various sample sizes obtained with the coring apparatus presently in use. Initial field tests indicate that ship motion has no effect on the system. Data collected from cores has been classified according to sediment type and displays good agreement with data presented by Hamilton (1970). The difference in compressional wave velocity, based on sediment type, for the two studies is 5 m per sec.     

Opto-acoustic feasibility design of a conical acoustic lens

The opto-acoustic design of a liquid-filled conical acoustic lens is discussed. An approximate model of stacked cylindrical elements is used to estimate the focal line. Final optimization is to be achieved experimentally. Diffraction enables point hydrophones to cover a vertical range of60deg. An iterative optimization procedure yields cone angle and window aperture size that give diffraction patterns to accomplish this with 3-dB maximum loss at the extreme elevations. Baffles are positioned by a ray-tracing analysis to minimize internal acoustic reflections. Reflection losses <1 dB are predicted above the critical angle (for total internal reflection) at the water-window boundary.     

声相关计程仪基阵设计方法

针对声相关计程仪的测速特点,研究了一维、二维基阵的设计方法。一维基阵采用基于约束最小冗余的设计方法,可以获得比均匀线阵大得多的阵列孔径,从而提高基阵的利用率,但约束最小冗余线阵(RMRLA)的设计方法计算量巨大,并不适合二维基阵的设计。在重新定义冗余因子,建立理想位置矢量图模型,提出位置矢量重合率等概念的基础上,实现了适用于声相关测速需求的二维基阵的快速设计。仿真结果证明了该设计方法的可行性。     

A high-frequency, shallow-water acoustic measurement system

The US Naval Ocean Research and Development Activity has developed a high-frequency acoustic measurement system for use in shallow water. The heart of this system is a pair of submersible towers supporting acoustic transmitting and receiving instrumentation. These towers are transported to an experimental staging area, assembled, and acoustic instrumentation installed. They are towed to a preselected measurement site, then the chambers on each tower are flooded, thereby settling slowly to the ocean bottom. Stability and dynamic response analyses were used to determine the towing and deployment stability envelopes for the towers. The acoustic transmitting system uses a pair of narrow-beam parametric acoustic sources operating at secondary frequencies ranging from 20 to 180 kHz. The acoustic receiving systems consist of a pair of 16-hydrophone, two-dimensional arrays with broadband capabilities up to 250 kHz. These systems have been used to make high-resolution bottom scattering measurements in shallow water off the coast, south of Panama City, Florida     

A high-resolution pulse-Doppler underwater acoustic navigation system

A high-resolution underwater acoustic pulse-Doppler navigation system has been developed and tested at sea. The system provides continuous, highly accurate tracking of underwater and ocean-surface platforms in a fixed 50-km²navigation net. Three reference buoys, moored 20 m from the ocean bottom, provide the navigation net used by shipboard processing equipment. Each reference buoy contains an acoustic transponder, used to obtain the acoustic travel times from the transponder to the platform, and a continuous-tone beacon, used to obtain the Doppler shift due to platform motion. The system is capable of determining the position of a platform with respect to the reference net with an error of 2-3 m. The relative position of the platform on a fix-to-fix basis can be determined within several centimeters over short time intervals (approx 10min).     

北极冰下水声通信中改进的最小均方/四次方直接自适应均衡器

本文提出了一种适用于北极冰下水声通信的最小均方/四次方直接自适应均衡器（LMS/F-DAE）。它能处理基带复信号,与LMS相比,具有更好的均衡效果。考虑到均衡器的稀疏特性,在其代价函数中加入自适应范数（AN）作为约束。它能根据均衡器系数的大小自适应变化：对于小系数,此约束项存在以加快收敛速度;对于大系数,此约束项不存在以减小均衡误差。利用第九次中国北极科学考察得到的实验数据验证AN-LMS/F-DAE的性能。结果表明,与传统的LMS/F-DAE相比,AN-LMS/F-DAE能提升均衡器的稀疏性且均衡性能更优。     

Coastal acoustic tomography system and its field application

The coastal acoustic tomography system (CATS), composed of five moored acoustic stations, has been constructed to measure current fields. The system is developed with special considerations in mind, including the use of Global Positioning System clock signals in the synchronization of the system clock timing among the multiple acoustic stations, and the use of the differently coded Gold sequences to identify the acoustic signals corresponding to individual stations from a received signal. The CATS was successfully applied to map the structure of strongly nonlinear tidal currents in the coastal sea. In spite of the limited spatial resolution caused by inadequate sound transmission data, the two-dimensional tidal vortices features of growth, translation, and decay processes are reconstructed through an inverse analysis of the acoustic travel time obtained among the station pairs. It is evident that the CATS is a powerful tool for measuring variable current fields generated in the coastal seas