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1.
《Oceanic Engineering, IEEE Journal of》2008,33(2):182-197
2.
In this paper, we investigate the performance of adaptive modulation （AM） orthogonal frequency division multiplexing （OFDM） system in underwater acoustic （UWA） communications. The aim is to solve the problem of large feedback overhead for channel state information （CSI） in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing （CS）. We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate （BER） and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying （QPSK） modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform （DCT） method. 相似文献
3.
Scattering functions from several experiments demonstrate that acoustic underwater channels are doubly spread. Receivers used on these channels to date have difficulty with large Doppler spreads. A receiver to perform coherent communication over Doppler spread channels is presented in this first paper of two. The receiver contains a channel tracker and a linear decoder. The tracker operates by means of a modified recursive least squares algorithm which makes use of frequency-domain filters called Doppler lines. The decoder makes use of the channel tracker coefficients in order to perform minimum mean square error decoding. This first paper treats theoretical aspects whereas the second part presents implementation issues and results 相似文献
4.
For pt. I see ibid., vol. 25, no. 1, p. 62-72 (2000). A receiver for coherent communication through underwater communication channels is analyzed. The receiver performance and stability versus delay spread, Doppler spread, and signal-to-noise ratio is quantified. The stability is governed by the ill-conditioning of a correlation matrix estimate and it sets the limit on how many taps should be used for a channel with a given number of degrees of freedom. The receiver is used extensively on both simulated and real data that are Doppler spread, and good performance in these channels is verified 相似文献
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6.
《Oceanic Engineering, IEEE Journal of》2009,34(4):624-633
7.
In this paper, we consider the use of multiple antennas and space-time coding for high data rate underwater acoustic (UWA) communications. Recent advances in information theory have shown that significant capacity gains can be achieved by using multiple-input-multiple-output (MIMO) systems and space-time coding techniques for rich scattering environments. This is especially significant for the UWA channel where the usable bandwidth is severely limited due to frequency-dependent attenuation. In this paper, we propose to use space-time coding and iterative decoding techniques to obtain high data rates and reliability over shallow-water, medium-range UWA channels. In particular, we propose to use space-time trellis codes (STTCs), layered space-time codes (LSTCs) and their combinations along with three low-complexity adaptive equalizer structures at the receiver. We consider multiband transmissions where the available bandwidth is divided into several subbands with guard bands in between them. We describe the theoretical basis of the proposed receivers along with a comprehensive set of experimental results obtained by processing data collected from real UWA communications experiments carried out in the Pacific Ocean. We demonstrate that by using space-time coding at the transmitter and sophisticated iterative processing at the receiver, we can obtain data rates and spectral efficiencies that are not possible with single transmitter systems at similar ranges and depths. In particular, we have demonstrated reliable transmission at a data rate of 48 kb/s in 23 kHz of bandwidth, and 12 kb/s in 3 kHz of bandwidth (a spectral efficiency of 4 bs-1Hz-1) at a 2-km range. 相似文献
8.
一种适用于水声通信的Doppler估计算法 总被引：1，自引：0，他引：1
在水下声通信中,收发双方相对运动产生的Doppler效应会导致信号的伸缩,引起信噪比的降级,需要采用Doppler补偿措施。在补偿前需要得到Doppler估计。文中提出了一种利用DFT进行Doppler估计的有效算法,该算法通过估计频率偏移来计算Doppler率。仿真结果验证了该算法的有效性。 相似文献
9.
Recent advances in high-speed underwater acoustic communications 总被引：4，自引：0，他引：4
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 相似文献
10.
Underwater acoustic networks 总被引：8，自引：0，他引：8
With the advances in acoustic modem technology that enabled high-rate reliable communications, current research focuses on communication between various remote instruments within a network environment. Underwater acoustic (UWA) networks are generally formed by acoustically connected ocean-bottom sensors, autonomous underwater vehicles, and a surface station, which provides a link to an on-shore control center. While many applications require long-term monitoring of the deployment area, the battery-powered network nodes limit the lifetime of UWA networks. In addition, shallow-water acoustic channel characteristics, such as low available bandwidth, highly varying multipath, and large propagation delays, restrict the efficiency of UWA networks. Within such an environment, designing an UWA network that maximizes throughput and reliability while minimizing the power consumption becomes a very difficult task. The goal of this paper is to survey the existing network technology and its applicability to underwater acoustic channels. In addition, we present a shallow-water acoustic network example and outline some future research directions 相似文献
11.
Milica Stojanovic Lee Freitag 《Oceanic Engineering, IEEE Journal of》2006,31(3):685-695
Direct-sequence (DS) code-division multiple access (CDMA) is considered for future wideband mobile underwater acoustic networks, where a typical configuration may include several autonomous underwater vehicles (AUVs) operating within a few kilometers of a central receiver. Two receivers that utilize multichannel (array) processing of asynchronous multiuser signals are proposed: the symbol decision feedback (SDF) receiver and the chip hypothesis feedback (CHF) receiver. Both receivers use a chip-resolution adaptive front end consisting of a many-to-few combiner and a bank of fractionally-spaced feedforward equalizers. In the SDF receiver, feedback equalization is implemented at symbol resolution, and receiver filters, including a decision-directed phase-locked loop, are adapted at the symbol rate. This limits its applicability to the channels whose time variation is slow compared to the symbol rate. In a wideband acoustic system, which transmits at maximal chip rate, the symbol rate is down-scaled by the spreading factor, and an inverse effect may occur by which increasing the spreading factor results in performance degradation. To eliminate this effect, feedback equalization, which is necessary for the majority of acoustic channels, is performed in the CHF receiver at chip resolution and receiver parameters are adjusted at the chip rate. At the price of increased computational complexity (there are as many adaptive filters as there are symbol values), this receiver provides improved performance for systems where time variation cannot be neglected with respect to the symbol rate [e.g., low probability of detection (LPD) acoustic systems]. Performance of the two receivers was demonstrated in a four-user scenario, using experimental data obtained over a 2-km shallow-water channel. At the chip rate of 19.2 kilochips per second (kc/s) with quaternary phase-shift keying (QPSK) modulation, excellent results were achieved at an aggregate data rate of up to 10 kb/s 相似文献
12.
High-speed phase coherent communications in the ocean channel are made difficult by the combined effects of large Doppler fluctuations and extended, time-varying multipath. In order to account for these effects, we consider a receiver which performs optimal phase synchronization and channel equalization jointly. Since the intersymbol interference in some underwater acoustic channels spans several tens of symbol intervals, making the optimal maximum-likelihood receiver unacceptably complex, we use a suboptimal, but low complexity, decision feedback equalizer. The mean squared error multiparameter optimization results in an adaptive algorithm which is a combination of recursive least squares and second-order digital phase and delay-locked loops. The use of a fractionally spaced equalizer eliminates the need for explicit symbol delay tracking. The proposed algorithm is applied to experimental data from three types of underwater acoustic channels: long-range deep water, long-range shallow water, and short-range shallow water channels. The modulation techniques used are 4- and 8-PSK. The results indicate the feasibility of achieving power-efficient communications in these channels and demonstrate the ability to coherently combine multiple arrivals, thus exploiting the diversity inherent in multipath propagation 相似文献
13.
To improve the transmission performance of XCTD channel, this paper proposes a method to measure directly and fit the channel transmission characteristics by using frequency sweeping method. Sinusoidal signals with a frequency range of 100 Hz to 10 k Hz and an interval of 100 Hz are used to measure transmission characteristics of channels with lengths of 300 m, 800 m, 1300 m, and 1800 m. The correctness of the fitted channel characteristics by transmitting square wave, composite waves of different frequencies, and ASK modulation are verified. The results show that when the frequency of the signal is below 1500 Hz, the channel has very little effect on the signal. The signal compensated for amplitude and phase at the receiver is not as good as the uncompensated signal.Alternatively, when the signal frequency is above 1500 Hz, the channel distorts the signal. The quality of signal compensated for amplitude and phase at receiver is better than that of the uncompensated signal. Thus, we can select the appropriate frequency for XCTD system and the appropriate way to process the received signals. Signals below1500 Hz can be directly used at the receiving end. Signals above 1500 Hz are used after amplitude and phase compensation at the receiving end. 相似文献
14.
Underwater acoustic communications (UAC) at the reverberation-limited range results in severely distorted information signals. Wide-band signals are subject to both intermodal and intramodal-type of dispersions. The underwater acoustic channel impulse response and the sidelobes strongly depend on the waveguide structure and the source and receiver positions. The motion and displacement from this position, as well as other environmental variabilities impose a real-time adaptivity for the receiver operation to keep track of the fluctuations. To increase the system's reliability and data rate, there is a need to employ adaptive equalizers and diversity techniques to improve the margins against noise, and intersymbol interference (ISI). Blind adaptive equalization (BAE) is the ideal adaptive compensation when operating point-to-multipoint networks, and centralized communication systems in general. Inherent optimum multiple resonant modes within the ocean acoustic waveguide can be exploited judiciously via a new proposed parallel data multicarrier modulation (MCM) scheme by sending data over the multiple subcarriers. MCM might eventually obviate equalization which introduces higher-order computational complexity to the receiver. The above modulation eliminates multipaths and allows operation at multiples of the single-carrier transmission rate. The system's immunity to distortions such as ISI, fast fades, and impulsive noises, is increased due to incorporation of symbol guard space. Direct comparisons with single carrier schemes (such as higher-order statistics (HOS)-based equalization) are of great interest, since the proposed new receiver configuration has low-complexity to provide a compact, portable and low-power practical acoustic modem. Finally, network topology issues are considered to determine optimum network architectures for underwater acoustic LANs. A central topology (CT) supported by BAE and MCM transmission is proposed 相似文献
15.
为了在恶劣的水声信道中确保数据的可靠传输,采用性能优异的Turbo码,以6711DSP为核心处理单元构建译码系统。系统采用戈泽尔算法进行跳频的软解调,迭代的软输出维特比译码算法(SOVA)进行译码。系统经过实验室水池的试验,证实能保证译码的实时性及其在恶劣信道中数据传输的正确性,具有相当优异的性能。 相似文献
16.
本文介绍了一种水声扩谱通讯中，应用子波对多普勒效应进行补偿的方法。针对多普勒效应本身是否会随时间变化，分别应用直接估计、自适应估计的方法。其中自适应多普勒补偿方法同自适应信道均衡算法结合在一起，对多普勒漂移、信道参数进行联合估计。两种方法都应用了子波变换对多普勒展宽程度进行估计并用线性插值进行补偿。计算机仿真结果令人满意。 相似文献
17.
Estimation of Rapidly Time-Varying Sparse Channels 总被引：2，自引：0，他引：2
The estimation of sparse shallow-water acoustic communication channels and the impact of estimation performance on the equalization of phase coherent communication signals are investigated. Given sufficiently wide transmission bandwidth, the impulse response of the shallow-water acoustic channel is often sparse as the multipath arrivals become resolvable. In the presence of significant surface waves, the multipath arrivals associated with surface scattering fluctuate rapidly over time, in the sense that the complex gain, the arrival time, and the Dopplers of each arrival all change dynamically. A sparse channel estimation technique is developed based on the delay-Doppler-spread function representation of the channel. The delay-Doppler-spread function may be considered as a first-order approximation to the rapidly time-varying channel in which each channel component is associated with Doppler shifts that are assumed constant over an averaging interval. The sparse structure of the delay-Doppler-spread function is then exploited by sequentially choosing the dominant components that minimize a least squares error. The advantage of this approach is that it captures both the channel structure as well as its dynamics without the need of explicit dynamic channel modeling. As the symbols are populated with the sample Dopplers, the increase in complexity depends on the channel Doppler spread and can be significant for a severely Doppler-spread channel. Comparison is made between nonsparse recursive least squares (RLS) channel estimation, sparse channel impulse response estimation, and estimation using the proposed approach. The results are demonstrated using experimental data. In training mode, the proposed approach shows a 3-dB reduction in signal prediction error. In decision-directed mode, it improves significantly the robustness of the performance of the channel-estimate-based equalizer against rapid channel fluctuations. 相似文献
18.
《Oceanic Engineering, IEEE Journal of》2009,34(4):603-609
19.
An underwater acoustic local area network (ALAN) provides multipoint-to-point telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10-30 kHz vertical acoustical channel. Ocean-bottom modems initiate the transmission process by requesting data channel time slots via a common narrow-band request channel. Request packets overlap in time and frequency in this channel, and the throughput and average transmission delay rely heavily on the successful resolution of the request packet collisions. This paper presents the design, analysis, and experimental demonstration of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude, and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions. An inwater experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links 相似文献
20.
Effective communication and echolocation depends strongly upon the coherence of the channel through which the signal is propagated. Under certain conditions, the average coherence or equivalently, the spreading of a random channel may be described by a scattering function (SF). This represents a second order (energy) measure of the average delay, Doppler, and more generally, the spatial (azimuthal) spread that the signal experiences. The SF is analogous to the point spread function (PSF) discussed in the image processing literature and likewise describes the amount of “blurring” imposed upon the signal or scene transmitted. The SF will be briefly reviewed and its measurement by both direct (high resolution channel probing) and indirect (deconvolution) methods will be discussed. A new direct method using specially designed waveform pairs and a twin or uncertainty product (UP) receiver structure is introduced. Unlike high resolution matched filter implementations for direct probing that are limited by the fixed volume constraint of ambiguity functions, the UP receiver produces vanishing sidelobes and hence more nearly approximates a desirable two-dimensional delta characteristic. The improvement gained in SF measurement is illustrated by the results of an experiment in which the UP receiver and traditional matched filter implementations were used to directly probe an ocean multipath channel 相似文献