浅海声信道中脉位信息检测的抗多途研究

简述了浅海声信道中多途干扰的主要特征及其对脉位信息检测的影响,探讨克服多途干扰的几种可能途径,并提出了一种有效的抗多途方法,以实现浅海声信道中脉位信息的可靠、精确检测。     

水声信道高速率数据传输技术

本文介绍近年来水声信道高速率数所传输技术的一些研究进展，并结合本所研究的水声数据遥测，数字语音通讯和视频图像传输实验样机，讨论了具有抗多途干扰的声传输系统在调制信号设计及信号处理上所采用的关键技术。     

频率编码水声遥控指令的抗多途对策

浅海信道中,多途干扰是水声遥控指令可靠检测的首要障碍,本文探讨频率编码遥控指令的抗多途对策,即通过阻塞多途来削落多途干扰。     

深海RAP下的水声信道特性与通信技术研究

可靠声路径(Reliable Acoustic Path,RAP)是深海声传播的重要通道之一,其受界面影响较小, 传播损失较低,可以传播到较远的距离,而且在临界深度以下,环境噪声较低;其次,可靠声路径有效避开了多途扩展现象,声线以结构稳定的直达声为主。在总结可靠声路径物理机理和声传播优势的基础上,对比分析了几种不同海洋参数条件下 RAP 声传播特性,然后采用射线模型仿真分析了 RAP 声信道内的接收声线结构,之后基于仿真的 RAP 信道进行了单载波通信性能的分析。仿真结果发现：在 RAP 声信道内,直达声能量高,传播损失低,声线结构稳定且多途扩展小,对环境变化不敏感,在 35 km 左右的中远程距离内具有很高的信噪比;相同仿真条件下,RAP 区域的误码率较同距离浅深度的接收低很多,而且 RAP 区域接收信号信噪比高出其他区域 10 dB 左右。该研究结果对于实现垂直方向上深海信息的跨域传输具有重要意义。     

宽码率Polar码浅海水声通信实验研究

浅海水声信道的随机时–空–频特性给数据的可靠传输带来了重大挑战,低复杂度和理论上证明能到达香农限的极化码(Polar code)可以增强水声通信系统的鲁棒性。水下传输的图像、语音、文本、海洋监测数据和遥控指令具有不等重要性的特点,宽码率 Polar 码能够适应不同水声信道和不等重要性的信息传输。 目前 Polar 码在水声通信中的实验研究多为仿真分析,设计了宽码率 Polar 码在厦门港海域海试验证,在不同信噪比的实录环境噪声下进行分析。海试结果表明：在良好的信道条件下,宽码率 Polar 码的性能优异,0.25 码率的 BPSK 和 QPSK 在实录环境噪声信噪比为–1 和 4 时实现零误码,其低复杂度信道编译码机制和宽码率与水声信道相匹配,可有效提高水声数据传输的可靠性和有效性,为基于 Polar 码的稳健可靠水声通信系统提供海试实验验证。     

浅海声信道信号时间相关特性研究

水声信道中信号的时间相关特性是水声微弱信号相关检测的理论基础和具体参数选择依据。文中以厦门港典型的浅海声信道中信号的时间自相关和互相关特性进行实验研究。结果指出,信号有强的时间相关性,无论自相关或互相关性均比海洋环境噪声强得多,因此在水声信道中采用时间相关检测可望达到优良的抗噪声效果。     

基于Visual C和PC的软件水声调制解调系统

介绍了一种DS/DBPSK调制解调技术方案,该方案采用了具有较强抗干扰、抗衰落和抗多径性能的扩频技术,并结合与信道适应性较好、不需载波恢复的差分相位调制技术,对浅海水声信道造成的干扰进行抑制.在此方法上,基于Visual C(VC)及PC平台设计了一个通用性好的软件水声调制解调系统,利用双缓冲邋和多线程技术机制实现算法实时处理,同时可通过串口通信提供上层应用接口.实验系统验证了扩频和差分调制技术在浅海水声通信中的性能,为水声组网提供了底层支持.海试实验结果表明了系统的有效性.     

海洋信道多途干扰下提高信息传输可靠度的一种编码方法

水声数据在海洋信道传输过程中．除受到海洋噪声干扰外还存在由于信道界面反射引起的多途干扰。多途干扰的性质不同于海洋噪声及其它类型噪声．在抗干扰理论中所归纳的几种抗干扰方法均不适用本文提出一种编码方法．它使多途干扰信号在某一特定时刻以相同相位同时抵达接收点．由于信号的相互迭加作用使信号幅度增大．有效地提高海洋信道中数据传输的可靠性。     

基于声学宽带扩频信号的抗多径测距技术研究

为研究宽带扩频信号在复杂水声信道下的测距精度,提高水声定位系统的抗多径性能,以BELLHOP模型的多径干扰冲激响应作为水声信道环境基础,通过构造不同传播距离、海洋环境特征的多径水声信道,对宽带扩频编码进行同步相关测距仿真;提出一种基于宽带扩频信号载波域同步的抗多径测距方法,利用载波携带的宽带特征信息进行相关同步以提高理论测距精度。实验结果表明：在有直达声且不考虑声线弯曲条件下,浅海多径信道条件测距精度可达厘米级,中深水信道维持分米级测距精度,较基带同步方法的抗干扰性能有所提升,基于宽带扩频信号载波域同步的抗多径测距方法,能够为声学定位系统的高性能测距技术提供理论依据。     

干涉谱法测量水下竖直运动目标轨迹

以被动测量竖直运动目标轨迹为目的,通过分析多途信号声压场模型,讨论了经过相干多途信道的目标辐射噪声在接收点产生相干干涉的现象。在目标水平距离已知的情形下,给出了干涉频率周期与目标深度的关系,提出了利用多途信道的相干干涉信息来解算目标深度轨迹的方法。时频分析可以得到干涉条纹、条纹粗细变化的规律与目标深度变化有关。通过对实测数据的分析,说明本方法的有效性。     

Performance analysis of digital acoustic communication in a shallowwater channel

The major obstacle to underwater acoustic communication is the interference of multi-path signals due to surface and bottom reflections. High speed acoustic transmission over a shallow water channel characterized by small grazing angles presents formidable difficulties. The reflection losses associated with such small angles are low, causing large amplitudes in multi-path signals. In this paper we propose a simple but effective model for multi-path interference, which is then used to assess the performance of a digital communication system operating in a shallow water channel. The results indicate that transmission rates in excess of 8 kbits/s are possible over a distance of 13 km and channel depth of only 20 meters. Such a system offers improved performance in applications such as data collection from underwater sensors     

时变水声信道中基于BEM的迭代信道估计技术

水声信道的多径时延扩展和时变特性对信道估计和均衡技术的研究带来了很大的挑战,同时也决定了水声信道是一种时频双扩展信道,提出一种水声OFDM通信系统中基于软信息的迭代信道估计技术,利用基于复指数基扩展模型(CE-BEM)进行信道估计。OFDM系统本身可以消除由于多径引起的符号间干扰(ISI)。基于导频的BEM信道估计,可以实现对时变信道的估计,结合基于软信息迭代的迭代均衡模块,将每次迭代生成的符号软判决信息作为辅助导频用于信道估计。同时,为了防止由于信道时变引起的信道子载波间干扰(ICI)对导频符号的影响,采用基于保护间隔的导频插入法插入导频。仿真结果显示基于BEM的软信息迭代信道估计性能较非迭代信道估计时明显提升。     

利用线性调频脉冲进行帧同步的方法及性能

针对水声信道的特点,提出了一种利用具有大的时宽带宽积的线性调频信号作为帧同步信号的方法。该方法首先在接收端进行滑动时频分析,从而实现线性调频信号的检测及粗同步,再进一步利用拷贝相关处理来实现细同步。计算机仿真和海上试验结果均表明,该方法能够在多途干扰下有效地实现同步。     

Robust source localization in shallow water based on vector optimization

Owing to the multipath effect, the source localization in shallow water has been an area of active interest. However, most methods for source localization in shallow water are sensitive to the assumed model of the underwater environment and have poor robustness against the underwater channel uncertainty, which limit their further application in practical engineering. In this paper, a new method of source localization in shallow water, based on vector optimization concept, is described, which is highly robust against environmental factors affecting the localization, such as the channel depth, the bottom reflection coefficients, and so on. Through constructing the uncertainty set of the source vector errors and extracting the multi-path sound rays from the sea surface and bottom, the proposed method can accurately localize one or more sources in shallow water dominated by multipath propagation. It turns out that the natural formulation of our approach involves minimization of two quadratic functions subject to infinitely many nonconvex quadratic constraints. It shows that this problem (originally intractable) can be reformulated in a convex form as the so-called second-order cone program (SOCP) and solved efficiently by using the well-established interior point method, such as the software tool, SeDuMi. Computer simulations show better performance of the proposed method as compared with existing algorithms and establish a theoretical foundation for the practical engineering application.     

Multichannel processing of broad-band multiuser communicationsignals in shallow water acoustic channels

High-throughout multiple-access communication networks are being considered for use in underwater acoustic channels. Bandwidth limitations of underwater acoustic channels require receivers to process broad-band communications signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operations. While greatly reducing intersymbol and multiple-access interference, spatial diversity implies high increase in adaptive multiuser receiver complexity. To reduce the complexity of the optimal multichannel combiner, spatial structure of multipath is exploited. The complexity of resulting adaptive decentralized multichannel multiuser receiver is reduced at almost no cost in performance. Comparison of proposed multichannel receivers in an experimental shallow water channel demonstrates superior performance of spatial signal combining. The use of multiple input channels is shown to provide high level of tolerance for the near-far effect in both centralized and decentralized receivers. Decentralized receiver with reduced-complexity combining is found to satisfy the performance/complexity trade-off required for practical receiver realization in shallow water networks     

Phase-coherent digital communications for underwater acousticchannels

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     

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     

BDRM理论在双轴海洋声道中的应用

由于表面声道与深海声道间的耦合效应,声波在双轴海洋声道中的传播比较复杂,因此求解双轴海洋声道中的声场就比较困难。在 WKBZ 本征函数的基础上,推导出了参考界面相位修正的一致表达式,并将浅海声传播的波束位移射线简正波(BDRM)理论应用于计算双轴海洋声道中的声场,进行了数值模拟并与传统简正波方法进行比较,结果表明应用 BDRM 理论计算的传播损失具有很高的精度和速度,可以对双轴海洋声道内声传播问题进行分析和预报。