虚拟时反镜在水声被动测距中的应用

水声信道多途干涉使被动测距产生偏差．接收信号的自相关输出波形含有信道冲激响应结构的信息，本文提出通过简化模型可估计出信道冲激响应，并将其时间反转构造一前置预处理器，可实现虚拟时间反转镜（VTRM）．虚拟时反镜可获得时间、空间上的自适应聚焦效果，克服多途干涉对被动测距的影响，并且实现简单，可实时对目标进行测距．通过计算机仿真研究，验证VTRM技术可匹配声信道，提高被动测距的精度．     

基于基追踪去噪的水声正交频分复用稀疏信道估计

针对传统的l₂-范数信道估计精度低的问题, 提出了一种基于基追踪去噪(BPDN)的水声正交频分复用稀疏信道估计方法, 该方法针对水声信道的稀疏特性, 利用少量的观测值即可以很高的精度估计出信道冲激响应. 与贪婪追踪类算法相比, 基于BPDN算法的稀疏信号估计具有全局最优解, 采用l₂-l₁范数准则估计信号, 同时考虑了观测值含噪情况, 通过调整正则化参数控制估计信号稀疏度和残余误差之间的平衡. 仿真分析了导频分布、正则化参数等对BPDN 算法的影响以及BPDN算法与最小平方(LS)、正交匹配追踪(OMP)信道估计算法的性能. 湖试结果表明, 在稀疏信道下, 基于BPDN的信道估计方法明显优于LS和OMP信道估计方法.     

起伏海面环境下水声信道特性及估计方法*

为了研究随机起伏海面的散射对水声信道特性的影响,以及不同信道估计算法在该信道条件下的性能问题,该文从利用Monte Carlo方法模拟的基于PM海浪谱的二维随机起伏海面出发,提取了与风向不同夹角的一维随机起伏海面,并利用Kirchhoff近似法计算了其散射强度。将得到的声散射特性和海面环境信息融入到信道模型中,建立了基于PM海浪谱的随机信道模型。通过仿真分析了不同风速、传播距离条件下,声波的传播损失和信道冲激响应变化,并利用l_0-最小均方误差法、匹配追踪算法和正交匹配追踪算法进行了信道估计。给出了不同环境条件下,三种算法估计性能的综合比较结果,验证了算法针对该信道模型的有效性。     

基于DFT-SAMP算法的MIMO-VLC系统压缩感知信道估计

多输入多输出(Multiple-input-multiple-output,MIMO)可见光通信(Visible Light Communications,VLC)系统接收端需精确的信道状态信息用以解调信号，而常用的最小二乘算法对噪声敏感，估计误差较大，难以保证可靠性。基于信道稀疏特性，利用压缩感知方法进行MIMO-VLC信道估计，提出一种基于离散傅里叶变换(Discrete Fourier Transform,DFT)的稀疏度预测自适应匹配追踪(DFT Based Prediction-sparsity Adaptive Matching Pursuit,DFT-SAMP)算法。首先，通过DFT的稀疏度预测方法对信道冲激响应的稀疏度进行预估计，将估计的稀疏度作为算法初始步长，以快速逼近真实稀疏度，提高算法效率；其次，采用SAMP算法重构信道冲激响应，提高信道估计准确性，保证通信可靠性。基于2×2的MIMO-VLC系统信道估计实验结果表明，导频数为32时，本文算法相较于最小二乘算法在误码率满足前向纠错的误码率阈值(3.8×10^-3)时所需的信噪比降低4.5 dB；...     

用于单载波频域均衡水声通信的可分近似稀疏信道估计

针对单载波频域均衡水声通信中信道估计易受噪声干扰的问题,提出了一种低复杂度的信道估计方法。考虑水声信道的时域稀疏特性,导出频域输入、输出信号与信道冲激响应的关系式,并引入稀疏正则项,构造稀疏信道估计的目标函数。然后利用可分近似对目标函数进行迭代优化,再经过稀疏化与去偏处理,得到信道传递函数的最终估计。最后,利用数值仿真和海试数据对所提出方法的性能和运算效率进行评估。较之传统信道估计方法,所提出的方法在估计精度和计算复杂度方面具有一定的优势。      

基于混合范数约束的非均匀稀疏水声信道估计方法*

水声信道具有明显的簇状稀疏特性,即稀疏的信道冲激响应大部分为零或接近零的小值系数,而非零值系数是以簇的形式非均匀分布于时延域。为此本文提出了一种基于非均匀混合范数约束仿射投影算法的水声信道估计方法。该方法首先根据信道簇状结构对其进行非均匀分组,基于此将范数约束规则加入仿射投影算法中,具体方法为：对簇状部分施加范数约束,有效提高系数间的相关性,而簇状结构与其他零值抽头之间利用范数约束实现了整体的稀疏特性。数值仿真以及深海远程水声通信实验数据处理结果表明了本文所提出的水声信道估计算法相较现有稀疏信道估计方法能够实现更快的收敛速度以及更高精度的信道估计结果。     

基于改进的统计信道模型与多天线系统性能分析

为提高电磁信号的到达角度以及多普勒效应等信道参数估计的准确性,在散射体非均匀分布下引入了散射概率和有效散射体两个物理概念,提出一种合理的改进型空间信道模型,该模型能准确地描述宏小区(macrocell)和微小区(microcell)等各种移动通信环境下的重要空时信道参数,并应用于多入多出系统(multiple input multiple output,MIMO)信道性能仿真中.数值仿真结果与早期多径衰落信道模型对比,表明本模型的信道参数估计结果符合理论和经验,拓展了空间统计信道模型的研究和应用,对评估多天线MIMO系统空时处理算法和仿真无线通信系统提供有力的工具.     

水声多途信道的维纳误差限信道质量量化分析方法

提出了一种定量分析不同多途结构的时不变水声信道对单载波水声通信系统性能影响程度的方法。该方法利用水声信道下信号线性均衡的残留均方误差与通信系统误比特率性能相关性强的特点,计算信号线性均衡的残留均方误差极限,并将其表示为只与信道冲激响应有关的变量,称之为维纳误差限(WEB),将此作为多途信道质量量化指标来评估多途信道对通信性能的影响。为验证方法性能,对比分析了大量海试实测水声信道下,线性均衡、判决反馈均衡以及Turbo均衡等单载波通信中3种常用信道均衡方法的误比特率与多种水声信道质量量化指标的相关性,利用Spearman相关系数对相关性进行量化分析,实验结果表明,相比于时延扩展、相干带宽、频谱衰落深度和信号多径比等传统信道多途特性量化指标,维纳误差限与均衡后误比特率的Spearman相关系数更高,平均相关系数可达0.71。利用维纳误差限可有效量化和分类不同质量的多途信道,定量体现不同水声信道多途结构对于通信性能的影响程度。     

基于LMS/SOLMS算法的时变多径水声信道估计方法

缓变信道下基于LMS算法的信道估计算法具有较好的跟踪性能,但对快变信道,LMS算法跟踪性能下降。SOLMS算法具有比LMS算法更好的跟踪性能,尤其是在快变的信道下。但由于SOLMS算法在收敛阶段的振荡性,这时收敛速度较LMS算法慢。本文提出一种收敛模式下用LMS算法获得信道的参数,收敛后则切换成SOLMS算法跟踪信道的变化的信道估计方法。新方法结合了LMS算法收敛快和SOLMS算法跟踪性能好的优点。对时变多径水声信道估计的计算机仿真实验证明了该方法的有效性。     

单模费米热噪声信道量子容量的估计

从量子信道的算符求和表象出发,对单模费米系统量子信道进行了参数化.参数化的结果得到另一类费米热噪声量子信道.给出了两类费米热噪声量子信道上相干信息的最大值,以估计信道的量子容量. 关键词： 量子容量 相干信息 费米热噪声信道     

Characterization of ultrasonic imaging systems using transfer functions

The transfer functions for focussed and defocussed, coherent and confocal optical imaging systems have been applied to the equivalent ultrasonic imaging systems. The transfer functions with varying degrees of defocus were calculated to show the defocus effects for ultrasonic imaging systems. Assuming that the acoustic waves are reflected perfectly on the surface of the step edge, the theoretical line-scans for small amplitude signals across a step edge, with various degrees of defocus, were generated. The first derivative of the line-scan for a step edge is shown theoretically to yield the same impulse response as that calculated using the inverse Fourier transform applied to the original transfer function. These results show how the real and imaginary parts of the transfer functions contribute to ultrasonic image formation. A method for the experimental determination of the impulse response, and the transfer functions for the characterization of an ultrasonic imaging system, such as an acoustic microscope, is provided.     

聚焦换能器自由场互易校准的冲击响应积分法衍射系数计算

在聚焦换能器的会聚球面波自由场互易校准法中，衍射损失修正系数是获得互易常数的一个关键参数。本文利用冲击响应函数积分法系统地计算了超声治疗头、机械扫描B超探头以及多普勒诊断探头的衍射损失系数，得出了一些典型参数下的计算曲线，可供不同类型的聚焦换能器会聚球面波互易校准时引用。与传统的瑞利积分法分析比较显示，本文中所采用的冲击响应函数积分法显著地提高了运算速度，同时基本保持了与瑞利积分算法计算结果的一致性。     

一种精确计算换能器空间脉冲响应的快速方法

空间脉冲响应被认为是最有效的瞬态声场计算方法,针对空间脉冲响应直接计算时需要很高的采样频率,导致数据量大、计算效率差的问题,推导了空间点与其在换能器平面的投影点脉冲响应之间的关系,提出了一种计算脉冲响应的快速算法,探讨了计算采样频率和插值采样频率对计算精度和计算效率的影响,研究发现,采用1000 MHz采样频率能够保证精度要求,采用500 MHz作为插值时的采样频率是较优的选择。该方法与直接求解相比,可提高计算效率18倍。      

An approximate treatment of reflection coefficient in the phased beam tracing method for the simulation of enclosed sound fields at medium frequencies

The phased beam tracing method (PBTM) was suggested as a medium-frequency simulation technique for the calculation of impulse response, although main assumptions of geometric acoustics still hold. The phased method needs the reflection coefficient for characterizing the acoustic property of a surface and the complex wave number for describing the propagation characteristics. In this study, two types of approximate real reflection coefficients derived from the measured absorption coefficient were tested for a practical applicability. As a test example, pressure impulse responses and energy impulse responses computed from the PBTM were compared with those from the measurement and the ordinary beam tracing method. The PBTM employing the approximate reflection coefficients greatly increased the accuracy of the prediction compared to the ordinary beam tracing method, in particular at the medium-frequency range in octave bands above the Schroeder cutoff frequency. A comparison was made between angle-dependent and angle-independent reflection coefficients in the calculation of acoustic measures. Although the angle-dependent reflection coefficient yielded best matched results with measured data, but the simple angle-independent reflection coefficient can be also used with a reasonably good precision.     

A fast implementation for computing spatial impulse response of ultrasonic transducers

The spatial impulse response(SIR) method is often used as the 'gold standard5 in simulation of transient acoustic wave fields due to its high accuracy in the linear domain.However, a high sampling frequency is often required in order to achieve the high accuracy. As a result, a large amount of data has to be processed. In this paper a fast approach for computing spatial impulse response is proposed to reduce the computation burden. The proposed approach is developed by employing the relationship of SIRs at observed points and SIRs of the projection points on the transducer surface. Two critical parameters used in the proposed approach, the calculation sampling frequency and the interpolation sampling frequency, are then analyzed.Results show that for a 2.25 MHz rectangular transducer with the size of 5 mmxlO mm,a calculation sampling frequency of 1000 MHz and an interpolation sampling frequency of500 MHz can achieve superior performance while improving the computation efficiency 18 times than the direct solving.     

Array gain for a conformal acoustic vector sensor array:An experimental study

An acoustic vector sensor can measure the components of particle velocity and the acoustic pressure at the same point simultaneously, which provides a larger array gain against the ambient noise and a higher angular resolution than the omnidirectional pressure sensor. This paper presents an experimental study of array gain for a conformal acoustic vector sensor array in a practical environment. First, the manifold vector is calculated using the real measured data so that the effects of array mismatches can be minimized. Second, an optimal beamformer with a specific spatial response on the basis of the stable directivity of the ambient noise is designed, which can effectively suppress the ambient noise.Experimental results show that this beamformer for the conformal acoustic vector sensor array provides good signal-tonoise ratio enhancement and is more advantageous than the delay-and-sum and minimum variance distortionless response beamformers.     

水声通信中基于信道辨识的盲turbo均衡方法

水声信道多途效应明显,造成接收信号存在严重的码间干扰(ISI,Intersymbol interference)。基于最小均方误差(MMSE,Minimum mean square error)准则的turbo均衡器级联了均衡和信道译码,能够有效去除ISI,并获得优良的性能。由于水声信道的时变性,传统MMSE-turbo均衡需要周期性的训练序列,以实现连续可靠的通信。训练序列虽然提高了通信的可靠性,但降低了信息的有效传输速率。因此,为提高通信效率,本文提出了一种盲turbo均衡方法,该方法通过引入新的盲信道辨识器来同时获得信道估计响应和已去除部分ISI的初步均衡输出信号,并为turbo均衡提供初始的响应参数和比特软信息。与水声通信中应用较多的盲判决反馈均衡器(DFE,Decision feedback equalizer)相比,海上实验结果证明本文提出的盲turbo均衡方法抗信道多途衰落的能力较强,并且与传统MMSE-turbo均衡相比无需训练序列,因此提高了信息的有效传输速率。     

Prediction of noise inside an acoustic cavity of elongated shape using statistical energy analyses with spatial decay consideration

In this paper, Statistical Energy Analysis (SEA) is used to predict the interior noise of an acoustic cavity of elongated shape. The disadvantage of the conventional SEA method, which quantifies the response in terms of the energy averaged over each subsystem, is overcome by introducing a one-dimensional spatial decay relation, through which information about the acoustic energy variation in the elongated direction is taken into account. The modified SEA is experimentally validated using a 1:5 scaled space station prototype, having the longitudinal dimension much larger than the cross-sectional dimension. It is also compared with a model reported in the literature. It is shown that, in the region where the acoustic pressure level decays at a constant rate, the two models agree well with each other and are capable of estimating the acoustic pressure variation along the space station cabin. However, near the end walls where the decay rate of the acoustic pressure level is not constant, the proposed model provides better accuracy.     

Automatic detection of bioacoustics impulses based on kurtosis under weak signal to noise ratio

Passive acoustic monitoring (PAM) of marine mammal vocalizations has been efficiently used in a wide set of applications ranging from marine wildlife surveys to risk mitigation of military sonar emissions. The primary use of PAM is for detecting bioemissions, a good proportion of which are impulse sounds or clicks. A click detection algorithm based on kurtosis estimation is proposed as a general automatic click detector. The detector works under the assumption that click trains are embedded in stochastic but Gaussian noise. Under this assumption, kurtosis is used as a statistical test for detection. The algorithm explores acoustic sequences with the optimal frequency bandwidth for focusing on impulse sounds. The detector is successfully applied to field observations, and operates under weak signal to noise ratios and in presence of stochastic background noise. The algorithm adapts to varying click center frequency. Kurtosis appears as a promising approach to detect click trains, alone or in combination with other clicks detector, and to isolate individual clicks.