浅海中舰船辐射噪声非均匀调制特征分析

真实海洋环境下,信道复杂的畸变作用使目标分类识别特征的实际应用效果并不理想,因此寻找能够在信道中稳定传播的目标分类识别特征显得尤为必要。文章提取出平均窄带包络相关和最佳窄带包络相关两个非均匀调制特征来衡量舰船噪声在频带上的调制分布差异,并构建浅海相干多途信道的射线模型,通过仿真实验分析非均匀调制特征的传播变化。结果表明,舰船噪声的这种非均匀调制特征在相干多途信道的传播过程中具有良好的稳定性,是目标分类识别的一种稳定特征。     

宽带多途信号仿真

文章介绍宽带多途信号软件仿真的实现方法。通过设计一个FIR滤波器拟合指定的目标信号功率谱,产生水下目标辐射噪声信号;再结合多途海洋信道模型,得到经过多途信道传输的接收信号。该方法实现简单,可以仿真具有一定频谱特性的水下运动目标的接收信号。所产生的仿真信号可用于声纳信号处理等方面的研究工作。     

MUSIC算法用于低信噪比下的包络分析

1引言 可将水卢目标螺旋桨噪卢建模为一个受窄带调制的宽带随机过程[1],其窄带调制信号(又称包络信号)是本文的分析对象.在声纳信号处理中把通过对接收的宽带信号进行解调(常采用平方律解调)以计算低频解调谱的算法称为DEMON(Detection of Envelope Modulation On Noise)分析,解调后的低频时域信号称为包络信号,其功率谱称为DEMON谱(又称包络谱).实际情况下,我们所期望的包络信号可能会淹没在噪声中而无法检测出来.     

基于MEEMD和GA-SVM的列车车轮多边形故障识别方法

根据列车车轮振动信号的非平稳特性,提出一种基于改进的集合经验模态分解(MEEMD)和遗传算法支持向量机(GA-SVM)的诊断方法,用于识别车轮多边形故障。该方法对车轮轴箱垂向振动信号进行MEEMD分解,依据各固有模态函数(IMF)分量的峭度值和能量值选取出主要IMF分量。利用希尔伯特变换求取主要IMF分量的包络谱,并计算包络谱熵。将包络谱熵值归一化后作为特征向量输入GA-SVM中进行训练和识别。对实测信号进行分析的结果表明该方法能有效识别出车轮多边形故障,识别准确率可达到95%。     

基于改进快速峭度图的轴承故障诊断方法研究

针对传统快速峭度图在轴承故障诊断中，通过计算滤波信号的包络峭度确定频带范围容易受到随机脉冲干扰的问题，提出一种基于改进快速峭度图的轴承故障诊断方法。首先运用自相关算法、改进谱幅值调制(improved spectral amplitude modulation, ISAM)算法对包络信号进行处理，获取修正信号并以其频谱峭度衡量冲击响应的强度；然后以修正信号谱峭度代替传统快速峭度图中的包络峭度，得到可降低各频带随机干扰的改进峭度图；最后通过对3组信噪比不同的仿真信号、2组轴承试验信号的平方包络谱故障诊断情况进行对比分析，验证该方法的有效性和稳定性。通过与原始快速峭度图、功率谱峭度图、ISAM法3种方法对比，验证该方法的优越性。对比结果表明，所提方法可有效降低随机脉冲的影响，实现轴承故障特征频率的准确提取。     

一种改进的峭度图方法及其在复杂干扰下轴承故障诊断中的应用

快速峭度图是一种常用的滚动轴承故障诊断方法,但由于峭度指标对冲击过于敏感,在干扰较复杂的工况中,该方法往往无法正确识别出最优的共振频带进行包络解调。然而,解调信号的包络谱对噪声具有一定的免疫能力,而且包络谱中通常会清晰的出现故障特征频率及其倍频成分,呈现出典型的周期性脉冲特点。因此,提出应用相关峭度定量地刻画窄带信号的包络谱幅值,即以频域相关峭度值生成峭度图,用于最优频带的自适应地识别。同时,基于相关峭度的指向性,可以将该方法应用于轴承的复合故障诊断。最后通过实验分析,验证了该方法对轴承微弱故障和复合故障诊断的有效性。     

基于子频带谱峭度平均的快速谱峭度图算法改进

快速谱峭度图(Fast Kurtogram)算法具有能自适应选取共振解调频带并实现包络解调提取的优点,在滚动轴承包络分析中有广阔的应用前景,但其在实际应用中,若被采集信号中包含有较高峰值的脉冲干扰时,将可能导致谱峭度图的自适应共振带确定失效,最终导致无法获得包含有效滚动轴承的故障特征信息的包络信号。为解决快速谱峭度图算法的上述不稳定问题,本文提出了一种基于子频带谱峭度平均的改进快速谱峭度图算法,其可有效消除或削弱脉冲干扰成分对谱峭度图结果的影响,提高了共振解调频带确定的鲁棒性,实现了基于快速谱峭度图算法的滚动轴承故障特征准确提取。仿真和试验结果验证了本方法的有效性。     

基于VMD和窄带包络相关的船舶辐射噪声改进DEMON分析

船舶辐射噪声的包络谱中蕴含着轴频和桨叶数等船舶固有特征信息,对船舶目标识别具有重要意义。为了提高船舶辐射噪声包络谱解调性能,提出了基于变分模态分解(Variational Mode Decomposition, VMD)和窄带包络相关的改进DEMON分析方法。首先利用VMD算法代替传统带通滤波器,将船舶辐射噪声信号分解为若干个子带;然后对各子带进行希尔伯特(Hilbert)检波并计算平均窄带包络相关系数,用于衡量信号的包络调制在频域上的非均匀性;最后提取各子带信号包络谱并按照平均窄带包络相关系数进行加权融合,从而得出宽带噪声信号的包络谱。利用该方法对实测不同类型和不同航速船舶辐射噪声信号进行了处理,结果均表明所提方法能有效提高包络谱解调效果,较传统方法更为有效。     

基于多天线分集接收的信号调制识别

针对多径衰落的信道,研究了基于多天线分集接收技术的信号调制识别,分析设计了基于多天线分集接收调制识别的分类器。计算机仿真实验结果表明的,对于多衰落信道,基于多天线分集接收调制识别可以提高识别参数估计精度,能有效地降低调制识别对观察数据长度和信噪比的要求,提高了调制识别的正确率。     

基于双树复小波包峭度图的轴承故障诊断研究

针对传统包络谱和峭度图分析技术的缺陷，提出了一种基于双树复小波包峭度图的轴承故障诊断方法。该方法综合利用了双树复小波包变换和峭度图分析技术，克服了原峭度图方法只采用FIR和短时傅立叶变换滤波器的缺点，提高了从强噪声环境中提取瞬态冲击特征的能力。首先利用双树复小波包变换，将振动信号分解成不同频带的分量，然后计算各小波分量的谱峭度，再利用谱峭度的滤波器作用，计算最大峭度值对应分量信号的包络谱，根据包络谱就可识别齿轮箱轴承的故障部位和类型。齿轮箱轴承故障振动实验信号的研究结果表明：该方法不仅提高了信噪比和频带选择的正确性，而且能有效地识别轴承的故障。     

Analysing the behaviour of partially coherent divergent Gaussian beams propagating through oceanic turbulence

Theoretical study of propagation behaviour of partially coherent divergent Gaussian beams through oceanic turbulence has been performed. Based on the previously developed knowledge of propagation of a partially coherent beam in atmosphere, the spatial power spectrum of the refractive index of ocean water, extended Huygens–Fresnel principle and the unified theory of coherence and polarization, analytical formulas for cross-spectral density matrix elements are derived. The analytical formulas for intensity distribution, beam width and spectral degree of coherence are determined by using cross-spectral density matrix elements. Then, the effects of some source factors and turbulent ocean parameters on statistical properties of divergent Gaussian beam propagating through turbulent water are analysed. It is found that beam’s statistical propagation behaviour is affected by both environmental and source parameters variations.     

Experimental demonstration of gray-scale sparse modulation codes in volume holographic storage

We discuss experimental results of a versatile nonbinary modulation and channel code appropriatefor two-dimentional page-oriented holographic memories. An enumerative permutation code is used to provide a modulation code that permits a simple maximum-likelihood detection scheme. Experimental results from the IBM Demon testbed are used to characterize the performance and feasibility of the proposed modulation and channel codes. A reverse coding technique is introduced to combat the effects of error propagation on the modulation-code performance. We find experimentally that level-3 pixels achieve the beet practical result, offering an 11-35% improvement in capacity and a 12% increase in readout rate as compared with local binary thresholding techniques.     

Non-line-of-sight identification in ultra-wideband systems based on received signal statistics

Non-line-of-sight (NLOS) propagation can severely degrade the reliability of communication and localisation accuracy in indoor ultra-wideband (UWB) 'location-aware' networks. Link adaptation and NLOS bias mitigation techniques have respectively been proposed to alleviate these effects, but implicitly rely on the ability to accurately distinguish between LOS and NLOS propagation scenarios. A statistical NLOS identification technique based on the hypothesis-testing of received signal parameters in UWB propagation channels is discussed. In contrast to narrowband and wideband signals, UWB signals possess higher temporal resolution and robustness to multipath fading. We show that these characteristics result in differences in the statistics of (a) the time-of-arrival (TOA), (b) the received signal strength (RSS) and (c) the root-mean-squared delay spread (RDS) of the received signals, between LOS and NLOS propagation scenarios, which can be exploited for accurate channel identification. We statistically characterise the ability of TOA, RSS and RDS estimates to distinguish between LOS and NLOS propagation based on an extensive indoor measurement campaign. Our measurement results suggest that the RDS of UWB signals can, even in isolation and without complete statistical information, serve as a robust and computationally efficient indicator of the LOS/NLOS nature of propagation. Finally, we demonstrate the efficacy of the discussed NLOS identification method in a location- tracking application based on indoor UWB measurements.     

Investigation of non-specular scattering by comparing directional channel characteristics from microcell measurement and simulation

Channel characteristics from microcell measurement and simulation are compared. In microcell environment, the dominant propagation mechanisms are not only from reflections and diffractions from buildings but can also come from non-specular scatterings. Propagation prediction tools such as ray-tracing algorithms which calculate only reflections and diffractions are hence not enough to completely predict the channel. To address this issue, an implementation called polygon meshed physical optics (polygon meshed PO) is used. Polygon meshed PO approximates the traditional PO by dividing the scatterer surface into triangular meshes to reduce the surface integral (for calculating the scattering field) into summation of mesh areas. This makes it flexible even for irregularly shaped objects. As PO utilises the whole illuminated scatterer surface to calculate the scattering field, it is not limited to specular directions and can be used to compute the contribution of non-specular scattering. To know the actual propagation channel, wideband measurement was performed inside a university campus. With antenna arrays on both the transmit and receive sides, wideband double-directional channel characteristics such as azimuth-delay power spectrum can be obtained after applying the conventional beamforming. Comparison of measurement and simulation results reveals that small metallic objects cause non-specular scattering and affect the propagation channel. These objects can be predicted by the proposed polygon meshed PO in cooperation with ray tracing.     

纠错编码技术在水声跳频通信中的性能研究

将水声跳频通信系统与纠错编码技术相结合进行了研究,针对复杂多变、强多途和起伏干扰的浅水水声信道,建立了基于BELLHOP射线模型的时变衰落水声信道模型,该信道模型建立起了信道特性与海洋环境参数之间的关系,在海洋环境参数给定的条件下可以精确仿真信道。在此基础上通过系统仿真的方法研究了卷积码、RS码和串行级联码三种纠错编码方案在水声跳频通信系统中的性能,给出了两种典型信道下的仿真结果并进行了对比分析,结果表明串行级联码具有更强的纠错能力,比较适合复杂多变的水声衰落信道。     

基于ADCP测量的海浪方向谱研究

海洋方向谱的研究对于海洋预报、海上建设等人类海洋活动有重要的意义。介绍了一种利用声学多普勒流速剖面仪(Acoustic Doppler Current Profiler, ADCP)测量海浪方向谱的方法。通过计算ADCP测量的流速之间的交叉谱,利用最大似然算法可以得到海浪的方向谱。给出了利用 ADCP 估计方向谱的算法流程,进行了仿真,结果表明所提出的方法可以较准确地估计海浪方向谱。     

Doppler spectrum of the multipath fading channel in mobile satellite systems with directional terminal antennas

A new approach for determining the Doppler spectrum of narrowband multipath channels by assuming a three-dimensional propagation model is proposed. The new derivation methodology is first applied to determine the Doppler spectrum of a Rayleigh channel, when an omni-directional mobile-terminal antenna is assumed. The obtained results are shown to agree with the existing analytical results, which are available for particular assumptions regarding the angular distribution of the received multipath power. The main contribution is the derivation of the Doppler spectrum of the mobile satellite channel, where directive tracking terminal antennas are assumed. The results, which are dependent on the antenna's beamwidth and bearing, apply to emerging types of commercial vehicular satellite systems.     

Analysis of multimode effects in the free-propagation region of a silicon-on-insulator-based arrayed-waveguide grating demultiplexer

Multimode effects in the free-propagation regions (FPRs) of an arrayed-waveguide grating (AWG) demultiplexer based on silicon-on-insulator are considered. Some undesired multimode effects, such as the increase of the insertion loss and the cross talk, are studied by use of a method of three-dimensional guided-mode propagation analysis. It is found that the multimode effects for the edge channels are more serious than those for the central channel. For an AWG demultiplexer with a small channel number, the multimode effects can be minimized by choosing appropriate FPR parameters such as the length and the thickness of the FPR. The coupling coefficient between the FPR and an arrayed waveguide is sensitive to the thickness of the FPR.     

Analytical solution of radiative transfer in the coupled atmosphere-ocean system with a rough surface

Using the computationally efficient discrete-ordinate method, we present an analytical solution for radiative transfer in the coupled atmosphere-ocean system with a rough air-water interface. The theoretical formulations of the radiative transfer equation and solution are described. The effects of surface roughness on the radiation field in the atmosphere and ocean are studied and compared with satellite and surface measurements. The results show that ocean surface roughness has significant effects on the upwelling radiation in the atmosphere and the downwelling radiation in the ocean. As wind speed increases, the angular domain of sunglint broadens, the surface albedo decreases, and the transmission to the ocean increases. The downward radiance field in the upper ocean is highly anisotropic, but this anisotropy decreases rapidly as surface wind increases and as ocean depth increases. The effects of surface roughness on radiation also depend greatly on both wavelength and angle of incidence (i.e., solar elevation); these effects are significantly smaller throughout the spectrum at high Sun. The model-observation discrepancies may indicate that the Cox-Munk surface roughness model is not sufficient for high wind conditions.     

基于卷积神经网络的超声金属材料辨识

传统的金属材料辨识方法会给被检测样品带来一定程度的损伤。文章通过采集金属材料的超声回波时域信号,采用短时傅里叶变换对其进行时频分析,得到包含金属材料微观组织信息的超声时频谱。将目标样品的超声时频谱预处理后作为训练样本,输入到构建好的卷积神经网络中进行训练。然后采集目标样品以及干扰样品的超声频谱图,分别将其输入网络进行辨识。结果表明,神经网络在训练时收敛较快,损失函数在迭代200次后收敛,在经过100次迭代后训练集准确率趋于100%。训练完成的网络模型记录着对应训练样本的特征信息,利用该训练好的网络对待测样本进行辨识,最终可实现超声金属材料辨识。