近场波束形成的约束最小平方和法

文章研究了近场情况下的波束形成问题。对于任意结构阵列,提出了近场波束形成的一种约束最小平方和法,用近场信号模型来设计近场波束。该方法借助于优化工具箱,在对波束旁瓣施加约束的条件下,使波束主瓣的误差平方和最小。该方法在主瓣区域和旁瓣区域都能得到满足设计要求的波束性能,可以方便地控制期望响应,简单易行。基于32元均匀离散圆阵的仿真设计结果表明了文中方法的有效性。     

基于凸优化方法的期望主瓣幅度响应波束图设计

将发射阵的期望主瓣幅度响应波束设计转化为凸优化问题,并利用cvx工具箱求解最优加权。首先,根据期望波束的主瓣范围,将空间区域分为主瓣区域和旁瓣区域,再在主瓣区域内将设计波束和期望波束之差的2-范数最小化,并将设计波束图旁瓣级控制在期望值之下。最后,利用cvx工具箱对该凸优化问题进行求解,获得满足要求的设计波束图。通过计算机仿真对所提波束图设计方法的有效性进行了验证。     

旁瓣约束方向不变恒定束宽波束自适应综合

将自适应加权方法应用于波束图的主瓣控制研究,提出了方向不变恒定束宽波束形成算法。在波束图旁瓣区域引入若干虚拟干扰源,自适应调整干扰强度,改变波束形状。以主瓣某一指向的波束宽度为参考,以每次迭代后波束图最低旁瓣峰值与主瓣峰值的差作为下次迭代运算的预设主旁瓣比,在保持主瓣区域宽度不变的情况下,用自适应综合的方法获得不同主瓣指向上的最低均匀旁瓣。将该算法应用于均匀线阵和半球面阵,均在±60°的范围内得到了方向不变恒定束宽波束。研究表明,该算法不仅适用于均匀线列阵,也可用于任意结构阵列。     

球谐域二次Dolph-Chebyshev波束形成的设计

提出了一种球谐域二次Dolph-Chebyshev波束形成方法,该方法在常规波束形成方法和Dolph-Chebyshev波束形成方法的基础上通过平方Chebyshev多项式来计算波束形成的权值.在保持主瓣最大增益不变的情况下,通过平方Chebyshev多项式可以降低旁瓣的最大增益.因此,这种改进的波束形成在保证与前两者具有相同主瓣宽度的同时,能有效地限制旁瓣,提高声场分辨率.且在低频时具有更高的白噪声增益,从而提高球阵处理噪声的能力.仿真实验通过将这三种波束形成方法进行比较,验证了球谐域二次Dolph-Chebyshev波束形成方法的有效性.     

全设计频段束宽恒定的低旁瓣时域波束形成

为了在空域无失真地接收宽带信号和有效地抑制环境干扰,提出了全设计频段束宽恒定的低旁瓣时域波束形成方法。首先,把宽带信号分为几个子带,应用半定规划的优化方法设计这些子带中心频率上的加权,使所形成的波束主瓣与设计带宽中最低频率上的波束相同,同时约束其具有低旁瓣特征。然后,设计FIR滤波器拟合这些离散频率点上恒定束宽加权所表示的幅相加权。显然,设计得到的FIR滤波器的幅相响应给出了全设计频段上的幅相加权。最后应用该方法,针对阵元具有方向性的12元均匀离散圆弧阵,设计覆盖一个倍频程的低旁瓣时域恒定束宽波束形成器,并使用线性调频信号作为测试信号,验证了所提方法的有效性。     

成像声呐中聚焦波束形成技术研究及实现

介绍了聚焦波束形成的基本原理,分析了一种基于半圆阵的相位补偿方法。通过Matlab仿真得出聚焦波束形成的波束图,相比远场方法,波束宽度减小,旁瓣得到抑制。设计了一种基于FPGA的数字聚焦波束形成器的实时处理结构,使用8组加权系数即可完成成像声呐近场范围内分辨力的改进。通过乒乓操作和并行结构提高处理速度,实时产生72个波束。实验结果表明,所设计的聚焦波束形成器使某型成像声呐近场分辨力得到了提高。     

二维MUSIC近场被动定位方法

介绍了一种高精度的近场被动定位方法——二维MUSIC被动定位方法。它是一种在距离和方位上进行二维联合搜索的高精度被动定位方法。将MUSIC（多重信号分类）算法与近场聚焦波束形成方法相结合,能大大提高对近场目标的定位精度。先推导了基于均匀线列阵的二维MUSIC近场被动定位方法的定位原理,通过仿真比较了二维MUSIC被动定位方法与常规聚焦波束形成的定位性能,仿真表明,二维MUSIC被动定位方法的定位性能要明显高于常规的聚焦波束形成被动定位方法。并仿真了该方法在不同的阵元间距以及不同的目标距离时的定位性能,验证了该方法的可行性和有效性。     

不等间距线列阵波束形成

介绍了不等间距波束形成的基本原理和分析方法,分别计算了10元、16元和32元线列阵阵元不等间距分布时的波束图。采用z变换的方法,将等间距幅度加权的波束形成的幅值分布转化为不等间距波束形成的距离分布。仿真结果表明该方法对于第一、第二旁瓣的抑制较好,并且主瓣并未明显展宽。     

矢量水听器阵列虚拟阵元波束形成

在信号频率一定的情况下,常规波束形成方法需要通过增大基阵孔径来提高目标方位的估计精度,但这会受到实际工程应用的限制。在研究矢量水听器波束形成的基础上,提出了基于Taylor级数展开的虚拟阵元波束形成方法。该方法针对有限尺度双十字阵型的矢量水听器基阵,根据已知阵元接收的数据,运用Taylor级数展开方法估计虚拟阵元上的接收数据,使基阵孔径在虚拟意义上得以扩大。从而改善了阵列的波束性能,窄化主瓣和抑制旁瓣,实现了空间分辨率的提高。仿真和试验数据结果证明了该方法的有效性。     

基于ac-MVDR高分辨算法的被动时反近场声源定位方法研究

为了克服多途效应对水下复杂结构声源的近场定位精度的影响,研究了基于幅度相位联合补偿的MVDR(ac-MVDR)算法的被动时反近场声源定位方法,该方法一方面将幅度补偿引入到MVDR近场聚焦波束形成中,充分利用声场的幅度相位信息重建声源平面上的噪声源空间位置分布,实现高分辨定位;另一方面可综合多途信息,对信号进行空时被动聚焦的同时,在信道输出总能量强的目标处实现选择性聚焦。通过仿真和实测数据的处理分析表明,该方法可有效的克服多途干扰,提高水下声源的近场定位精度。     

Numerical analysis of the radiated fields of ultrasonic transducers

A finite element model is implemented for simulating the radiated fields of both planar and curved transducers in acoustic media. The approach is based on general finite element analysis developed for solving the governing equations of elastic wave propagation. The distributions of the wave fields are presented for both the nearfield and farfield regions of the transducer. Three excitation pulses with the same center frequency but different bandwidths are examined and the accuracy is indicated by a comparison of the simulation results for the axial and transverse fields with the analytical results for continuous wave excitation.     

纳米声学及近场声成像技术

纳米声学是近年来迅速发展的新的学科领域,旨在亚微米和纳米尺度上来"听到"和"看到"我们尚未发现的物质世界。而近场声成像技术,像扫描探针声显微术(Scanning ProbeAcoustic Microscopy,SPAM)和压电响应力显微术(Piezoresponce Force Microscopy,PFM)等,不仅具有亚微米和纳米分辨力,而且能方便地对试样微区的表面形貌,材料的力学和电学等性质进行成像,是开展纳米声学研究的有效手段。文章结合实验结果,对SPAM和PFM等近场声成像技术作了简要介绍。     

Enhanced optical absorption in semiconductor nanoparticles enabled by nearfield dielectric scattering

The optical absorption of semiconducfing AgBr nanocubes is significantly increased by up to 5 times in the measured spectral range when they are bonded to the surface of dielectric SiO2 nanospheres through electrostatic interaction.The absorption enhancement factor depends on the wavelength and the size of the SiO2 nanoparticles (NPs).Finite-difference time-domain calculations provide the nearfield intensity mapping of a heterostructure that is composed of a AgBr nanocube in close contact with a SiO2 nanosphere.The electric-field distributions indicate the field enhancement near the SiOdAgBr interface due to light scattering and absorption enhancement in the AgBr nanocube,implying that the enhanced scattering nearfield increases the absorption cross section of the AgBr nanocube.The absorption cross-section spectra calculated using Mie theory agree with the experimental observations.This discovery sheds light on the utilization of dielectric spherical particles to increase the absorption in semiconductor NPs,thus improving the light-harvesting efficiency for solar-energy conversion.     

Optimal apodization design for medical ultrasound using constrained least squares part II: simulation results

In the first part of this work, we introduced a novel general ultrasound apodization design method using constrained least squares (CLS). The technique allows for the design of system spatial impulse responses with narrow mainlobes and low sidelobes. In the linear constrained least squares (LCLS) formulation, the energy of the point spread function (PSF) outside a certain mainlobe boundary was minimized while maintaining a peak gain at the focus. In the quadratic constrained least squares (QCLS) formulation, the energy of the PSF outside a certain boundary was minimized, and the energy of the PSF inside the boundary was held constant. In this paper, we present simulation results that demonstrate the application of the CLS methods to obtain optimal system responses. We investigate the stability of the CLS apodization design methods with respect to errors in the assumed wave propagation speed. We also present simulation results that implement the CLS design techniques to improve cystic resolution. According to novel performance metrics, our apodization profiles improve cystic resolution by 3 dB to 10 dB over conventional apodizations such as the flat, Hamming, and Nuttall windows. We also show results using the CLS techniques to improve conventional depth of field (DOF).     

一种新型的OFDM智能天线

根据自适应天线阵列理论，结合给定的参考波束的误差，引入虚拟干扰的概念，对目标波束图形状进行调整，提出一种新的可以应用于任意类型天线阵列的波束综舍算法．应用提出的新算法，在主辩和旁瓣位置都可以对波束进行有效的调节．最终获得阵列的最优权矢量，能够最小化目标波束图与参考波束图间的差异．理论分析与仿真结果表明，与现有的同类算法相比，该算法能更有效地获得与参考波束基本相符的波束．应用于OFDM智能天线系统时，对不同子载波频率上信号进行单独处理，利用该算法进行波束综合，能够在整个有效频段，使所有子载波上获得基本一致的阵列输出．     

A Fourier transform-based sidelobe reduction method in ultrasound imaging

Focusing is widely used to increase the resolution in medical ultrasound imaging systems. Focusing increases signal levels returning from the mainlobe direction and decreases those from sidelobe directions. The sidelobes, when not completely cancelled, deteriorate the resulting image resolution. This paper proposes a method of improving the resolution by scaling the received signal according to the ratio between the mainlobe and the sidelobe levels computed in the frequency domain by the use of Fourier transform. The proposed method is verified by computer simulation and experiment and is shown to be highly effective in narrowing the mainlobe width and decreasing the sidelobe levels at the same time.     

Deaberration of incoherent wavefront distortion: An approach towardinverse filtering

Techniques for the correction of ultrasonic wavefront distortion are compared using measured pulse transmission through human breast specimens. The measured data were obtained by recording the pulse signals on each element of a linear array that was moved in elevation to synthesize a two-dimensional aperture. The one-way point spread functions were reconstructed using matched filtering technique, phase conjugation, time-delay compensation, and backpropagation followed by phase conjugation, each with and without amplitude compression. Two measures of performance were compared, the mainlobe diffraction shape and the ratio of the energy outside the mainlobe to the energy inside the mainlobe. Matched filtering, which compensates the phase distortion but also increases the variation in the modulus, performs more poorly than other techniques with respect to both measures. Phase conjugation and time-delay compensation, which leave the magnitude of the wavefront unchanged, have similar beamwidths but phase conjugation is consistently superior with respect to energy ratio. The backpropagation method, which models wavefront distortion using a phase screen at a computed position between the source and aperture, is shown to perform better. The use of a novel amplitude compression that approaches inverse filtering improves the performance of the compensation techniques significantly. This is because inverse filtering optimizes image fidelity, in contrast, for example, to matched filtering, which optimizes SNR. With the amplitude compression method, the results of the one-way experiments show that the mainlobe shape can be recovered down to -30 dB     

Constrained optimization using a multipoint type chaotic Lagrangian method with a coupling structure

This article proposes a new constrained optimization method using a multipoint type chaotic Lagrangian method that utilizes chaotic search trajectories generated by Lagrangian gradient dynamics with a coupling structure. In the proposed method, multiple search points autonomously implement global search using the chaotic search trajectory generated by the coupled Lagrangian gradient dynamics. These points are advected to elite points (which are chosen by considering their objective function values and their feasibility) by the coupling in order to explore promising regions intensively. In this way, the proposed method successfully provides diversification and intensification for constrained optimization problems. The effectiveness of the proposed method is confirmed through application to various types of benchmark problem, including the coil spring design problem, the benchmark problems used in the special session on constrained real parameter optimization in CEC2006, and a high-dimensional and multi-peaked constrained optimization problem.