截断阶梯Legendre函数加权阵列恒定束宽波束特性分析*

连续Legendre函数加权的恒定束宽换能器(CBT)阵列可以在一定频率范围内采用与频率无关的简单阵元权重实现恒定束宽的波束特性。然而常规CBT阵列需要为每个阵元配置相应的阵元权重,当阵元数量较大时会增加复杂度。该文分析了截断阶梯Legendre函数加权的面阵列,包括球面阵、柱面阵和平面阵,并且分别分析了Legendre函数球面加权方法和Legendre函数柱面加权方法应用于上述阵列的性能。结果表明,截断阶梯Legendre函数加权对波束方向的影响可以忽略,而主要影响波束宽度。对于面阵列,当采样阶梯不大于3 dB,截断点不大于-21 dB时,截断阶梯Legendre函数加权能很好近似连续Legendre函数加权。因此截断阶梯Legendre函数加权可以在保证CBT阵列性能的前提下有效减少阵元加权系数的数量。     

声矢量传感器阵宽带相干信号子空间最优波束形成

提出了基于矢量传感器阵的宽带相干信号子空间最优波束形成。建立了矢量传感器阵的宽带阵列信号模型,将宽带聚焦思想引入到了矢量传感器阵宽带处理,证明了矢量传感器阵与声压传感器阵宽带聚焦矩阵之间的关系,为矢量传感器阵宽带聚焦矩阵的构造提供了一种方法。推导了基于矢量传感器阵宽带最优波束形成的空间谱,通过空间重采样方法实现矢量传感器阵宽带聚焦,避免了对目标源方位的预估。通过仿真验证了矢量传感器阵具有更优越的性能:可以解宽带相干源;能够实现全空间无模糊定向;可以空间欠采样,因而在不增加计算量的基础上,能够提高阵列处理的性能;克服了传统声压传感器线阵端射方向上,目标方位估计性能下降的缺点。     

圆阵宽带恒定束宽波束形成的实验研究

为了检验“任意结构阵列宽带恒定束宽波束形成新方法”(声学学报, 2001;26(1):55—58)一文中提出的方法是否在实际系统中可行,我们针对24元均匀圆阵进行了湖上实验。基于实验数据,在覆盖一个倍频程带宽的8个频率点上形成了恒定束宽波束。实验圆阵的实测阵列流形和理论阵列流形存在较大误差,这表明该文采用的可适用于任意结构阵列的宽带恒定束宽波束形成方法具有良好的宽容性。     

基于Kaiser窗的矢量阵近场聚焦恒定束宽方法

本文针对宽带信号矢量阵近场聚焦的恒定束宽问题,提出了基于Kaiser窗函数改进的矢量阵近场聚焦恒定束宽方法。主要在常规Kaiser窗恒定束宽基础上,根据Kaiser窗的特点,使得窗参数随指数变化,提高了恒定束宽的计算精度及效果;利用束宽随频率的变化规律及窗函数本身特点,以低频率恒定束宽参数为先验信息,确定较高频率的恒定束宽参数,缩小了参数搜寻范围,提高了计算效率。仿真实验结果表明,本文建立的恒定束宽方法具有较好的效果。     

基于Kaiser窗函数的矢量阵近场聚焦恒定束宽方法

本文针对宽带信号矢量阵近场聚焦的恒定束宽问题,提出了基于Kaiser窗函数改进的矢量阵近场聚焦恒定束宽方法。主要在常规Kaiser窗恒定束宽基础上,根据Kaiser窗的特点,使得窗参数随指数变化,提高了恒定束宽的计算精度及效果;利用束宽随频率的变化规律及窗函数本身特点,以低频率恒定束宽参数为先验信息,确定较高频率的恒定束宽参数,缩小了参数搜寻范围,提高了计算效率。仿真实验结果表明本文建立的恒定束宽方法具有较好的效果。     

基于声压振速联合信息处理的声矢量阵相干信号子空间方法

为解决水下宽带源的远程测向问题,提出了一种基于声压P与振速V的互谱矩阵的声矢量阵相干信号子空间方法。与现有的将声矢量传感器的振速信息仅仅作为独立阵元来处理的声矢量阵测向方法不同,新方法完全基于声压与振速联合信息处理,充分利用了声矢量阵中P-V互谱的抗噪能力,能将相干信号子空间方法的宽带高分辨能力及去相干能力与声矢量阵的抗噪能力有机地结合起来,实现对宽带源的远程、高分辨方位估计。理论分析给出了基于P-V互谱矩阵的宽带聚焦原理和特征分解原理,以及信源数检测准则。基于湖试数据的仿真实验结果显示,采用3元声矢量阵,在信噪比为-10dB和观测时间为20s时,新方法方位估计的均方根误差约为5°,明显强于现有方法。     

Legendre函数加权的声纳宽频带恒定束宽波束特性分析*

为了处理水声信号,声纳阵列通常需要形成宽频带恒定束宽的波束。采用两种阵元加权方法分析二维圆弧形恒定束宽换能器声纳阵列:球面Legendre函数加权方法和柱面Legendre函数加权方法。分别对球面阵、柱面阵和平面阵这三种几何结构的声纳阵列进行分析,并且计算波束宽度和波束方向。结果表明,在宽频带范围内,除了柱面Legendre函数加权的球面阵之外,其余Legendre函数加权的声纳阵列均能利用简单的、不随频率变化的阵元权重和阵元延时,形成恒定束宽的波束,并且具有较小的旁瓣,此外波束方向与预设方向也较为一致。相对于其他恒定束宽波束形成方法,Legendre函数加权方法能利用较低的计算复杂度来实现良好的宽频带恒定束宽的波束特性。     

声矢量圆阵宽带相干目标MVDR方位估计*

针对宽带相干目标的远程探测问题，本文提出一种基于声压振速联合处理和矢量重构的声矢量圆阵MVDR波束形成方法。该方法利用相位模态变换技术，将声矢量圆阵变换为与信号频率无关的虚拟线阵，并构建虚拟线阵声压与组合振速的互协方差矩阵，利用声压与振速各分量间的空间相关性有效地抑制各向同性环境噪声；并对宽带相干信号的互协方差矩阵进行矢量重构，即将最大特征值对应的特征向量划分为相互重叠的子向量，从而构建前/后向Hermitian矩阵；最后，基于MVDR波束形成器实现宽带相干目标的方位估计。仿真计算和实验数据处理结果表明，该方法具较强的解相干能力和噪声抑制能力以及较高的方位估计性能。     

基于二阶锥规划的任意传感器阵列时域恒定束宽波束形成

提出了基于FIR滤波器的适用于任意几何形状和阵元方向性的传感器阵列的时域恒定束宽波束形成方法。首先将设计频带分成若干窄带,采用优化方法,通过对各窄带波束施加约束,设计出具有设定主瓣形状要求的各窄带波束。然后针对每个传感器,根据其各频率波束形成加权向量,采用约束优化方法,设计出具有要求幅度和相位响应的FIR滤波器。各滤波器输出相加即得到时域恒定束宽波束输出时间序列。波束图主瓣形状设计与期望频率响应FIR滤波器的设计问题都可以转化为二阶锥规划(Second-Order Cone programming)的形式,然后利用已有的内点方法(Interior-Point,Methods)求出其数值解。计算机仿真结果显示,设计出的各子带波束主瓣宽度比较接近,FIR滤波器设计精度高。湖上实验数据处理结果表明,该方法可以适用于实际水声系统,时域宽带波束能够满足恒定束宽要求。     

基于期望响应内插技术的宽带波束优化设计

在以往窄带波束优化设计研究工作的基础上,提出了一种基于期望响应内插技术的宽带波束优化设计方法,目的在于避免宽带问题中由于频带划分过细带来的过重的设计负担,并使之可应用于稳健波束优化设计当中。该方法首先在设计频段内选取几个频率点,在每个频率点上设计得到相应的窄带波束优化权系数,并且根据每个阵元在不同频率点上的权系数构造一个宽带期望响应函数;然后用基于期望响应内插的FIR滤波器设计方法设计合适的FIR滤波器组,以使整个频段上的宽带波束优化得以实现。采用这种方法可以避免宽带波束优化设计中繁重的设计负担,并且在对实际声呐基阵进行宽带波束优化设计时只需少数几个频率点的测量值即可获得好的效果。仿真设计结果和实验研究表明了本文方法的有效性。     

Design of broadband beamformers based on an expected response interpolation technique

A method for designing broadband beamformers based on an expected response interpolation technique was proposed, with the aim of reducing the heavy optimization burden in the broadband beamformer design problem. In the method, some typical frequencies in the designing frequency band are selected and the optimal array weights are designed on these typical frequencies, based on which a broadband expected response function is constructed. Then, FIR filters are designed to realize the broadband beamforming over the whole designing frequency band, via the interpolation technique which is utilized to the expected responses of the FIR filters. Broadband beamforming can be fulfilled by applying these filters to each of the sensors in the array. By using the proposed method, designers need not to divide the whole designing frequency band into narrow subbands so as to make the narrow band assumption valid, which not only reduces the heavy designing burden, but also releases the requirement for measurements at fine frequency grids in the robust beamformer design. Results of both computer simulations and watertank experiments verify the effectiveness of the proposed method.     

特定频率响应FIR滤波器的设计及其在水声中的应用

在水声信号处理中，有时普通滤波器不能够满足应用要求，需要利用具有任意的给定频率响应的滤波器，本文利用自适应方法设计具有给定频率响应的FIR滤波器，将将该方法设计的滤波器用于产生宽带信号的精确时延信号和进行时域宽带波速形成，使用效果较好。该类滤波器使用非常有效。而且计算量小，易于工程实现。     

宽带波束形成器的设计与实现（英文）

The designs and implementations of broadband beamformers are reviewed and discussed.Broadband beamformer includes discrete Fourier transform(DFT) implementation of a frequency-domain beamformer and finite impulse response(FIR) implementation of a timedomain beamformer.The designs of weights at each frequency bin for the DFT beamformer and that of FIR filters associated with each sensor for the FIR beamformer are both introduced.Performance analysis and comparisons for several typical approaches to them are presented.A unified approach,i.e.,multiple-constrained approach,to narrowband beamformer is provided and is extended to the FIR implementation of broadband case.The unified approaches include the existing individual approaches as their special cases,which leads to much more flexible designs.Design examples of broadband robust adaptive beamformer with constant mainlobe response and controlled sidelobes are given to illustrate the behavior.     

Broadband optimal contrast resolution beamforming

This paper proposes a novel receive beamformer architecture for broadband imaging systems that uses unique finite impulse response (FIR) filters on each channel. The conventional delay-and-sum (DAS) beamformer applies receive apodization by weighting the signal on each receive channel prior to beam summation. Our proposed FIR beamformer passes the focused receive radio frequency (RF) signals through multi-tap FIR filters on each receive channel prior to summation. The receive FIR filters are constructed to maximize the contrast resolution of the system’s spatial response. The broadband FIR beamformer produces spatial point spread functions (PSFs) with narrower mainlobe widths and lower sidelobe levels than spatial PSFs produced by the conventional DAS beamformer.We present simulation results showing that FIR filters of modest tap lengths (3-7) can yield marked improvement in image contrast and point resolution. Specifically we show that 7-tap FIR filters can reduce sidelobe and grating lobe energy by 30 dB and improve contrast resolution by as much as 20 dB compared to conventional apodization profiles. This improvement in contrast resolution comes at the expense of a decrease in beamformer sensitivity. We investigate the effects of phase aberration and show in simulation results that the multi-tap FIR beamformer outperforms the unaberrated DAS beamformer by 8-12 dB even in the presence of moderate aberration characterized by a root-mean-square strength of 28 ns and a full-width at half-maximum correlation length of 3.6 mm. We show experimental results wherein multi-tap FIR filters decrease sidelobe energy in the resulting 2D spatial response while achieving a narrow mainlobe. We also show results where the FIR beamformer improves the contrast to noise ratio (CNR) in simulated B-mode cyst images by more than 4 dB. Our algorithm has the potential to significantly improve ultrasound beamforming in any application where the system response is reasonably well characterized. Furthermore, this algorithm can be used to increase contrast and resolution in one-way beamforming systems such as acousto-optic and opto-acoustic imaging.     

线列阵甚宽频带恒定束宽的数字实现方法

线性嵌套阵可以很好地在很宽的频率（N个倍频程，N=7）上实现恒定束宽。这种方法设计简单，又节省线阵阵元。本文给出了线性嵌套阵具体的数字化实现方法，解决了FIR滤波器设计，滤波器时间延迟等问题。由于数字化实现过程中使用了很多组FIR滤波器，计算量很大，文章采用了DSP信号处理板实现数字化算法，利用DSP处理板可以多任务同时处理优势，解决了计算量大的问题。经过实验验证，在DSP上实现的数字化的线性嵌套阵宽带恒定束宽方法满足了工程上实时处理的需要。     

Nearfield broadband array design using a radially invariant modal expansion

This paper introduces an efficient parameterization for the nearfield broadband beamforming problem with a single parameter to focus the beamformer to a desired operating radius and another set of parameters to control the actual broadband beampattern shape. The parameterization is based on an orthogonal basis set of elementary beampatterns by which an arbitrary beampattern can be constructed. A set of elementary beamformers are then designed for each elementary beampattern and the desired beamformer is constructed by summing the elementary beamformers with frequency and source-array distance dependent weights. An important consequence of our result is that the beamformer can be factored into three levels of filtering: (i) beampattern independent elementary beamformers; (ii) beampattern shape dependent filters; and (iii) radial focusing filters where a single parameter can be adjusted to focus the array to a desired radial distance from the array origin. As an illustration the method is applied to the problem of producing a practical array design that achieves a frequency invariant beampattern over the frequency range of 1:10 (which is suitable for speech acquisition using a microphone array), and with the array focused either to farfield or nearfield where at the lowest frequency the radial distance to the source is only three wavelengths.     

时域宽带恒定束宽波束形成的实验研究

由于实际水声系统中的阵列模型往往与理想阵列模型失配,导致基于理想阵列模型设计的时域恒定束宽波束形成器所形成的波束图在实际应用中严重畸变,从而不能有效地抑制旁瓣干扰和无失真地接收宽带信号。本文利用水池实验说明阵列模型失配对基于理想阵列模型设计的时域恒定束宽波束形成器的影响,并提出一种有效的设计方法以减小这种影响,即通过实际测量,获得包含失配信息在内的实际阵列模型,并将该模型应用到时域恒定束宽波束器的设计当中。实验结果表明,基于实际阵列模型的设计可以有效地减小由于阵列模型失配带来的影响,这对设计实际系统可以使用的时域恒定束宽波束形成器具有重要的意义。     

Optimal design of FIR beamformer with frequency invariant patterns

Two approaches to the optimal design of FIR beamformers with frequency invariant patterns using second-order cone programming (SOCP) are proposed. The first approach is a two-step method, which is implemented via separately optimal array pattern synthesis and optimal FIR filters design. The array weights for each frequency bin within the working frequency band are designed to insure that the array patterns approximate the reference ones. And the FIR filter corresponding to each sensor is designed to insure that the frequency responses approximate the array weights. The second approach is a direct method, in which the beam response is expressed as a linear function of FIR filter tap weights and the filters are designed by jointly optimizing the spatial and frequency responses to achieve the desired array patterns. All the optimal design problems (array pattern synthesis, FIR filter design and joint optimization) are formulated as the SOCP, which can be solved efficiently using the well-developed interior-point methods. Results of computer simulations and lake experiment for a twelve-element semicircular array confirm satisfactory performance of the two approaches proposed in this paper.     

A weighting orthogonal method for constant beamwidth beamforming matrices

A weighting orthogonal method for constant beamwidth beamforming matrices is proposed. This method multiplies weighting factors to each orthogonal beamforming matrix corresponding to different frequency bins. The method proposed doesn't cause waveform aberration, and doesn't cause additional loss of array signal-to-noise ratio when the sources have uniform spectrum. The waveform aberration and additional loss of array signal-to-noise ratio can not be avoided simultaneously by ordinary orthogonal method. So we can get good detection and estimation performances at the same time by the weightmg method. Simulation results and water tank experiments are presented to confirm the conclusion above.     

Optimal array pattern synthesis for broadband arrays

Broadband beamformers with constant mainlobe response over the frequency of interest are desirable in many applications including underwater acoustics, ultrasonics, acoustic imaging and communications, and so on. Solutions to this problem have been presented for specific array geometry often requiring a larger number of sensors. And the array pattern synthesis error minimization is employed for the whole field of view, which leads to suboptimal designs. In this paper, a broadband array pattern synthesis approach to designing time-domain constant mainlobe response beamformer is proposed. By imposing constraints both on the mainlobe spatial response variation over frequency and on the sidelobes of the beamformer, several optimization criteria are presented and the corresponding convex second-order cone programming implementations are given. In this approach, no preliminary desired beampattern is required and the beam responses variation minimization is employed only in the mainlobe region and not in the sidelobe regions, which improves the beamformer mainlobe synthesis accuracy. Equally, one can obtain lower sidelobes at the same mainlobe synthesis accuracy. This approach is applicable to arrays with arbitrary geometry. Simulation and experimental results are presented to illustrate the effectiveness of this approach. Performance comparisons of the proposed beamformers and the existing beamformer are also provided.