基于波束特征值分析的单双目标分辨技术方法

目标源数估计是基于特征值分析的高分辨谱估计技术中用来划分信号子空间和噪声子空间的关键步骤,现有的估计方法可以工作的最低信噪比大约在0dB左右。通过分析直线阵空间功率谱以及相关函数,给出了波束特征值的概念,并说明了波束特征值是由直线阵阵列流形所决定的,利用这一规律设计了波束特征值分辨波束内多目标的方法,可以在波束内分辨单、双目标,通过仿真分析说明可以在-10dB信噪比条件下工作。     

非相干与相干宽带信号高分辨方位估计方法的性能比较

通过仿真实验,对相干法和非相干法两种宽带高分辨波束形成的方法在性能上进行了比较。非相干法原理简单,易于实现,不需要对方位进行预估,但其分辨性能不如相干法,尤其是在当目标为相干信号时,非相干信号子空间法失效;相干法无论目标是否相干,均具有良好的分辨性能,特别是在低信噪比的条件下仍有一定的分辨能力。但是,它需要预估方位,当预估方位的误差较大时,相干法的性能急剧下降。     

基于正交匹配追踪算法的虚拟内插空间平滑DOA估计

针对正交匹配追踪(Orthogonal Matching Pursuit, OMP)算法在实际的波达方向(Direction of Arrival, DOA)估计应用中,测向分辨率较低且在相干信号环境下估计性能较差的问题,提出了一种虚拟阵列空间平滑DOA估计算法.以互质阵列为模型,通过在正交匹配追踪算法重构信号的基础上,使用虚拟内插的方法,构造均匀虚拟阵列,能够提高波达方向的估计精度,加入空间平滑算法,可以分辨相干信号.仿真结果表明,使用虚拟内插算法之后,空间谱图波峰更尖锐,且在信噪比为0 dB时,最高探测精度提高到2°,测向分辨率明显有所提升,同时加入空间平滑算法,可以有效地分辨相干信号,且在信噪比为0 dB时,最高探测精度提高为4.7°,验证了本文方法的有效性.     

基于恒定束宽波束输出的宽带相干源高分辨测向

本文给出了基于恒定束宽波束输出的宽带相干源分辨测向方法。计算机仿真表明本方法在降低运算量的同时,得到了优于相干信号子空间方法的方位估计性能。     

Research on the resolution in coherent sources using a MUSIC algorithm based on a dual vector-sensor array

研究了单矢量传感器多重信号分类(SV-MUSIC)算法对非相干源的分辨性能,推导了声源等功率情况下的信噪比分辨门限表达式.针对相干源的分辨问题,提出了一种基于空间平滑的二元矢量阵多重信号分类(DV-MUSIC)算法.仿真实验表明,SV-MUSIC算法对非相干源的分辨能力随着信噪比的增加而提高,在相应的判定准则下其结果与理论值基本吻合,而基于二元矢量传感器的DV-MUSIC算法能够实现相干源的分辨,其分辨性能曲线一定程度上类似于SV-MUSIC算法对非相干源的分辨性能曲线.     

一种基于广义互相关的水声直扩信号检测方法

直扩信号因其低功率谱密度特性使得检测十分困难,针对传统自相关法在低信噪比条件下检测性能急剧下降的问题,在分析了直扩信号自相关特性的前提下,提出一种基于广义互相关估计的直扩信号检测方法。首先对接收信号分段并对相邻信号段分别进行广义互相关估计,估计结果采用二阶矩非相干积累,提取相关峰获得检测统计量,将检测统计量与门限比较,判定信号是否存在。通过计算机仿真对比了改进自相关检测法与文中方法的检测性能,仿真结果表明：该方法相比于改进自相关检测法信噪比容限降低了5 dB,适用于低信噪比条件下的水声直扩信号检测。水池试验处理结果也验证了该方法的实用性。     

基于独立分量分析的波束空间波达方向估计算法

波束空间变换是波达方向估计算法中重要的预处理方法,该方法适合大阵列、小信号源数的场合,可以有效降低运算量。但是目前存在的波束空间类DOA估计算法大多需要预知信号的大致来向,同时对阵型有要求。为了解决上述问题,提出一种基于独立分量分析的波束空间DOA估计算法,使用独立分量分析方法获得波束形成变换矩阵。该算法对阵型无要求、无需知道信号来向并且具备较强的角度分辨能力。仿真实验验证了有效性。     

基于空间平滑的多波束测深声呐相干分布源方位估计

在多波束测深声呐的工作环境中,若海底反向散射信号不满足点源假设,方位估计精度将严重下降,而基于分布源模型的方位估计算法可以适应这种环境。目前,大多数分布源算法要求分布源之间不相干,有人提出采用Toeplitz方法估计相干分布源,但该方法精度不高并且忽略了角度扩展参数。为解决多波束测深声呐相干分布源的方位估计问题,提出了基于空间平滑的广义MUSIC方法,公式推导证明了算法的有效性,通过计算机仿真给出算法方位估计的精度以及不同信噪比条件下的性能,最后采用多波束测深系统的实验数据对算法进行了验证。     

可利用波束输出信息的高分辨方位估计算法

本文介绍一类可以利用波束输出信息实现高分辨方位估计的算法,这类算法由于所利用的波束输出比阵元个数少得多,计算量得以降低,计算机仿真结果表明,对常见声纳接收阵波束输出信息进行高分辨方位估计不仅是可行的,而且还可获得比常规阵元域高分辨算法更低的分辨信噪比门限。     

一种新的被动水声探测盲自适应波束形成器

提出了一种适合拖曳线列阵鱼雷自导系统的新的被动水声探测盲自适应波束形成器。介绍了该方法的设计思路和推导原理,然后通过计算机仿真验证了该方法的收敛性,并在不同信噪比、快拍数条件下与单独使用自适应MVDR波束形成器在角度估计方面做了对比。与单独使用盲源分离算法在源信号估计方面作了比较。最后证明该方法是一种稳健的、对信噪比变化不敏感的适合被动水声探测的新方法。     

杂乱背景中隐藏信号的三重相关检测技术

针对噪声中检测微弱信号难的问题,提出了一种实用的三重相关检测方法。该方法利用有信号和无信号噪声背景下,三重自相关集平均值的差异,排除噪声干扰,判断出信号的存在;采用似三维图形法计算峰值,实现杂乱背景中隐藏信号的检测,获得信号的确定位置。实验表明,采用三重相关峰值检测方法,在峰值信噪比为0.8702313、功率信噪比为-0.9257607dB时,可获得的信噪比改善约为23dB。在背景为随机噪声的二维图像序列中,也能判断和检测出微弱的点目标,为防伪机器识别提供了一种可供选择的方法。     

A digital beamformer for high-frequency annular arrays

Digital transmit and receive beamformers for a 45-MHz, 7-element annular array are described. The transmit beamformer produces 0- to 80-Vpp monocycle pulses with a timing error of less than +/-125 ps. Up to four adjustable transmit focal zones can be selected. The dynamic receive beamformer uses a variable frequency sampling technique in which the frequency of analog-to-digital conversion on each channel is adjusted as the signals are received. The variable frequency clock signals required to trigger analog-to-digital conversion are obtained using a pair of high-frequency field-programmable gate arrays and a precision quartz oscillator. The gate arrays are also used to sum the digitized signals. A maximum receive beamformer timing error of less than +/-900 ps was obtained on each channel. The performance of the combined transmit and receive beamformer was tested by imaging wire phantoms. Images of CD-1 mice were also generated. The system produced images with a dynamic range of 60 dB.     

ADC characterization based on singular value decomposition

A new method for ADC characterization, based on singular value decomposition, is introduced in this paper. Here, the singular values of the sampled data matrix, directly derived from the measured input data, are used to characterize the signal-to-noise ratio (SNR), and further to estimate the number of effective bits. Various input signals, such as single-tone, dual-tone, or multitone, can be used to obtain accurate estimation results. In addition, the new method avoids the difficulties and problems of the earlier characterization methods such as the sensitivity to the applied sinewave frequency and sampled data sizes, and the spectral leakage. Extensive simulations indicate that the proposed method provides excellent performance with single-tone, dual-tone, and multitone test signals. The proposed method also shows remarkable robustness over a truly wide SNR range: from 5 dB to 200 dB     

Heterodyning technique to improve performance of delta-sigma-based beamformers

Delta-sigma (DeltaSigma) modulators can implement a simpler digital ultrasound beamformer than can traditional architectures based on multi-bit analog-to-digital converters (A/D). The signal-to-noise ratio (SNR) of the DeltaSigma modulators, however, suffers from limited oversampling ratios. To improve the SNR of each channel, a mixing signal heterodynes narrowband signals to lower frequencies where the baseband DeltaSigma modulator performs better. Noise figure analyses are presented that illustrate the effectiveness of this technique in improving noise performance. Also, spectral Doppler and color flow simulations are presented that realistically emulate a 32 channel oversampled beamformer and compare these results with traditional and ideal systems.     

Ultrasonic imaging using a 5-MHz multilayer/single-layer hybrid array for increased signal-to-noise ratio

Conventional diagnostic ultrasound scanners are bulky and require significant amounts of electrical power during operation. Reducing the size, weight, and consumption of electrical power is made easier through the use of highly integrated compact transmit and receive electronics that may be incorporated in the transducer handle. This necessitates the use of low voltage transmitters and low power receive preamplifiers. Conventional scanners typically use approximately 100-V pulses during transmit; therefore, decreasing the transmit voltage to 15 V decreases the transmit sensitivity. Conventional receive electronics that are located at the scanner degrade the received signal-to-noise ratio (SNR) because the array element cannot efficiently drive the coaxial cable. Transmit sensitivity and received SNR can be radically improved using a multilayer/single-layer hybrid array making integration of electronics into the transducer handle more feasible. In this paper, we discuss the design, fabrication, and testing of a 5-MHz hybrid linear array. The hybrid array included 16 multilayer transmit elements (10 Omega impedance) and 24 single-layer receive elements at a half wavelength element pitch. Low voltage transmitters with an output resistance of 7 Omega and high impedance JFET preamplifiers using 15 V for biasing were located adjacent to the hybrid array in the transducer handle. The transmit sensitivity and received SNR of the hybrid array were compared with a conventional array using 50-Omega transmitters and receive preamplifiers at the scanner. The transmit sensitivity improved by 12.8 dB, and the received SNR improved by 7.8 dB, yielding an overall improvement of 20.6 dB, which compared well with predictions from the KLM model. Images of phantoms and in vivo images of the kidney obtained with the Siemens Model 1200 phased array system showed the increased SNR using the hybrid array. Estimates of penetration in tissue mimicking phantoms (alpha=0.5 dB/(cm MHz)) improved by 7 cm compared with the control.     

低信噪比环境下语音识别的鲁棒性方法研究

针对噪声环境下语音识别率急剧下降的问题，提出了一种基于语音时频域稀疏性原理的改进最小方差无畸变响应波束形成与改进维纳滤波结合的算法。该算法首先利用麦克风阵列语音信号的空间信息，通过基于时频掩蔽的改进最小方差无畸变响应波束形成器，增强目标声源方向的语音信号，抑制其他方向噪声的干扰，然后再使用改进的维纳滤波器去除残留噪声并提高语音可懂度，对增强后的语音信号提取梅尔频率倒谱系数作为特征参数，使用隐马尔可夫模型搭建语音识别系统。实验结果表明，该方法能够有效提高低信噪比环境下的语音识别率，具有较好的鲁棒性。     

Ultrasonic data compression via parameter estimation

Ultrasonic imaging in medical and industrial applications often requires a large amount of data collection. Consequently, it is desirable to use data compression techniques to reduce data and to facilitate the analysis and remote access of ultrasonic information. The precise data representation is paramount to the accurate analysis of the shape, size, and orientation of ultrasonic reflectors, as well as to the determination of the properties of the propagation path. In this study, a successive parameter estimation algorithm based on a modified version of the continuous wavelet transform (CWT) to compress and denoise ultrasonic signals is presented. It has been shown analytically that the CWT (i.e., time x frequency representation) yields an exact solution for the time-of-arrival and a biased solution for the center frequency. Consequently, a modified CWT (MCWT) based on the Gabor-Helstrom transform is introduced as a means to exactly estimate both time-of-arrival and center frequency of ultrasonic echoes. Furthermore, the MCWT also has been used to generate a phase x bandwidth representation of the ultrasonic echo. This representation allows the exact estimation of the phase and the bandwidth. The performance of this algorithm for data compression and signal analysis is studied using simulated and experimental ultrasonic signals. The successive parameter estimation algorithm achieves a data compression ratio of (1-5N/J), where J is the number of samples and N is the number of echoes in the signal. For a signal with 10 echoes and 2048 samples, a compression ratio of 96% is achieved with a signal-to-noise ratio (SNR) improvement above 20 dB. Furthermore, this algorithm performs robustly, yields accurate echo estimation, and results in SNR enhancements ranging from 10 to 60 dB for composite signals having SNR as low as -10 dB.     

Direct frequency counting with enhanced beat signal-to-noise ratio for absolute frequency measurement of a He-Ne/I2 laser at 633 nm

We obtain a heterodyne beat signal between an iodine-stabilized He-Ne laser at 633 nm and a low power comb mode of an optical frequency synthesizer with an enhanced signal-to-noise ratio (SNR) up to approximately 35 dB. This is accomplished by the adoption of a fiber coupler and the reduction of the shot noise induced by adjacent comb modes. SNR is improved more than 15 dB compared with that of conventional methods. We measure the absolute frequency of the He-Ne laser mentioned above directly utilizing the enhanced SNR. A quantitative analysis of SNR and some information on experimental criteria for the correct frequency counting are given.     

MVDR自适应波束形成技术在水声中的研究进展

高分辨波束形成器比常规波束形成具有更好的方位分辨力与干扰抑制能力。该波束形成器能够提高阵列输出信干噪比,从而提高声呐的探测性能。与多重信号分类、旋转不变子空间等方法相比,最小方差无畸变响应(Minimum Variance Distortionless Response,MVDR)波束形成器输出真实反映了观察方向的信号功率,同时可提供波束时间序列做后置处理,在水声阵列处理领域得到了快速发展和深入研究。对高分辨MVDR技术在水声阵列处理中的研究进展进行了回顾,重点介绍了其宽带处理、稳健性、运动补偿、解相干等国内外的研究热点和最新成果,同时给出其在各种水声阵列处理领域的应用前景。