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。     

稳健协方差矩阵重构波束形成算法

针对信号导向向量失配以及接收数据协方差矩阵存在误差会导致传统的自适应波束形成器产生能损失的问题,提出了一种基于干扰加噪声协方差矩阵重构的稳健波束形成算法.该算法通过对信源来波角度范围进行Capon谱估计得出重构信源协方差矩阵,并通过特征分解以及子空间性质得出信源的导向向量,然后利用重构所得信源导向向量计算出信源功率以及...     

非一致性噪声条件下的特征空间波束形成

由于水声环境的复杂性,阵列的噪声分布可能是非一致性的。当阵元噪声功率各不相同时,阵列协方差矩阵特征分解得到的特征子空间与真实目标的特征子空间之间存在误差,导致特征子空间波束形成算法的性能衰减。文章提出了一种新的非一致性噪声条件下特征子空间的估计方法,将阵列协方差矩阵对角线置0,进行特征分解估计的特征子空间将不受阵元噪声非一致性的影响。将该方法应用到特征空间波束形成算法,提高了非一致性噪声条件下特征空间波束形成算法的方位分辨能力。仿真和实验结果验证了所提方法的可行性和有效性。     

基于子空间空间谱极值起伏特性加权的未知目标检测方法

针对同一频带中不等强度未知目标检测问题,依据不同目标辐射信号对各子空间空间谱贡献大小的不同,提出一种基于子空间空间谱极值起伏特性加权的未知目标检测方法。对线列阵接收信号协方差矩阵进行特征分解得到各子空间相应子矩阵;对各子矩阵进行波束形成得到各子空间空间谱,并利用各子空间空间谱极值起伏差异形成加权因子;利用该加权因子对归一化的各子空间空间谱进行加权统计得到最终空间谱,以提升弱目标子空间空间谱在最终空间谱的比例。理论分析、数值仿真和实测数据处理结果均证明了,该方法可以有效抑制了噪声子空间对目标子空间的干扰,增强了弱目标子空间空间谱在最终空间谱的比例,使其比例由原先的σ_(s,w)~2/(σ_(s,all)~2+σ_n~2/N变为1/L(N为阵元数,L为目标数,σ_(s,w)~2为弱目标能量,σ_(s,all)~2为目标信号能量和,σ_n~2背景噪声能量);减小了不同强度目标子空间空间谱幅值差异,使强、弱目标空间谱幅度值比由原先的σ_(s,s)~2/σ_(s,w)~2近似变为1(σ_(s,s)~2为强目标能量),改善了常规波束形成和常规子空间重构法对同一频带中不等强度未知目标的检测性能,并在同一波束图中,同时对不等强度未知目标实现有效检测。     

一种基于时空瞬时稳定性的方位谱估计方法

针对基于子空间分解的空间方位谱估计方法稳健性问题,提出了一种基于时空瞬时稳定的方位谱估计方法。该方法首先依据空间目标信号时域瞬时稳定性,将协方差矩阵频域求取过程转换为经相参补偿的复域求取,降低空间数据稳定性对协方差矩阵估计的影响;然后依据各子空间输出方位谱峰值索引离散度差异,提取各子空间方位谱加权因子;最后根据加权因子实现对各子空间方位谱加权处理,降低背景噪声对最终合成空间方位谱影响。数值仿真及实测数据处理结果表明,相比子空间分解方法,在不损失空间分辨率情况下,能够有效降低空间数据稳定性对协方差矩阵估计产生的影响,在无目标先验信息情况下,对最低信噪比的需求得到了6 dB以上的降低。     

空域预滤波的稳健自适应波束形成方法

假定的期望信号方向向量与真实信号方向向量存在误差时,常规自适应波束形成性能将严重降低,针对该问题,提出了一种稳健的自适应波束形成算法.首先利用凹槽空域矩阵滤波器对基阵接收数据进行空域预滤波,消除协方差矩阵中的期望信号分量,然后重构协方差矩阵同时反变换到阵元域,再用重构的协方差矩阵形成自适应波束权向量.由于方法消除了期望信号分量的影响,极大地提高了自适应波束形成算法对系统误差的稳健性.计算机仿真验证了所提方法的有效性.     

基于协方差矩阵拟合的宽带无网格波达角估计

宽带波达角(Direction of Arrival,DOA)估计是声呐系统阵列信号处理中一个重要的研究方向。文章提出了一种基于相干子空间的改进稀疏与参数方法(Coherent Signal-subspace based Modified Sparse and Parameter Approach,C-MSPA),以实现高精度和高空间方位分辨能力的宽带DOA估计。算法利用聚焦矩阵将各子带上的采样协方差矩阵投影至聚焦频率上。完成聚焦后,文章基于频率选择的范德蒙分解理论对协方差矩阵拟合准则进行改进,使重构的协方差矩阵中包含的DOA信息严格限制在聚焦区域内,最终对重构的协方差矩阵进行范德蒙分解,得到DOA估计值。所提出的算法无需选取正则参数,同时避免了基不匹配问题。仿真和湖上实测数据分析结果表明,所提出的方法实现了高空间方位分辨能力且提高了DOA估计精度。     

三种宽带自适应波束形成器比较研究

1引言 在被动声呐系统中,目标信号通常是宽带的,因此必须利用DFT将信号分解成若干个窄带,然后再每个窄带上利用MVDR波束形成技术.对于MVDR波束形成算法而言,是将宽带分为若干个窄带,在每个窄带上求出协方差矩阵,再把每个窄带的结果能量相加得到宽带结果,这就假定了各个窄带互不相关,所示是非相干累积,非相干累积要损失信息.事实上,各个窄带并非完全不相关,因此我们将研究针对宽带信号的相干波束形成或空间能量谱估计.     

基于高阶次Cross Sensor处理的波束形成方法

为得到鲁棒性、高分辨波束形成,提出了一种高阶次Cross Sensor处理方法。该方法依据阵元间协方差矩阵同一斜对角线上不同元素具有相同相位差的特点,对协方差矩阵进行M阶次Cross Sensor处理后可虚拟出近(2~M-1)(N-1)个虚拟阵元(N为原始阵元数)。虚拟阵元的增加可扩大线阵有效孔径,降低波束形成主瓣宽度,提高方位分辨率。在Cross Sensor处理过程中,该方法对协方差矩阵同一斜对角线上不同元素进行了叠加运算,可进一步削弱噪声对波束形成的影响,提高了波束形成鲁棒性。数值仿真和海上试验结果表明,该方法既能有效降低波束形成主瓣宽度,提高方位分辨率,又可削弱方位历程图干扰背景。相比已有的逆波束形成,该方法具有较好的方位分辨率;相比已有的基于AR模型的高分辨逆波束形成,该方法对最低门限信噪比的要求较低,方位估计均方误差较小。     

特征波束形成及其模拟实验研究

本文论述一种新的波束形成方法——特征波束形成。对于完全空间相关的平面波信号,基阵协方差矩阵的特征值集中。但在许多实际情况下,波前只有部分相关,从而造成协方差矩阵的特征值分散。本文对完全相关和部分相关的波前信号都进行了处理,研究了DOA(信号到达方向)估计和产生瞬时波束输出波形的问题,并给出了计算机模拟实验结果。     

Implementation of broadband low-sidelobe beamforming in time domain

In modern active and passive sonar systems, broadband beamforming for acoustic arrays is widely used to suppress unwanted interference and to detect target signals of interest. A broadband low sidelobe beamforming scheme in time domain is proposed in this paper. The first step of this scheme is to delay the outputs of each element in the acoustic array by a tapped-delay-line (TDL) to accomplish the integer part of the time delay need to form a beam. Then, finite impulse response (FIR) digital filters are used to implement the fractional part of the time delay. The weighting coefficients for all array elements at different frequencies to realize the low sidelobe beams are also implemented with the FIR digital filters. Finally, the outputs of the digital filters are summed up to yield the time domain beam output. The design of low sidelobe beam pattern and that of the FIR digital filters are two crucial technical issues in this beamforming procedure. The low sidelobe beams of each sub-band are designed using the optimized beam synthesis approach based on the principle of MVDR beamforming. An improved adaptive approach are used for the design of FIR digital filters, and the design requirements of these filters were specified by the weights of low sidelobe beams of each sub-band over the broad frequency band. Results of computer simulation for a twelve-element arc array show that the beamforming scheme is very effective in forming low sidelobe broadband beam.     

An approach to multibeam covariance matrices for adaptive beamforming in ultrasonography

Medical ultrasound imaging systems are often based on transmitting, and recording the backscatter from, a series of focused broadband beams with overlapping coverage areas. When applying adaptive beamforming, a separate array covariance matrix for each image sample is usually formed. The data used to estimate any one of these covariance matrices is often limited to the recorded backscatter from a single transmitted beam, or that of some adjacent beams through additional focusing at reception. We propose to form, for each radial distance, a single covariance matrix covering all of the beams. The covariance matrix is estimated by combining the array samples after a sequenced time delay and phase shift. The time delay is identical to that performed in conventional delay-and-sum beamforming. The performance of the proposed approach in conjunction with the Capon beamformer is studied on both simulated data of scenes consisting of point targets and recorded ultrasound phantom data from a specially adapted commercial scanner. The results show that the proposed approach is more capable of resolving point targets and gives better defined cyst-like structures in speckle images compared with the conventional delay-and-sum approach. Furthermore, it shows both an increased robustness to noise and an increased ability to resolve point-like targets compared with the more traditional per-beam Capon beamformer.     

基于宽带稳健STMV波束形成的相关检测方法

采用一种宽带相关结合稳健自适应波束形成的检测方法,来提高弱目标在强干扰背景下的检测能力。宽带相关检测具有多目标方位分辨力高的优点,它基于常规波束形成得到,但由于其旁瓣级有限,存在强干扰时会导致其旁瓣区域的相关检测产生凹陷,因此直接影响弱目标的检测能力。通过分析常规宽带相关检测中目标旁瓣区域产生凹陷原因,利用宽带稳健自适应波束形成的干扰抑制能力,提出了一种将宽带稳健导向最小方差波（Steered Minimum Variance,STMV）与宽带相关检测结合的宽带检测新方法。该方法能有效消除相关检测中强干扰的影响,提高被动声呐的宽带检测性能。仿真和试验数据处理结果验证了该方法的有效性。     

Improved Hybrid Beamforming for mmWave Multi-User Massive MIMO

Massive multiple input multiple output (MIMO) has become essential for the increase of capacity as the millimeter-wave (mmWave) communication is considered. Also, hybrid beamforming systems have been studied since full-digital beamforming is impractical due to high cost and power consumption of the radio frequency (RF) chains. This paper proposes a hybrid beamforming scheme to improve the spectral efficiency for multi-user MIMO (MU-MIMO) systems. In a frequency selective fading environment, hybrid beamforming schemes suffer from performance degradation since the analog precoder performs the same precoding for all subcarriers. To mitigate performance degradation, this paper uses the average channel covariance matrix for all subcarriers and considers an iterative algorithm to design analog precoder using approximation techniques. The analog precoder is iteratively updated for each column until it converges. The proposed scheme can reduce errors in the approximating process of the overall spectral efficiency. This scheme can be applied to fully-connected and partially-connected structures. The simulation results show that spectral efficiency performance for the proposed scheme is better than the conventional schemes. The proposed scheme can achieve similar performance with full-digital beamforming by using a sufficiently large number of RF chains. Also, this paper shows that the proposed scheme outperforms other schemes in the frequency selective fading environment. This performance improvement can be achieved in both structures.     

滤波求和恒定束宽波束形成器设计

文章在深入分析连续阵的恒定束宽波束形成理论基础上，将阵元权系数等效为两级滤波器，第一级只该与基阵的孔径分布有关，第二级只与基阵的维数有关，给出了滤波器的设计方法。通过时域的滤波求和实现恒定束宽波束形成，避免了频域分解，降低了运算量。该方法可用于一维、二维和三维情况，并且适用于任意带宽。基于非均匀线阵的计算机仿真表明该方法的恒定束宽效果是令人满意的。     

稳健宽带自适应波束形成

文章提出了宽带自适应波束形成及稳健设计方法，建立了二次约束宽带波束形成稳健设计模型，从理论上解决了自适应阵观察方向的稳健设计问题，彻底抛弃了自适应阵需要时延补偿的环节。所建立的阵处理器能抵消40dB以上的干扰，工作带宽达到两个倍频程，在理论上为被动跟踪系统设计最佳宽带接收阵和阵处理器作了准备。     

一种改进的窄带信号降噪算法及其应用

提出一种改进的基于FFT窄带信号频域降噪算法,并研究了其在宽带信号分段滤波中的应用。针对传统的基于FFT窄带信号频域去噪方法的不足,首先估计出信号频率与量化频率点的偏离程度,然后对信号进行频移,使信号频率尽量接近量化频率点,再进行频域滤波,并对滤波后信号逆向频移恢复原信号。最后,采用这种基于频移的窄带信号降噪算法对宽带信号进行分段滤波处理。Matlab仿真表明,改进的算法在不同的频率段内性能,且明显优于传统的窄带信号频域去噪算法。     

Minimum Variance Beamforming Combined with Adaptive Coherence Weighting Applied to Medical Ultrasound Imaging

Currently, the nonadaptive delay-and-sum (DAS) beamformer is used in medical ultrasound imaging. However, due to its data-independent nature, DAS leads to images with limited resolution and contrast. In this paper, an adaptive minimum variance (MV)-based beamformer that combines the MV and coherence factor (CF) weighting is introduced and adapted to medical ultrasound imaging. MV-adaptive beamformers can improve the image quality in terms of resolution and sidelobes by suppressing off-axis signals, while keeping onaxis ones. In addition, CF weighting can improve contrast and sidelobes by emphasizing the in-phase signals and reducing the out-of-phase ones. Combining MV and CF weighting results in simultaneous improvement of imaging resolution and contrast, outperforming both DAS and MV beamformers. In addition, because of the power of CF in reducing the focusing errors, the proposed method presents satisfactory robustness against sound velocity inhomogeneities, outperforming the regularized MV beamformer. The excellent performance of the proposed beamforming approach is demonstrated by several simulated examples.