Weight vector fluctuations in adaptive antenna arrays with nonlinear function in a feedback loop

We present the results of statistical analysis of adaptive antenna arrays with nonlinear function in the feedback loop, which are adjusted by an algorithm for recurrent inversion of a sample estimate of the input-signal correlation matrix with allowance for the weight-coefficient fluctuations. Using the perturbation-theory methods in the first (Born) approximation, we obtain expressions for the correlation function and power of the output signal of an adaptive antenna array and a formula for the covariance matrix of the weight-coefficient vector. The weight-coefficient fluctuations are shown to result in signal distortions at the antenna-array output and appearance of additional terms in the statistical characteristics of the antenna array. These terms are second-order infinitesimals with respect to adaptation coefficient. Fluctuations are first-order infinitesimals and, thus, are much greater in an antenna array without nonlinear function in the feedback loop. It is also shown that the weight-vector fluctuations are anisotropic, i.e., they are absent in the constraint direction, minimal in the interference direction, and maximal in all other directions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 2, pp. 164–172, February 2006.     

Spectral correlation characteristics of an adaptive Antenna array tuned by the Hebb algorithm with allowance for the weight-vector fluctuations

In this paper, we present the results of statistical analysis of the characteristics of an adaptive antenna array tuned by the Hebb algorithm with allowance for the weight-vector fluctuations in the case where signals with different levels of autocorrelatedness of the input-signal readouts arrive at the antenna array. Expressions for the correlation function of the output signal and the power spectral density at the antenna-array output are obtained. It is shown that fluctuations lead to distortions of the output signal of the adaptive antenna array and appearance of additional terms in the formulas for these characteristics.     

The Influence of Weight-Vector Fluctuations on Statistical Properties of an Adaptive Antenna Array with a Fast Recurrent Tuning Algorithm

We present the results of statistical analysis of an adaptive antenna array tuned in accordance with the fast recurrent algorithm of inversion of a sample estimate of the correlation matrix with allowance for weight-vector fluctuations. Using the perturbation-theory methods in the first (Born) approximation, we obtain the spectral-correlation characteristics of the output signal of an antenna array. Fluctuations are shown to result in signal distortions at the output of an adaptive antenna array. Comparing the obtained expressions with the corresponding formulas for the antenna array with a gradient tuning algorithm, we see that for equal convergence rates of the above algorithms, the output-signal distortions are more pronounced for the fast recurrent algorithm. It is also shown that the weight-vector fluctuations are anisotropic for the fast algorithm, i.e., they are absent in the restriction direction, minimal in the interference direction, and maximal in all other directions. The weight-vector fluctuations in an adaptive antenna array with a gradient tuning algorithm are isotropic in the restriction subspace, i.e., they are the same in all directions and are equal in magnitude to fluctuations in the interference direction for the fast algorithm. The weight-vector fluctuations are also absent in the restriction direction for the gradient tuning algorithm.     

Analysis of the operation of adaptive antenna arrays of communication systems maximizing the output signal-to-noise ratio

The statistical characteristics of adaptive antenna arrays of communication systems with amplitude and phase modulated signals are investigated theoretically and by means of computer simulation for Gaussian interference and noise. An adaptive array antenna is considered, in which the host signal is formed by a nonlinear noninertial transformation of the output array signal. It is shown that the optimum vector of weight coefficients, minimizing the mean square deviation between the output and host signals, leads at the same time to a maximum output signal-to-noise ratio.Nizhegorod State University. Translated from Izvestiya Vyssikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 3, pp. 340–350, March, 1994.     

Influence of fluctuations of the weight coefficients on the characteristics of adaptive antenna arrays

We analyze theoretically the influence of the weight-coefficient fluctuations on the characteristics of adaptive antenna arrays (AAAs) with gradient adjustment algorithms under the assumption that the useful and noise signals have different correlation times. We obtain expressions for the correlation function and the spectral power density of the AAA output signal, find the average radiation pattern (RP), and derive an expression for the correlation matrix of the weight-coefficient fluctuations. It is shown that the weight-coefficient fluctuations bring about distortions of the useful signal and decrease its output power compared with the case of no fluctuations. In the frequency spectrum, this “overcompensation” phenomenon results in the depression of the output signal spectrum of the antenna array in the frequency band where high-power noise exists. As an example, we calculate the spectral power density of the output signal for an AAA with single linear constraints and plot the fluctuation RP and the variance of the weight-coefficient fluctuations versus the noise arrival angle. N. I. Lobachevsky State University, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 1, pp. 83–92, September 2000.     

Analysis of the effect of various physical and technical factors on the efficiency of adaptive algorithms of processing of hydroacoustic signals

This paper presents results on evaluating the impact of a set of physical and technical factors on the effectiveness of classical and projection fast adaptive algorithms. The factors include the conditions of signal propagation, including multipath; the noise-signal situation; signal fluctuations due to scattering in the medium; the parameters of the receiving antenna, including the amplitude-phase value of element dispersion; and the parameters of the spectral signal analysis at the output of the antenna elements.     

Analysis of statistical characteristics of adaptive antenna array with independent reference-signal generation

The steady-state statistical characteristics of an adaptive antenna array whose weight-factor vector is adjusted by means of a least-mean-square algorithm are studied using a nonlinear inertialess transformation of the array output as a reference signal. The case of phase-shift keying of the desired signal with Gaussian interference and noise is examined. An optimal nonlinear reference-signal transformation is found for which mean-square error of signal estimation is minimized and the output signal-to-noise ratio is maximized. The theoretical results are confirmed by computer modeling.     

采用空间谱加权稀疏约束的稳健Capon波束形成方法

针对标准Capon波束形成器旁瓣级高以及期望信号方向存在误差时,性能会严重下降的问题,提出一种基于噪声及干扰空间谱加权的稀疏约束Capon波束形成方法。该方法利用波束响应本身具有的稀疏特性,以及在稀疏重构算法中l₁范数具有抑制较大值保护较小值的特性,使用噪声及干扰的空间谱对不同角度的波束响应进行加权,然后施以稀疏约束。仿真、水池试验与湖上试验中,与其它几种方法比较表明该方法可以降低波束形成器的旁瓣级,获得较深的零陷,提高了阵列输出信干噪比与抗导向向量误差的能力。      

自适应直达波抑制方法在前向散射宽带信号中的应用（英文）

The basic principles of target detection by forward acoustic scattering are presented.A direct blast suppression approach based on adaptive filtering(DBS-AF) is proposed to suppress the direct blast.The DBS-AF technique is extended to the linear frequency modulation(LFM) signal,where the envelope of the signal is regarded as a "general waveform" and imported into the adaptive filter.Application of the DBS-AF method to the data collected from a lake trial yields an output detection curve,in which the direct blast is mapped to the background while the acoustic field aberration is represented by the peak value fluctuation.The inhibitory effect in single hydrophone is approximately- 5 dB,and is then enhanced by exploiting the mean value removal approach as a preprocessing technique.The direct blast is further suppressed to a level of-10 dB by making full use of multichannel receptions.The main factors affecting the algorithm performance are as follows:the fluctuation degree of the receptions during the weighting vector training period and the power ratio of the forward scattered wave to the direct blast when the target is present.     

宽带恒定束宽波束形成器的设计与实现

本文提出了一种设计宽带恒定束宽波束形成器的方法,这对应用于海底探测和分类问题的声纳系统来说十分重要,对于给定频带宽度的信号,选择一定数量的频率点来表达其频率响应,对于一个给定的频点,可以通过现有方法导出满足恒定宽要求的加权矢量,例如对于一个线阵列,可用切比雪夫多项多得到其加权矢量,因此对于一个具有Ｎ个阵元的宽带阵,如果选择Ｍ个频率点,可以得到一个ＮＭ的加权矩阵,这个矩阵的每一行代表一个阵元的频率响     

Constrained adaptation for feedback cancellation in hearing aids.

In feedback cancellation in hearing aids, an adaptive filter is used to model the feedback path. The output of the adaptive filter is subtracted from the microphone signal to cancel the acoustic and mechanical feedback picked up by the microphone, thus allowing more gain in the hearing aid. In general, the feedback-cancellation filter adapts on the hearing-aid input signal, and signal cancellation and coloration artifacts can occur for a narrow-band input. In this paper, two procedures for LMS adaptation with a constraint on the magnitude of the adaptive weight vector are derived. The constraints greatly reduce the probability that the adaptive filter will cancel a narrow-band input. Simulation results are used to demonstrate the efficacy of the constrained adaptation.     

自适应直达波抑制方法在前向散射宽带信号中的应用

介绍了前向声散射目标探测的基本原理,并提出了基于自适应滤波的直达波抑制(DBS-AF)方法。将DBS-AF方法扩展到线性调频(LFM)信号中,把信号的包络视为一种广义"波形"并输入自适应滤波器。利用该方法分析了湖试数据,并得到了随观测时间变化的检测输出曲线。在曲线上直达波对应了曲线的背景输出而目标引起的接收声场畸变则对应了曲线的峰值起伏。单个通道的直达波抑制效果可以达到-5 dB左右;引入去均值化预处理能够增强直达波抑制效果,再利用多通道的接收信息,直达波被抑制到-10 dB以下。影响算法性能的主要因素是训练权值时段内接收信号的起伏和信直比。      

相位加权的矢量全聚焦超声阵列成像方法研究

常规矢量全聚焦成像仅利用检测信号的幅值信息,其成像质量受噪声、栅瓣和旁瓣等的影响大。综合利用检测信号的幅值和相位信息,本文提出两种相位加权的矢量全聚焦成像方法。首先,对全矩阵数据的相位信息进行分析,提取出两种相位特征参数:相位一致因子(Phase Coherence Factor:PCF)和极性一致因子(Sign Coherence Factor:SCF);然后,将全阵列划分为若干子阵列,分别利用两种相位特征参数对各个子阵列的成像幅值进行加权,求取加权幅值特征向量;最后,对所有子阵列的加权特征向量进行合成,得到两种加权的矢量全聚焦成像,并从中提取出裂纹方向及尺寸等特征信息。将三种矢量全聚焦成像方法应用于不同缺陷检测仿真及实验验证,结果表明,3种方法均可以实现缺陷方向识别与长度定量测量;但相位加权矢量全聚焦成像效果明显优于常规矢量全聚焦成像结果,其成像信噪比及分辨率更高,缺陷角度及长度测量结果更准确。本文研究工作为缺陷无损评价提供了可行的技术手段。      

Feedback method of the noise suppression in wave-mixing amplifiers based on non-linear materials with photorefractive response in a reflection grating configuration

In the photorefractive wave-mixing system, fluctuation in the signal beam intensity of the photorefractive output with a reflection grating has been analyzed by employing pump feedback method. In this method, fluctuations of the photorefractive wave-mixing process not only induce the intensity fluctuation of the mixing waves but also induce phase fluctuation of the mixing waves. Thus, the phase of the pump and signal beams at the output surface fluctuates in time around a mean value. Using such a positive feedback method of a pump beams, the relative fluctuation in the photorefractive output signal beam intensity with respect to its mean intensity can be minimized significantly without reducing its mean intensity. The factors that control the fluctuation in the signal beam intensity, such as the phase fluctuation of the output pump beam, absorption strength of the material and the feedback reflectivity of the cavity mirrors, on the relative fluctuation of the output signal intensity in the photorefractive wave-mixing systems have been studied in detail. It has been found that the fluctuation of the output signal intensity relative to its mean intensity in the photorefractive wave-mixing system can be suppressed to larger extent by taking lower value of feedback reflectivity of the cavity mirrors which could exist at a higher value of absorption strength of the photorefractive materials.     

Analysis of interference cancellation techniques in OFDMA systems employing adaptive antenna arrays

We analyze the problem of interference cancellation in OFDMA cellular communication systems employing adaptive antenna arrays at the receiver and operating in the channels with frequency-selective fading and interference from the neighboring base stations. This problem is very important for modern broadband cellular systems operating with a low coefficient of multiple use of frequencies. Under such conditions of the network deployment, the system efficiency is restricted by the level of intrachannel interference from the neighboring base stations transmitting in the same frequency channel. In this work, we analyze different methods for measuring the interference characteristics and using them for spatial processing of received signals. Five different methods for signal processing in an adaptive antenna array of the receiver are discussed. Each approach (reception method) has its own interference immunity and complexity degree. Comparative analysis of the interference immunity of the considered receivers has been performed for different frequency-channel loads and different numbers of interfering stations. Recommendations for using the considered receivers are given.     

Characteristics of sound signals on a quasi-stationary track between the Josephine and Ampere Seamounts

Experimental data are presented on the measurement of the cross-correlation coefficients and the differences in the arrival times (temporal spectra) of acoustic signals transmitted through a 210-km-long quasi-stationary track in the Atlantic Ocean. At the summit of the Josephine Seamount, a continuous pseudonoise signal was emitted in the one-third-octave band with a mean frequency of 630 Hz. The signal was received at the Ampere Seamount by a 40-m flexible vertical array with a directivity pattern in the form of a static fan of 32 lobes. The width of each lobe was ～3°. At a signal-to-noise ratio of about 3 dB and with an incomplete resolution of multipath signals by the array, the correlation coefficients reached a value of 0.48. The rms fluctuations of the differences in the arrival times varied from 1.8 to 3.1 ms depending on the signal arrival angles. The factors responsible for the low correlation coefficients and the relatively high fluctuations of the arrival time differences are discussed. Apparently, one of the factors is the presence of short-period internal waves.     

一种加权稀疏约束稳健Capon波束形成方法

为了克服标准Capon波束形成器旁瓣级高以及存在角度失配时性能急剧下降等缺点, 在稀疏约束Capon波束形成器的基础上, 提出了一种加权稀疏约束Capon波束形成器. 该方法利用波束响应的稀疏分布特性, 在标准Capon波束形成优化模型中加入旁瓣区域波束响应稀疏约束(l₁ 范数约束), 使旁瓣区域波束响应向量中非零元素的个数最小化; 通过阵列采样数据协方差矩阵特征分解得到信号子空间及噪声子空间, 利用信号子空间与噪声子空间的正交特性, 构造加权矩阵对稀疏约束进行加权, 使得稀疏重构时波束响应向量中不同角度对应的元素得到不同程度的约束. 该方法有效地抑制了Capon波束形成器的高旁瓣级, 加深了干扰方位零陷, 提高了阵列输出信干噪比. 由于稀疏约束, 波束响应向主瓣集中, 期望信号方向附近的波束响应都较大, 从而也提高了阵列抗导向矢量角度失配的能力. 数值仿真和水池实验验证了所提方法的有效性.     

基于半定规划和秩-1分解的稳健波束形成

针对标准Capon波束形成器在存在导向矢量失配时性能急剧下降问题, 提出了一种基于半定规划和秩-1分解的稳健波束形成算法. 该方法通过对实际导向矢量的估计提高自适应波束形成算法稳健性. 首先分别从干扰抑制和噪声抑制两个方面推导了新导向矢量应满足的约束条件, 并证明了利用矩阵滤波器构造约束条件的合理性; 构造了估计最优导向矢量的优化问题并将其转化为易于求解的松弛半定规划问题, 同时引入秩-1分解理论用于优化问题的求解. 仿真分析表明, 与目前较为常见的算法相比, 本文算法只需利用期望信号可能入射区间这一先验信息, 能获得更高输出信干噪比和功率估计精度. 关键词： 稳健自适应波束形成 半定规划 秩-1分解 导向矢量估计     

Antenna array in the sound field of an arctic waveguide

An antenna array placed in an arctic-type acoustic waveguide is considered. The guided sound field consists of a signal generated by a point source and an isotropic interfering noise produced by the ice cover. The array is operated in a specific regime: the output signals correspond to individual modes of the sound field. The signal received by the array is subjected to correlation processing with a finite averaging time. It is shown that, depending on the processing method, the signal-to-noise ratio can reach 40–60 dB for a realization duration of 1 min, and the quality of the signal detection is increased.     

Optimization of transmitting beam patterns of a conformal transducer array

A method is presented to calculate the driving-voltage weighting vector of a conformal array of underwater acoustic transmitting transducers to obtain a low-sidelobe beam pattern based on the measured receiving array manifold. The relationship among three quantities is given, which are, respectively, the radiated acoustic field, the measured receiving array manifold matrix and the driving-voltage weighting vector of the transducer array. Then, the driving-voltage weighting vector of the array is calculated using the optimization method to obtain a low-sidelobe transmitting beam pattern. At the frequency of 12.5 kHz, the receiving array manifold matrix of a 27-element conformal array is measured in an anechoic water tank. The driving-voltage weighting vector of the array is calculated using the proposed method. In addition, the computer simulation and experiments are carried out. The results agree well and show that the proposed method can obtain a low-sidelobe transmitting beam pattern and at the same time provide the largest amplitude of pressure in the axial direction when the maximum amplitude of the driving voltages of the array elements keeps unchanged.