Covariance matrix estimation errors and diagonal loading inadaptive arrays

Simulations were used to investigate the effect of covariance matrix sample size on the system performance of adaptive arrays using the sample matrix inversion (SMI) algorithm. Inadequate estimation of the covariance matrix results in adapted antenna patterns with high sidelobes and distorted mainbeams. A technique to reduce these effects by modifying the covariance matrix estimate is described from the point of view of eigenvector decomposition. This diagonal loading technique reduces the system nulling capability against low-level interference, but parametric studies show that it is an effective approach in many situations     

基于FPGA的自适应阵列协方差矩阵特征分解的实现

卫星导航抗干扰的过程中，对空间信号波达方向估计、干扰个数检测、最优权矢量的求解直接影响着导航接收机的抗干扰性能，而协方差矩阵的特征分解是这些算法实现的核心部分。根据自适应阵列天线获得的协方差矩阵的特性，基于双边并行Jacobi算法，实现了基于FPGA的协方差矩阵特征值和特征向量的求解，并通过在信号波达方向估计的应用进行了验证。另外，在实现的过程中对直接调用CORDIC IP核的方式进行了精度误差分析，并用一种双精度浮点的方式进行修正，提高了矩阵特征分解FPGA的实现精度，为导航抗干扰接收机性能的提升提供了有效的工程基础。     

一种基于协方差矩阵加权的阵元误差补偿STAP处理

机载多通道阵列雷达天线在工程实践中不可避免地存在各类阵元误差,所产生的通道失配问题会对空时二维自适应处理的性能造成大的影响。对存在阵元误差时的阵列信号模型进行了分析,提出了一种基于协方差矩阵加权（CMT）的阵元误差补偿空时自适应处理（STAP）方法,在工程应用中该加权矩阵可通过地面天线定标及校飞过程确定,通过对总干扰协方差矩阵估计的加权预处理,可将实际阵元误差对STAP性能的影响控制在测量误差的影响范围,最后通过仿真验证了算法的有效性。     

Proposals for Simplifying Envelope Normalization in Adaptive Antenna Arrays

In adaptive antenna arrays using control loops the processing is determined under certain conditions by the covariance matrix Bmn = E(f(v*m)vn)?n), where ?1,..., ?N are N-complex Gaussian noise video signals and f is some function of the phase of ?m. Half of the storages and real multiplications could be saved by taking only the real part of ?n. It will be shown that this can be done without changing the performance of the adaptive antenna array significantly.     

Estimating the Angles of Arrival of Multiple Plane Waves

The problem of estimating the angles of arrival of M plane waves incident simultaneously on a line array with L + 1 (L?M) sensors utilizing the special eigenstructure of the covariance matrix C of the signal plus noise at the output of the array is addressed. A polynomial D(z) with special properties is constructed from the eigenvectors of C, the zeros of which give estimates of the angle of arrival. Although the procedure turns out to be essentially the same as that developed by Reddi, the development presented here provides insight into the estimation problem.     

Control-Loop Noise in Adaptive Array Antennas

Adaptive array receiving antennas can be designed to sense the external noise field and to optimize the array illumination function. A substantial improvement in signal-to-noise ratio can be obtained with adaptive arrays when the external noise field is nonuniformly distributed in angle. The external noise process may be time varying and contain both discrete sources and continuously distributed sources. Two adaptive array implementations which maximize the signal-to-noise ratio are described in this paper. Expressions are derived for control-loop noise, i.e., the variance of the array element weights, and for the additional noise in the array output due to this element weight noise. It is shown that both the element weight noise and the array convergence rate are determined by the eigenvalues of the noise covariance matrix.     

Rapid convergence by cascading Applebaum adaptive arrays

An adaptive array architecture is described which has improved convergence speed over the conventional Applebaum array when the eigenvalue spread of the input signal covariance matrix is large. The architecture uses N+1 Applebaum adaptive arrays in a two-layer cascaded configuration. The gain constants in the first layer are set so that large interfering sources are quickly nulled, but small interfering sources are suppressed more slowly. Since the first layer removes the large interfering signals, the gain constant for the second layer can be set to a large value to quickly null the smaller interferers. The adaptation time is examined for several combinations of signal levels and array sizes. It is shown that, in many signal environments, the computational requirements for the cascaded array compare favorably with those of conventional sample matrix inversion (SMI) methods for large arrays     

New correlated MIMO radar covariance matrix design with low side lobe levels and much lower complexity

In this paper, a new correlated covariance matrix for Multi-Input Multi-Output (MIMO) radar is proposed, which has lower SideLobe Levels (SLLs) compared to the new covariance matrix designs and the well-known multi-antenna radar designs including phased-array, MIMO radar and phased-MIMO radar schemes. It is shown that Binary Phased-Shift Keying (BPSK) waveforms that have constant envelope can be used in a closed-form to realize the proposed covariance matrix. Therefore, there is no need to deploy different types of radio amplifiers in the transmitter which will reduce the cost, considerably. The proposed design allows the same transmit power from each antenna in contrast to the phased-MIMO radar. Moreover, the proposed covariance matrix is full-rank and has the same capability as MIMO radar to identify more targets, simultaneously. Performance of the proposed transmit covariance matrix including receive beampattern and output Signal-to-Interference plus Noise Ratio (SINR) is simulated, which validates analytical results.     

Direction-of-arrival estimation using signal subspace modeling

A computationally viable algorithm for estimating the direction-of-arrival (DOA) of multiple wavefields that are incident on an array of sensors is developed. The geometry of the array is unrestricted and the incident wavefields may be generated by mixtures of incoherent and coherent emitting sources. In the approach taken, the sensor signals are modeled as a noise-contaminated linear combination of steering vectors which are functionally dependent on a set of DOA parameters. These parameters are to be chosen so that this sensor signal model is most compatible with empirically measured data (i.e., snapshot data). An iterative procedure is developed for selecting the most data compatible set of DOA parameters in both the snapshot domain and the array covariance matrix domain. Critical to the success of such iterative solution procedures is the generation of quality DOA parameters to initialize the algorithm. A sequential beamforming method for this initialization is presented. Numerical examples to illustrate the effectiveness of the proposed algorithmic approach are given     

Phase Control in Multibeam Arrays

The phases of the radiation fields in a multibeam antenna are analyzed and their relations to the feed network are established. A method of phase control is proposed and applied to the case of cophasal beams radiated from an array for which the feed network is a lossless Butler matrix.     

未知互耦条件下混合信号波达方向估计

阵列天线互耦对导向矢量的扰动以及信号相干性对数据协方差矩阵造成的秩损,使得基于子空间正交性原理的超分辨波达方向估计(Direction-of-Arrival,DOA)算法性能恶化,甚至失效。针对这一问题,提出一种在相干与非相干信号混合状态下无需阵列互耦补偿的特征矢量平滑DOA估计算法。该算法对部分阵元接收数据的协方差矩阵特征分解,将得到的特征矢量平滑处理后构造等效协方差矩阵,抑制阵列互耦影响的同时完成混合信号DOA估计。在阵列互耦和信号相干性均未知的条件下,正确估计了信号DOA,无需互耦参数估计或补偿。计算机仿真结果验证了算法的有效性。     

Sensitivity analysis of the maximum likelihood direction-findingalgorithm

A first-order analysis is performed of the sensitivity of the maximum likelihood (ML) direction-finding algorithm to system errors which cause differences between the array manifold used by the algorithm and the true array manifold. The effect of such errors on the directions-of-arrival (DOA) estimates is investigated. The ability of the ML algorithm to resolve two closely spaced sources in the presence of phase and gain errors in the array elements or in the receivers, or errors in the element locations, is analyzed. A formula for computing the failure threshold of the algorithm as a function of source separation and other system parameters is derived and tested by simulation. The analysis assumes that the exact covariance matrix of array element outputs is known     

机载共形阵天线阵列信号处理的关键技术分析

机载共形阵是机载有源相控阵天线发展的下一个目标。共形阵天线的阵元具有不同的方向性,空间分辨率算法必须考虑来波的极化问题。本文研究了空间信号波达方向(Direction of Arrival,DOA)估计算法的现状,分析了一种适用于共形阵天线空域自适应波束形成的高速滤波算法。     

基于约束最小冗余线阵与干扰对消的测向方法

针对有源干扰背景下信号源和干扰源的个数超过线阵的自由度而产生线阵饱和现象,提出一种将约束最小冗余线阵与干扰对消技术相结合的测向方法。通过将无源状态和有源状态下线阵输出数据的协方差矩阵进行对消运算去除有源干扰和噪声分量,并对约束最小冗余线阵的波达方向(DOA)估计算法进行改进,构造了新的协方差Toeplitz矩阵,有效抑制了由阵列非均匀性导致的伪峰,提高了阵列的DOA估计性能。仿真结果表明:该算法在低信噪比背景下具有抗有源干扰能力,扩展了阵列孔径,并具有较高的测向精度和鲁棒性。     

基于VLBI的上行组阵标校技术研究

天线组阵能否完全替代大口径天线有一个关键性难题,就是天线阵是否支持上行链路组阵。深空航天器无法将不同地面天线的上行信号对齐,所以上行链路信号的调整必须在地面完成。针对上行组阵发射机相位调整问题,提出一种基于VLBI(Very Long Baseline Interferometry,甚长基线干涉测量)技术的接收模式天线上行组阵标校方案,并对标校精度进行了简要分析。将上行链路时延分解为几何时延和发射系统时延,建立了几何时延模型,通过标定接收时延和发射时延,便可以得到天线阵元间的相位标校值。理论分析结果表明,该方案具有一定的可行性,对上行组阵相位标校的研究具有一定的借鉴意义。     

A Generalized Clutter Computation Procedure for Airborne Pulse Doppler Radars

A general procedure for analyzing ground clutter effects in airborne pulse Doppler radars is described. The quantity computed is the expected clutter power at the output of any specified range gate/ Doppler filter processing cell. The procedure has been computerized and is quite general with respect to antenna gain pattern, clutter cross section variation, PRF, pulse and range gate shapes, and the various receiver processing functions. It is applicable only to distributed ground clutter and linear processing, and excludes the dynamic effects of continuous antenna scanning. To exemplify the use of the procedure, two studies conducted for a postulated high PRF radar are described, and the results are presented.     

Adaptive array CFAR detection

Presented here is a large class of adaptive array detection algorithms with constant false alarm rate (CFAR), so that the false alarm rate can be set to any preassigned number without knowledge of the noise covariance matrix. This class map incorporate any usual method of cell averaging and any method for array weight vector synthesis. A sufficient condition for CFAR is derived, which is easy to satisfy in practice. Basic system parameters are discussed. An example of detection performance for a simple cell-averaging detector, in which the array weight vector is synthesized by the method of diagonal loading, is provided using Monte Carlo simulations     

The Segmented Aperture Synthetic Aperture Radar (SASAR)

A new concept in synthetic aperture radar, called SASAR, which uses a segmented aperture, is described. Use of the segmented aperture allows appreciable extra receiving antenna gain to be realized. Each subarray of the receive antenna is equal in length to the transmit antenna; the system performance is increased approximately by a factor equal to the number of subarrays. To allow array combination of the subarray signal outputs requires a phase-shift factor (varying with azimuth) to be applied to each subarray signal. A digital implementation of this preprocessor is sketched out; it uses a push-down storage stack to store the range histories for a synthetic aperture from each subarray. Appropriate phase shifts are added to the stacks and a sum of stack values then provides the combined output range history sequence. Possibilities of using analog delay lines for preprocessing are also discussed. Pattern errors due to subarray size and receive array near field are examined and constraints are given.     

扩展酉矩阵束算法实现稀疏可重构天线阵的优化设计

针对方向图可重构天线阵列的稀疏布阵问题,提出一种基于扩展酉矩阵束算法的优化设计算法。首先建立以阵元位置和多组激励为变量的多方向图联合稀疏优化模型,并利用期望方向图采样数据构建Hankel块矩阵。然后通过centro-Hermitian化处理和酉变换将采样矩阵从复数域转换到实数域,舍弃实矩阵中较小的奇异值对可重构线阵进行稀疏。最后通过对等价矩阵束的广义特征值分解估计稀布阵元位置,进而得到每个方向图对应的激励。仿真验证了该方法能够以阵元非均匀稀疏分布的阵列形式有效实现多个方向图的精确重构。