Virtual Uniform Linear Array Research Articles

D3LS STAP Approach on Wideband Signals using Uniformly Spaced Real Elements

In this paper, Space Time Adaptive Processing (STAP) applied on wideband signals from uniformly spaced real elements using Direct Data Domain Least Squares (DLS). Two antenna configurations are used: an array of dipole antenna (narrowband antenna) and an array of patch antenna (wideband antenna). A frequency transformation technique is used only for the case of narrowband antenna which transforms the steering vector at a wideband frequency to another steering vector at the desired frequency. The mutual coupling between elements will affect on the estimation of complex amplitude of Signal of Interest (SOI). It is necessary to compensate for the strong mutual coupling that exists between the real antenna elements. This is done by using coupling transformation matrix which converts the voltages that are induced at the real antenna elements to an equivalent set of voltages that will be induced by the same incident wave in uniform linear virtual array (ULVA). Then, we will apply STAP DLS on the compensated voltages. Numerical simulations are done using the three main methods of DLS namely the forward, backward, and the forward–backward methods. General Terms Adaptive signal Processing algorithm, Beamforming algorithm

Modeling the ultra-wideband outdoor channel: Measurements and parameter extraction method

This paper presents results from an outdoor measurement campaign for ultra-wideband channels at gas stations. The results are particularly relevant for "infostations" where large amounts of data are downloaded to a user within a short period of time. We describe the measurement setup and present a novel high-resolution algorithm that allows the identification of the scatterers that give rise to multipath components. As input, the algorithm uses measurements of the transfer function between a single-antenna transmitter and a long uniform linear virtual array as receiver. The size of the array ensures that the incoming waves are spherical, which improves the estimation accuracy of scatterer locations. Insight is given on how these components can be tracked in the impulse response of a spatially varying terminal. We then group the detected scatterers into clusters, and investigate the angular power variations of waves arriving at the receiver from the clusters. This defines the cluster's "radiation pattern." Using sample measurements we show how obstacles obstruct the line-of-sight component -- a phenomenon commonly referred to as "shadowing." We compare the measurement data in the shadowing regions (locations of the receiver experiencing shadowing) with the theoretical results predicted by diffraction theory and find a good match between the two.

DOA ESTIMATION WITH SUB-ARRAY DIVIDED TECHNIQUE AND INTERPORLATED ESPRIT ALGORITHM ON A CYLINDRICAL CONFORMAL ARRAY ANTENNA

A novel DOA flnding method for conformal array applications is proposed. By using sub-array divided and interpolation technique, ESPRIT-based algorithms can be used on conformal arrays for 1-D and 2-D DOA estimation. In this paper, the circular array mounted on a metallic cylindrical platform is divided to several sub- arrays, and each sub-array is transformed to virtual uniform linear array or virtual uniform planar array through interpolation technique. 1-D and 2-D direction of arrivals can be estimated accurately and quickly by using LS-ESPRIT and 2-D DFT-ESPRIT algorithms, respectively. This method can be applied not only to cylindrical conformal array but also to any other arbitrary curved conformal arrays. Validity of this method is proved by simulation results.

ROBUST ADAPTIVE BEAMFORMER USING INTERPOLATION TECHNIQUE FOR CONFORMAL ANTENNA ARRAY

A novel robust adaptive beamforming method for conformal array is proposed. By using interpolation technique, the cylindrical conformal array with directional antenna elements is transformed to a virtual uniform linear array with omni-directional elements. This method can compensate the amplitude and mutual coupling errors as well as desired signal point errors of the conformal array e-ciently. It is a universal method and can be applied to other curved conformal arrays. After the transformation, most of the existing adaptive beamforming algorithms can be applied to conformal array directly. The e-ciency of the proposed scheme is assessed through numerical simulations.

Preestimation-Based Array Interpolation Approach to Coherent Source Localization Using Multiple Sparse Subarrays

In application to coherent source localization using multiple sparse uniform linear subarrays (ULSAs), the array interpolation technique with Lau or Pesavento formulation causes unacceptable interpolation errors over the entire field of view because of high sparseness of ULSAs. In this letter, we propose a modified array interpolation approach by specifying a union of nonoverlapping narrow subsectors as the in-sector to cover only the source locations preestimated roughly on an assumption that at least a single ULSA is available for coherent source localization. Furthermore, we skip noise prewhitening and employ more subarrays of the virtual uniform linear array (VULA) for forward-backward spatial smoothing (FBSS) that plays a key role in noise floor reduction as well as coherent source decorrelation.

Direction of Arrival (DOA) Estimation Using Electrically Small Tuned Dipole Antennas

We present a methodology for the direction of arrival (DOA) estimation using the induced voltages that are measured at the loads connected to electrically small tuned dipole antenna arrays illuminated by the signal of interest (SOI). The matrix pencil method is applied directly to the induced voltages to estimate the DOA of the various signals. Using electrically small tuned antennas can be advantageous as they can be placed in close proximity of each other saving the real estate and, thus, making it possible to deploy phased arrays on small footprints. When dealing with closely spaced tuned electrically small antennas, it is necessary to use a transformation matrix to compensate for the strong mutual coupling that may exist between the antenna elements. The transformation matrix converts the voltages that are induced at the loads corresponding to the feed point of the array operating in the presence of mutual coupling and other near field scatterers to an equivalent set of voltages that will be induced by the same incident wave in an uniform linear virtual array (ULVA) consisting of omnidirectional isotropic point radiators equally spaced and operating in free space. For any given incident field, the open circuit voltage developed across the feed-point of the small dipole will always be less than the open circuit voltage developed across the feed-point of the half-wavelength dipole. The difference is in the voltage developed across the loads connected to the dipole's feed-point. With the small dipole antenna, the voltage developed across the load impedance will be orders of magnitude greater than the voltage developed across the load connected to the half-wavelength dipole, even though the power captured by any conjugately matched antenna is approximately the same irrespective of their lengths. Three different scenarios are presented to illustrate the methodology. First, we consider resonant dipole elements spaced half wavelength apart, electrically small tuned antenna elements spaced half wavelength apart and electrically small tuned antenna elements placed in close proximity of each other to reduce the footprint without affecting the performance of the phased array. In addition, we consider the possibility of DOA estimation using a combination of different type of electrically small antennas both uniformly and nonuniformly spaced. Numerical examples are presented to illustrate the principles of this methodology

Adaptive processing using a single snapshot for a nonuniformly spaced array in the presence of mutual coupling and near-field scatterers

This paper presents an adaptive technique to extract the signal of interest (SOI) arriving from a known direction in the presence of strong interferers using a single snapshot of data. The antenna elements in this method can be nonuniformly spaced and there can be mutual coupling between them. In addition, near-field scatterers can also be present. First, the voltages induced in the antenna elements of the array due to interferers, mutual coupling between the elements, and near-field scatterers is preprocessed by applying a transformation matrix to these voltages through a rigorous electromagnetic analysis tool. This electromagnetic preprocessing technique transforms the voltages that are induced in a nonuniformly spaced array containing real antenna elements to a set of voltages that will be produced in a uniform linear virtual array (ULVA) containing omnidirectional isotropic point radiators. In the transformation matrix we would like to include various electromagnetic effects like mutual coupling between the antenna elements, presence of near-field scatterers and the platform effects on which the antenna array is mounted. This transformation matrix when applied to the actual measured voltages yields an equivalent set of voltages that will be induced in the ULVA. A direct data domain least squares adaptive algorithm is then applied to the processed voltages to extract the SOI in the presence of interferers. Limited numerical examples are presented to illustrate the novelty of the proposed method.

Direction‐of‐arrival (DOA) estimation using a single snapshot of voltages induced in a real array operating in any environment

AbstractThis paper investigates an electromagnetic preprocessing technique that transforms the voltages that are induced in a nonuniformly spaced array containing real antenna elements to a set of voltages that will be produced in a uniform linear virtual array (ULVA) containing omnidirectional isotropic point radiators. The objective here is that, in the processing methodology, we would like to include various electromagnetic effects like mutual coupling between the antenna elements, the presence of near‐field scatterers, and the platform effects on which the antenna array is mounted. The preprocessing is carried out using a least squares method, which generates a transformation matrix for the set of induced voltages in the real array. This transformation matrix, when applied to the actual measured voltages, yields an equivalent set of voltages that will be induced in the ULVA. Then, a direct data domain superresolution technique like the matrix pencil method is applied to the processed voltages to yield the DOA for the various signals of interest. Numerical results are presented to illustrate the efficiency and accuracy of this method. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 335–340, 2002.

Mode space smoothing algorithm to estimate DOA of coherent signals impinging on uniform circular array

A novel method to estimate DOA of coherent signals impinging on a uniform circular array(UCA) is presented in this paper. A virtual uniform linear array (VULA) is first derived by using spatial DFT technique, transforming the UCA from element space to phase mode space to obtain the properties of ordinary ULA, and then the well known spatial smoothing technique is applied to the VULA so that the lost rank of covariance matrix due to signal coherence can be retrieved. This method makes it feasible to use the simple MUSIC algorithm to estimate DOA of coherent signals impinging on a UCA without heavy computation burden. Simulation results strongly verify the effectiveness of the algorithm.

A note on most favorable array geometries for DOA estimation and array interpolation

Given an n-element linear array with the fixed positions x/sub 1/ and x/sub n/ of the leftmost and rightmost array sensors, it is shown that the stochastic Cramer-Rao bound (CRB) and MUSIC performance depend on positions of the remaining n-2 sensors within the interval [x/sub 1/, x/sub n/]. The asymptotic performance of the interpolated array approach shows similar dependence. The most favorable geometries are unrealizable for q<n-1 because the array sensors tend to form q+1 point clusters, where q is the number of sources. An interesting consequence of these facts is that for certain realizable nonuniform linear array (NULA) geometries, interpolated root-MUSIC with a virtual uniform linear array (ULA) of length x/sub n/-x/sub 1/ has better asymptotic performance than conventional root-MUSIC applied to a real ULA of the same length.

Direction finding of coherent sources with nonlinear arrays

This paper presents an approach to estimate the directions-of-arrival (DOAs) of narrowband coherent (completely corrected) signals by an arbitrary geometry sensor array. The concept of focussing matrices is introduced to estimate the outputs of a virtual uniform linear array from the outputs of the real array in order to use spatial smoothing techniques. Unlike the work presented in [5], the proposed approach avoids multiple singular value decomposition (SVD) computations in the various sectors because it is not necessary to divide the field of view of the array into sectors. Simulation results are given to support the approach.

Design of virtual uniform linear arrays for efficient direction finding

The problem considered in this paper is to interpolate a virtual uniform array from a real two-dimensional array with arbitrary geometry via an interpolation matrix. The key to this problem is how to arrange these virtual sensors. It is shown that the virtual uniform linear array should have the same main-lobe beam-pattern as the real array over an angular sector of interest. Simulation results are presented to illustrate the application of virtual array in direction finding.

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This work presents a Kronecker product based methodology of frequency-domain beamforming of large sensor arrays for far-field broadband speech signals. The principal idea involves splitting up a given uniform linear array (ULA) into two smaller virtual ULAs (VULAs), using the Kronecker product. The linear system of the original ULA is bifurcated into two smaller linear systems of the VULAs. Henceforth, traditional adaptive beamformers such as the minimum-variance-distortionless-response (MVDR) beamformer may be obtained for each of the VULAs, using lesser data to estimate the statistics. The short-length beamformers, obtained from the VULAs, are finally combined by the Kronecker product to derive the full-length Kronecker product beamformer. Additionally, the VULAs allow fixed and adaptive beamforming to be implemented separately on each of them. As fixed beamformers do not employ statistical information, the Kronecker product hybrid beamformers reduce the original linear system to just a small linear system involving one VULA. Accordingly, hybrid beamformers may be implemented using traditional fixed beamformers, such as the delay-and-sum (DS) beamformer, on one VULA, and traditional adaptive beamformers, such as the MVDR, on the other. The proposed Kronecker product beamformers are observed to provide faster convergence and superior robustness with respect to the traditional beamformers.