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Antennas and Propagation, IEEE Transactions on

Issue 3 • Date March 1986

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Displaying Results 1 - 25 of 26
  • [Front cover and table of contents]

    Page(s): 0
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    Freely Available from IEEE
  • Introduction

    Page(s): 273 - 275
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    Freely Available from IEEE
  • Steady state analysis of the generalized sidelobe canceller by adaptive noise cancelling techniques

    Page(s): 330 - 337
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    Narrow-band adaptive noise cancelling techniques are used to study the generalized sidelobe canceller (GSC), a general form of linearly constrained adaptive beamforming structure. In an environment which consists of a look-direction signal, one jammer, and additive receiver noise, exact expressions are derived for the Wiener solution, the steady state output signal-to-interference-plus-noise ratio (SINR0), and performance improvement due to adaptation (PIA), defined as the ratio of SINR0after adaptation to SINR0 before adaptation. These expressions are in terms of the signal directions and power levels, an arbitrary array geometry, and a general signal blocking matrix. Next, easily evaluated scalar equations for PIA are given for two classes of constraints. The first is constant gain in the look direction, and the second is constant gain plus a main beam zero first derivative in the look direction. Under the further assumption of an equally spaced line array, even simpler equations for PIA result, and are used to show that for jammers arriving outside the heamwidth between first nulls (BWFN) region of the unadapted beampattern, the introduction of the additional main beam zero first derivative constraint leads to negligible degradation in PIA. View full abstract»

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  • Methods for null control and their effects on the radiation pattern

    Page(s): 404 - 409
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    Several methods for synthesis of array antenna patterns with prescribed nulls are reviewed. Methods based on full amplitude/phase control at each array element and methods with a restricted number of degrees of freedom are compared, with attention focused on the characteristic features of the resultant patterns. These features are largely independent of any algorithm for achieving the nulls, and therefore they also provide a perspective on the performance of adaptive antenna systems, which employ these various control architectures. View full abstract»

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  • A coherent perturbation algorithm

    Page(s): 380 - 388
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    A coherent perturbation (CP) adaptive technique which eliminates the need to implement a separate receiver on each adaptive element is described. The new algorithm supports more implementation options than other perturbation techniques and produces an estimate of the power in a single element. A hardlimited version of the new algorithm can also be implemented to reduce the dynamic range requirement as well as the variations in transient response with jamming power. An approximation for the steady-state misadjustment is obtained. Simulation results are presented and compared with theoretical results. Timing synchronization requirements derived from computer runs are also presented. View full abstract»

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  • Tamed adaptive antenna array

    Page(s): 388 - 394
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    The adaptive array under directionaily constrained minimization of power (DCMP) algorithm is improved by adding a "pseudonoise." It is effective to protect the desired signal from cancellation or distortion in such cases as 1) where a coherent interference is incident, or 2) where the desired signal direction for the constraint contains some pointing error, or 3) when the desired signal is broad band, The optimum amount of pseudonoise to be added is also discussed and its formula is given. This system is named "tamed adaptive antenna" since its killing capability is somewhat moderated so as not to hurt the desired signal. View full abstract»

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  • Analysis of constrained LMS algorithm with application to adaptive beamforming using perturbation sequences

    Page(s): 368 - 379
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    Adaptive antenna array processing employing a constrained least mean square (LMS) algorithm requires an unbiased estimate of the gradient of the output power with respect to the array weights. There are a number of schemes for obtaining an unbiased estimate of this gradient. Though in each case the estimated gradient is unbiased, the covariance of the estimated gradient with each method is different and thus the transient and the steady state behavior of the constrained algorithm is different in each case. The transient and the steady state behavior of the weight covariancc matrix is analyzed, exact expressions for the misadjustment are derived, and a comparison of the performance of the algorithm is presented when the required gradient is estimated by different schemes. The schemes considered include gradient estimation when all the array signals are accessible as well as gradient estimation using perturbation sequences for eases when the array signals are inaccessible. The necessary and the sufficient condition for the diagonlization of the weight covarience matrix is also derived. View full abstract»

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  • A practical method of obtaining constant convergence rates in LMS adaptive arrays

    Page(s): 440 - 446
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    A practical, readily realizeable method of eliminating the old problem of widely disparate modal convergence rates in least mean squares (LMS) adaptive arrays is analyzed. The method is mathematically based on the Newton-Raphson iteration technique of finding zeros of a function. The practical realization is based on an extension of Compton's improved-feedback adaptive loop. It is shown how this modification results in constant and equal modal convergence rates in an adaptive array in conditions which cause widely disparate modal convergence rates in "standard" gradient-descent LMS algorithms, even in the presence of common circuit imperfections. The improved algorithm is compared to Compton's original improved-feedback loop and to a "standard" LMS adaptive array, all with equal open-loop time constants, then with individually optimized time constants. Preliminary experimental results are also shown to substantiate some of the analysis. View full abstract»

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  • An adaptive generalized sidelobe canceller with derivative constraints

    Page(s): 311 - 319
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    An adaptive broad-band beamforming structure is presented which employs a gradient-based weight adjustment algorithm to minimize output variance subject to a set ofJlinear constraints on broadband directional derivatives in the desired look direction. A generalized sidelobe-cancelling structure is employed in which a nonadaptive (conventional) beamformer operates in parallel with an adaptive beamformer. The conventional portion has a broad-band beampattern which adheres to the specified constraints while the adaptive path is a cascade of a fixed signal blocking matrix and a set of tapped-delay line filters. Blocking is employed to ensure that all incident waveforms which meet the specified constraints do not reach the tapped-delay lines. As a result, an unconstrained least mean square (LMS) power minimization algorithm is employed to adapt the delay line weights. It is shown that with the addition of the directional derivative constraints, the beamformer quiescent bcampattern becomes a function of the location of the phase center used to specify the constraints. A design criterion for choosing this location is suggested and simulation experiments which illustrate the performance of this new adaptive beamformer are presented. View full abstract»

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  • Area coverage adaptive nulling from geosynchronous satellites: Phased arrays versus multiple-beam antennas

    Page(s): 410 - 419
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    Antenna design trade-offs associated with providing an area coverage communication link via a geosynchronous satellite, where the geographical area to be serviced by the associated adaptive antenna system is characterized by a narrow-cone angle directed at an a priori known region on the earth's surface, are considered. Interference sources within and just surrounding the coverage area are adaptively nulled; interference sources located far from the coverage area are minimized by designing the antenna system to have low sidelobes outside the coverage area. This minimizes the number of adaptive channels which must be processed, with subsequent decrease in satellite weight and power requirements. The antenna design trade-off is between the phased array (PA) and the multiple-beam antenna (MBA). Three specific performance factors are considered in the MBA/PA design trade-off: susceptibility to out-of-coverage-area interference sources which use up the adaptive degrees of freedom allocated to the desired coverage area; effects of reduced link margin for low-gain thinned arrays; and nulling bandwidth restrictions imposed on spread spectrum communication systems. Three classes of PA's are considered: filled arrays, high gain thinned arrays, and low-gain thinned arrays. View full abstract»

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  • Considerations on a scanned adaptive superresolution array

    Page(s): 447 - 453
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    The behavior of a scanned Howells-Applebaum (H-A) adaptive superresolution array (SRA) in a steady-state after convergence is derived analytically. The steering signals are scanned (swept) with time in order to observe the signal source locations. However, the SRA performance deteriorates seriously when scanning is fast. First, we study the causes of degradation of the scanned SRA performance at a high scanning rate. Second, we derive the conditions for the low pass filter in an H-A loop that gives a better SRA performance at a certain high scanning rate. Finally, we propose a new filter configuration in the H-A loop which permits faster scanning with less degradation. View full abstract»

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  • Using spectral estimation techniques in adaptive processing antenna systems

    Page(s): 291 - 300
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    Improved spectral estimation techniques hold promise for becoming a valuable asset in adaptive processing array antenna systems. Their value lies in the considerable amount of additional useful information which they can provide about the interference environment, utilizing a relatively small number of degrees of freedom (DOF). The "superresolution" capabilities, estimation of coherence, and relative power level determination serve to complement and refine the data from faster conventional estimation techniques. Two conceptual application area examples for using such techniques are discussed; partially adaptive low-sidelobe arrays, and fully adaptive tracking arrays. For the partially adaptive area the information is utilized for efficient assignment of a limited number of DOF in a beamspace constrained adaptive system in order to obtain a stable main beam, retention of low sidelobes, considerably faster response, and reduction in overall cost. These benefits are demonstrated via simulation examples computed for a 16-element linear array. For the fully adaptive tracking array area the information is utilized in an all-digital processing system concept to permit stable hulling of coherent interference sources in the main beam region, efficient assignment/control of the available DOF, and greater flexibility in time-domain adaptive filtering strategy. View full abstract»

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  • Exact solution for control loop adaptive antenna weights in band-limited noise

    Page(s): 395 - 403
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    A technique is presented that yields exact solutions for the noise analysis of the Applebaum adaptive algorithm. Past researchers have derived results under the assumption that the input noise is rapidly varying with respect to the output weighting vector of the Applebaum algorithm. In this paper, this work has been extended to include noise that can vary at any rate. External noise sources are modeled as continuous state jump Markov processes (CSJMP) which results in exact first moment equations for the weighting vector that are solvable. Specifically, the case where the adaptive algorithm is subjected to a single external noise source is examined in detail. Results in adaptive processing convergence times and average null depth are presented. View full abstract»

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  • Adaptive antenna arrays for weak interfering signals

    Page(s): 420 - 426
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    The performance of adaptive antenna arrays in the presence of weak interfering signals (below noise level) is studied. It is shown that conventional adaptive arrays are unable to suppress such interfering signals. To overcome this problem, the feedback loops controlling the array weights are modified. In the modified feedback loops, the noise level in the feedback loops is reduced by reducing the correlation between the noise components of the two inputs to the loop correlator. Two techniques to decorrelate these noise components are discussed. It is shown that adaptive arrays with the modified feedback loops provide the desired interference suppression. An expression is given for the amount of noise decorrelation required to achieve a specified interference suppression. View full abstract»

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  • An adaptive algorithm for array processing

    Page(s): 454 - 458
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    A mathematical summary of the joint complex least squares lattice (JCLSL) adaptive algorithm is presented. The algorithm has as inputs two scalar discrete time series (primary. and reference channels). Output consists of the filtered reference channel subtracted from the primary channel. The convergence characteristics of the algorithm are illustrated experimentally for a problem where the reference channel time series consists of dual constant frequency sinusoids which undergo an instantaneous step in frequency. The primary channel time series consists of dual constant frequency sinusoids whose frequencies coincide with those of the reference channel after the step. Lastly, an application of the JCLSL algorithm to the rejection of ocean acoustic boundary reverberation is described. View full abstract»

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  • Parallel spatial processing: A cure for signal cancellation in adaptive arrays

    Page(s): 347 - 355
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    A spatial processing algorithm with parallel structure is presented for the prevention of signal caneellation phenomena in conventional adaptive arrays. This algorithm basically uses a parallel structure with a spatial averaging effect to combat coherent jamming. It results in a spatially smoothed maximum-likelihood estimate of the desired signal when the adaptive beamformer converges. Simulations have been conducted which verify the effectiveness of the proposed structure. View full abstract»

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  • Spatial spectrum estimation in a coherent signal environment using an array in motion

    Page(s): 301 - 310
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    Spatial spectrum estimation utilizing an array in motion is here investigated for dealing with coherent arrivals in a multiple signal environment. The effect of estimating the spatial correlations while the array is moving is studied in terms of the decorrelation it produces, the change it causes in the eigenvalues of the correlation matrix, and the improvements obtained in the measured spectrum. Cases of both fixed and varying angle of arrival are investigated, The former arises with distant sources and will, with a sufficiently long estimation interval, emulate uncorrelated sources and given correspondingly sharp spectra. The latter arises with nearby sources and will allow them to be distinguished but will be attended by spectral shift and broadening, and loss of resolution. It is shown that meaningful estimates of the arrival angles can nevertheless be made. Specific illustrations are worked out using a seven-element, sparse, nonuniformly spaced linear array utilizing the well-known superresolution spectral estimators-the maximum likelihood (ML) method, the method of linear prediction (LP), and the method of multiple signal classification (MUSIC). View full abstract»

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  • A planar version of a 4.0 GHz reactively steered adaptive array

    Page(s): 427 - 431
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    -A planar (two-dimensional) reactively steered adaptive array (RESAA) including a single active microstrip element and eight closely coupled parasitic microstrip elements has been designed and tested. Steering of a null toward a single incident jammer is accomplished by adaptive control of reactive terminations on the parasitic elements using a guided random search algorithm. The planar array can steer a null in both the elevation and azimuth directions with a depth of 30 dB. View full abstract»

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  • A new set of linear constraints for broad-band time domain element space processors

    Page(s): 320 - 329
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    A new set of linear constraints for designing broad-band time domain element space antenna array processors is presented. The set of linear constraints is used to ensure that a desired look-direction response of the processor over a frequency band of interest can be closely approximated. The design technique is posed in such a way that three types of presteering can be handled: no presteering, coarse presteering, and exact presteering. The elimination of presteering time delays or the possibility to use coarse presteering is an attractive feature in a digital implementation of antenna array processors. The relationship that the new processor has to other broad-band processors is also established. Furthermore, the approach enables various types of errors and mismatches between signal model and actual scenario to be incorporated in the problem formulation. View full abstract»

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  • An alternate derivation of the distribution of the conditioned signal-to-noise ratio

    Page(s): 463 - 464
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    A result in multiple regression analysis is used to derive the probability distribution of the conditioned signal-to-noise ratio. View full abstract»

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  • Wide-band adaptive array processing using pole-zero digital filters

    Page(s): 355 - 367
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    Conventional broad-band array processing is accomplished by linearly combining the outputs of tapped-delay lines attached to each sensor of an array. This type of processing can be interpreted as using an all-zero digital filter at each sensor to generate a frequency-dependent magnitude and phase shift (weighting) over the array operating bandwidth. A new array processing method is presented which uses digital filters having both poles and zeros to perform the frequency-dependent array weighting. Several algorithms for adapting the pole-zero array filters are introduced. Computer simulations are then presented demonstrating the potential for substantial improvement in broad-band interference nulling provided by pole-zero array processing. View full abstract»

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  • Multiple source DF signal processing: An experimental system

    Page(s): 281 - 290
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    The multiple signal characterization (MUSIC) algorithm is an implementation of the signal subspace approach to compute parameter estimates of multiple point-source signals from the observed voltages received on an array ofMantennas. In it, the solution to the multiple source direction finding (DF) problem is provided by the intersection of the signal subspace (obtained from the received data) and the array manifold (obtained via array calibration or prior knowledge of array directional characteristics). The MUSIC algorithm was implemented to experimentally verify the performance of the signal subspace approach to DF under very general scenarios and conditions which are regarded as difficult to impossible in traditional systems. The results of those experiments are described herein. The experimental system consisted of an eight-element antenna array 13 wavelengths in diameter, an eight-channel receiver and digitizer, and a minicomputer with disk storage to process the digitized data. With ideal instrumentation, the MUSIC algorithm provides performance that, as the amount of data collected increases without limit, is asymptotically ideal. However, with finite precision and finite data collection, the performance of even an ideal system can be a sensitive function of source and scenario parameters. Tests demonstrated the resolution of three sources all within one beamwidth (5deg), even when the closer two were spaced less than 0.2 beamwidths. Sources that were polarized differently could be resolved at closer spacings. Experimental DF accuracy was limited by the oncalibrated scattering of source energy from the test range support tower and from the ground. The measured direction of arrival of one source changed by less than 0.01 beamwidths as the other two sources were switched on and off in all combinations. In general, results indicated that all parameters of a source can be measured and the signal waveform can be recovered as well in the presence of other sources less than a beamwidth away. View full abstract»

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  • The effects of reflector antenna diffraction on the interference cancellation performance of coherent sidelobe cancellers

    Page(s): 432 - 439
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    While adaptive antenna technology has undergone significant development, little attention has been given to the impact of antenna design on the performance of the adaptive system. A need exists to factor the details of the antenna system response into the analysis of adaptive system performance, particularly in the case where adaptive cancellation is required over a broad bandwidth. At extremely high frequency (EHF) where wide bandwidth allocations exist, reflector antenna technology used with an adaptive sidelobe canceller design is appropriate. This paper uses a simple diffraction model to compare the adaptive performance differences between Cassegrain and offset reflector designs. The reduced diffraction of the offset reflector design results in improved cancellation performance. These analyses also provide the opportunity to explore the impacts of antenna design parameters and interference power levels and arrival directions. View full abstract»

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  • Fast orthogonalization networks

    Page(s): 458 - 462
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    A numerically efficient and stable algorithm is developed to adaptively filter multiple input channels into desired multiple output channels. The algorithm is a type of adaptive lattice filter that employs a fast orthogonalization network (FON) algorithm for numerical efficiency. Past researchers have concentrated on developing efficient lattice algorithms to process stationary input channels; i.e., the input covariance matrix is Toeplitz in form. The algorithm developed herein is designed to adaptively filter nonstationary input channels. Various implementations of the FON algorithm are given. View full abstract»

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  • A novel algorithm and architecture for adaptive digital beamforming

    Page(s): 338 - 346
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    A novel algorithm and architecture are described which have specific application to high performance, digital, adaptive beamforming. It is shown how a simple, linearly constrained adaptive combiner forms the basis for a wide range of adaptive antenna subsystems. The function of such an adaptive combiner is formulated as a recursive least squares minimization operation and the corresponding weight vector is obtained by means of theQ-Rdecomposition algorithm using Givens rotations. An efficient pipelined architecture to implement this algorithm is also described. It takes the form of a triangular systolic/wavefront array and has many desirable features for very large scale integration (VLSI) system design. View full abstract»

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IEEE Transactions on Antennas and Propagation includes theoretical and experimental advances in antennas.

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Editor-in-Chief                                                 Kwok W. Leung