Showing papers on "Adaptive beamformer published in 1976"

Adaptive arrays

Abstract: A method for adaptively optimizing the signal-to-noise ratio of an array antenna is presented. Optimum element weights are derived for a prescribed environment and a given signal direction. The derivation is extended to the optimization of a "generalized" signal-to-noise ratio which permits specification of preferred weights for the normal quiescent environment. The relation of the adaptive array to sidelobe cancellation is shown, and a real-time adaptive implementation is discussed. For illustration, the performance of an adaptive linear array is presented for various jammer configurations.

Adaptive arrays with main beam constraints

Abstract: Initial applications of adaptive array theory to the radar sidelobe jamming problem ignored the problem of incidental cancellation of the desired signal returns. In more recent applications, longer transmitted waveforms have combined with returns from extended clutter and/or strong targets to create a more serious signal cancellation problem. There are several ways in which the adaptive processor can be constrained from responding to desired main lobe target returns while maintaining good cancellation of interference in the sidelobes. This paper examines the major techniques for constraining the response of the adaptive processor, including methods of controlling the response of the array in the absence of external interference. Time domain and frequency domain techniques are discussed. The majority of the discussion is devoted to angle domain techniques such as pilot signals, preadaption spacial filtering, and control loop spatial filtering. Analysis is presented showing the relationship between these techniques. Finally, examples are given showing the effects of these constraints as well as control of the quiescent array pattern.

Principles of aperture and array system design: Including random and adaptive arrays

Abstract: Antennas and Radiowave PropagationPhased Array AntennasAdaptive Phase-Shifter Nulling Techniques for LargeAperture Phased ArraysPrinciples of LasersMedical Devices and SystemsModern AntennasAn Introduction to Underwater AcousticsBiomedical Engineering HandbookFourier Transforms in Radar and Signal Processing, Second EditionGovernment-wide Index to Federal Research & Development ReportsElectronically Scanned Arrays (ESAs) and K-Space Gain FormulationAdvanced Array Systems, Applications and RF TechnologiesPrinciples of Planar Near-Field Antenna MeasurementsPrinciples of Aperture and Array System DesignThe Handbook of Antenna DesignTomographic Methods in Nuclear MedicinePrinciples of Stellar InterferometryPrinciples of Microwave MeasurementsAcoustical ImagingGeophysical AbstractsFundamentals of Aperture Antennas and ArraysUltrasound Imaging and TherapyMicrowave Imaging with Large Antenna ArraysObservational AstrophysicsPrinciples and Practice of UltrasonographyPhysical Principles of Medical UltrasonicsPrinciples of Aperture and Array System DesignAdaptive ArraysPrinciples and Practice of Clinical Electrophysiology of Vision, second editionInverse Problems in Underwater AcousticsGeophysical AbstractsModal Array Signal Processing: Principles and Applications of Acoustic Wavefield DecompositionTechniques and Principles in Three-Dimensional Imaging: An Introductory ApproachElectronic Microwave Imaging with Planar Multistatic ArraysPrinciples of Underwater SoundBasic Principles of Fresnel Antenna ArraysThe Principles of

A comparison of adaptive algorithms based on the methods of steepest descent and random search

Abstract: This paper compares the performance characteristics of three algorithms useful in adjusting the parameters of adaptive systems: the differential (DSD) and least-mean-square (LMS) algorithms, both based on the method of steepest descent, and the linear random search (LRS) algorithm, based on a random search procedure derived from the Darwinian concept of "natural selection." The LRS algorithm is presented here for the first time. Analytical expressions are developed that define the relationship between rate of adaptation and "misadjustment," a dimensionless measure of the difference between actual and optimal performance due to noise in the adaptive process. For a fixed rate of adaptation it is shown that the LMS algorithm, which is the most efficient, has a misadjustment proportional to the number of adaptive parameters, while the DSD and LRS algorithms have misadjustments proportional to the square of the number of adaptive parameters. The expressions developed are verified by computer simulations that demonstrate the application of the three algorithms to system modeling problems, of the LMS algorithm to the cancelling of broadband interference in the sidelobes of a receiving antenna array, and of the DSD and LRS algorithms to the phase control of a transmitting antenna array. The second application introduces a new method of constrained adaptive beamforming whose performance is not significantly affected by element nonuniformity. The third application represents a class of problems to which the LMS algorithm in the basic form described in this paper is not applicable.

An experimental four-element adaptive array

Abstract: The experimental pattern behavior of a four-element adaptive array based on a steepest-descent feedback algorithm is described. The array uses four multiturn loop elements on an aircraft fuselage mock-up. Operational patterns have been measured as a function of various parameters: frequency, element placement, desired signal and interference angles of arrival, signal powers, etc. Typical patterns are presented and the performance characteristics of the array are described.

Time-domain adaptive beamforming of HF backscatter radar signals

Abstract: Experimental results relating to the use of time-domain, continuously adapting beamformers in an HF bistatic FM/CW backscatter radar system are presented. Data for the study were obtained using the Wide Aperture HF Radio Research Facility (WARF) which is located in the central valley of California and is operated by Stanford Research Institute, Menlo Park, CA. Eastward-looking transmissions were employed with an operating frequency chosen so as to provide single-hop ionospheric propagation on both the forward and backscatter paths. Digital real-time recordings were taken at eight received subarray outputs. These recordings were then processed off-line using a large general purpose computer. The beamforming methods studied, however, are computationally simple and may be readily implemented in real-time using a commercially available minicomputer. Two adaptive algorithms were studied and in both cases it was shown that signal-to-noise ratio improvements of 10 to 15 dB are readily achieved when adaptive beamforming is compared with conventional, Dolph taper beamforming methods using identical received data in an HF backscatter environment. It was also demonstrated that the time scale of coefficient variation in an adaptive processor operating in this environment is the order of 1 s. Successful tracking of the adaptive algorithm under these conditions was demonstrated. The use of moving target indication (MTI) clutter suppression filters at the subarray outputs, prior to adaptation, was investigated. No significant improvement was observed with the use of these filters on experimental data. Finally, it was shown that the presence of fading nulls can significantly affect the determination of optimal subarray location and spacing in an HF environment. In general, the adaptive beamformer performance was found to be less dependent upon array geometry than was the case for conventional processing.

Adaptive monopulse beamforming

Abstract: A new receive-array adaptive beamformer configuration is presented. The array output signal consists of the difference between a conventionally weighted beam and an adaptive beam that is constrained to have a spatial null in the direction of interest. Adaptation then provides minimum total array output power.

The transient response of adaptive arrays in TDMA systems

Abstract: : This report investigates the performance which can be obtained by utilizing adaptive arrays to suppress undesired signals in coded communications systems. Emphasis is placed on determining the factors which limit the rate at which the array can respond to a time-varying signal environment, e.g., the environment which exists in a time-division multiple access (TDMA) communication system. The performance of adaptive arrays implemented with the LMS algorithm is evaluated analytically and experimentally. The direct matrix inversion algorithm is also investigated analytically to compare LMS algorithm response with the response obtained when estimates of the input signal parameters required to implement an optimal array processor are used in a direct manner to calculate the adaptive array weights.

An on-line adaptive beamforming capability for HF backscatter radar

Abstract: An adaptive-array beamforming capability has been implemented on-line in an existing over-the-horizon (OTH) backscatter radar. Inputs to the beamformer consisted of signals from eight 32- element subarrays of the 256-element, 2.5-km-long receiving array at the Wide Aperture Research Facility (WARF) located in California. Both conventional and adaptive beamforming were performed prior to the usual range/Doppler analysis used to extract radar targets from noise and clutter. The Griffiths P -vector algorithm, a recursive, time-domain adaptive technique, was implemented in all-digital fashion using fixed-point arithmetic on a 16-bit minicomputer. Desired signals utilized were aircraft targets and a fixed, ground-based radar repeater simulating a moving target, while unwanted signals were other-user interference and signals from a separate ground-based radar repeater. It was found that adaptive rejection of unwanted signals was dependent on pointing-angle alignment and that rejection was often increased by removal of the clutter by moving target indicator (MTI) filtering prior to adaptation. For some conditions, Doppler broadening can he produced by the time modulation imposed by continuous adaptation, unwanted-signal rejection with the adaptive beamformer is variable, but side-by-side comparisons obtained at WARF show that adaptive beamforming can reject off-azimuth signals up to 20 dB better than conventional beamforming with a -25 dB Dolph taper.

A fast Fourier transform (FFT) based sonar signal processor

Abstract: The design for a convolution processor is presented, which employs a single highly parallel implementation of the FFT algorithm. This processor is eminently suited for real-time matched filtering of linear FM (LFM) signals encountered in sonar systems. Computer simulations have shown that this processor, which uses fixed point arithmetic and modest word sizes can efficiently handle signals with multiple targets and relatively large doppler shifts. The parallel architecture provides a throughput rate sufficient for computing both forward and inverse transforms in the one processor. This retains for the system the flexibility for frequency domain adaptive beamforming, attractive in many sonar applications.

Fast-Adapting Matrix Filtering

Abstract: Implementation and convergence of adaptive processors are of continuing interest. To this end, a fast-adapting, linear, general recursive matrix-filtering algorithm is described. Two complex xoefficients are optimized at each recursion. A signal and interference estimation procedure is included. Results for some low-resolution, frequency domain beamformer trials with two targets, and using a single sample set, are presented. Considerable adaptive improvement occurs, along with target resolution, compared with a correlation beamformer. The algorithm shows rapid and direct convergence.

An experimental, TDMA-system compatible, LMS adaptive array

Abstract: An experimental L -band satellite simulator has been developed which is equipped with a four-element adaptive (null-steering) receiving array and operates in conjunction with prototype time division multiple access (TDMA) modems developed previously. This simulator performs signal processing operations which result in both the accommodation of multiple (pulsed-envelope) desired signals in time sequence and the suppression of undesired signals when the sources of desired and undesired signals are adequately displaced. Most notably, the composite signals received by the four array elements are processed within an adaptive spatial processor so that the amplitude of a suitable error signal is minimized in a least mean square sense. Overviews of the configuration and performance of the experimental system are presented in this paper.

Adaptive optimum array tracking detectors (Ph.D. Thesis abstr.)

Abstract: echoes from a known number of scatterers random in location and acoustic cross section. The scatterers of chief interest here are a variety of small fish, all about the same length (10 cm). Other origins of backscattering, which form the signals for competing categories, are seaweed (aquarium grass), surface reverberation (mechanically driven), and an artifact (glass bottle). From these data, it is possible to formulate three illustrative ,classification problems. The three problems require the classification of received signals which result from scattering by: a) fish versus surface, seaweed, and/or artifacts; b) fish and surface versus surface; and c) a low, versus a medium, versus a high density of fish. Classifiers are designed and tested with and without estimates of the environmental state. The usefulness of the approach is demonstrated quantitatively by the improvement in expected performance which results from monitoring the environmental state.