Showing papers on "Adaptive beamformer published in 1977"

A comparative performance study of adaptive array processors

Abstract: In this paper a comparative performance analysis of the time and frequency domain techniques in element or beam space adaptive array processing is presented. Only the steady-state performance in representative background conditions is considered. Signal-to-background improvement, interference cancellation and bearing response patterns are used as the primary performance measures. The problem of signal preservation with various types and orders of constraints for signals of various strength is analyzed. Examples of partially adaptive modes of operation are presented to determine the number of adaptive elements or beams that should be employed for satisfactory operation under given conditions.

An Adaptive Beamformer which Implements Constraints Using an Auxiliary Array Preprocessor

Abstract: This paper presents a method of adaptive beamforming which makes use of a fixed, conventionally-weighted array and an auxiliary, adaptive array. The beamformed output is taken as the difference between the conventional and adaptive outputs. It is assumed that the sensor outputs, which are common to both arrays have been time-delay steered to the direction of interest. Similar structures have been described previously in which the auxiliary array is adapted using linearly-constrained adaptive algorithms. In the present work, it is shown that linear constraints can also be implemented by preceding the auxiliary adaptive array with a fixed preprocessor Ws. The adaptive algorithm then reduces to the simple; unconstrained LMS procedure. Thus, the proposed implementation separates out, in a natural manner, the three basic components of a linearly constrained adaptive beamformer: the conventional beamformer, the linear constraints, and the adaptive algorithm. This separation has significant advantages in practical implementations and suggests extensions to other beamformer structures.

The Forgotten Algorithm in Adaptive Beamforming

Abstract: Many quite different algorithms can be used in adaptive beamforming, but the one mostly treated in the literature, is the gradient descent (GD) algorithm. Its behaviour, stability and performance are well known. In this paper, emphasis is put on the Woodbury identity (Wl) algorithm, whose name is motivated by the algorithm’s use of the Woodbury identity (matrix inversion lemma). The GD algorithm will be used only as a reference.

A Constrained Adaptive Beamformer Tolerant of Array Gain and Phase Errors

Abstract: This paper describes a new method of constrained adaptive beamforming whose performance is relatively unaffected by array gain and phase errors. The method consists of setting certain of the adaptive parameters of a conventional adaptive beamformer to zero. It provides a means of rejecting large signals incident on the side lobes of an antenna array while preserving small broadband and narrowband and large broadband signals incident in the steering direction. In exchange for error tolerance broadband superdirectivity and large narrowband signal reception are lost.

A comparison of quadrature and single-channel receiver processing in adaptive beamforming

Abstract: A performance comparison between two different types of receivers in an HF adaptive array application is presented. The data used for the study were recorded using a bistatic HF radar located in the central valley of California. Eastword-looking transmissions were generated using a continuous sweep-frequency (chirp) format with a sweep repetition rate of 60 Hz. Backscatter signals, produced by a one-hop forward ionospheric path followed by a one-hop ionospheric return path were received using eight adjacent 32-element linear subarrays spanning a total linear aperture of 2.5 km. Single-channel receivers at each of the eight subarrays were employed for deramping and conversion to baseband. The overall system bandwidth was 960 Hz, and a common local oscillator was used for the receivers. All adaptive processing was carried out off-line using digitized data recorded simultaneously at the eight receiver outputs. The objective of the study described in this paper was to compare the results obtained using the receiving system described above with those which would be obtained using quadrature receivers at the subarray outputs. In the latter case, dual common local oscillator injection, with a 90\deg phase difference, is employed to obtain two receiver outputs, (in-phase and quadrature components) for each receiver. The component outputs then each have a bandwidth equal to one-half that of the single-channel receiver or 480 Hz for both the in-phase and quadrature signals. One advantage of quadrature reception is that a single real multiplying weight can be used at the two outputs to obtain a frequency-independent gain and phase-shift across the received bandwidth, a property which is not easily obtained for all azimuths in the single-channel receiver structure. The study described in this paper was undertaken to quantitatively determine the importance of this difference in an adaptive beamforming system. The approach taken was to synthesize quadrature receiver signals from the recorded single-channel data using Hilbert transform methods. Comparisons between the two receiving systems were then carried out using identical data sets. The results obtained illustrate that minor differences (less than 5 dB) exist in the processed outputs provided that the same number of adaptive degrees of freedom are used in the two processors. As a result, decisions as to the specific form of a receiver to be used in an HF radar should be based on considerations relating to ease of implementation rather than the overall performance of the adaptive processor.

Aspects of signal processing with emphasis on underwater acoustics : proceedings of the NATO Advanced Study Institute held at Portovenere, La Spezia, Italy, 30 August-11 September 1976

Abstract: Subject 3.- Optimum Antenna Processing: A Modular Approach.- The Forgotten Algorithm in Adaptive Beamforming.- The Effects of Signal and Weight Coefficient Quantisation in Adaptive Array Processors.- Adaptive Beamforming in an Active Doppler Sonar.- Adaptive Array Processing Experiments at HF.- Separation and Representation of Sources and Signals by a Linear Antenna.- Complementarity of Propagation Model Design with Array Processing.- Spectral Signal Set Extraction.- A Constrained Adaptive Beamformer Tolerant of Array Gain and Phase Errors.- Signal Extraction Algorithms for Adaptive Processing of Array Data.- Properties of the Discrete Fourier Transform (DFT).- An Adaptive Beamformer Which Implements Constraints Using an Auxiliary Array Preprocessor.- Subject 4 - Displays, Pattern Recognition, Human Decision.- Error and Reject Tradeoff for Nearest Neighbor Decision Rules.- Two-Dimensional Spectral Analysis for Image Coding, Processing and Recognition: An Approach Based on Some Properties of the Human Visual System.- Subject 5 - Relevant Inputs from Other Fields.- Spectrum Analysis.- Autoregressive and Maximum Likelihood Spectral Analysis.- Confidence Intervals for Maximum Entropy Spectral Estimates.- Team Decision Theory in the Optimal Control of a Communication Network.- Topological Structure in Group Transforms.- Some Signal Processing Aspects in Medical Ultrasound.- Recherche ET Caracterisation De Dependances Lineares Entre Signaux Par Analyse Interspectrale. Applications.- Subject 6 - Modern Processor Architecture and Techniques.- Linear Signal Processing Architectures.- Surface-Acoustic-Wave Devices for Signal Processing.- One/Two Dimensional Digital Signal Processors.- Surface-Acoustic-Wave Fourier-Transform Processors.- High Resolution Sonar Simulation Techniques.- Summaries of Workshops.- Properties of time/space Variant Channel.- Autoregressive Spectral Analysis.- Adaptive Array Processing.- Index of Names.

Adapt i ve array process i ng experi ments at hf1

Abstract: This paper summarizes experimental results which demon­ strate that significant improvements in signal-to-noise ratio SNR can be achieved by utilizing adaptive array methods in a 15 MHz, electromagnetic, high frequency (HF), radar system. The experi­ ments were conducted with a bistatic radar which employed adaptive beamforming at the receive array elements only. 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 one second. Successful track­ ing of the adaptive algorithm under these conditions was demon­ strated. The use of MTI clutter suppression filters at the sub­ array outputs, prior to adaptation, was investigated. No signifi­ cant 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.

Solving signal processing algorithms with a multi-microprocessor network

Abstract: This paper presents a state-of-the-art approach to a classic sonar signal processing problem. First a theoretical foundation for Source Oriented Beam Forming (SOBF) is introduced. A distributed processing element is then introduced and discussed which is offered as one solution to the platform implementation of high-powered signal processing such as SOBF. While many of the details of the implementation are beyond the scope or intent of this paper they may be found in the references.

Adaptive Array Processing Experiments at HF

Abstract: This paper summarizes experimental results which demonstrate that significant improvements in signal-to-noise ratio SNR can be achieved by utilizing adaptive array methods in a 15 MHz, electromagnetic, high frequency (HF), radar system. The experiments were conducted with a bistatic radar which employed adaptive beamforming at the receive array elements only. 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, Dolphaper 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 one second. Successful tracking of the adaptive algorithm under these conditions was demonstrated. The use of 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.

Energy-responsive intruder detection system

Abstract: A shielded two-wire cable used to transmit signals from one or more remote receivers to a central signal processing unit in an energy-responsive system is simultaneously used to transmit from the signal processing unit to the receivers (a) a signal proportional to the system noise level, (b) a pulsating signal when the system noise exceeds a level indicative of a desired margin of safety, and/or (c) a steady-state signal when an alarm signal is generated by the signal processing unit.