Showing papers on "Adaptive beamformer published in 1989"

Adaptive beamforming for noise reduction

Abstract: The invention provides an adaptive noise cancelling apparatus which operates to overcome a problem encountered in conventional noise cancelling circuitry when the signal-to-noise ratio at the sensor array is high--to wit, that the target signal is degraded, leading to poorer intelligibility. An apparatus constructed in accord with the invention selectively inhibits the adaptive filter from changing its filter values in these instances and, thereby, prevents it from generating a noise-approximating signal that will degrade the target component of the output signal.

An analysis of several partially adaptive beamformer designs

Abstract: A general analysis of broadband interference cancellation in adaptive beamformers is provided. The results are then used to analyze beam, eigenstructure, and power-minimization approaches to the design of linearly constrained partially adaptive beamformers. It is shown that although beam-based designs are capable of good performance in narrowband environments, they are not appropriate for broadband environments. Eigenstructure-based designs are capable of excellent performance in narrow- or broadband environments. However, performance degrades if the number of adaptive weights is less than the eigenstructure dimension. Power-minimization designs are based on directly minimizing interference output power. Simulations indicate that the power-minimization design is capable of the best cancellation performance with a limited number of adaptive weights. >

Improved time-delay estimates of underwater acoustic signals using beamforming and prefiltering techniques

Abstract: Passive sonar systems that localize broadband sources of acoustic energy estimate the difference in arrival times (or time delays) of an acoustic wavefront at spatially separated hydrophones, The output amplitudes from a given pair of hydrophones are cross-correlated, and an estimate of the time delay is given by the time lag that maximizes the cross correlation function. Often the time-delay estimates are corrupted by the presence of noise. By replacing each of the omnidirectional hydrophones with an array of hydrophones, and then cross-correlating the beamformed outputs of the arrays, the author shows that the effect of noise on the time-delay estimation process is reduced greatly. Both conventional and adaptive beamforming methods are implemented in the frequency domain and the advantages of array beamforming (prior to cross-correlation) are highlighted using both simulated and real noise-field data. Further improvement in the performance of the broadband cross-correlation processor occurs when various prefiltering algorithms are invoked. >

Enhanced Minimum Variance Beamforming

Abstract: The MVDR criterion for achieving maximum wavefront Signal-to-Noise Ratio (SNR) gain with a spatial sensor array beamformer is well established. The MVDR beamformer has its technical origins in the early 1960’s (see [1] for bibliography). Eigenstructure based, signal subspace wavenumber spectrum analysis procedures appeared in the early 1970’s [2, 3] and have evolved to the present day high resolution procedures typified by the MUSIC algorithm [4]. The difference between a beamformer, which can produce a directional waveform estimate, and a wavenumber analyzer which provides a source direction estimate only is emphasized. Accordingly, the evaluation of MVDR and eigenstructure techniques has been along two parallel paths. This dual perspective has precluded the establishment of common theoretical and practical approaches which could result in the exploitation of the advantages of both techniques. The enhanced minimum variance (EMV) beamformer represents an attempt to find such a common viewpoint and to subsequently address current space-time processing technical issues from this unified perspective. Such issues as detection of threshold signals, the determination of the number of sources present, reducing computational burden, coherent multipath arrivals and wideband signals are of particular concern.

Fast acquisition of burst and transient signals using a predictive adaptive beamformer

Abstract: The author presents a simple prediction algorithm for blindly adapting a narrowband beamformer to rapidly acquire burst or transient signals by exploiting only the abrupt ON/OFF transitions of the transient waveforms, that is without using knowledge of the frequency or DOA (direction of arrival) of the signal of interest or knowledge of the manifold, geometry, or noise covariance of the array. It is shown via analysis and computer simulation that the algorithm is able to detect burst and transient signals, and to capture those signals with maximum attainable output SINR (signal-to-interference and -noise ratio) in environments containing severe stationary cochannel interference. In particular, it is shown that the algorithm can adapt a four-element antenna to blindly detect and capture FSK (frequency shift keying) bursts with 10-20-ms burst durations in less than 2 ms, even at low power levels ( approximately 4-dB signal-to-white-noise ratio) and in the presence of strong ( approximately 50-dB SIR) stationary interference. >

Interference source noise cancelling beamformer

Abstract: A beam is directed to an interfering source to obtain the best estimate of this signal. The beamformer output is fed into an inverse beamformer in such a way that the total delay experienced by a signal from any element of the array is the same as that of other elements in the array. Each output from the inverse beamformer is a spatially filtered replica of the signal received by the corresponding array element except for a fixed time delay constant for each element. The same delay is imparted in a second channel to each of the array outputs which are substituted from the corresponding inverse beamformer outputs. If the amplitude and delay of each signal to the subtracter are equal, the outputs from each of the subtracters will no longer contain signal components from the direction of the interfering signal.

A structured gradient algorithm for adaptive beamforming

Abstract: The constrained least‐mean‐square (LMS) algorithm uses a noisy estimate of the required gradient to adaptively estimate the weights of an optimal antenna array The gradient is estimated by multiplying the array output with the array receiver outputs An alternative scheme for estimating the required gradient is proposed in this paper The new scheme uses a structured estimate of the array correlation matrix to estimate the gradient This structure reflects the structure of the exact array correlation matrix and is obtained by a spatial averaging of the elements of the noisy array correlation matrix used in the standard algorithm It is shown that the estimated gradient is unbiased and the array weights converge to the optimal weights in the mean sense Furthermore, the weights estimated by the structured gradient algorithm are less noisy than those estimated by the standard algorithm

Models for The Application of Kalman Filtering to the Estimation of the Shape of a Towed Array

Abstract: Thin flexible arrays of hydrophones play an important role in sonar ASW and in seismic exploration. As arrays become longer, and are towed at lower speeds to reduce self-noise, there is an increased tendency for the shape of the array to deviate from linearity. Such deviations are caused by transverse motion of the towing vessel, oceanic currents and hydrodynamic effects and can cause severe performance degradations to sophisticated signal processing algorithms such as adaptive beamforming.

Comparison of resolving power of array processing methods by using an analytical criterion

Abstract: Resolving power is very important when comparing localization methods in underwater passive listening. The authors introduce a simple criterion that is based on a second-order analysis of different localization functions usually available in array processing: conventional and adaptive beamforming and the MUSIC method. A general procedure is presented in both finite- and infinite-time observation cases, providing a system of equations. Its solutions are shown to describe areas as functions of the spectral levels and bearings of the two sources to be resolved. Explicit analytical formula are given in the asymptotic case. The angular resolving powers of the three methods have been derived by approximations and compared with an ultimate resolution bound. It is found that the resolving power of the classical approach depends only on array geometry, whereas that of the adaptive beamforming and MUSIC methods is varies as the fourth root of the input-signal-to-noise ratio. >

Adaptive beamforming as an inverse problem

Abstract: In previous work, the authors developed a minimum-mean-square-error beamformer (MMSE-BF). When they compared it to the minimum-variance distortionless-response beamformer (MVDR-BF), they concluded that it is especially suited for correlated returns. This improvement is at the cost of some frequency distortion in wideband applications. To circumvent this problem, the minimum-mean-square-error distortionless-response beamformer (MMSEDR-BF) is introduced. Its behavior is compared with the MVDR-BF and the MMSE-BF. Adaptive beamforming is discussed as an inverse problem. Within this framework, the authors suggest the use of alternative norms, e.g. the L/sub 1/ norm. Preliminary results for L/sub 1/ adaptive beamforming are presented. >

An experimental SMI adaptive antenna array for weak interfering signals

Abstract: A description is given of the modified SMI (sample matrix inversion) adaptive arrays in which the sample covariance matrix is redefined to reduce the effect of thermal noise on the weights of an adaptive array. This is accomplished by subtracting a fraction of the smallest eigenvalue of the original covariance matrix from its diagonal terms. In situations where the number of degrees of freedom of a modified SMI adaptive array is larger than the number of interfering signals incident on the array, the adaptive array provides the required interference suppression. An experimental modified SMI adaptive antenna array is described. Performance results are presented, and the experimental system is a three-element SMI adaptive antenna array and operates in a signal scenario consisting of a desired signal and an interfering signal whose source may be located at an arbitrary angular separation from the desired signal. >

Spatial pseudorandom array processing

Abstract: A pseudorandom permuting procedure and the associated array signal processing are introduced for resolving multiple coherent signal sources. Conventional adaptive beamforming algorithms fail to operate in such a situation, as their performance will degrade. In addition, when applied to an irregularly spaced array or when background noise is colored, most existing adaptive algorithms do not work. The method presented can handle irregularly spaced arrays, especially when some of the sensors have been damaged and even when the background noise is colored. The scheme is extended to a multidimensional irregularly spaced array for broadband signals, and it is verified by large-scale simulation. >

On a systolic implementation and the numerical properties of a multiple constrained adaptive beamformer

Abstract: An efficient algorithm for the multiple linearly constrained adaptive beamformer is developed. It is based on rank-one updating and downdating Cholesky factorizations. These operations can be realized by a sequence of complex Givens and hyperbolic rotations. A linear systolic array using CORDIC processors as the processing elements for implementing the algorithm is presented. The authors also investigate the numerical properties of the algorithm. >

A novel approach for robust adaptive beamforming

Abstract: A robust technique for adaptive array beamforming in the presence of steering vector error is proposed. It is based on the fact that an optimized steered-beam adaptive beamformer possesses the maximum output SNR when the steering vector coincides with the signal phase vector. The steering vector is adjusted iteratively to meet the actual signal phase vector. The resulting beamformer provides optimum performance in the sense of satisfying the single-look direction constraint without any tradeoff between the array performance and robustness. Thus the difficulties of using conventional algorithms can be overcome. Results of several computer simulations are given for illustration. >

Detection of targets in presence of strong jammers by adaptive beamforming

Abstract: A study is made of a matched system for detecting a target in the presence of background noise and strong jammers. The ideal receiver is analytically computed using classical decision theory, and its performances, which gives the upper bounds of the problem, is evaluated for a given scenario. Adaptive beamforming is then shown to be a good approximation of the unrealizable ideal receiver, and its performance is measured in simulation using the same scenario as for the ideal receiver. A comparison is made of the performance of the two systems. >

Novel multiple beam microstrip patch array with integrated beamformer

Abstract: A novel multiple beam microstrip antenna is described in which the bcamforming network and array are combined into a planar module. Results for a small 5-beam array are given that illustrate the action and indicate typical performance. The beamformer's low profile and low loss potential are attractive features applicable to modern communication terminals.

Localization of multiple broadband targets via frequency domain adaptive beamforming for planar arrays

Abstract: Computer simulation studies of a frequency domain adaptive beamforming algorithm for planar arrays are presented. The algorithm, which can localize multiple broadband targets, is a modified complex least‐mean‐square (LMS) adaptive algorithm, and can process an arbitrary number of harmonics. The algorithm provides estimates of both the depression and bearing angles of incident plane‐wave fields. Computer simulation results are presented comparing the average depression and bearing angle estimation errors as a function of the input signal‐to‐noise ratio (SNR) at a single element in the array, sampling rate, and harmonic number. The ‘‘full angular coverage’’ capability of the algorithm was also tested.

U4.10 a novel approach for robust adaptive beamforming

Abstract: This paper concerns the adaptive array beamforming in the presence of steering vector emr. A robust technique is proposed based on the fact that an optimized steered beam adaptive beamformer possesses the maximum output SINR when the steering vector coincides with the signal phax vector to iteratively adjust the steering vector to meet the actual signal phase vector. The resulting robust adaptive beamformer can provide the optimum performance in the sense of satisfying single-look direction constraint without making any tradeoff between the array performance and robustness Thus the difficulties of u$ing conventional algorithms can be overcome. Several computer simulations are given for illustration.

Systolic preprocessors for linearly constrained beamforming

Abstract: Systolic preprocessors for linearly constrained beamforming are developed using factorizations of the generalized sidelobe canceller. These preprocessors transform linearly constrained beamformers into unconstrained form, which permits unconstrained adaptive processors to be used for computing the beamformer output. Two preprocessors are developed for small numbers of constraints relative to the data dimension, and two for partially adaptive beamforming where the number of effective constraints is often large relative to the data dimension. All four preprocessors are applicable to arbitrary constraint sets. Tradeoffs between the various preprocessors are discussed. >

A novel accelerated convergence technique for adaptive antenna applications

Abstract: The authors introduce a novel adaptive antenna scheme. The technique involves the use of a fast digital processing algorithm in an adaptive feedback loop arrangement to compute updates for an analogue beamformer weighting circuit. This ensures a scenario-independent convergence performance, and because of the feedback path, can accommodate beamformer circuit errors and nonlinearities without undue degradation of the achievable cancellation performance.

Reducing the effects of target misalignment in an adaptive beamformer for hearing aids

Abstract: Previous work [Peterson et al., Acta Otolaryngol. (in press)] has shown that a multimicrophone adaptive beamforming system for hearing aids reduced noise effectively at input target‐to‐jammer ratios (TJRs) up to about 0 dB. However, that beamforming algorithm assumed that the target signal was identical at the microphones, and violation of this assumption caused degrading target cancellation when input TJR was greater than 0 dB. The present system uses a measure of intermicrophone correlation to estimate short‐time TJR and inhibits adaptation dynamically when TJR exceeds a selected threshold. Computer simulations of a two‐microphone system in anechoic and moderately reverberant environments show that this method reduces (speech) target cancellation when the identical target assumption is violated. The improved system reduces noise at both positive and negative input TJRs (±20 dB), with target misalignments up to 10 deg. Additionally, this processing scheme reduces the misadjustment noise caused by strong target signals in the adaptive feedback loop. [Work supported by NIH.]

as an Inverse Problem

Abstract: In previous work, we developed a minimum mean square error beanifornier (MMSE-BF). When compared to the minimum variance distortionless response beamformer (MVDR-BF), we concluded that it is specially suited for correlated returns. This improvement is at the cost of some frequency distortion in wideband applications. To circumvent this problem, we introduce the minimum mean square error distortionless response beamformer (MMSEDR-BF). Its behavior is compared with the MVDR-BF and the MMSE-BF. Adaptive beamforming is discussed as an inverse problem. Within this framework, we suggest the use of alternative norms, e.g., the L1 norm. Preliminary results for L, adaptive beamforniing are presented.

Systolic array for adaptive estimation of the eigenstructure of correlation matrices

Abstract: Calculations of eigcnyalues and eigenvectors of correlation matrices in modern spatial and temporal signal processing problems incvitabcly require intensive computations. Applications include spectral estimation, beamforming, direction finding, adaptive antenna arrays. and parameter estimation. In many c a m , thc environmcnt is non-stationary and the estimation of the eigenvalucs and the cigenvcctors has to he updated in an adaptive manner. In this paper, a new systolic array for adaptivc estimation of all or the dominant cigenvcctors of the correlation matrix is proposed. The operation of the array is bascd on the Simultaneous Iteration Method (SIM) algorithm which has good convergence rate and residual noise properties. After a brief review of existing adaptivc algorithms for eigenstructue estimation, the SljM algorithm and its desirable properties are presented. The new systolic array is introduced and the different architectures needed to implcment the Sl,V algorithm on the array are describcd.

Adaptive beamforming using multimode feed horn antennas

Abstract: The authors develop a new model for implementing adaptive nulling of interfering signals which is based solely on a single-feed horn antenna. The model uses a multimode feed antenna in which various modes are generated or extracted by means of slot coupling. Adaptive nulling is then carried out by treating each mode as an element of a phased-array antenna. The algorithm that is used to implement the adaptive nulling relies quite heavily on the QR decomposition method, a recursive least-squares algorithm which is amenable to systolic array processing. With proper choice of modes and design parameters for the feed horn antenna, simulation studies show that wide beamwidths (bandwidth), low sidelobe levels, and high signal-to-noise and interference ratio can be achieved with the proposed adaptive beamforming technique. >

Evaluation of a real‐time adaptive‐beamforming hearing aid

Abstract: A real‐time two‐microphone monaural hearing aid, based on the constrained adaptive beamformer proposed by Griffiths and Jim [IEEE Trans. Antennas Propag. AP‐30, 27–34 (1982)], has been implemented and evaluated. The beamformer adapts to preserve the target signal (assumed to be straight ahead) and to minimize the power of jammer signals arriving from all other directions. The basic Griffiths‐Jim algorithm is augmented with a method to inhibit adaptation in the presence of strong target signals [Greenberg et al., J. Acoust. Soc. Am. Suppl. 1 85, S26 (1989)]. The real‐time system employs the Motorola DSP56001 and was evaluated using a speech target and a single jammer for a variety of free‐space and head‐mounted microphone configurations in anechoic and reverberant environments. The system demonstrates very good performance (30‐ to 40‐dB gain from input to output in an intelligibility‐weighted spectral average of target‐to‐jammer ratio) for several broadside microphone configurations in the anechoic environment. Adaptation inhibition provides a robust insensitivity to target misalignment at high‐input target‐to‐jammer ratios. As expected, performance degrades with increasing reverberation; at the critical distance, the system provides 5‐ to 10‐dB gain. In extreme reverberation, the system performs at least as well as a delay‐and‐sum beamformer. [Work supported by NIH.]

Performance sensitivity of optimum beamformers

Abstract: The objective of this work is to study the effect of arbitrary and small look-direction errors on the gain-pattern of the Capon (1969) beamformer and on the optimum interference notch for the general case any number of sensors and sources, and to compare their relative performance. In this analysis, explicit expressions for the sensitivity functions, defined as the derivative of the array response with respect to look-direction errors, were derived. Using these expressions, the perturbations in the array gains at the source directions were evaluated and compared with the actual ones and they were found to be very close. >