Showing papers on "Adaptive beamformer published in 1991"

Application of Toeplitz covariance estimation to adaptive beamforming and detection

Abstract: Array processing algorithms for adaptive beamforming and the adaptive detection of radar targets in unknown interference are proposed and simulated. These algorithms rely on estimates of the interference covariance which are constrained to be Toeplitz. It is shown that the incorporation of this constraint into the covariance estimation has a significant impact on the adaptive beamformer and adaptive detector performance. >

Microwave imaging techniques

Abstract: The Mathematics of Microwave Imaging Wideband Effects Problems in Microwave Imaging Diversity Transmitter-Location Diversity Deconvolution Adaptive Beamforming I Adaptive Beamforming II Data Reduction Superresolution Display Techniques References Index

Minimum variance beamforming with soft response constraints

Abstract: Soft constraints on the beamformer response to the signal are examined in the context of minimum variance beamforming. A quadratic constraint on the beamformer weights is used to control the mean-squared error between a desired response and the actual response in the signal direction. The constraint is purposely chosen to permit distortion of the signal with the goal of achieving improved interference cancellation. Under the assumptions of known signal direction and spectral shape the signal-to-noise ratio is shown to be a nondecreasing function of the mean-square distortion. The distortion presented to the signal is easily computed from the beamformer weights and can be equalized after beamforming if desired. Properties of this beamforming method and its relationship to linear constrained beamforming are discussed. Simulations verify analytic results and illustrate the utility of the soft constrained approach. >

Adaptive beamforming with conjugate symmetric weights

Abstract: The Hermitian persymmetric maximum likelihood estimator (MLE) is better than the conventional MLE in estimating the covariance matrix of a symmetrically distributed antenna array. Though using the Hermitian persymmetric MLE improves the performance of adaptive beamforming, it increases the computational load due to the forward-backward averaging of array input sample data. By observing that the weight vector obtained from the Hermitian persymmetric MLE is conjugate symmetric if the phase origin is chosen at the array geometry center, methods are developed to reduce the computational load of the direct-form beamformer and the generalized sidelobe canceler (GSC). The weights of both beamformers can be adapted with real computations, with significant savings in computation time. >

A constraint projection approach for robust adaptive beamforming

Abstract: When the optimal signal-to-interference-plus-noise-power ratio (SINR) at the output of the linearly constrained minimum variance beamformer is large, small perturbation errors in the steering constraint can cause substantial degradation of the SINR. Even if there is no perturbation, an excessive number of data snapshots can be required for the SINR to converge to its optimal value. To alleviate both the perturbation and sample data problems, a novel approach is proposed-projecting the nominal steering vector onto the signal-plus-interference subspace to obtain a new steering constraint. Analysis suggests that the problems of perturbation error and insufficient data are closely related. >

A multi-DSP implementation of a broad-band adaptive beamformer for use in a hands-free mobile radio telephone

Abstract: An implementation of a broadband adaptive array on a multiprocessor digital signal processing system for use in a hands free mobile radio telephone is described. This implementation of a five-microphone adaptive Griffiths-Jim array can handle FIR filters with up to 128 taps behind each microphone at a sampling rate of 8 kHz. The filter structure makes it possible to combine an adaptive array with a noise canceler. The near-field problem has been solved by using focusing, a speech-controlled adaptive algorithm, and a short hourglass. Preliminary measurements indicate a considerable potential for this technique in hands-free mobile telephony. The array gives a 20-30 dB suppression of a broadband jammer covering 300-1100 Hz, even with three reflecting walls surrounding the microphone. >

Experimental and theoretical comparison of some algorithms for beamforming in single receiver adaptive arrays

Abstract: Adaptive null steering in single receiver adaptive arrays is discussed. The single receiver structure allows only output power for a given set of weights to be measured. The problem, then, is to adaptively adjust the weights of the antenna array based on output power measurements only so as to reject interference signals while maintaining a fixed response in a given look direction. The authors determine the optimal beamformer weights in a single step, by estimating the covariance matrix of the array sensor outputs using a weight perturbation technique. Based on this covariance matrix estimate, three different approaches for finding the beamformer weights are studied. The first corresponds to a sample matrix inversion scheme, with the sample covariance matrix replaced by the one obtained from the perturbation estimation method, while in the second approach the weights are determined using an eigenvalue decomposition of the covariance matrix estimate. In the third approach the directions-of-arrival (DOAs) of the incoming wavefronts are first estimated, and then, in a second step, the beamformer weights are calculated from the DOA estimates. The advantage of the third approach is that this method is not affected by correlation between the different sources. >

Applications of minimum redundancy arrays in adaptive beamforming

Abstract: It is shown, through analysis and simulation, that the use of a minimum redundancy array (MRA) in conjunction with an adaptive beamformer results in performance superior to that attained by a comparable system based on an array with uniformly spaced elements, or uniform array (UA) in terms of rejecting interferences located in close angular proximity to the look direction. Further, it is demonstrated that choosing the adaptive elements of a thinned adaptive array (TAA) based on a minimum spatial redundancy criterion, rather than spacing them uniformly, results in improved rejection of main lobe interferences, with negligible degradation in sidelobe interference rejection capabilities.

The PDF of adaptive beamforming weights

Abstract: The author derives a closed-form expression for the marginal probability density function (PDF) for the weight vector coefficients in a minimum-variance distortionless response (MVDR) adaptive beamformer, when the snapshots are independently identically distributed (IID) normal and the weights are computed via sample matrix inversion. The marginal PDF allows one to determine the dynamic range required to avoid saturation (with a specified degree of probability) in digital and/or analog implementation of beamforming weights. >

A beamformer for the acquisition of evoked potentials

Abstract: Evoked potential (EP) measurements using a delay and sun beamformer acquisition system are described. This system has the potential for significantly improving the signal/noise ratio (SNR) in EP measurements. The performance of the beamformer is characterized in terms of the number of electrodes and cross-channel correlation. When compared to conventional ensemble averaging, the beamformer reduces the number of response repetitions required to achieve a given SNR by a factor which approaches the number of channels in the acquisition system. >

On the number of signals whose directions can be estimated by an array

Abstract: The dependence of direction finding accuracy and sensitivity on the number of signals and their separation is investigated. Using the Cramer-Rao lower bound on the direction-of-arrival estimates, it is shown that accuracy degrades rapidly with the number of signals if the signals are separated by less than a beamwidth. Using results on the sensitivity of the direction estimates to calibration errors, it is demonstrated that the sensitivity to calibration errors increases rapidly with the number of signals if the signals are separated by less than a beamwidth. >

Adaptive Beamformer Performance In Reverberation

Abstract: Introduction. Recent applications of adaptive filtering to hearing aids [4-81 have shown that very simple two-microphone systems can provide large improvements in target-to-jammer ratio under anechoic conditions. Some of these studies also considered non-anechoic conditions and showed that the presence of reverberation has a strong effect on performance. Because of differences among the acoustic and signal-processing conditions of these studies, a more detailed summary cannot be given. The present study illustrates , for a particular adaptive beamformer, the effects of reverberation on performance and the interactions among reverberation, target-to-jammer ratio (TJR) and filter length (L). System Description and Methods. The system used here (Figure 1) is a modified, two-microphone version of the Griffiths-Jim (21 constnained adaptive beamformer. The modifications were developed to deal with the problems of misadjustnient and misalignment at high TJRs and do so by exploiting the fluctuations in speech to allow adaptation during intervals of low TJR [l]. The first method employs the input correlation p between bandpass-filtered microphone signals as a measure of TJR and inhibits adaptation when p exceeds a threshold. The second method includes output power in the normalization of the weight update: Aw&z + 1) = 2ay[n]d[n-k]/{L(P,,[n] + Pd[n])}, where Py and P d are running estimates of power in the system output and the adaptive filter' The study employed computer simulations of this beamformer with 7-cm spacing between microphones in free-space. Input signals were generated by convolving single-talker target and babble jammer sources with synthetic source-to-microphone impulse responses [3]. For all conditions, the target was located at 0", broadside to the array, the jammer was at 45", and both sources were 0.9 m from the center of the array. Output target and jammer were measured separately through use of a master and two slave processors. The master processed target and jammer summed together, while the slave systems processed the target and jammer separately using adaptive filter weights copied from the master. The performance metric, GI, is a spectrally-weighted gain in target-to-jammer ratio from input to output, measured in the steady-state [6]. Results. A sampling of rooms and source/array geometries was simulated to study the joint effects of TJR, degree of reverberation, and filter length. Condition A employed a room with dimensions 5.2 x 3.4 x 2.8 mtders and a uniform absorption coefficient of 1.0 (anechoic), 0.6, or 0.2, resulting in direct-to-reverberant energy ratios at the array of 00, 5.7,oI-2.4 dB, respectively. The …

Receiving method and receiver for discrete signals

Abstract: The invention relates to an adaptive receiving method and a receiver comprising an adaptive equalizing device for processing a received signal. The receiver further comprises an adaptive detector utilizing the self-organizing map principle and operationally connected after the adaptive equalizer. The adaptive equalizer is controlled on the basis of an error between a processed signal and a detected signal.

A robust method in adaptive array processing for random phase errors

Abstract: Robust constraints have been developed that protect signal-to-noise performance of a desired signal in phase perturbed adaptive beamformers. Linearly constrained adaptive beamforming is powerful in receiving desired signals while nulling unwanted jammers. Unfortunately, it is very sensitive to phase perturbations in high signal-to-noise ratio environments. Such perturbations are caused by channel phase errors, array element placement errors, steering errors, and frequency errors. The addition of robust constraints increases performance in these perturbed environments. The subsequent algorithm was tested on data produced by a thirty-six element linear RF array and shown to enhance signal-to-noise performance. >

Exact least squares adaptive beamforming using an orthogonalization network

Abstract: The pros and cons of various classical and state-of-the-art methods in adaptive array processing are discussed, and the relevant concepts and historical developments are pointed out. A set of easy-to-understand equations for facilitating derivation of any least-squares-based algorithm is derived. Using this set of equations and incorporating all of the useful properties associated with various techniques, an efficient solution to the real-time adaptive beamforming problem is developed. >

High resolution passive array processing : An overview of principles

Abstract: High resolution array processing has been an important field of interest. This is due to the fact that often, and this is particularly true for passive sonar, the array aperture is small, which results in a weak resolving power, and also secondary lobes are difficult to maintain at a sufficiently low-level to reduce the masking effect due to high level jammers.

Fault-Tolerance for Parallel Adaptive Beamforming

Abstract: Beamforming is used in performing spatial filtering from an array of sensors so as to minimize the undesired interferences [23]. Under nonstationary conditions, adaptive beamforming is definitely necessary if certain target performance is demanded. However, the required massive computation makes it very difficult for real-time applications. Seeking fast algorithms for adaptive beamforming is therefore of great interest for signal processing researchers. In particular, the advent of maturing VLSI technologies has triggered many researches on parallel algorithms and architectures. Among them, application specific array processors [17] are most attractive for high speed adaptive beamforming.

Broadband adaptive beamforming based on coherent signal subspace using spatial interpolation preprocessing

Abstract: The coherent signal-subspace (CSS) approach to broadband adaptive beamforming for correlated signal and interferences requires preprocessing by transformation matrices to focus signal subspaces at various frequencies into a common one. The transformation matrices often call for preliminary estimates of the directions of arrival of impinging signals through a spatial periodogram, and these preliminary angle estimates, if they are inappropriately selected, especially in the multigroup multiple signals, often result in an estimation error in the transformation matrices, and the array beamforming performance can be then degraded. Therefore an alternative method of preprocessing, called spatial interpolation, which does not require preliminary angle estimates, has been used instead of transformation matrices to estimate the arrival angles of signals impinging on a linear uniform array. The paper extends this idea to broadband adaptive beamforming. Performance comparisons of nulling coherent interferences using CSS with the two sorts of preprocessing are presented. Simulation results show the effectiveness of the spatial interpolation in adaptive beamforming.

Parallel and Fully-Pinelined Instantaneous Optimal Weight Extraction for Adaptive Beamforming Using Systolic Arrays

Abstract: Systolic algorithms and architectures for parallel and fully pipelined instantaneous optimal weight extraction for multiple sidelobe canceller (MSC) and minimum variance distortionless response (MVDR) beamformer are presented The proposed systolic parallelogram array processors are parallel and fully pipelined, and they can extract the optimal weights instantaneously without the need for forward or backward substitution. We also show that the square-root-free Givens method can be easily incorporated to improve the throughput rate and speed up the system. As a result these MSC and MVDR systolic array weight extraction system are suitable for real-time very large scale integration (VLSI) implementation in practical radar/sonar system. >

Representation of external ear transfer function via a beamforming model

Abstract: The modeling of the behavior of the external ear with a beamformer is proposed. The output of a beamformer is a weighted combination of the data received at any array of spatially distributed sensors. The beamformer weights and array geometry determine its spatial and temporal filtering characteristics. A least squares procedure for choosing the weights so the beamformer response approximates the measured response of the external ear is described. The effectiveness of this model is demonstrated by designing a beamformer of several hundred weights which duplicates and interpolates the measured external ear response of a cat over broad ranges of frequency and direction. >

Adaptive beamforming using recursive eigenstructure updating with subspace constraint

Abstract: An algorithm is presented for updating the adaptive beamformer weights using recursive eigenvalue decomposition (EVD) of a covariance matrix and subspace constraint. This algorithm exploits the subspace structure that the covariance matrix of the interference sources and the noise is a low-rank matrix plus a diagonal matrix. This eigenspace characterization approach avoids the numerically unstable recursive procedure based on the matrix inversion lemma. Moreover, the subspace property makes it possible to develop a fast algorithm by monitoring only the principal eigenvalues and eigenvectors and the noise eigenvalue.

An ASIC design for adaptive beamforming

Abstract: Adaptive beamforming is based on the RLS (recursive least squares) algorithm realized by the RLS systolic array (RLSSA) proposed by McWhirter in 1983. The authors use CORDIC (CO-ordinate Rotation Digital Computer) algorithm to implement RLSSA. Only shifting and adding operations are employed to realize all the primitive array's complex procedure. The authors have specially designed some proposed logic and circuits. After all levels simulation, the layouts based on CMOS 1.2 um technology. The system can reach 2 Mhz sampling rate with robust performance. >

A customized beamformer system for acquisition of speech signals

Abstract: The authors describe algorithms and hardware used to implement an acoustic beamformer for the purpose of speech acquisition. Beamforming algorithms studied include conventional delay and sum, Frost, and the generalized sidelobe canceler. In each case, parameters related to the algorithm such as the number of sensors, the number of tops, and the rate of adaptation were studied in order to optimize performance with speech signals yet maintain feasibility of real-time implementation on a single digital signal processing (DSP) chip. Results with simulated and real data as well as with an audio tape are presented. Eight channels of microphone data were simultaneously digitized using TLC32044 analog-digital converters, multiplexed in a T1 data frame format, and sent to a TMS320C30 serial port for subsequent processing. >

An adaptive minimum redundancy array for digital communications

Abstract: The performance of a minimum redundancy array (MRA) used in conjunction with an adaptive combiner for receiving digital communications signals under co-channel interference and multipath conditions is compared with a similar system based on a uniform array. Simulation and analysis of four-element array-based systems indicate that for interference in close angular proximity to the desired signal, or for multiple paths which show small angular separation, the MRA-based system is superior. Using numerical integration techniques, it is shown that the overall bit error probability is smaller using the MRA-based system than with the uniform-array-based system.

Design considerations for an adaptive phased array radar

Abstract: The author discusses the implications of alternative adaptive nulling architectures for the design of a space-based radar, taking into account typical practical constraints. Two different classes of adaptive processing architecture are considered: the sidelobe canceler and the fully adaptive array. These are shown to achieve different tradeoffs between inherent complexity, requirements upon the system specification, flexibility, and performance. The design of the 'front-end' signal processing (from antenna design through ADC (analog-to-digital converter) design and clutter processing) has an impact on the use of an adaptive algorithm. Conversely, adaptivity places constraints and performance criteria on other parts of the system. >

A Systolic Array for Recursive Least Squares Minimisation and Adaptive Beamforming

Abstract: A systolic/wavefront array for linearly constrained recursive least squares minimisation is described in the context of adaptive antenna array beamforming. The architectural and numerical advantages of this signal processing technique are discussed.

Noise Reduction For Hearing Aids: Evaluation Of The Adaptive Beamformer approach

Abstract: The adaptive beamformer is a promising approach for noise reduction in hearing aids. In order to explore the requirements for a useful realization, the complex behaviour of the adaptive beamformer was investigated by theoretical analysis and experiments in different acoustic environments. It was found, that an adaptation inhibition in the presence of strong target signals is indispensable in order to ensure satisfactory performance at high signal-tonoise ratios. If notable attenuations of noise in moderately reverberant rooms are to be achieved, adaptive filters of at least 500 taps in length and a proper choice of the delay in the signal path have shown to be essential.