Showing papers on "Beamforming published in 1993"

Blind beamforming for non-gaussian signals

Abstract: The paper considers an application of blind identification to beamforming. The key point is to use estimates of directional vectors rather than resort to their hypothesised value. By using estimates of the directional vectors obtained via blind identification, i.e. without knowing the array manifold, beamforming is made robust with respect to array deformations, distortion of the wave front, pointing errors etc., so that neither array calibration nor physical modelling is necessary. Rather suprisingly, ‘blind beamformers’ may outperform ‘informed beamformers’ in a plausible range of parameters, even when the array is perfectly known to the informed beamformer. The key assumption on which blind identification relies is the statistical independence of the sources, which is exploited using fourth-order cumulants. A computationally efficient technique is presented for the blind estimation of directional vectors, based on joint diagonalisation of fourth-order cumulant matrices; its implementation is described, and its performance is investigated by numerical experiments.

Phased Array Antenna Handbook

Abstract: Phased Arrays in Radar and Communication Systems. Pattern Characteristics and Synthesis of Linear and Planar Arrays. Patterns of Nonplanar Arrays. Elements, Transmission Lines, and Feed Architectures for Phased Arrays. Summary of Element Pattern and Mutual Impedance Effects. Array Error Effects. Special Array Feeds for Limited Field of View and Wideband Arrays.

On the potential of fixed arrays for hearing aids

Abstract: Microphone arrays with fixed (time‐invariant) weights are directed at enhancing a desired signal from one direction (straight ahead) while attenuating spatially distributed interference and reverberation. Using the theory of sensitivity‐constrained optimal beamforming [Cox et al., IEEE Trans. Acoust. Speech Signal Process. ASSP‐34, 393–398 (1986)], free‐field arrays of head‐sized extents were studied. The key parameters affecting array design and performance are the set of transfer functions from the target direction to each array microphone and the intermicrophone cross‐spectral densities for isotropic noise. Design variables included the orientation of the array, the number and [as motivated by Soede, Ph.D. thesis, Delft Univ. of Technol. (1990)] the directionality of the microphones within the array, and the complexity and robustness of the required processing. For broadside orientation, a variety of arrays based on cardioid and hypercardioid microphones gave very similar performance. They can provide ...

A photonic integrated-optic RF phase shifter for phased array antenna beam-forming applications

Abstract: A photonic in-phase/quadrature (I-Q) RF phase shifter utilizing two integrated optic Mach-Zehnder modulators has been tested. This phase shifter demonstrates the capability of 360 degrees phase control, >

Constant beamwidth beamforming

Abstract: The beamwidth of a linear array decreases as frequency increases. For wideband beamformers such as microphone arrays intended for teleconferencing, this frequency dependence implies that signals incident on the outer portions of the main beam are subject to the undesirable effects of lowpass filtering. The authors discuss several ways of attaining beamwidth constancy and present a method based on superimposing several marginally steered beams to form a constant beamwidth multi-beam. This method provides an analytically tractable framework for designing constant beamwidth beamformers. >

An implementation of a CMA adaptive array for high speed GMSK transmission in mobile communications

Abstract: The hardware implementation of an adaptive array as a technique for compensating multipath fading in mobile communications is described. The number of the antenna elements is four. The target communication system is modulated by 256 kbps Gaussian-filtered minimum shift keying (MSK) and has a time-division multiplexing (TDM) architecture with 24 time slots. Based on the digital beamforming concept, all of the signals and the array weights are digital-signal processed. The constant modulus algorithm (CMA) is employed for weight optimizing. In an additive white Gaussian noise channel, this system has 5.6-dB gain in an energy-per-bit-to-noise-density ratio at a bit error rate (BER) of 1.0*10/sup -3/, compared with a single antenna system. The result of the basic field test shows that the gain at a BER of 1.0*10/sup -3/ reaches 22.3 dB in a nonselective, slow Rayleigh fading channel given a 5 Hz maximum Doppler shift. >

Monopulse estimation with adaptive arrays

Abstract: The angle estimation problem with an adaptive phased array radar is considered. Because of the adaptation, the shape of the sum and difference beam is perturbed, therefore the usual monopulse formula produces errors if applied to the adaptive sum and difference beam outputs without correction. Correction values are derived for the slope and bias of the adaptive monopulse ratio for a planar array, and it is shown by example that this corrected adaptive monopulse procedure may be applied to the subarray outputs of a large array with partial digital beamforming. The bias can be further reduced by iterating the procedure, resulting in a ‘multistep corrected adaptive monopulse procedure’. This technique is also applicable to subarray output data. The method is applicable to arbitrary planar or volume arrays.

Superdirective arrays for hearing aids.

Abstract: Microphone arrays are the most effective of the techniques that have been proposed for improving speech intelligibility in noise for the hearing impaired. However, classical delay‐and‐sum beamforming provides very small amounts of array gain at low frequencies, while adaptive array processing has been shown to cancel the desired signal in the presence of strong room reflections. Superdirective arrays offer a heretofore overlooked solution in which optimal performance can be obtained for a stationary random noise field, but where the desired signal will not be canceled. A short constrained superdirective array suitable for hearing‐aid applications is proposed in this paper, and its theoretical performance is evaluated.

Broadband matched‐field processing

Abstract: Acoustic energy in the ocean propagates from a source to an array of sensors via multiple pathways. Conventional time‐delay‐and‐sum beamforming does not utilize energy contained in the multipath arrivals. In this paper, a generalized beamformer is presented which coherently recombines the multipaths to provide enhanced detectability of broadband transients as well as range and depth localization of the source. Modeling of the wave field at the array due to a broadband source is accomplished with a normal mode model using 1.0‐Hz increments across the 1‐ to 100‐Hz band. These results are combined to yield the impulse response from a specified source location to each of the array elements. The set of calculated impulse responses is used in the generalized beamformer to coherently recombine energy arriving along multipaths that exist from that source range and depth to the array. The location of a source is determined by examining the beamformer output at a number of candidate range/depth cells; a peak in the...

Base station transmitter antenna arrays with mobile to base feedback

Abstract: A central base station simultaneously communicates with several mobile users by allocating a separate time or frequency channel for each mobile to prevent undesired crosstalk. However, each time or frequency channel may be reused among several mobiles by means of an antenna array at the base station which points a separate beam at each user. The downlink beamformer would normally operate in an "open loop" mode, in which the base steers a mainlobe in the direction of each mobile. Such a system may operate effectively in a free space environment with no multipath. In the presence of scattering, open loop methods will not perform adequately. A new "closed loop" technique is presented in which each mobile user feeds back to the base estimates of the received signal amplitudes. Using feedback, the base station can achieve precision beamforming resulting in lower crosstalk and improved signal separation even in the presence of strong scattering environments. >

Blocked element compensation in phased array imaging

Abstract: In clinical applications using large apertures, a significant number of phased array elements may be blocked due to discontinuous acoustic windows into the body. These blocked elements produce undesired beamforming artifacts, degrading spatial and contrast resolution. To minimize these artifacts, an algorithm using multiple receive beams and the total-least-squares method is proposed. Simulations and experimental results show that this algorithm can effectively reduce imperfections in the point spread function of the imager. Combined with first-and second-order scatterer statistics derived from multiple receive beams, the algorithm is modified for blocked element compensation on distributed scattering sources. Results also indicate that compensated images are comparable to full array images, and that even full array images can be improved by removing undesired sidelobe contributions. This method, therefore, can enhance detection of low contrast lesions using large phased-array apertures. >

VLSI circuits for adaptive digital beamforming in ultrasound imaging

Abstract: For phased-array ultrasound imaging, alternative beamforming techniques and their VLSI circuits are studied to form a fully digital receive front-end hardware. In order to increase the timing accuracy in beamforming, a computationally efficient interpolation scheme to increase the sampling rate is examined. For adaptive beamforming, a phase aberration correction method with very low computational complexity is described. Image quality performance of the method is examined by processing the non-aberrated and aberrated phased-array experimental data sets of an ultrasound resolution phantom. A digital beamforming scheme based on receive focusing at the raster focal points is examined. The sector images of the resolution phantom, reconstructed from the phased-array experimental data by beamforming at the radial and raster focal points, are presented for comparison of the image resolution performances of the two beamforming schemes. VLSI circuits and their implementations for the proposed techniques are presented. >

Sonar sequences from Costas arrays and the best known sonar sequences with up to 100 symbols

Abstract: The authors construct new and improved sonar sequences by applying rotation, multiplication, and shearing transformations to Costas sequence constructions. A catalog of the best known sonar sequences with up to 100 symbols is given. >

Signal Processing for Swath Bathymetry and Concurrent Seafloor Acoustic Imaging

Abstract: Whether they use the multibeam or sidescan geometries, swath bathymetry sonar systems aspire to provide the widest cross-track coverage with the highest spatial resolution possible. Although these two requirements are often contradictory, a fair compromise can be achieved through a combination of array design and signal processing techniques. To this end, a version of the split-aperture correlator is used in sidescan sonar systems to obtain estimates of differential phase as a function of time of arrival of seafloor echoes. These estimates are subsequently converted to sequences of angle of arrival versus time and then to bathymetry. The same technique is used in beamformed systems to detect the time of arrival of the echoes at the zero crossing of the differential phase sequence. However, this technique is unreliable in the near-specular direction and other beamformed echo detection methods working in the time domain or the spatial frequency domain are considered. FFT beamforming techniques offer some echo detection implementation advantages allowing a choice between any of the aforementioned techniques. For each ping, once signals backscatterred by the seafloor have been processed for bathymetry, the magnitudes of the returns can be positioned at their corresponding horizontal distances in an amplitude cross-track profile. Accumulation over many pings of such profiles as lines of a raster image forms a sidescanned acoustic image of the seafloor. This article reviews the signal processing issues related to these echo detection techniques, and their implication for bathymetric resolution and swath coverage.

Compact deployable acoustic sensor

Abstract: A sensor for detecting acoustic energy emitted by a target is disclosed. The sensor comprises a housing including a plurality of transducers for receiving acoustic energy. The transducers are randomly deployed within a predetermined area around the housing, forming an array and a self-survey technique is utilized to determine the positions of the sensors. Beamforming techniques are utilized to analyze the acoustic energy detected by the transducers to provide the azimuth of a target and a class estimate of the target. A plurality of sensors may be utilized to provide target location, course and velocity.

Calibration of a distorted towed array using a propagation operator

Abstract: In this paper, a new method for array shape calibration is presented. This method can be applied to any antenna distortion, it does not require any a priori knowledge about the sources, and it is relatively simple to implement. A number of unknown remote point sources, propagating plane waves, are required to implement this method. An equal number of sensors having known relative positions are used as a geometric reference. The method relies on a propagation operator, called the propagator, which can be estimated from the cross spectral matrix of the received signals through a simple least‐square process. The performances of the proposed method are illustrated through simulations by showing how good accuracy in source bearing estimation, by both the classical beamforming method and MUSIC, can be recovered when the array shape is estimated.

Effect of pointing errors on the performance of the projection beamformer

Abstract: The projection beamformer generates a steering constraint vector for beamforming by projecting the original steering vector onto the signal-plus-interference subspace of the correlation matrix. This work investigates the effect of pointing errors on the performance of the projection beamformer. A general expression for the output SINR is first derived for the projection beamformer with multiple interferers present. The expression can also be applied to the directional-constraint beamformer except that the steering constraint vector is replaced by the steering vector. Based on the expression for the output SINR, the sensitivity of the projection and the directional-constraint beamformers to pointing errors are examined for the cases of no interferer and a single interferer. Moreover, the performance of the projection beamformer with the source number overestimated is evaluated. Theoretical analysis validated by computer simulations indicates that the projection beamformer performs better than the directional-constraint beamformer, even when the source number is overestimated. >

BER performance of CMA adaptive array for high-speed GMSK mobile communication-a description of measurements in central Tokyo

Abstract: This paper describes the performance of an adaptive array as a countermeasure to multipath fading for a 256 kbps Gaussian-filtered minimum shift keying (GMSK) mobile communication system operating in the 1.5 GHz band. An adaptive array having four antenna elements is implemented using the digital beam forming concept. The constant modulus algorithm (CMA) is employed for the adaptation process to ease the implementation. Measurements in central Tokyo of the bit error rate (BER) performance and an array pattern arising in the multipath environment are presented. Analysis of the array pattern confirms that the array succeeds in directing nulls to the delayed signals. BER performance shows an improvement in E/sub b//N/sub 0/, compared with that of a single antenna system, of 17.5 to 22 dB at a BER of 1.0*10/sup -2/ in a frequency-selective fading channel. >

Towed array shape estimation using frequency-wavenumber data

Abstract: Towed array beamforming algorithms require accurate array shape information in order to perform properly. Very often, these algorithms assume the array is linear. Unfortunately, the mechanical forces on the array due to ship motion and sea dynamics can change the shape of the array, which degrades the performance of the beamforming algorithm. A data-driven approach to estimating the relative shape of a nominally linear array is presented. The algorithm is robust in that it optimally combines information contained in a wide band of frequencies and source bearings. At the heart of the algorithm is a maximum-likelihood (ML) estimation scheme. The Cramer-Rao lower bound is derived and compared to the performance of the ML estimator. The utility of the algorithm is verified using both simulated and actual towed array data experiments. >

Spatial equalization of a radio-mobile channel without beamforming using the constant modulus algorithm (CMA)

Abstract: The author considers the problem of signal recovery by a multisensor receiver in a multipath propagation channel. She shows that a spatial filtering can recover the transmitted signals under two conditions, namely that both the number of sensors and the number of paths be larger than the length of the intersymbol interference, which is assumed to be finite. Thereby, signal recovery is realized without beamforming, i.e. without forming a beam towards one of the paths while cancelling the others, which would require a number of sensors much larger than the number of paths. Spatial filtering is able to cancel a multipath jammer, provided that an additional number of sensors, equal to the ISI (intersymbol interference) length of the jammer, is available. The spatial filtering is obtained adaptively by a blind algorithm, the CMA, which relies on the property of the transmitted signal to have a constant modulus. Simulation results on the present spatial CMA were compared with results for a temporal single-sensor CMA. The latter failed to recover a correct signal constellation while spatial CMA succeeded, even in a noisy environment plus a jammer. >

Adaptive beam forming using split-polarity transformation for coherent signal and interference

Abstract: Split-polarity transformation (SPT), which is incorporated into conventional linearly constrained minimum variance (LCMV) beamformers to decorrelate the desired signal from interference, is presented. The SPT processor does not distort the direction vectors associated with wave fronts impinging on the array, but it does reverse the phase of the signal coming from a specified direction. With the aid of SPT processing, the signal cancellation due to correlation between the desired signal and interference is almost eliminated. The design of a robust SPT processor for combating the effects of mismatch between idealized array model and actual scenario arising from causes such as sensor location, amplitude, and phase errors is discussed. A detailed performance study of the SPT-LCMV beamformer shows it to be robust against direction uncertainty in the assumed look direction. Numerical results are presented to demonstrate the effectiveness of the SPT-LCMV beamforming scheme is a coherent interference environment. >

Synthetic aperture processing for a towed array and a moving source

Abstract: In this paper the use of synthetic aperture processing is examined for simultaneously estimating a source’s unknown rest frequency, relative radial speed, and bearing from towed array hydrophone data. By examining this three‐dimensional estimation problem, it is concluded that for the situation where the velocity and/or rest frequency of the source are unknown there is no advantage in using coherent synthetic aperture techniques instead of incoherent methods to estimate the source’s bearing. The case of multiple sources is also examined and it is concluded that although long‐time coherent processing can improve bearing resolution for two closely spaced targets, the same results can be obtained by taking a long‐time Fourier transform and then using conventional beamforming methods.

Remedying the effects of array shape distortion on the spatial filtering of acoustic data from a line array of hydrophones

Abstract: The effects of both small perturbations and large deformations to the array's shape on both conventional and adaptive beamformers are shown for two frequencies: the spatial Nyquist frequency (or design frequency) of the array and a frequency about three times greater. Large shape deformations lead to a decrease in the conventional beamformer's output power for a beam steered in the direction of the signal source, together with an increase in the sidelobe levels (or secondary maxima), while small perturbations in the array shape have little effect. Signal suppression is observed to be far greater for the adaptive beamformer because it is very sensitive to system errors. The imposition of a weight norm constraint on the adaptive beamformer reduces the signal suppression only for small shape perturbations array shape estimation techniques are needed to reduce signal suppression for large shape deformations. The adverse effects of a nonlinear array shape on both conventional and adaptive beamforming are shown to be substantially reduced by applying techniques that estimate the coordinates of the hydrophones prior to beamforming. >

Reduced-complexity simultaneous beamforming and equalization for underwater acoustic communications

Abstract: Multichannel spatial signal processing of high-speed underwater acoustic communication signals is associated with computationally intensive receiver algorithms. Besides requirements in computational time, large adaptive filters operating under computationally efficient algorithms imply increased sensitivity to numerical errors and large noise enhancement, which limit their performance at high signalling rates. To overcome these difficulties, a receiver is proposed which consists of a multi-input multi-output combiner in which many input channels are coherently combined into a smaller number, suitable for subsequent multichannel decision-feedback equalization. Receiver operations are optimized jointly to ensure minimum mean-squared error detector performance. Experimental results obtained on a long-range shallow water channel with data rates up to 2000 bits per second demonstrate the receiver capability to fully exploit the spatial diversity of underwater multipath while keeping the receiver complexity at a minimum, thus allowing both fast convergence and little noise enhancement. >

Wavefront simulator for evaluating RF communication array signal processors

Abstract: A wavefront simulator that emulates plane wave propagation from multiple nsmitting antennas is used to evaluate, in both a static and a dynamic manner, an array processor used in RF communications to determine the location of transmitting antennas and possibly to perform beamforming for cancelling the energy of an interfering transmitter. The wavefront simulator generates time delay signals, giving the appearance of being emitted from different transmitters or sources, and simulates those signals as being received by an antenna array associated with the array processor. The array processor utilizes the time delay signals to calculate, e.g., the angle of arrival of the signals from the emitting antennas.

Analytical comparison of sensor signal processing enhancements for NDT synthetic aperture ultrasonic imaging

Abstract: The results of a detailed analytical study of the effects of sensor processing techniques on clutter suppression and image enhancement for nondestructive testing (NDT) systems are presented. A relatively simple beamforming/diffraction model is developed for near-field, wideband, synthetic aperture ultrasonic imaging in NDT systems. The physical model is used to quantitatively evaluate a variety of front-end sensor signal processing tradeoffs for the enhanced detection and sizing of defects. It is shown using statistical microscopic scattering calculations that a combination of increased spatial sampling and rectangular windowing can increase the signal-to-clutter ratio by approximately 10 dB while maintaining crack size resolutions well below future projected specifications. The sensor signal processing image enhancements are demonstrated by the construction of simulated strip-map SAFT (synthetic aperture focusing technique) images of metallic crack defects in the presence of large numbers of randomly distributed clutter (simulated grain boundary) scatterers. >

Optical system for microwave beamforming using intensity summing

Abstract: An optically based feed structure is used to distribute appropriately phased signals from a central signal generator to the individual elements of a phased array antenna. Any phase can be generated by adding together four phased signals (phased by 90 degree increments) if the amplitudes of the individual phased signals are appropriately controlled. By appropriately controlling the amplitude of the individual phased signals the amplitude of the resultant can also be controlled. In many cases it is desired to keep this amplitude constant. In some cases an amplitude taper across the phased array is desired.

Spatial rejection of direct blast interference in multistatic sonars

Abstract: A multistatic sonar system (50) employing a short duration automatic gain control (AGC) following beamforming of the return signal to reject spatial transients at the sonar waveform leading and trailing edges. The output of the beamformer (58) is delayed by a delay line (60A) before passing through a programmable gain (60B). The magnitude of an undelayed version of the beamformer output is also squared (60D) and averaged over a time window (60E). This average is used to normalize the delayed value of the beamformer output. The normalized output of the beamformer is compared (60C) with a threshold having a value much greater than unity, and the comparator output controls the gain circuit (60B). When the normalized value of the beamformer output exceeds the threshold value, the gain of the AGC is set to a value much smaller than unity. When the normalized value of the beamformer output does not exceed the threshold, the gain of the AGC is set to unity.

Dipole detection and localization processing

Abstract: Dipole detection and localization systems and methods employing improved processing techniques. The first processing technique provides for higher spatial resolution by implementing maximum likelihood beamforming processing to detect and locate a dipole in a manner analogous to the processing of wave propagation phenomena. The high resolution technique is comprised of using data derived from an array of magnetic sensors that is arranged in the form of a vector of coefficients in lieu of a matrix. This vector can be either the magnetic field components or the Anderson function expansion coefficients and is used to form a dyadic matrix, to which a multiple of an identity matrix is added to prevent singularity. The second improvement uses more than three Anderson function expansions to achieve detection and localization of the dipole. The use of more than three Anderson functions more effectively normalizes the noise dependence by providing better noise representations, and provides for detection and localization of multipoles. The third improvement provides for multiple-pass multiple-target processing of data measured by the array of sensors. First, the location of the largest dipole in the field-space of the array is determined, and its moment and orientation are determined. The field due to a dipole at that location and orientation is then removed from the measurement data and the process is repeated. The field is removed by subtracting out its contribution at each of the sensors in the array, and then estimating the location of the next largest dipole. Simulations have shown that this process provides the ability to resolve multiple dipoles, when applied as many times as is needed.