Showing papers on "Beamforming published in 1989"

Self-cohering large antenna arrays using the spatial correlation properties of radar clutter

Abstract: A technique for self-calibrating a large antenna array system in the absence of a beamforming point source is presented that uses the spatial correlation properties of radar clutter. The array could be real or synthetic. It is shown that if R(X), the spatial autocorrelation function of the field (as measured by adjacent element pairs), is ensured to be real and positive in the neighborhood of the origin, both periodic and aperiodic arrays can be synchronized, forming retrodirective beams pointing at the axis of symmetry of the radar transmitter, provided that the interelement spacing does not exceed some limit (the order of the size of the transmitting aperture). If the spatial autocorrelation function is complex but has a linear phase, it is shown that one can still synchronize both periodic and aperiodic arrays, while if the phase of R(X) is nonlinear, only periodic arrays can be synchronized. In both cases of complex R(X), a residual beam-pointing error occurs. Computer simulations and airborne sea clutter data are reported that verify the theory and practicality of the algorithm. >

Iterative design of a holographic beamformer.

Abstract: A design method is presented for computing the phase functions of an energy efficient system using two holographic elements for converting a Gaussian beam into a uniform beam with rectangular support in the far field of the source. The method is based on a modification of the Gerchberg-Saxton algorithm which includes an x-y separability constraint on the phase of one of the holographic elements. A beamforming system was fabricated using this method, and experimental results were obtained which support the design approach.

A multi-channel processing approach to real time network detection, phase association, and threshold monitoring

Abstract: This paper presents an approach to multi-channel processing of data recorded by a network of stations which represents an extension of the delay-and-sum beamforming traditionally applied in array processing. A number of geographical beam-steering points are defined, and for each beam a set of time-aligned traces derived from the network stations are processed together so as to extract, for each step in time, a set of features corresponding to that particular beam. Applying this approach to the network of the three regional arrays, NORESS, ARCESS, and FINESA, in Fennoscandia, we demonstrate its usefulness for associating regional phases detected at individual arrays and originating from the same event. We also address the problem of using a network to continously monitor the seismic noise field. The purpose here is to determine to what extent interfering events affect the monitoring of events within a target region. In this regard, we show that one can obtain, at a given confidence level, a continuous assessment of the upper limit of magnitudes of seismic events in the target region that would go undetected by such a network.

Line-array beamforming using linear prediction for aperture interpolation and extrapolation

Abstract: It is demonstrated that linear prediction can be successfully utilized in conjunction with otherwise conventional narrow-band line-array beamforming for aperture extrapolation (APEX), where there are significant SNR (signal-to-noise ratio) and resolution advantages compared to the conventional approach, aperture interpolation, where faulty sensor data are replaced, and a combined technique (ALPINEX). The performance of all three methods is investigated in connection with both simulated and real data. The APEX technique is shown to be amenable to a simple theoretical treatment. Further, the approach is demonstrated to offer a practical performance comparable to the well-known high-resolution technique, MUSIC, when the observation time is short. >

Multiple-beam array antenna

Abstract: An antenna capable of transmitting and/or receiving multiple beams and particularly adapted for use with satellites. In a preferred embodiment, the antenna includes a plurality of transmit channels interconnected with a first beamforming matrix. The first beamforming matrix communicates with channel preamplifiers such that two or more transmit signals can be inputted to the first beamforming matrix. The first beamforming matrix communicates with an array of transmit elements, which is preferably divided into two or more transmit subarrays. In one example of operation, two or more beams can be contemporaneously transmitted from the transmit subarrays with the at least one of the transmit subarrays contributing to the formation of at least two of the beams. In the sme preferred embodiment, as described immediately above, the antenna includes a plurality of receive channels interconnected with a second beamforming matrix such that two or more receive signals can be outputted from the second beamforming matrix to the channel receivers. The second beamforming matrix communicates with an array of receive antenna elements which is preferably divided into two or more receive subarrays. In one example of operation, two or more beams can be contemporaneously received by the receive subarrays with at least one of the receive subarrays contributing to the reception of at least two of the beams. In another embodiment, circulators and/or diplexers can be utilized so that common antenna elements and a common beamforming matrix can be employed for both transmission and reception. Consequently, two or more beams can be contemporaneously transmitted from and/or received at the transmit/receive antenna elements, with at least one of the subarrays contributing to the formation and/or reception of at least two of the beams.

Efficient digital beamforming in the frequency domain

Abstract: The formation of steered beams for a linear array of hydrophones is a necessary task in many sonar systems. In this paper, algorithms for beamforming digital signals in the frequency domain are compared for their computational efficiency and suitability for implementation on an array processor. Methods based on the chirp z transform (CZT) and on interpolation using finite‐impulse response (FIR) filters are shown to be the most efficient, although beamforming based on Goertzel’s algorithm is also competitive when the beams are formed in conjugate pairs.

Systolic array processor for MVDR beamforming

Abstract: An efficient systolic array for computing the minimum variance distortionless response (MVDR) from an adaptive antenna array is described. It is fully pipelined and based on a numerically stable algorithm which requires O(p2 + Kp) arithmetic operations per sample time, where p is the number of antenna elements and K is the number of look direction constraints.

Leaky-wave multiple dichroic beamformers

Abstract: The beamforming action of three planar frequency-selective surfaces (FSSs) placed in front of a waveguide aperture in a ground plane is demonstrated experimentally, together with the application to printed antennas. A simple ray theory illustrates the leaky-wave action for one FSS in position and the advantages and disadvantages of this beamforming technique are noted.

Multifunction array radar

Abstract: This definitive book supplies the information needed to specify and design a multifunction array radar system. With minimal mathematics, the book shows how radars smaller in aperture and power can meet demands formerly conceived for the larger rotating and phased array radars.

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. >

Adaptive antenna arrays for HF radio beamforming communications

Abstract: HF aerials for ship-shore communications consist of spaced dipole arrays. By appropriate adaptive phasing very high gain HF aerials are formed. On transmission a feedback signal is required from the receiver, otherwise similar algorithms are used to control the beamforming. A random phase algorithm has been devised for the phases applied to the array aerials, operating in four tranches of 100 iteration steps with progressively reduced maximum phase variation. The initial step has a phase variation in the range ±180°. The algorithm has the advantage that there is a high probability that a relatively high gain beam will be immediately formed in the required direction and thus the system can quickly settle towards a direction where the signal is weak. When in the transmit mode the receiver returns a signal to the transmitter giving the step number of the random phases which gives the maximum received signal.

Seafloor profiling by a wideband sonar: simulation, frequency-response optimization, and results of a brief sea test

Abstract: An ahead-looking probe of some kind, optical or acoustic, is critical when one is attempting seafloor exploration from a mobile platform. A single-frequency, split aperture sonar system can be used for this purpose, but a wideband monopulse sonar offers many advantages. It computes a running estimate of the vertical directional cosine of the source of the echo, and can thus reveal the positions of multiple wave scatterers as long as their echoes can still be time resolved. Theoretical studies of its performance have been made previously, but were directly applicable only to extremely simple seafloor geometries. A new time-domain digital simulation that largely circumvents this limitation has been developed. The simulation also provides a means for testing the theory and optimizing system parameters. The reverberation model does not account for some features of acoustic backscattering such as diffraction, but it is believed to be adequate for the investigation of most signal processing aspects of the sonar system. The theory of the simulation is developed and several examples are presented and discussed. In addition, some preliminary results are presented from a sea test that used the air-sea interface as a surrogate seafloor. >

Coherence effects on the detection performance of quadratic array processors, with applications to large-array matched-field beamforming

Abstract: The effect of coherence is examined using an exponential-power-law model for the signal wavefront correlation. Detection performance is quantified by the small-signal deflection criterion, which is first reviewed in the general setting of quadratic detectors and then applied to matched-field and optimal quadratic beamformers. It is shown that the detection performance of matched-field beamforming is substantially degraded for large arrays and typical coherence lengths. The optimal quadratic processor reduces this coherence loss, but at a cost of greatly increased processor complexity. Several suboptimal beamformers with reduced computational load are also developed, and their detection performance is compared to that of the matched-field and optimal beamformers. One suboptimal processor, the subarray beamformer, has proved outstanding for this application in three respects: it can realize within 1 dB of optimal performance; it is robust over a class of correlation functions; and it entails a computational burden no greater than that of full-array matched-field beamforming. >

A broadcast reference technique for self-calibrating of large antenna phased arrays

Abstract: A modified phase-multilateration technique is proposed for calibrating the element locations of a flexible phased array The technique requires several auxiliary beacons The calibrating system measures the phase, relative to a broadcast reference, of each beacon signal at each array element Automatic calibration of the differential phase delay in each antenna element signal path is included in the self-survey process Digital beamforming is used for array signal processing The beacon locations can be accurately or loosely known For the system with loosely known beacon locations, an additional baseline measurement is required, and the beacon locations are treated as unknown variables It is shown that a simple variable transformation can transform the mathematical equations of this technique into a form identical to that of the corresponding equations of previous cabled reference systems The mathematical properties of the algorithm and its associated tolerance analysis are therefore readily available A combination of the versions of this technique with accurately known and loosely known beacon locations is shown to have better performance than the individual version >

Underwater Acoustic Data Processing

Abstract: I Propagation and Noise.- Fundamentals of bistatic active sonar.- Sources of low-frequency sound in the sea.- Application of the output error system identification method to the calibration of underwater acoustic transducers.- Low frequency sector scanner using NLA.- Acoustic coherence loss due to ocean boundary interactions.- Advanced methods for the investigation of the underwater channel.- A new algorithm for the identification of distorted wavefronts.- The multipath coherence function for correlated random channels and a moving source.- Vertical directionality of ambient noise at 32 N as a function of longitude and wind speed.- The influence of bubbles on acoustic propagation and scattering.- In-situ measurement of elastic properties of sea ice.- Generalized mixture noise models for U. W. A..- Some aspects of sound propagation in shallow water: estimation of source and sound channel parameters.- Mediterranean underwater ambient noise model.- Determination of the acoustic properties of the sea floor by measuring the angle dependency of the reflection coefficient.- The influence of random thermocline displacements on shallow water transmission loss.- A review of target strength estimation techniques.- Near field target strength measurements.- Efficient processing and displaying of active systems data.- Performance of incoherent pulse compression of costas signals.- A numerical and analytical approach for pulse propagation in refracting and random media.- II Signal Processing.- Future trends in sonar signal processing.- Active and passive localization: similarities and differences.- State estimation of moving active targets by reverberation analysis.- Signal processing in the linear statistical model.- Parameter estimation of signals corrupted by noise using a matrix of divided differences.- Factor analysis and estimation of covariance matrix.- Rayleigh estimates for high resolution direction finding.- High discrimination target detection algorithms and estimation of parameters.- Parametric methods for estimation of signals and noise in wavefields.- Enhanced minimum variance beamforming.- Optimal estimation and beamforming.- Adaptive processing of broadband acoustic signals.- Beamforming on linear antennas with optical processors.- Adaptive methods in temporal processing.- Passive array processing: from conventional to high resolution concepts.- Beamforming in the presence of correlated arrivals.- High-resolution spatial processing with short observation times.- Estimation for array processing of spatial noise correlations in the presence of sources.- Spatial array processing by the method CLEAN.- A robust adaptive array structure using the soft constrained LMS algorithm.- Design and development of an acoustic antenna system for industrial noise source identification.- A small aperture acoustic direction finder.- Inverse problems: a tutorial survey.- Applied modelling to underwater vehicles identification.- The effect of mismatch on array processors with normal mode replica vectors.- Robust beamforming for matched field processing under realistic environmental conditions.- Threshold extension by nonlinear techniques.- Application of acoustics in the land environment.- Systolic array implementation of parallel Kalman filtering for heave compensation in underwater acoustic data processing.- TMA performance for towed arrays of low manoeuvrability.- Multi-tracks association for underwater passive listening.- New observability criterion in target motion analysis.- Bearings only target motion analysis.- Models for the application of Kalman filtering to the estimation of the shape of a towed array.- Frequency line tracking algorithms.- Localization of far-field sources with an array of unknown geometry.- Passive localization.- Inter-array data association and target motion analysis.- Joint delay and signal determination.- III Post Processing.- Use of multicolor displays for sonar detection.- Artificial intelligence and signal understanding.- An artificial intelligence approach to multipath localization and tracking.- 3D reconstruction and recognition from multiple views and with different acquisition constraints.- Expert system applications in underwater acoustics.- Classification of ships using underwater radiated noise.- Practical experience gained during the building of an expert system for the interpretation of underwater signals.- Knowledge-based interpretation of passive sonar data.- "Constant capacity," DSP architecture - an historical perspective.- Parallel computing you can do.- Practical graph partitioning algorithms for SONAR.- Concurrency in digital signal processing.- An experimental sonar system using transputers.- Summaries of Workshops.

Postbeamformer interference canceler with improved performance

Abstract: A postbeamformer interference canceler (PIC) processes the signals derived from an antenna array by forming two beams using fixed beamforming weights. The weighted output of one beam, referred to as the interference beam, is subtracted from the other beam, referred to as the target beam. In a conventional beamforming method, both beams are formed using equal weighting on each element. For this case the PIC processor suppresses a substantial amount of the signal; its output contains a large amount of residual interference when the weight of the interference channel is adjusted to minimize the mean output power of the PIC. This article presents and analyzes two methods to reduce the signal suppression by the PIC. The analysis shows that in one case the signal suppression has been completely eliminated, while in the second case the output of the PIC does not contain any residual interference. The examples presented show that the output signal to noise ratio (SNR) of the PIC using these beamforming methods is larger than the output SNR of the PIC using conventional beamforming methods.

Digital beamforming calibration for FMCW radar

Abstract: Three sources of beamforming deterioration are identified for an FMCW radar. These all relate to time-of-arrival and cable delays. There are small range-dependent phase errors at the receiver outputs. The delays cause frequency shifts, which produce phase errors at the discrete Fourier transform (DFT) outputs after ranging. These frequency shifts cause amplitude errors when the signals are resolved into the same range bin. Two methods of compensating for phase and amplitude errors are proposed. The first corrects for phase errors prior to beamforming. Amplitude errors are ignored but the residual error is usually small. The second method aligns the receiver frequencies digitally before ranging. This eliminates the phase errors and the amplitude errors at the DFT range estimates. >

Sonar testing apparatus

Abstract: Test apparatus for a sonar system utilizing beam former circuitry wherein a quantitative test of the signal channels of the sonar system, including the transducers themselves may be automatically tested in situ utilizing the beamformer circuitry of the sonar system itself.

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

Analysis of a recursive least-squares signal-processing algorithm

Abstract: This paper concerns a popular recursive least-squares algorithm for beamforming. The way in which the method's stability depends on the condition of a special matrix is analyzed in detail, and a new procedure for estimating the error in the computed solution is presented.

Directional constrained adaptive array using symmetric amplitude control

Abstract: A single directional constrained adaptive array using symmetric amplitude control is presented. The adaptive algorithm is based on the method of projection gradient. The steady-state and transient performances of the array are investigated. Detailed transient analysis for the case of one interference source is performed to get further insight. Results are compared with those obtained with conventional complex weights. It is shown that the new array has a faster real-time convergent rate and its optimal output SNIR is less susceptible to pointing errors.

Evaluation of a Reconfigurable Architecture for Digital Beamforming Using the OODRA Workbench

Abstract: This paper describes the application of the Object-Oriented Design of Reliable/Reconfigurable Architectures (OODRA) workbench to the performance simulation of an adaptive digital beamforming architecture. The performance effects due to chip partitioning and reconfiguration for fault tolerance and yield enhancement are presented. The experiments described illustrate the flexibility of the OODRA work-bench in architectural-level performance evaluation of algorithm-specific reconfigurable architectures.

Holographic Beamformer Designed By An Iterative Technique

Abstract: A fundamentally energy efficient, two element holographic beamformer for converting a Gaussian beam into a rectangular uniform beam in the far-field was designed, fabricated, and tested. The design method is based on a modification of the Gerchberg-Saxton iterative Fourier transform algorithm to include an x-y separability constraint on the phase of one of the holographic elements. This modification gave a design with improved beamforming performance. The two required holographic elements were fabricated using a combination of optical and computer-generation techniques. The experimental results verify the effectiveness of the design approach.

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. >

DFT processing of bistatic arrays operating in sequential mode

Abstract: A multiple-target-detection technique based on phase processing of data collected in a sequential mode operation is presented. The elements are assumed to be circular dishes. No tapering or phase-shifting hardware is needed. The firing of the elements is sequential with all or some of the elements receiving in parallel. The sequential operation and phase processing applied to a real array show a significant improvement in the array pattern. However, the attenuation of the secondary patterns is not sufficient because the diameter of the dish is limited by the element spacing. the secondary patterns can be reduced to an acceptable level using bistatic arrays. In the present application, the bistatic array consists of two adjacent linear arrays: one transmitting array with large dishes and spacing, and a receiving array with small dishes and spacing. >

Phased array fundamentals

Abstract: Phased arrays are a major class of antennas which offer many important features such as electronic beam scanning. Their flexibility has to be assessed against their complexity and cost. The fundamental principles of a phased array are described. The properties of radiating apertures are given first, as these are basic to both aperture antennas and arrays as sampled apertures. Linear array analysis then introduces beam steering and null steering. Array beamforming shows how array excitation is controlled. This is followed by discussion of the effects of systematic or random excitation errors. Planar array analysis includes different array grid structures and the important topic of mutual coupling. Finally active arrays are briefly introduced.

The effects of ADC nonlinearities on digital beamformers

Abstract: The output beam pattern of a digital beamformer (DBF) using the full dynamic range of a high-speed (HS) analog-to-digital converter (ADC) was investigated. A simple model for the full-range HS ADC nonlinearity was developed. It is shown that the nonlinearities in an HS ADC can cause substantial degradation in a digital beamforming array when interference is present. This degradation is caused by the appearance of 'phantom' interferers created by the nonlinearity of the ADCs. These phantom interferers are harmonics of the original interferer and signal. >

Elliptical boundary arrays for coherent and incoherent imaging

Abstract: In the imaging of far-field source distributions by a planar array of antenna elements, the quality of the image is determined by the beam pattern of the array. The physical size of the aperture ultimately determines its resolution capability, and the excitation or weighting imparted to the aperture controls the sidelobe characteristics. For a given number and spacing of antenna elements, the largest physical apertures are formed if the elements are distributed only around the boundaries of the apertures. This, however, results in beam patterns with rather high sidelobes. Thus it is of clear practical advantage to have available a technique for effectively filling in the interior of such a boundary aperture through appropriate signal processing applied to the boundary element signals. The paper builds on early results concerning such a technique, which made use of incoherent (receive-only) imaging systems. It is shown that for both incoherent imaging, the latter implemented using a transmit/receive scheme, it is possible to obtain such a filling-in or interpolation effect when imaging with an array composed of an elliptical (or circular) arrangement of array elements. Explicit synthesis theorems which give a specific constructive approach to implementing this effective interpolation to any desired degree of accuracy are given, and this approach is illustrated by giving the results of simulations, both with and without element location errors.