Showing papers on "Adaptive beamformer published in 1979"

Adaptive structures for multiple-input noise cancelling applications

Abstract: This paper is concerned with problems in which the interference present in a primary signal is reduced using a sum of M linearly-filtered reference signals. These latter signals contain interference components which are correlated with that present in the primary. Examples occur in antenna array processing and in multiple-axis seismometer recordings of geophysical data. In the structures of interest, the linear filters are adaptive and employ a lattice configuration. Previous work in this area has been restricted to the case of a single reference signal. The multiple-reference case is shown to have unique characteristics which do not appear in the single-dimensional case. Examples illustrating this difference are presented.

Shifted sideband beamformer

Abstract: A fundamentally different time domain beamformer structure is described which can be used to process bandpass sensor signals efficiently. The beamformer operates directly on complex, frequency translated, single sideband representations of the input signals to obtain a similar representation of the beam output. Such representations are typically obtained by complex demodulation of the signals to facilitate the use of bandwidth sampling procedures. This new technique, which is referred to as the shifted sideband beamformer, is functionally a time-domain beamformer but it combines attributes of both time-domain and frequency-domain beamforming. Shifted sideband beam-forming has the advantage that beamformer vernier delay and throughput requirements depend on the frequency content of the translated band rather than of the original band. This paper discusses the potential hardware savings associated with shifted sideband beamforming in terms of analog to digital conversion, cable bandwidth, digital processing and, also, signal conditioning hardware. The impact of delay quantization on beam-pattern structure is compared for a shifted sideband and a conventional digital implementation. Beamformer throughput is also analyzed for both implementations. A further reduction in the beamformer throughput requirement is demonstrated by the use of digital interpolation in conjunction with the shifted sideband beam-forming concept.

A constrained minimum power adaptive beamformer with time-varying adaptation rate

Abstract: A linear adaptive algorithm was developed for array beamforming purposes. The design goal for the algorithm is to minimize the squared filter output subject to filter constraints which allow energy propagating from the array steering direction to pass without being distorted. The adaptive filter coefficients were initialized to satisfy the constraints which were preserved during the iterations. The adaptation rate is inversely varied with filter output and total input channel power. Performance of the algorithm was studied using the recorded short‐period array data from the Korean Seismic Research Station. Processed were a high‐amplitude signal from Kamchatka, a medium‐amplitude signal from eastern Kazakh, and a number of low‐amplitude signals from central Eurasia. Results of signal‐to‐noise ratio gain relative to a conventional beamformer among the events tested were consistent and were in the range of 4.5 to 6.5 dB in the wide passband. Much better signal‐to‐noise ratio improvement was obtained in the l...

Adaptive array processor and processing method for communication system

Abstract: A processing circuit (50) in a communication system performs multi-channel adaptive array processing using a summed reference technique A plurality of adaptive modules (56 and 58) modify the magnitude and phase of antenna element signals from a plurality of antenna elements The outputs of the adaptive modules are combined by a combiner (60) to produce a summation signal A modem (64) processes and divides the summation signal into a plurality of reference channel signals on channels 1 through K A summer (70) adds the reference channel signals together to produce a composite reference signal that is subtracted by a subtractor (74) from the summation signal to produce an error signal The error signal is applied to the adaptive modules (56 and 58) which respond to nullify interfering and noise signals and to enhance the signal-to-interference ratio for the summation signal

Linear antenna arrays with broad nulls with applications to adaptive arrays

Abstract: The problem of designing a class of linear antenna arrays with broad nulls against interfering signals of variable direction of arrival is considered. The problem is formulated in an appropriate signal space selected for an efficient representation of array signals of specified angular occupancy. The array-weight vector is then chosen to be orthogonal to this space. The approach is seen to yield a min-max array in a sense to be defined in this paper. The application of such arrays to maintain the beamshape of an adaptive antenna array is also illustrated.

A review of adaptive antennas

Abstract: Most adaptive receiving arrays permit beam steering in a selected "look" direction while rejecting or nulling strong interferences which arrive at angles other than the look direction. Nulling is accomplished by adjusting parameters of a signal processor connected to the array sensing elements (whether at rf, audio, or seismic frequencies) to minimize total output power. A signal arriving in the look direction would not be nulled because the adaptive process is constrained to maintain a predetermined sensitivity in the look direction. A substantial literature exists in the field. The September 1976 issue of the IEEE Transactions on Antennas and Propagation was dedicated to the subject of adaptive antennas. It is the purpose of this presentation to compare the works of Howells and Applebaum, Widrow, and Frost.

A rapid approximation to optimal array processing for the case of strong localized interferences

Abstract: The application of adaptive beamforming to the problem of a finite number of strong localized interferences is addressed. It is shown that (a) the general, complex optimal array processing problem reduces to a simpler problem in this case: specifically, the problem of optimizing J(N−1) free parameters (where N is the number of array elements and J is the number of frequency bands of interest) is formally reduced to one of optimizing only n free parameters, where n is the number of interferences; (b) the problem of signal cancellation which exists in classical optimal processing, where the filter output power is minimized, can be avoided by minimizing the total power in all directions, instead; (c) simulations using an existing target scenario (the Frost scenario)[Proc. IEEE 60 926–935 (1972)] indicate that methods based upon the above findings are at least as good as classical optimal processing for appropriate target scenarios. This paper can also be regarded as providing a relatively rigorous basis for methods which were developed earlier [J. Acoust Soc. Am. 59 106–111 (1976)] on more intuitive grounds, and as relating these methods to the better‐known classical optimal processing techniques.

An Experimental Adaptive Array for Radar Applications

Abstract: This paper concerns an experimental adaptive array facility at ASWE. The array operates at X-band and comprises a linear arrangement of 8 dipoles spaced approximately 0.6 ? apart. The signal processor is an analogue implementation of a simple gradient-descent algorithm operating at an IF of 60 MHz. Radar waveforms are usually well suited to this adaptive implementation in that target returns are sufficiently low power and duration that the control circuitry does not respond to them. Some preliminary experimental results include transient response and steady state adapted patterns for several interference situations. The results are compared with those predicted by theory and shown to be in reasonable agreement.

A measurement system and analysis procedure for determining the spatial phase structure function of ionospherically reflected waves

Abstract: A 40-km sparse high-frequency (HF) receiving array is described which was used to measure the instantaneous phase at eight sites within the array of a pulsed 9-MHz transmission from Boulder, CO, to the Philadelphia, PA, area. The experimental program was designed to produce data on the statistical properties of the phase-front of a wave after passage through the ionosphere for the purpose of assessing the design problems of an array large enough to focus a 1-km cross-section beam at transoceanic distance (array size the order of 100 km). The problem of the effect of phase-front fluctuations due to turbulence induced scattering and internal ionospheric waves is formulated. The adaptive beamforming and scanning program of the Valley Forge Research Center, University of Pennsylvania, Philadelphia, is expected to yield solutions to this problem through self-cohering procedures. The nature of the desired measurements and a measuring system for accomplishing the task are also described.

Adaptive linear prediction filtering for airborne underwater acoustic signal processors

Abstract: The behavior of a particular implementation of a Widrow-Hoff, or iterative adaptive version of the classical Least Mean Square Wiener Filter, is evaluated by means of tests on the Adaptive Filter in conjunction with two different airborne under-water acoustic signal processors. The ability of the adaptive mechanism to enhance spectral estimates, detection and tracking of narrow-band signals in noise is illustrated by initial test results which are presented as graphs of detection indices and photographs of airborne acoustic displays. A summary review of the adaptive filter theory is included. Certain peculiarities in the performance of the adaptive mechanism also are noted as they relate to integration of such equipment with existing airborne acoustic processors and displays.