Showing papers on "Adaptive beamformer published in 1978"

An adaptive array in a spread-spectrum communication system

Abstract: This paper describes the integration of an LMS adaptive array into a pseudonoise (PN) coded biphase modulated communication system. The paper explains how these systems my be combined and presents a systems overview of the interaction between the adaptive array and the signaling waveform. An experimental system is described, and typical performance results are presented. The hybrid system requires only very modest spectrum-spreading ratios, such as 5:1, for the adaptive array to null interference. The combined system provides the interference protection of the adaptive array during the code timing acquisition phase as well as after code lockup.

A novel approach to digital beamforming

Abstract: For many sonar applications, the sensor outputs of a hydrophone array are sampled at a rate significantly higher than that required for waveform reconstruction when digital beamforming is used. The reason for this is that the number of synchronous, or ’’natural,’’ beampointing directions is proportional to the beamformer input rate. This paper presents an implementation of a digital beamformer that achieves the desired synchronous beams while minimizing the sensor channel sampling rate requirement. The technique employs zero padding of sensor data followed by digital interpolation filters to achieve vernier beamformer delays. Interpolation filtering can be done either at the beamformer input or output to minimize processing requirements. The resulting structure realizes a hardware savings since both A/D converter and cable bandwith requirements can be traded off against digital processing complexity to achieve an optimal partitioning.

Channel estimating reference signal processor for communication system adaptive antennas

Abstract: A spread spectrum communication adaptive array antenna processor is disclosed which can acquire and remain synchronized to a pseudo-noise (PN) signal transmitted in a multipath signal environment. The plurality of antennas which receive rf signals are individually associated with mixing circuitry which reduces the received signals to IF frequencies. The IF signals are fed into the adaptive filtering portion of the adaptive signal processor which contains circuits to generate an adaptive weight corresponding to each antenna element. An array signal is formed by summing the products of each IF signal with a filter weight corresponding to each antenna element generated within each respective adaptive loop. The adaptive signal processor utilizes the complex conjugate of the error feedback signal which is then multiplied by each respective IF signal. The complex conjugate of this integrated product forms each filter weight. A channel estimator generates an adaptive reference signal which inclues the essential multipath characteristics of the received signal. By using this reference signal in conjunction with the array signal generated by the adaptive filtering portion of the processor, the adaptive array can form an appropriate main beam without prior knowledge of the signal propagation direction.

A Generalized Sidelobe Cancelling Structure for Adaptive Arrays

Abstract: : This report introduces a new formulation, termed generalized sidelobe cancelling (GSC), for adaptive arrays. This formulation offers an alternative method or implementing older adaptive structures, as well as a method usable in formulating a new, wider class of adaptive beamformers. The GSC approach provides an implementation advantage in that the conventional beamformer is separated out as a distinct element in the overall processor. In addition, the linear constraints which ensure the absence of distortion in the main array lobe can be implemented as simple analog hardware elements. This approach is in contrast to previous approaches in which the constraints have been incorporated into the adaptive algorithm. Another advantage of the GSC formulation is that it clearly delineates the similarities and differences between P-vector and linearly constrained adaptive beamforming. To assess further the performance characteristics of adaptive processing of HF radar data, the GSC beamformer was used to study data collected at the Wide Aperture Research Facility operated by SRI International. Findings of this study are included.

Detection performance of time-varying adaptation rate adaptive beamformer

Abstract: : The previously developed time-varying adaptation rate adaptive beamformer (ABF) was modified for the purpose of using a fixed set of design parameters in different levels of noise and signals. The ABF performance was evaluated in terms of its capability to suppress seismic coda wavetrains of strong earthquakes, accuracy of its bodywave magnitude (m sub b) measurements and detection capability, as compared with those of single sites and the conventional beamsteer using recorded data from the Korean Seismic Research Station (KSRS) short-period array. A total of 129 events recorded in winter were processed. On the average, the ABF yielded the same magnitude difference (0.1 m sub b units smaller) as the beamsteer, when compared with the NEIS and NORSAR bulletin mb. However, preliminary results indicated that the ABF may have improved the array detection capability by 0.5 mb units from the conventional beamsteer based on subjective analyst interpretation of the detection status of processed events. More data with automatic detection criteria applied are needed to more accurately estimate the detection gains which are possible by applying the ABF as a field operating front-end detector. (Author)

The root-trajectory method--A tool for analyzing linear adaptive arrays

Abstract: A root-trajectory method is described that seeks to obviate the need for an extensive set of antenna patterns to describe the behavior of an adaptive array during its adaptation process. This aim is achieved by use of Schelkunoff's idea of describing a linear array in terms of an array polynomial. The roots of this polynomial have a direct interpretation in obtaining the array gain, and the trajectories of these roots during the adaptation process give a simple visualization of the dynamic behavior of the adaptive array.