Showing papers on "Adaptive beamformer published in 1986"

The CM array: An adaptive beamformer for constant modulus signals

Abstract: This paper introduces a new adaptive beam-forming algorithm called the CM Array. Unlike existing adaptive beamformers, the adaptive CM Array exploits the constant modulus (cm) property of the signal of interest (soi) to steer a beam in the direction of the soi while steering nulls in the directions of interference.

An adaptive generalized sidelobe canceller with derivative constraints

Abstract: An adaptive broad-band beamforming structure is presented which employs a gradient-based weight adjustment algorithm to minimize output variance subject to a set of J linear constraints on broadband directional derivatives in the desired look direction. A generalized sidelobe-cancelling structure is employed in which a nonadaptive (conventional) beamformer operates in parallel with an adaptive beamformer. The conventional portion has a broad-band beampattern which adheres to the specified constraints while the adaptive path is a cascade of a fixed signal blocking matrix and a set of tapped-delay line filters. Blocking is employed to ensure that all incident waveforms which meet the specified constraints do not reach the tapped-delay lines. As a result, an unconstrained least mean square (LMS) power minimization algorithm is employed to adapt the delay line weights. It is shown that with the addition of the directional derivative constraints, the beamformer quiescent bcampattern becomes a function of the location of the phase center used to specify the constraints. A design criterion for choosing this location is suggested and simulation experiments which illustrate the performance of this new adaptive beamformer are presented.

Adaptive beamforming with the generalized sidelobe canceller in the presence of array imperfections

Abstract: Antenna designers often employ linearly constrained adaptive beamforming as an antijamming measure. With minimal a priori knowledge of the signal environment, this technique nulls out jammers while simultaneously preserving the quality of the main lobe so that a friendly look-direction signal can be received with unity gain. Unfortunately, in the absence of special strategies, linearly constrained adaptive beamforming is hypersensitive to array imperfections when the input signal-to-noise ratio exceeds a certain threshold. This hypersensitivity manifests itself as a nailing of the friendly signal as if it were a jammer. Luckily, the signal nulling problem can be easily remedied by artificial receiver noise injection. A particularly simple and general structure for linearly constrained adaptive beamforming was proposed during the 1970's, and is known as the generalized sidelobe canceller. A detailed analysis of the generalized sidelobe canceller in the presence of array imperfections is discussed, and two new artificial receiver noise injection algorithms are proposed. Computer simulations are included to demonstrate that use of these new algorithms alleviates the signal nailing problem without seriously compromising jammer nulling. For the special case of the Capon maximum-likelihood beamformer, simple approximations are presented for: 1) the Wiener output signal-to-interference-plus-noise ratio ( SINR_{0}\astr ), 2) the antenna element error variance that causes a 3 dB loss of SINR_{0}\astr from its value for an ideal array, and 3) the optimal artificial receiver noise that maximizes SINR_{0}\astr .

A novel algorithm and architecture for adaptive digital beamforming

Abstract: A novel algorithm and architecture are described which have specific application to high performance, digital, adaptive beamforming. It is shown how a simple, linearly constrained adaptive combiner forms the basis for a wide range of adaptive antenna subsystems. The function of such an adaptive combiner is formulated as a recursive least squares minimization operation and the corresponding weight vector is obtained by means of the Q-R decomposition algorithm using Givens rotations. An efficient pipelined architecture to implement this algorithm is also described. It takes the form of a triangular systolic/wavefront array and has many desirable features for very large scale integration (VLSI) system design.

Steady state analysis of the generalized sidelobe canceller by adaptive noise cancelling techniques

Abstract: Narrow-band adaptive noise cancelling techniques are used to study the generalized sidelobe canceller (GSC), a general form of linearly constrained adaptive beamforming structure In an environment which consists of a look-direction signal, one jammer, and additive receiver noise, exact expressions are derived for the Wiener solution, the steady state output signal-to-interference-plus-noise ratio (SINR 0 ), and performance improvement due to adaptation (PIA), defined as the ratio of SINR 0 after adaptation to SINR0 before adaptation These expressions are in terms of the signal directions and power levels, an arbitrary array geometry, and a general signal blocking matrix Next, easily evaluated scalar equations for PIA are given for two classes of constraints The first is constant gain in the look direction, and the second is constant gain plus a main beam zero first derivative in the look direction Under the further assumption of an equally spaced line array, even simpler equations for PIA result, and are used to show that for jammers arriving outside the heamwidth between first nulls (BWFN) region of the unadapted beampattern, the introduction of the additional main beam zero first derivative constraint leads to negligible degradation in PIA

The Partial-Rank Algorithm for Adaptive Beamforming

Abstract: This paper describes a new family of algorithms for adaptive weight computation for sensor arrays with arbitrary geometries. This family includes existing algorithms of the steepest-descent and matrix-factorization types, plus a range of new algorithms that are intermediate between these two classes, in terms of performance and computational complexity. This approach is particularly valuable for applications where steepest descent is too slow, but the standard matrix methods require too much computation.

Anti-multipath signal processor

Abstract: Signal processing that reduces multipath in communication receivers is implemented with an adaptive array. The invention address a signal environment in which the directions of arrival and the time of arrival of the signal of interest and the unwanted multipath are unknown. A reference signal is generated by amplitude limiting the adaptive array output signal. The system uses the strongest received signal as the signal of interest and rejects the others by placing an antenna pattern null in their direction of arrival.

Parallel spatial processing: A cure for signal cancellation in adaptive arrays

Abstract: A spatial processing algorithm with parallel structure is presented for the prevention of signal caneellation phenomena in conventional adaptive arrays. This algorithm basically uses a parallel structure with a spatial averaging effect to combat coherent jamming. It results in a spatially smoothed maximum-likelihood estimate of the desired signal when the adaptive beamformer converges. Simulations have been conducted which verify the effectiveness of the proposed structure.

Adaptive array processing using the Davies beamformer

Abstract: The paper presents and analyses a simple and fast adaptive array based on the Davies null-steering beamformer [1]. The use of perturbation algorithms on this beamformer is investigated. The convergence time constants and misadjustment are derived. It is shown that, if the feedback factors for these algorithms are appropriately normalised, the time constants can be roughly equalised, resulting in fast convergence behaviour.

Dynamic allocation of degrees of freedom in an adaptive antenna

Abstract: An algorithm to acquire a desired signal of unknown direction and waveform and to steer nulls to jammers by a process of sequential selection and testing of auxiliary beams in a sidelobe canceller is proposed. The receiver must be able to recognise the desired signal when its input SNR exceeds some given threshold of around 10 dB. For weak desired signals, the achievable output SNRs approach those obtained by the beamformer in the absence of jamming, but a saturation effect occurs as input signal power increases; output SNRs saturate relatively early and become dispersed in value, although remaining quite useable. Performance against jamming in diffuse multipath conditions typical of the urban environment is evaluated with fairly satisfactory results. Comparison is made with the competitive techniques of eigenvector steering and the use of a linearly constrained adaptive beamformer with an augmented sample correlation matrix diagonal.

A One Third Gigaflop Systolic Adaptive Beamformer

Abstract: ESL is currently building a 350 MFLOP Systolic Adaptive Beamformer. The beamformer implements the Minimum Variance Distortionless Response (MVDR) algorithm. The Systolic Adaptive Beamformer can process over 100 sensors, and 280 frequencies in real time. This frequency domain adaptive beamformer is being developed using a systolic architecture processor implemented with custom VLSI chips.

The application of linear constraints to an adaptive beamformer

Abstract: This paper describes a novel technique for implementing the constrained adaptive array solution. It involves the use of a simple full-rank transformation network to pre-process the received data prior to using a canonical "end element clamped" adaptive combiner. It will be shown that the form of the transformation is nonunique and how a number of particularly efficient realisations can be devised. The resulting preprocessors only require O(N) arithmetic operations per time sample for an N channel array and can be implemented using a simple linear array of processing elements. It will also be shown how the pre-processor technique can be generalised to encompass multiple linear constraints and that these can also be implemented using efficient pre-processor networks of low computational complexity.

An adaptive recursive algorithm for array processing of coherent signal

Abstract: In this paper we presented a novel on-line adaptive recursive algorithm which is capable of resolving multiple coherent spatial sources, as is often the case in radar situation. Most of the adaptive beamforming algorithms fail to operate in such situation or their performance will be degraded. Our algorithm is based upon the "travelling antenna" concept introduced by Gabriel [1]. We test the performance of this algorithm using computer simulation. An eight-element array involving coherent sources of equal and unequal strength are used for the test. We compared the performance of this algorithm with the performance of the SMI algorithm. We find that this algorithm can successfully seperate two coherent sources.

Time delay estimation via generalized correlation with adaptive spatial prefiltering

Abstract: In this paper, the use of adaptive beamforming prior to generalized correlation (GC) is demonstrated as a means of time delay estimate (TDE) bias reduction in interference dominated environments. Analysis of GC TDE bias with both minimum variance distortionless response and conventional delay-and-sum beamforming is given. At high interference-to-noise ratios, theoretical and simulation results indicate that the adaptive structure can facilitate improved delay estimation performance compared to conventional methods.

A simple fast adaptive array based on a null steering beamformer

Abstract: This paper describes and analyses a simple adaptive array based on the use of perturbation algorithms on a null steering beamformer proposed by Davies [1]. The convergence time constants and misadjustments of using these algorithms are derived. It is shown that, from choosing the feedback factors appropriately, the time constants can be roughly equalized, resulting in faster convergence behaviour.

Optical Filtering Techniques.

Abstract: : The purpose of this study was to examine the potential of using optical information processing technology for adaptive antenna beamforming and null steering. The adaptive beamforming/null steering problem consists of estimation of the covariance matrix of the noise field and inversion of the covariance matrix to obtain the antenna element weights which optimize the antenna's of the covariance matrix to obtain the antenna element weights which optimize the antenna's directional characteristics (gain pattern). This report examines the adaptive beamforming/nulling problem in view of the capabilities of optics and identifies areas where optics can be used to benefit. Keywords included: Optical processing, Signal processing, and Adaptive processing.

The Role Of Digital Adaptive Beamforming In High Resolution Microwave Imaging

Abstract: The user of a high resolution microwave imaging system views it as a large phased array radar. To the designer, however, it is a huge signal processing system with a multiport input (the phased antenna array) and a single output port ( the image display) . Between the input and the output, there is a great deal of signal handling and processing. Of course, there is a transmitter and receivers with local oscillators, mixers, IF amplifiers and quadrature demodulators. These are standard items required for frequency translation, amplification and demodulation; the real action as well as the intellectual depth in the creation of the system lies in the signal processing.

A partitionable distributed beamformer and signal conditioner architecture

Abstract: VLSI circuit technology provides at least an order of magnitude increase in signal processing density; this greater processing density and the associated reduction in cost per function has led to larger systems with significant increases in front-end signal conditioning and signal data transfer capability. Increased signal conditioning can be provided by distributed signal processing using both parallel and pipelined operations. Increased signal data transfer requirements can be satisfied using local, nearest-neighbor communication instead of global communication. The combination of parallel processing and parallel signal data paths offers an approach for VLSI signal conditioners with much higher processing throughput and signal bandwidth; this combination of attributes is typical of systolic array processor organizations [1]. An interconnection method and two distributed processing elements, one multi-function programmable and the other single-function programmable, are described.