Showing papers on "Adaptive beamformer published in 2000"

A Bayesian approach to robust adaptive beamforming

Abstract: An adaptive beamformer that is robust to uncertainty in source direction-of-arrival (DOA) is derived using a Bayesian approach. The DOA is assumed to be a discrete random variable with a known a priori probability density function (PDF) that reflects the level of uncertainty in the source DOA. The resulting beamformer is a weighted sum of minimum variance distortionless response (MVDR) beamformers pointed at a set of candidate DOAs, where the relative contribution of each MVDR beamformer is determined from the a posteriori PDF of the DOA conditioned on previously observed data. A simple approximation to the a posteriori PDF results in a straightforward implementation. Performance of the approximate Bayesian beamformer is compared with linearly constrained minimum variance (LCMV) beamformers and data-driven approaches that attempt to estimate signal characteristics or the steering vector from the data.

Electronically steerable passive array radiator antennas for low-cost analog adaptive beamforming

Abstract: Electronically steerable passive array radiator antennas are proposed for low-cost analog adaptive beamforming. Unlike conventional digital beamforming, they require only one transmitter or receiver. For 2-, 3-, 4- and 5-element antennas, beam and null steering characteristics are estimated by the method of moments. It is numerically shown that full 360-degree continuous azimuthal beam/null steering is possible by at least 4 elements. Also shown is that a directive gain of 9 dBi /spl plusmn/1 dB and null depth of -50 dBi /spl plusmn/10 dB are obtained by 5 elements, along with an associated return loss of better than 20 dB at the feeding port. Their drastically low cost and low power consumption could help to achieve adaptive antennas for portable wireless user terminals.

A novel adaptive beamforming algorithm for a smart antenna system in a CDMA mobile communication environment

Abstract: An alternative way of adaptive beamforming is presented. The main contribution of the new technique is in its simplicity with a minimal loss of accuracy. The total computational load for computing a suboptimal weight vector from each new signal vector is about O(2N/sup 2/+5N). It can further be reduced down to O(3N) by approximating the autocorrelation matrix with the instantaneous signal vector at each snapshot. The required condition on the adaptive gain for the proposed algorithm to converge is derived analytically. The proposed beamforming algorithm is applied to the base station of a code-division-multiple access (CDMA) mobile communication system. The performance of the proposed method is shown in multipath fading communication channels in terms of the signal-to-interference+noise ratio (SINR), the bit error rate (BER), and the achievable capacity of a given CDMA cell/sector.

Adaptive beamforming algorithm for OFDM systems with antenna arrays

Abstract: This paper presents an adaptive beamforming algorithm for an OFDM system with an adaptive array antenna. The proposed algorithm for adaptive beamforming in the OFDM system is derived by (1) calculating the pilot error signals in the frequency domain, (2) transforming the frequency-domain error signals into time-domain error signals, (3) updating the filter coefficients of the adaptive beamformer in the direction of minimizing the MSE. The convergence behavior and performance improvement of the proposed approach are investigated through computer simulation by applying it to the conventional OFDM system.

Relationships between adaptive minimum variance beamforming and optimal source localization

Abstract: For many years, the popular minimum variance (MV) adaptive beamformer has been well known for not having been derived as a maximum likelihood (ML) estimator. This paper demonstrates that by use of a judicious decomposition of the signal and noise, the log-likelihood function of source location is, in fact, directly proportional to the adaptive MV beamformer output power. In the proposed model, the measurement consists of an unknown temporal signal whose spatial wavefront is known as a function of its unknown location, which is embedded in complex Gaussian noise with unknown but positive definite covariance. Further, in cases where the available observation time is insufficient, a constrained ML estimator is derived here that is closely related to MV beamforming with a diagonally loaded data covariance matrix estimate. The performance of the constrained ML estimator compares favorably with robust MV techniques, giving slightly better root-mean-square error (RMSE) angle-of-arrival estimation of a plane-wave signal in interference. More importantly, however, the fact that such optimal ML techniques are closely related to conventional robust MV methods, such as diagonal loading, lends theoretical justification to the use of these practical approaches.

Experimental performance of adaptive beamforming in a sonar environment with a towed array and moving interfering sources

Abstract: The performances of adaptive array algorithms are known to suffer from a strong degradation in scenarios with moving interfering sources. In this article, basic adaptive beamforming techniques are compared using shallow sea sonar data recorded in a towed horizontal array environment with moving interfering sources originated from shipping noise. Our experimental results show the relationship between the practical performances of adaptive and conventional beamforming techniques compared in terms of output SINR or a related measure given by the noncompensated postbeamforming interference power. These results demonstrate noticeable performance improvements that can be achieved using several robust algorithms relative to traditional adaptive beamforming schemes.

Adaptive array antenna beamforming architectures as viewed by a microwave circuit designer

Abstract: Five alternative architectures are introduced for adaptive beamforming in array antennas. The beamforming process is carried out in the baseband, local, radio, spatial, and optical stage, respectively, for the five architectures. Their pros and cons are discussed from the viewpoint of microwave circuit design. As practical hardware implementations of these architectures, some state-of-the-art examples of recently developed microwave signal processor devices and antennas are also shown.

Method and system for estimating adaptive array weights used to transmit a signal to a receiver in a wireless communication system

Abstract: A transmitter uses adaptive array weights to modify a gain and a phase of a communication signal to produce a plurality of element communication signals coupled to antenna elements in an adaptive array antenna. The communication signal is transmitted along with an element pilot signal that is coupled to one of the elements in the adaptive array antenna. In a receiver, the communication signal is received, and the element pilot signal is received. Thereafter, the adaptive array weights used at the transmitter are estimated in response to comparing characteristics of the received element pilot signal to characteristics of the received communication signal.

Adaptive Arrays and Diversity Antenna Configurations for Handheld Wireless Communication Terminals

Abstract: This dissertation reports results of an investigation into the performance of adaptive beamforming and diversity combining using antenna arrays that can be mounted on handheld radios. Handheld arrays show great promise for improving the coverage, capacity, and power efficiency of wireless communication systems. Diversity experiments using a handheld antenna array testbed (HAAT) are reported here. These experiments indicate that signals received by the antennas in two-element handheld antenna arrays with spacing of 0.15 wavelength or greater can be combined to provide 7-9 dB diversity gain against fading at the 99% reliability level in non line-of-sight multipath channels. Thus, peer-to-peer systems of handheld transceivers that use antenna arrays can achieve reliability comparable to systems of single-antenna handheld units, with only one-fifth the transmitter power, resulting in lower overall power consumption and increased battery life. Similar gains were observed for spatial, polarization, and pattern diversity. Adaptive beamforming with single- and multi-polarized four-element arrays of closely spaced elements was investigated by experiment using the HAAT, and by computer simulation using a polarization-sensitive vector multipath propagation simulator developed for this purpose. Small and handheld adaptive arrays were shown to provide 25 to 40 dB or more of interference rejection in the presence of a single interferer in rural, suburban, and urban channels including line-of-sight and non line-of-sight cases. In multipath channels, these performance levels were achieved even when there was no separation between the transmitters in azimuth angle as seen from the receiver, and no difference in the orientations of the two transmitting antennas. This interference rejection capability potentially allows two separate spatial channels to coexist in the same time/frequency channel, doubling system capacity.

Convergence analysis of the least squares constant modulus algorithm in interference cancellation applications

Abstract: The convergence behavior of the least squares constant modulus (CM) algorithm in an adaptive beamforming application is examined. It is assumed that the desired signal and the interference are uncorrelated. The improvement in output signal-to-interference ratio (SIR) with each iteration of the algorithm is predicted for several different signal environments. Deterministic results are presented for an environment containing two complete sinusoids. Probabilistic results are presented for a CM desired signal with a CM interferer and with a Gaussian interferer. The asymptotic improvement in output SIR as the output SIR becomes high is also derived. The results of Monte Carlo simulations using sinusoidal, frequency modulation, and quadrature phase-shift keying signals are included to support the derivations.

Comments on "Theory and application of covariance matrix tapers for robust adaptive beamforming" [with reply]

Abstract: For original paper see Guerci (IEEE Trans. Signal Processing, vol.47, p.977-85, 1999 April). Covariance matrix tapers and derivative constraints in the directions of jammers have been proposed for broadening the null in adaptive processing, thereby improving the algorithms' robustness. In this correspondence, the relationship between these two methods is explored. A reply is given in which Guerci makes some further points.

Annihilation-reordering look-ahead pipelined CORDIC-based RLS adaptive filters and their application to adaptive beamforming

Abstract: The novel annihilation-reordering look-ahead technique is proposed as an attractive technique for pipelining of Givens rotation (or CORDIC)-based adaptive filters. Unlike the existing relaxed look-ahead, the annihilation-reordering look-ahead does not depend on the statistical properties of the input samples. It is an exact look-ahead based on CORDIC arithmetic, which is known to be numerically stable. The conventional look-ahead is based on multiply-add arithmetic. The annihilation-reordering look-ahead technique transforms an orthogonal sequential adaptive filtering algorithm into an equivalent orthogonal concurrent one by creating additional concurrency in the algorithm. Parallelism in the transformed algorithm is explored and different implementation styles including pipelining, block processing, and incremental block processing are presented. Their complexities are also studied and compared. The annihilation-reordering look-ahead is employed to develop fine-grain pipelined QR decomposition-based RLS adaptive filters. Both QRD-RLS and inverse QRD-RLS algorithms are considered. The proposed pipelined architectures can be operated at arbitrarily high sample rate without degrading the filter convergence behavior. Stability under finite-precision arithmetic are studied and proved for the proposed architectures. The pipelined CORDIC-based RLS adaptive filters are then employed to develop high-speed linear constraint minimum variance (LCMV) adaptive beamforming algorithms. Both QR decomposition-based minimum variance distortionless response (MVDR) realization and generalized sidelobe canceller (GSC) realization are presented. The complexity of the pipelined architectures are analyzed and compared. The proposed architectures can be operated at arbitrarily high sample rate and consist of only Givens rotations, which can be scheduled onto CORDIC arithmetic-based processors.

Adaptive single-user receivers for direct-sequence spread-spectrum CDMA systems

Abstract: The capacity of a direct-sequence spread-spectrum code-division multiple-access (DSSS-CDMA) system is limited by multiple-access interference (MAI) and the near-far problem. Multiuser receivers provide a solution to these problems, but they require knowledge of parameters of the MAI and are computationally complex. Adaptive single-user receivers, however, do not require knowledge of MAI parameters and need fewer computations. This paper discusses a wide range of adaptive single-user receivers found in the literature and presents their performance results under a unified framework to provide a basis of comparison. Results indicate that, compared to the conventional receiver, adaptive single-user receivers provide large gains in system capacity and are near-far resistant. It is shown that fractionally spaced adaptive receivers, which exploit spectral correlation due to the cyclostationary nature of the DSSS signal, perform better than adaptive receivers that cannot exploit this correlation. Multipath results presented for two-ray and urban channels indicate that fractionally spaced adaptive receivers act as RAKE receivers.

Linear QR Architecture for a Single Chip Adaptive Beamformer

Abstract: This paper presents the design of a novel single chip adaptive beamformer capable of performing 50 Gflops, (Giga-floating-point operations/second). The core processor is a QR array implemented on a fully efficient linear systolic architecture, derived using a mapping that allows individual processors for boundary and internal cell operations. In addition, the paper highlights a number of rapid design techniques that have been used to realise this system. These include an architecture synthesis tool for quickly developing the circuit architecture and the utilisation of a library of parameterisable silicon intellectual property (IP) cores, to rapidly develop detailed silicon designs.

Multi-rate adaptive beamforming (MRABF)

Abstract: An important trend in underwater acoustics is the use of large arrays with many elements and small resolution cells in an effort to detect weak signals. Frequently, the background noise is dominated by multiple moving surface ships that act like discrete interferers. These interferers may move through many spatial resolution cells in the time required to obtain sufficient "snapshots" for covariance matrix estimation. The non-stationarity of the background limits the performance of adaptive algorithms. Reduced degree-of-freedom approaches that can track and rapidly null interferers before they move too far are of great interest. The paper presents a new class of algorithms in which a low-rank, dynamically changing interference sub-space is estimated and tracked using a short-term average. An orthogonal projection is then applied to the data to null components in the interference sub-space. A second stage of adaptive beamforming that uses a longer averaging time is then applied to the projected data. Unlike most sub-space approaches that work for strong signals, there is no requirement that the signal be contained in the interference sub-space. It is shown how multiple interferers moving rapidly across beams can be tracked and nulled to reveal the presence of a weak target not visible with conventional, adaptive, or usual sub-space methods. The effectiveness of multi-rate adaptive beamforming is illustrated by examples. It is shown that moving interferers are effectively nulled to allow detection of weak signals when other methods including MVDR, conventional, and signal subspace fail.

Spatio-temporal processing of coherent acoustic communication data in shallow water

Abstract: Achieving reliable underwater communication in shallow water is a difficult task because of the random time-varying nature of multipath propagation. When the product of Doppler-related signal bandwidth spread and multipath-related time spread of the channel is larger than one, some types of adaptive signal processing may not work very well. In this paper, various methods of coherent space-time processing are compared for a condition of a marginally overspread channel operating at 50 kHz. Various combinations of suboptimal spatially adaptive and time adaptive methods are considered. The coherent path beamformer (CPB) and recursive least squares (RLS) adaptive beamformer, both in combination with RLS time filtering, are analyzed. Also considered in the analysis is the combined RLS space-time optimal adaptive processor. Many experiments using broad-band phase-shift-keyed transmissions in shallow water have been conducted to provide data for testing these various processing methods. Because of the rapid time variation of the multipath, the product of bandwidth spread and time spread at this test site approached unity. In this environment, a suboptimal approach consisting of the adaptive beamformer followed by RLS equalization reduced reverberation and transmission errors.

Efficient robust adaptive beamforming for cyclostationary signals

Abstract: This paper deals with the problem of robust adaptive array beamforming for cyclostationary signals. By exploiting the signal cyclostationarity, the LS-SCORE algorithm presented in a paper by Agee et al. (1990) has been shown to be effective in performing adaptive beamforming without requiring the direction vector of the desired signal. However, this algorithm suffers from severe performance degradation even if there is a small mismatch in the cycle frequency of the desired signal. In this paper, we first evaluate the performance of the LS-SCORE algorithm in the presence of cycle frequency error (CFE). An analytical formula is derived to show the behavior of the performance degradation due to CFE. An efficient method is then developed based on the fact that the array output power approaches its maximum as the CFE is reduced. We formulate the problem as an optimization problem for reducing the CFE iteratively to achieve robust adaptive beamforming against the CFE. Simulation examples for confirming the theoretical analysis and showing the effectiveness of the proposed method are provided.

PN code acquisition with adaptive antenna array and adaptive threshold for DS-CDMA wireless communications

Abstract: An efficient PN code acquisition scheme with an adaptive antenna array and adaptive threshold is proposed. The basic structure is the combination of a conventional PN correlation searcher, an adaptive beamformer and an adaptive threshold setting circuit. The proposed scheme employs all the array element outputs in the PN correlation searcher and also uses an adaptive threshold for each PN code phase hypothesis testing by estimating the instantaneous power prior to PN code despreading. The performance of the proposed scheme is theoretically analyzed and also simulated in terms of the false alarm probability P/sub f/, detection probability P/sub d/ and average acquisition time T/sub acq/ under a Rayleigh mobile fading environment. Theoretical and simulation results show that the proposed scheme is robust against received signal power variations and can significantly improve the PN code acquisition performance.

System and method for adaptive beamformer apodization

Abstract: An ultrasound array imaging system ( 50 ) and method for improving beamforming quality and ultimately for improving an ultrasound image are disclosed. The ultrasound array imaging system ( 50 ) includes a transducer ( 52 ), a first beamformer ( 54 ), a second beamformer ( 56 ) and a comparator ( 58 ). The first beamformer ( 44 ) receives a first electronic echo signal from the transducer ( 52 ) and produces a first apodized beam signal in accordance with a first beamforming apodization function. The second beamformer ( 56 ) receives the same electronic echo signal from the transducer ( 52 ) and produces a second apodized beam signal in accordance with a second beamforming apodization function. The first and second apodized beam signals are compared in the comparator ( 58 ) which then combines the first and second apodized beam signals to create a combined apodized beam signal for producing an image that has its lateral and contrast resolution simultaneously optimized.

Statistical performance of the MVDR beamformer in the presence of diagonal loading

Abstract: New results are derived that describe the statistical performance of the MVDR (minimum variance distortionless response) adaptive beamformer when the weights are computed utilizing the sample covariance matrix and diagonal loading. It is well known that undesirably high sidelobes are generated when the unaugmented sample covariance matrix is used as an estimate of the true covariance matrix in adaptive beamforming algorithms. In practice, diagonal loading is frequently employed to reduce this deleterious effect. However, surprisingly little information is available in the technical literature that enables the user to predict the statistical performance of the MVDR algorithm when the sample covariance matrix and diagonal loading are employed. We help fill this gap by generating an approximate probability density function for the adaptive beam response for the diagonally loaded MVDR algorithm. The approximations are valid for an important class of signal scenarios. The utility of the new analysis is demonstrated via simulation. In particular, the new results are used to estimate the probability density function of the range sidelobe level for a wideband radar system in which adaptive beamforming and pulse compression are implemented in subbands.

Speech enhancement using nonlinear microphone array with noise adaptive complementary beamforming

Abstract: This paper describes an improved complementary beamforming microphone array with a new noise adaptation. Complementary beamforming is based on two types of beamformers designed to obtain complementary directivity patterns. In this system, two directivity patterns of the beamformers are adapted to the noise directions so that the expectation values of each noise power spectrum are minimized. Using this technique, we can realize the directional nulls for each noise even when the number of sound sources exceeds that of microphones. To evaluate the effectiveness, speech enhancement experiments are performed based on computer simulations with a two-element array and three sound sources. Compared with the conventional spectral subtraction method cascaded with the adaptive beamformer, it is shown that the proposed array improves the signal-to-noise ratio of degraded speech by more than 6 dB and performs more than 18% better in word recognition rates when the interfering noise is two speakers.

Principle and applications of asymmetric crosstalk-resistant adaptive noise canceler

Abstract: This paper analyzes the performance of the crosstalk-resistant adaptive noise canceler (CTRANC). The CTRANC system's symmetric structure causes inconsistent performance when treating noise coming from different angles. To solve this problem, an asymmetric CTRANC (ACTRANC) structure using a delay unit on the primary channel to guarantee causality of the system is proposed. This system is analyzed in steady state assuming that the primary and reference signals are uncorrelated. The ACTRANC allows flexible alignment of the noise source with the sensor array in advanced communication applications such as adaptive noise cancellation, acoustic echo cancellation, and adaptive beamforming. Experiments are presented to demonstrate ACTRANC's performance improvement for various noise orientations.

Neural MCA for robust beamforming

Abstract: This paper aims at recalling recent results about neural Minor Component Analysis and to apply them to spatial adaptive array filtering (adaptive beamforming). The constrained beamformer power optimization principle is employed, which allows us to improve the performances of simpler beamforming algorithms by emphasizing white noise sensitivity control and prior knowledge about the disturbances.

Adaptive beamforming at very low frequencies in spatially coherent, cluttered noise environments with low signal-to-noise ratio and finite-averaging times

Abstract: Realistic simulations with spatially coherent noise have been run in order to compare the performance of adaptive beamforming (ABF), inverse beamforming (IBF), and conventional beamforming (CBF) for the case of finite-averaging times, where the actual spatial coherence of the acoustic field, or covariance matrix, is not known a priori, but must be estimated. These estimation errors cause large errors in the ABF estimate of the directionality of the acoustic field, partly because ABF is a highly nonlinear algorithm. In addition, it is shown that ABF is fundamentally limited in its suppression capability at very low frequency (VLF), based on the sidelobe level of the conventional beampattern in the direction of the noise interferer [G. L. Mohnkern, “Effects of Errors and Limitations on Interference Suppression,” NOSC Technical Document 1478, Naval Ocean Systems Center (1989)]. The simulations include a low-level plane wave signal of interest, a stronger noise plane wave interferer, and spatially random back...

Space-time adaptive processing and adaptive arrays: special collection of papers

Abstract: Space-time adaptive processing (STAP) and related adaptive array techniques hold tremendous potential for improving sensor performance by exploiting signal diversity. Such methods have important application in radar, sonar, and communication systems. Recent advances in digital signal processing technology now provide the computational means to field STAP-based systems. The objective of this special collection of papers is to examine the current state-of-the art in STAP technology and explore the remaining obstacles, practical issues and novel techniques required to implement STAP-based radar, sonar or communication systems.

Traffic improvements in wireless communication networks using antenna arrays

Abstract: A wireless network with beamforming capabilities at the receiver is considered that allows two or more transmitters to share the same channel to communicate with the base station. A novel approach is introduced, which combines the effects of the digital signal processing (adaptive beamforming) at the physical layer with the traffic policies at the network layer on the overall queuing model of a cell. The effect of signal processing on the queuing model of the cell is represented by a parameter in the final cell model. Each cell is modeled by a multiuser/multiserver service facility, where each server is a beamformed channel formed by the cell's base station. From this effective cell model, we find the closed form solutions for blocking probabilities of the calls and total carried traffic in a wireless network with adaptive arrays. Our analytical as well as numerical results show that adaptive beamforming at the receiver reduces the blocking probability of the calls and increases the total carried traffic in the system.

The properties of adaptive algorithms with time varying weights

Abstract: Most adaptive array processing schemes assume that the interference is 'wide sense stationary'. However, in practice this assumption is often violated. For example there may be motion of either the array and/or the interference during the processing interval. The asymptotic performance of two adaptive beamforming algorithms which compensate for this type of changing interference scenario are analyzed in this paper.

Two-channel adaptive microphone array with target tracking

Abstract: This paper proposes a new robust adaptive microphone array suitable for a two-channel audio input system. This microphone array tracks a target speaker by using two adaptive beamformers and can reduce the target signal distortion, which is caused when the target signal is canceled in the case where the target direction differs from the initial look-direction of the microphone array. To enable target tracking, the directional responses of the beamformers are exploited to estimate the directions of arrival (DOA) of the target and the interference. In addition to the target tracking, a new arrangement of directional microphones is proposed to reduce the degradation of interference suppression caused by spatial aliasing. Simulation results show that the proposed microphone array can extract the signal from a moving target with high accuracy. © 2000 Scripta Technica, Electron Comm Jpn Pt 3, 83(12): 19–24, 2000