Robust vector sensor array processing and performance analysis

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.Includes bibliographical references (p. 179-186).Acoustic vector sensors, which measure scalar pressure along with particle motion (a vector quantity), feature many advantages over omnidirectional hydrophone sensors. A sizable literature exists on the theory of processing signals for many vector sensor array applications. In practice, however, mismatch (the difference between the assumed and actual system configurations), several noise processes and low sample support can pose significant problems. Processing techniques should be robust to these system imperfections and practical complexities. This thesis presents analytical results which quantify the effect of system mismatch and low sample support on acoustic vector sensor array performance. All arrays are susceptible to perturbations in array element locations; vector sensor arrays, however, are also sensitive to changes in sensor orientation. This is due to the fact that the particle motion vector measurement must be placed in a global reference frame. Gilbert and Morgan (1955) developed a statistical analysis with system mismatch for an array of scalar, omnidirectional elements. This thesis includes a vector sensor extension to their analysis by including sensor orientation perturbations. Theoretical expressions for the mean and variance of the vector sensor array spatial response are derived using a Gaussian perturbation model, with excellent comparisons between theory and simulation. Such analysis leads to insight into theoretical limits of both conventional and adaptive processing in the presence of system imperfections. One noteworthy result is that the vector aspect of the array "dampens" the effect of array mismatch, enabling deeper true nulls. This is accomplished because the variance of the vector sensor array spatial response (due to rotational, positional and filter gain/phase perturbations) decreases in the side lobes, unlike arrays of omnidirectional hydrophones.(cont.) As long as sensor orientation is measured within a reasonable tolerance, the beampattern variance dominates the average side lobe power response. Results from random matrix theory are used to characterize the effect of low sample support on signal detection using a vector sensor array. When using vector sensors, the effects of low sample support potentially increase by a factor of four since each element in a vector sensor array consists of a scalar hydrophone and up to three spatially orthogonal particle motion sensors. Also presented is an analysis of vector sensor array performance in ocean noise given an arbitrary spatial array configuration, sensor orientation and particle motion sensor type (velocity or acceleration). Several different ocean noise models exist, including isotropic noise, directional noise and realistic surface generated noise. Theoretical expressions are derived for array data covariance matrices in these different noise models for arbitrary array configuration and sensor orientation, which can in turn be used with optimal MVDR beamforming weights to analyze array gain. Using Monte Carlo simulations, we present examples of signal, noise and array gain variability as a function of mismatch intensity. Our analysis suggests that vector sensor array gain performance is less sensitive to rotational than to positional perturbations in the regions of interest. Hydrophones and particle motion sensors have very different response and noise characteristics. For instance, particle motion sensors are more sensitive to non acoustic, motion-induced noise than hydrophones. In a towed line array configuration, those sensors orthogonal to the direction of motion are exposed to higher intensities of flow noise at low frequencies than those coincident to the array axis.(cont.) Similarly, different dipole sensors may be exposed to varying degrees of rotational mismatch. Sensors may also rest on the seafloor, creating asymmetries. Recognizing these practical issues, we derive a new adaptive processing method customized to the unique characteristics of vector sensors and robust to mismatch and finite sample support. This new approach involves using multiple white noise gain constraints. During the past couple of decades, stationary vector sensor arrays have been built and tested, demonstrating improved gain and ambiguity lobe attenuation. Up until recently, however, very few towed vector sensor arrays had been built and tested. As such, many of the advantages of vector sensor arrays had only previously been shown in theory and/or with stationary arrays. We present results from sea trials in Monterey Bay, CA (2006) and Dabob Bay, WA (2007) towing a relatively short vector sensor array. Results highlight several of the distinct practical advantages of vector sensor arrays: resolution of spatial ambiguity (e.g., port/starboard and conical ambiguity), the ability to "undersample" an acoustic wave without spatial aliasing, quiet target recovery via clutter reduction, immunity to mismatch, improved array gain and enhanced detection performance.by Andrew Joseph Poulsen.Ph.D

About the Use of Adaptive Antennas in 60 GHz UWB-OFDM Personal Area Network Transceivers

The recent opening of unlicensed spectrum around 60 GHz has raised the interest in designing gigabit Wireless Personal Area Networks (WPANs). Since at 60 GHz the signal attenuation is strong, this band is basically suitable for short range wireless communications. It is understood that directional antennas can be employed to compensate for the path loss and combat the waste of power due to the scatter phenomena characteristic of these high frequencies. This thesis studies the use of adaptive array systems in 60 GHz Ultra Wide Band-Orthogonal Frequency Division Multiplexing (UWB-OFDM) personal area network transceivers. The study has been conducted by simulations and theoretical analysis. Two sensor arrangements have been considered, the Uniform Linear Arrays (ULA) and the Uniform Circular Arrays (UCA), in the simple case of the Line of Sight (LOS) transmission scenario. On the one hand we have designed a IEEE 802.15.3c Medium Access Control (MAC) phased-array controller throughput using Direction of Arrival (DOA) estimation to perform beamsteering. We have simulated the MAC controller with the network simulator ns-2. The impact of the array controller performance onto the achievable throughput of the wireless links has been studied to draw the requirements about the standard deviation of the DOA estimator. On the other hand, we have found the Cramér-Rao Bound (CRB) for DOA estimation of impinging 60 GHz OFDM sources. The requirements of the standard deviation of the DOA estimator are analysed against the CRB for DOA to validate the design of the directional 60 GHz UWB-OFDM transceivers. The comparison reveals that directional 60 GHz UWB-OFDM transceivers can achieve high wireless throughput with a number of pilot subcarriers and for a Signal to Noise Ratio (SNR) operating range typical of next generation WPAN

Bayesian reconstruction of binary media with unresolved fine-scale spatial structures

We present a Bayesian technique to estimate the fine-scale properties of a binary medium from multiscale observations. The binary medium of interest consists of spatially varying proportions of low and high permeability material with an isotropic structure. Inclusions of one material within the other are far smaller than the domain sizes of interest, and thus are never explicitly resolved. We consider the problem of estimating the spatial distribution of the inclusion proportion, F(x), and a characteristic length-scale of the inclusions, δ, from sparse multiscale measurements. The observations consist of coarse-scale (of the order of the domain size) measurements of the effective permeability of the medium (i.e., static data) and tracer breakthrough times (i.e., dynamic data), which interrogate the fine scale, at a sparsely distributed set of locations. This ill-posed problem is regularized by specifying a Gaussian process model for the unknown field F(x) and expressing it as a superposition of Karhunen–Loève modes. The effect of the fine-scale structures on the coarse-scale effective permeability i.e., upscaling, is performed using a subgrid-model which includes δ as one of its parameters. A statistical inverse problem is posed to infer the weights of the Karhunen–Loève modes and δ, which is then solved using an adaptive Markov Chain Monte Carlo method. The solution yields non-parametric distributions for the objects of interest, thus providing most probable estimates and uncertainty bounds on latent structures at coarse and fine scales. The technique is tested using synthetic data. The individual contributions of the static and dynamic data to the inference are also analyzed.United States. Dept. of Energy. National Nuclear Security Administration (Contract DE-AC04_94AL85000

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium