<p> Periodic arrays of Au patterns (dots and lines) were produced via electron beam lithography (EBL). GaAs mesas were produced by using the Au structures as a mask and wet etching the GaAs (lll)B substrates, leaving Au resting above GaAs pillars. Annealing experiments at typical nanowire growth temperatures (550°C) were performed on both mesa-supported samples and a control sample without mesas, and were later characterized by scanning electron microscopy (SEM). From SEM images, a model is proposed to describe the evolution of the Au seed particle during exposure to typical growth conditions. The Au particle is subject to not only a melting process but is also modified by a volume increase due to incorporating Ga atoms and a subsequent crystal structure change. Palpable discrepancies between the mesa-supported and control samples were observed after annealing experiments, suggesting the mesas were effective in confining the migration of the Au. NW s were then grown via gas source molecular beam epitaxy (GS-MBE). Discemable variation amongst the results was evident when a comparison between annealed samples and the grown counterpart was made. The inconsistency is ascribed to the NW growth process beginning only after supersaturation at the growth interface. This saturation took place only after 2-D film growth on the substrate surpassed the height of the mesas rendering the structures less functional. </p> / Thesis / Master of Applied Science (MASc)

An analysis of waveforms, power spectral density and array responses was performed using geophones and broadband seismometers, co-deployed as part of a geologically motivated study. Broadband seismometers record excellent waveforms but, due to cost and deployment effort, wavefields are usually spatially aliased above ~0.1 Hz. Industry rapidly deploys many thousands of inexpensive, passive geophones to record full, unaliased seismic wavefields; however, waveform quality is limited below the instrument's natural frequency of ≥2 Hz. In 2012, coincident passive and controlled-source seismic surveys were deployed to investigate tectonics in Idaho and Oregon. Broadband stations were deployed at quiet sites every 15 km, taking experienced professionals >1 person-days per station. Fifty 4.5 Hz geophones and "Texan" seismographs at 200-m spacing were deployed per person-day by inexperienced students. Geophone data were continuously recorded for 3 nights and 1 day, while broadband seismometers were deployed for ~2 years. The spectral and array responses of these real deployments were compared. For a M7.7 teleseismic event, the broadband seismometer and geophone recorded nearly identical waveforms down to <0.03 Hz (32 s) and matching power spectral density down to 0.02 Hz (50 s). For quiet ambient noise, the waveforms strongly correlate down to <0.25 Hz (4 s) and the power spectral density match to the low-frequency side of the microseismic peak at ~0.15 Hz (~7 s). By deploying a much larger number of geophones, waveforms can be stacked to reduce instrument self-noise and beamforming can be used to identify wavefield azimuth and apparent velocity. Geophones can be an effective tool in ambient noise seismology down to ~7 seconds and can be used to record large seismic events effectively down to tens of seconds, well below the natural frequency of the instruments. A well-designed deployment of broadbands and geophones can enable full wavefield studies from long period to short period. Scientific and societal applications that could benefit from the improved unaliased wavefield bandwidth include local to regional seismicity, strong ground motion, magma migration, nuclear source discrimination, and crustal studies. / Master of Science / An analysis of seismic responses was performed using common seismology sensors, codeployed as part of a geologically motivated study. Broadband seismometers record seismic activity extremely well, however, due to cost and deployment effort, are less effective above ~0.1 Hz. Industry rapidly deploys many thousands of inexpensive, geophones, to record effectively above ~2 Hz; however, quality of the signal is limited below 2 Hz. In 2012, coincident seismic surveys were deployed to investigate earth structures in Idaho and Oregon. Broadband stations were deployed at every 15 km, taking experienced professionals >1 person-days per station. Fifty geophones and “Texan” seismographs at 200-m spacing were deployed per person-day by inexperienced students. Geophone data were continuously recorded for 3 nights and 1 day, while broadband seismometers were deployed for ~2 years. The seismic responses of these real deployments were compared. For a M7.7 earthquake, the broadband seismometer and geophone recorded nearly identical waveforms down to <0.03 Hz (32 s) and had similar characteristics down to 0.02 Hz (50 s). For low energy seismic signal, the waveforms were comparable down to <0.25 Hz (4 s) and had similar characteristics at ~0.15 Hz (~7 s). By deploying a much larger number of geophones, waveforms can be added together to improve signal quality and determine where the seismic source is located. Geophones can be an effective tool for low energy seismic signal down to ~7 seconds in period and can be used to record large seismic events effectively down to tens of seconds in period. A well-designed deployment of broadbands and geophones can enable full seismic studies from low and high frequencies which would have many scientific and societal benefits.

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. / Ph. D.

Wireless communication has enjoyed explosive growth over the past decade. As demands for increased capacity and quality grow, improved methods for harnessing the multipath wireless channel must be developed. The use of adaptive antenna arrays is one area that shows promise for improving capacity of wireless systems and providing improved safety through position location capabilities. These arrays can be used for interference rejection through spatial filtering, position location through direction finding measurements, and developing improved channel models through angle of arrival channel sounding measurements. This thesis provides an overview of the technical challenges involved in position location of wireless users and details the hardware development of a multi-sensor testbed at the Mobile and Portable Radio Research Group at Virginia Tech. This testbed is to be used for position location experiments as well as a host of other adaptive signal processing applications. / Master of Science

Please read the abstract in the section 00front of this document / Thesis (PhD (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / PhD / unrestricted

A k × n array with entries from a q-letter alphabet is called a t-covering array if each t × n submatrix contains amongst its columns each one of the gt different words of length t that can be produced by the q letters. In the present article we use a probabilistic approach based on an appropriate Markov chain embedding technique, to study a t-covering problem where, instead of looking at all possible t ×n submatrices, we consider only submatrices of dimension t ×n with its rows being consecutive rows of the original k × n array. Moreover, an exact formula is established for the probability distribution function of the random variable, which enumerates the number of deficient submatrices (i.e., submatrices with at least one missing word, amongst their columns), in the case of a k × n binary matrix (q = 2) obtained by realizing kn Bernoulli variables.

No description available.

No description available.

Numerical techniques for designing planar arrays with low side lobe level often require memory intensive optimization algorithms and also initialization in the form of some known values of radiation pattern parameters beforehand - information that may not be available when designing arrays. A few analytical methods exist in the literature that can be used to design rectangular lattices of isotropic elements for desired half-power beamwidth and side lobe level, but the number of elements of the array often should be known before the design process. Some array designs based on analytical techniques may suffer from severe performance limitations, an example is the uniformly excited array which cannot produce side lobe levels below ̶13.3 dB. The goal of this study is to present an analytical technique for synthesis of planar arrays that, for specified radiation pattern requirements, not only provides quick solutions for the required number of elements and its distribution along the length and width of the array rectangular lattice, but also produces low side lobes without any limitation. A new class of non-uniformly excited equally spaced planar arrays is introduced and investigated in this study. The new array uses the patterns of uniformly excited linear arrays as its building blocks and has a separable element current distribution, hence making it mathematically convenient to analyze its radiation properties in terms of those of its constituent linear arrays. The proposed planar array does not suffer from the side lobe level limitation of uniformly excited planar arrays, and its synthesis, due to the analytical nature of description of its radiation properties, does not require iterative procedures that are inherent to numerical techniques. Radiation characteristics of the proposed planar array, including directivity, side lobe level, half-power beamwidths, far-field three dimensional radiation patterns, and element excitation currents, are examined and simulation results for several example cases are presented. The analysis culminates with successfully mapping a continuous radiation pattern to discrete element currents in a rectangular lattice geometry. The synthesis procedure is validated by successfully designing various planar arrays with desired requirements in terms of side lobe level and half-power beamwidths in the principal planes. Several design examples are presented. Radiation characteristics of the synthesized arrays are compared with the desired design requirements which were used as input information in the synthesis process. For the cases studied, the achieved performance characteristics are close to the desired ones. / MS

abstract: Modern software and hardware systems are composed of a large number of components. Often different components of a system interact with each other in unforeseen and undesired ways to cause failures. Covering arrays are a useful mathematical tool for testing all possible t-way interactions among the components of a system. The two major issues concerning covering arrays are explicit construction of a covering array, and exact or approximate determination of the covering array number---the minimum size of a covering array. Although these problems have been investigated extensively for the last couple of decades, in this thesis we present significant improvements on both of these questions using tools from the probabilistic method and randomized algorithms. First, a series of improvements is developed on the previously known upper bounds on covering array numbers. An estimate for the discrete Stein-Lovász-Johnson bound is derived and the Stein- Lovász -Johnson bound is improved upon using an alteration strategy. Then group actions on the set of symbols are explored to establish two asymptotic upper bounds on covering array numbers that are tighter than any of the presently known bounds. Second, an algorithmic paradigm, called the two-stage framework, is introduced for covering array construction. A number of concrete algorithms from this framework are analyzed, and it is shown that they outperform current methods in the range of parameter values that are of practical relevance. In some cases, a reduction in the number of tests by more than 50% is achieved. Third, the Lovász local lemma is applied on covering perfect hash families to obtain an upper bound on covering array numbers that is tightest of all known bounds. This bound leads to a Moser-Tardos type algorithm that employs linear algebraic computation over finite fields to construct covering arrays. In some cases, this algorithm outperforms currently used methods by more than an 80% margin. Finally, partial covering arrays are introduced to investigate a few practically relevant relaxations of the covering requirement. Using probabilistic methods, bounds are obtained on partial covering arrays that are significantly smaller than for covering arrays. Also, randomized algorithms are provided that construct such arrays in expected polynomial time. / Dissertation/Thesis / Doctoral Dissertation Computer Science 2016