Thesis (M.S.)--Rochester Institute of Technology, 1993. / "February 1992." Typescript. Includes bibliographical references (leaves 94-95).

Thesis (M.S.)--Rochester Institute of Technology, 1994. / Typescript. Includes bibliographical references (leaves 52-53).

Typescript, etc. / Digitized by Kansas State University Libraries

LD2668 .T4 1948 C65 / Master of Science

Call number: LD2668 .T4 1948 M65 / Master of Science

No description available.

No description available.

Over the past decade, infrared has attracted a considerable amount of interest as an alternative medium to radio for short-range indoor wireless local area networks. Infrared offers a number of significant advantages over its radio frequency counterpart, such as the abundance of bandwidth that is currently unregulated worldwide, the availability of low cost emitters and detectors, inherent security and resistance to multi path fading. The work presented in this thesis focuses on modulation techniques, the fundamental aim being to assess the suitability of digital pulse interval modulation (DPIM) for use in indoor optical wireless communication systems. Infrared transceivers are subject to eye safety regulations, and consequently power efficiency is an important criterion when evaluating modulation techniques. From the error probability analysis carried out on the non-distorting additive white Gaussian noise channel, it is shown that DPIM is able to trade off power efficiency against bandwidth efficiency by increasing the number of bits per symbol. Furthermore, by encoding an additional bit per symbol, DPIM can outperform pulse position modulation (PPM) both in terms of power efficiency and bandwidth efficiency when simple threshold detection is employed. Indoor optical wireless systems generally operate in the presence of intense ambient light, emanating from both natural and artificial sources. Along with contributing to the generation of shot noise, artificial ambient light sources also introduce a periodic interference signal which can have a detrimental effect on link performance. Original analysis is presented which examines the error performance of DPIM in the presence of interference from a fluorescent lamp driven by a high-frequency electronic ballast, which is potentially the most degrading source of ambient light. It is found that such interference results in an average optical power requirement that is almost independent of the bit rate. The analysis then goes on to consider the effectiveness of electrical high-pass filtering as a simple means of mitigating the effect of the interference, taking into account the baseline wander introduced by the high-pass filter. DPIM was found to be more susceptible to the effects of baseline wander compared with PPM, a finding which is supported by the original analysis carried out on the power spectral density of the scheme. Consequently, whilst electrical high-pass filtering was found to be very effective at high bit rates, significant power penalties are still incurred at low to medium bit rates. In non-directed line of sight and diffuse link configurations, multipath propagation gives rise to intersymbol interference (ISI), which must be taken into account for data rates above 10 Mbit/s. Original analysis is presented which examines the unequalized performance of DPIM in the presence of ISI. From this analysis, it is found that on any given channel, the improved bandwidth efficiency of DPIM results in lower average optical power penalties, compared with PPM. One novel technique which can be used to make DPIM more resistant to the effects of ISI is to add a guard band to each symbol, immediately following the pulse. Original contributions are presented which evaluate the effectiveness of this technique. To quantify the results obtained, analysis is also carried out on DPIM using a zero-forcing decision feedback equalizer (ZF-DFE), which represents a more traditional approach to mitigating the effects of ISI. It is shown that the guard band technique offers a similar level of performance to the ZF-DFE on all but the most severe channels, and has the advantage of reduced cost and complexity compared with implementing a ZF-DFE. To support the theoretical and simulated performance of DPIM carried out in this thesis, details are given of a prototype 2.5 Mbit/s diffuse infrared link employing 16-DPIM which has been designed and constructed. The error performance of the link is measured under a variety of ambient light conditions, and the effectiveness of electrical high-pass filtering in mitigating the resulting interference is assessed. It is shown that whilst a fluorescent lamp driven by a high frequency electronic ballast has the potential to significantly degrade link performance, the power penalty introduced by this source can be made manageable by careful selection of the high-pass filter cut-on frequency.

IRAS observations of extragalactic objects are analyzed, supplemented by optical spectroscopy and 10 μm photometry. The relationship between various forms of activity in the nuclei of spiral galaxies and their mid- to far-infrared spectral energy distributions is explored. It is shown that more than 70% of galaxies with F₆₀/F₂₅ ≤ 3 are Seyferts, while the remainder have bright optical emission lines in their nuclear spectra. It is argued that most Seyferts are powered by their active nuclei at 25 μm, while there is some indication that Seyferts with large F₆₀/F₂₅ flux ratios are undergoing starbursts in the vicinity of their nuclei. The properties of a sample of bright, extragalactic IRAS sources are studied. A catalog containing total infrared and blue fluxes, distance estimates, recession velocities, and morphological classifications for these objects is presented. The brightest sources at mid- to far-infrared wavelengths are (in order of frequency) nearby, normal spiral galaxies; galaxies with disturbed or irregular morphology (often known as interacting galaxies); type 2 Seyferts; and dust-embedded type 1 Seyferts. All of these sources are dominated by thermal emission from dust. The dust in the peculiar, irregular, and Seyfert galaxies is exposed to a higher mean intensity of radiation. Moreover, these IR-active galaxies tend to have strong, compact nuclear sources at 11 μm, whether or not they contain a known Seyfert nucleus. The distinctive spectral behavior of IR-luminous galaxies is shown to result from the presence of compact, dust-dominated IR nuclear sources, which are the predominant cause of IR luminosities above 10¹¹ L(⊙). Compact IR sources are always associated with a dust-embedded region of ionized gas in the galaxy's nucleus, which may exhibit Seyfert, LINER, or H II region characteristics. The luminosity of the compact nuclear source is well correlated with its 60/100 μm color. This relationship is a vital clue to the nature of these sources and has potential application as a standard candle.

The uses of the bispectrum for recovering the images of one-dimensional infrared astronomical speckle data are examined in detail. An analytic model for the bispectral transfer function, the variance, and the covariance of the bispectrum are developed. The models are evaluated by Monte Carlo integration and the results are compared to sample estimates of the same quantities obtained from simulated data. For comparison, the same sample quantities are computed from observed data. The bispectrum is shown to be useful for determining estimates of the object phase. A recursive method which is used to obtain the object phase estimates is introduced. Since the bispectrum provides multiple estimates of each object phase, a number of methods for combining the multiple estimates are developed and compared. Many techniques have been proposed to determine the phase of images which have been atmospherically distorted. Among these techniques are the Knox-Thompson, and the Simple Shift-and-Add algorithms. These methods are compared to the bispectrum via an objective measure which is developed. Optimization techniques are used to great success. A model for the bispectrum of a binary star is developed and fit to the image bispectrum by the Levenberg-Marquardt algorithm for non-linear least squares. The ability of the algorithm to determine binary star parameters from the bispectrum is tested with both simulated and observed data. Since the bispectrum may not always be available, a method is developed which determines binary star parameters from the image Fourier transform. The full set of object phases and moduli are determined by use of the conjugate gradient and conjugate direction algorithms in the last section. Two starting points for each algorithm are employed. The first starting point uses the estimates of the object phases obtained from the recursive bispectrum technique. The second assumes no information is known about the object. The speed of convergence of each algorithm is analyzed and recommendations are made for future use.