Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item ULTRA LOW POWER FSK RECEIVER AND RF ENERGY HARVESTER(2012) Li, Bo; Goldsman, Neil; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis focuses on low power receiver design and energy harvesting techniques as methods for intelligently managing energy usage and energy sources. The goal is to build an inexhaustibly powered communication system that can be widely applied, such as through wireless sensor networks (WSNs). Low power circuit design and smart power management are techniques that are often used to extend the lifetime of such mobile devices. Both methods are utilized here to optimize power usage and sources. RF energy is a promising ambient energy source that is widely available in urban areas and which we investigate in detail. A harvester circuit is modeled and analyzed in detail at low power input. Based on the circuit analysis, a design procedure is given for a narrowband energy harvester. The antenna and harvester co-design methodology improves RF to DC energy conversion efficiency. The strategy of co-design of the antenna and the harvester creates opportunities to optimize the system power conversion efficiency. Previous surveys have found that ambient RF energy is spread broadly over the frequency domain; however, here it is demonstrated that it is theoretically impossible to harvest RF energy over a wide frequency band if the ambient RF energy source(s) are weak, owing to the voltage requirements. It is found that most of the ambient RF energy lies in a series of narrow bands. Two different versions of harvesters have been designed, fabricated, and tested. The simulated and measured results demonstrate a dual-band energy harvester that obtains over 9% efficiency for two different bands (900MHz and 1800MHz) at an input power as low as -19dBm. The DC output voltage of this harvester is over 1V, which can be used to recharge the battery to form an inexhaustibly powered communication system. A new phase locked loop based receiver architecture is developed to avoid the significant conversion losses associated with OOK architectures. This also helps to minimize power consumption. A new low power mixer circuit has also been designed, and a detailed analysis is provided. Based on the mixer, a low power phase locked loop (PLL) based receiver has been designed, fabricated and measured. A power management circuit and a low power transceiver system have also been co-designed to provide a system on chip solution. The low power voltage regulator is designed to handle a variety of battery voltage, environmental temperature, and load conditions. The whole system can work with a battery and an application specific integrated circuit (ASIC) as a sensor node of a WSN network.Item Nonparametric Estimation and Testing of Interaction in Generalized Additive Models(2011) Li, Bo; Smith, Paul J; Mathematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The additive model overcomes the "curse of dimensionality" in general nonparametric regression problems, in the sense that it achieves the optimal rate of convergence for a one-dimensional smoother. Meanwhile, compared to the classical linear regression model, it is more flexible in defining an arbitrary smooth functional relationship between the individual regressor and the conditional mean of the response variable Y given X. However, if the true model is not additive, the estimates may be seriously biased by assuming the additive structure. In this dissertation, generalized additive models (with a known link function) are considered when containing second order interaction terms. We present an extension of the existing marginal integration estimation approach for additive models with the identity link. The corresponding asymptotic normality of the estimators is derived for the univariate component functions and interaction functions. A test statistic for testing significance of the interaction terms is developed. We obtained the asymptotics for the test functional and local power results. Monte Carlo simulations are conducted to examine the finite sample performance of the estimation and testing procedures. We code our own local polynomial pre-smoother with fixed bandwidths and apply it in the integration method. The widely used LOESS function with fixed spans is also used as a pre-smoother. Both methods provide comparable results in estimation and are shown to work well with properly chosen smoothing parameters. With a small and moderate sample size, the implementation of the test procedure based on the asymptotics may produce inaccurate results. Hence a wild bootstrap procedure is provided to get empirical critical values for the test. The test procedure performs well in fitting the correct quantiles under the null hypothesis and shows strong power against the alternative.