Browsing by Author "Hans D. Hallen, Committee Member"

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    Thermionic Emission from Doped and Nanocrystalline Diamond
    (2003-04-21) Koeck, Franz Alexander; Robert J. Nemanich, Committee Chair; Hans D. Hallen, Committee Member; Zlatko Sitar, Committee Member
    Microwave Plasma assisted Chemical Vapor Deposition (MPCVD) has been utilized to synthesize nitrogen doped and intrinsic nanocrystalline diamond films to investigate thermionic field emission behavior. Sulfur-doped nanocrystalline diamond films prepared by hot filament chemical vapor deposition (HFCVD) have been included in the thermionic field emission measurements. The samples were imaged in UHV by photo electron emission microscopy (PEEM) using a UV Hg lamp for photoemission excitation. The same instrument was used to obtain the thermionic-field emission electron microscopy images (T-FEEM) at temperatures up to 900°C. The Raman spectra of the films showed a strong diamond peak at 1332cm-1 and weaker signal from the graphitic regions in the sample. Field emission could not be measured at room temperature, but the PEEM images showed relatively uniform emission. The PEEM images showed little change as the temperature is increased. At temperatures as low as 640°C the T-FEEM images exhibited strongly enhanced electron emission with increasing temperature. Doped and undoped nanocrystalline diamond films showed localized emission from small emission sites with a significant temperature dependence of the electron emission for the sulfur doped films at around 600°C. This thesis focuses on developing a consistent model of thermionic emission from doped and nanocrystalline diamond films.
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    Transmitter Diversity and Multiuser Precoding for Rayleigh Fading Code Division Multiple Access Channels
    (2003-06-04) Guncavdi, Secin; Brian L. Hughes, Committee Member; J. Keith Townsend, Committee Member; Alexandra Duel-Hallen, Committee Chair; Hans D. Hallen, Committee Member
    Transmitter diversity in the downlink for Code Division Multiple Access (CDMA) systems provides a means to achieve similar performance gains as for the mobile station (MS) receiver diversity without the complexity of a MS receiver antenna array. Transmitter based methods enable to shift signal processing to the transmitter where power and computational complexity are more abundant, thus simplifying receiver units. We examine feasibility of several transmitter diversity techniques for the Wideband CDMA (W-CDMA) systems. We also investigate an optimal method to combine transmitter diversity and precoding that achieves the gain of maximum ratio combining of all space and frequency diversity branches. Under severe channel conditions (i.e. multipath), the multiple access interference (MAI) becomes the major source of performance degradation for direct sequence CDMA (DS/CDMA) systems. This is because of the loss of orthogonality between the spreading codes used by each user due to the multipath channel effects. To overcome this problem, many receiver based multiuser detection (MUD) techniques have been proposed. These techniques demand high computational complexity, power and knowledge of spreading codes of all users. As a result, in the downlink of a CDMA system it is not feasible to employ such methods at the MS. Alternatively, transmitter based techniques were proposed to shift computational complexity and power consumption to the BS, where they can be afforded. It was shown that these methods are very effective in removing the MAI. Although these methods are powerful, they are high in complexity and require the accurate knowledge of the channel. since MAI cancellation filters need to be updated continuously as fading coefficients vary. We propose a less complex method with similar performance improvements. In the proposed method, the functions of multipath combining and MAI cancellation are separated. Thus the MAI cancellation matrix does not depend on rapidly time-varying fading coefficients. Transmitter diversity and multiuser precoding can be combined to further improve the performance. Multiuser precoding preserves the multipath diversity while removing the MAI. Extending multiuser precoding to multiple antennas results in space diversity in addition to multipath diversity. Both transmitter diversity and multiuser precoding require the knowledge of the channel state information (CSI). The CSI can be estimated at the receiver and sent to the transmitter via a feedback channel. To enable the studied adaptive techniques for practical systems, we employ the long range prediction (LRP) algorithm, which characterizes the fading channel using the autoregressive (AR) model and computes the Minimum Mean Squared Error (MMSE) estimate of a future fading coefficient sample based on a number of past observations. Numerical, simulation and theoretical results are presented to show that transmitter diversity and multiuser precoding can be used to remove MAI and achieve frequency and space diversity through multipath channels and multiple antennas.