Browsing by Author "Liberti, Joseph C."

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    Analysis of CDMA cellular radio systems employing adaptive antennas
    Liberti, Joseph C. (Virginia Polytechnic Institute and State University, 1995)
    This research investigates the performance of Code Division Multiple Access (CDMA) cellular communication systems employing adaptive antennas at the base station. First, a simple analysis is presented which shows that significant improvements in performance can be obtained by using directional antennas in the mobile-to-base station link of CDMA cellular radio systems. Next, steps are taken to improve the analysis of CDMA systems employing adaptive antennas. The first step in improving the analysis is to develop new analytical bit error rate estimation tools which are much more accurate than previous expressions when power control is imperfect, when intercell interference degrades system performance, and when directional antennas are used at one end of the CDMA radio link. These expressions are further extended to accommodate multipath radio channels when omni-directional antennas, fixed directional antennas, or adaptive antennas are applied at either end of the link. This dissertation also develops a channel model which may be used to simulate the direction-of-arrival (DOA) as well as the time-of-arrival (TOA) and power level of multipath components. These analysis techniques are used to develop a simulation framework which can model the performance of CDMA systems, incorporating the effects of the multipath channel, optimal adaptive antenna arrays, and more accurate bit error rate expressions. Results from these simulations, and the impact of these results on future CDMA cellular systems, are described in this dissertation.
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    Evaluation of several techniques for enhancing speech degraded by additive noise in mobile radio environments
    Liberti, Joseph C. (Virginia Tech, 1991-05-05)
    This thesis presents a study of several algorithms for enhancing speech degraded by additive noise in mobile cellular communications. The primary goal of this multi-stage study was to examine adaptive noise cancellation techniques in which one microphone is used to measure the speech plus noise signal and another microphone is used to form an estimate of the interfering background noise. The first stage of this research project involved the design and operation of a measurement system to gather dual channel audio samples in mobile radio environments for use in testing adaptive noise cancellation algorithms developed at Northeastern University. In the second phase of this research, several adaptive algorithms were used to implement noise cancellation systems which were applied to the measured speech signals. In the third phase of this research, several of the adaptive noise cancellation algorithms are compared and additional speech enhancement techniques are investigated.