Thesis (Ph. D.)--University of California, San Diego, 2001. / Vita. Includes bibliographical references.

Radio spectrum is becoming scarce resource due to increase in the usage of wireless communication devices. However studies have revealed that most of the allotted spectrum is not used effectively. Given the demand for more bandwidth and the amount of underutilized spectrum, DSA (Dynamic Spectrum Access) networks employing cognitive radios are a solution that can revolutionize the telecommunications industry, significantly changing the way we use spectrum resources, and design wireless systems and services. Cognitive radio has improve the spectral efficiency of licensed radio frequency bands by accessing unused part of the band opportunistically without interfering with a license primary user PU. In this thesis we investigate the effects on the quality of service (QoS) performance of spectrum management techniques for the connection-based channel usage behavior for Secondary user (SU). This study also consider other factors such as spectrum sensing time, spectrum handoff and generally distributed service time and channel contention for different SUs. The preemptive resume priority M/G/1 queuing theory is used to characterize the above mentioned effects. The proposed structure of the model can integrate various system parameters such spectrum sensing, spectrum decision, spectrum sharing and spectrum handoff. / Sajid Mahmood 0046-762788990 Mujeeb Abdullah 0046-760908069

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The U.S. Environmental Protection Agency is in the process of developing a nationwide standard for turbidity in construction site runoff. It is widely expected that this standard cannot be met with conventional erosion and sediment control measures; consequently, innovative practices for managing sediment on construction sites must be developed. The objective of this research was to develop an understanding of how soil characteristics and polymer properties affect the amount of turbidity reduction that can be achieved through flocculation. The polymers used were PAMs, a proprietary product, and chitosan. The charge density of the PAMs ranged from 0% to 50% and the molecular weights ranged from 0.2 to 14 Mg/mol. A protocol for creating modified synthetic stormwater runoff for soil samples was developed and used on soils from seven construction sites. Particle size distributions were used to compare the modified synthetic stormwater runoff with grab samples of stormwater from one site and showed the synthetic runoff was representative of the actual runoff. Flocculation tests were performed on the synthetic runoffs with PAM and chitosan doses from 0.03 to 10 mg/L. The non-ionic PAM, proprietary product, and chitosan were found to be the most effective at reducing the turbidity of all the synthetic runoff below 200 NTU. The high molecular weight anionic PAMs were effective on only two of the seven synthetic runoff samples. Hardness tests were performed indicating interparticle bridging to be the bonding mechanism of the PAM. Electrophoretic mobility tests were performed on two of the soil suspensions and indicated the bonding mechanism of PAM to be interparticle bridging, and the bonding mechanism of chitosan to be a combination of charge neutralization and interparticle bridging. Tests showed as the charge density of the PAM increased, their effectiveness decreased. / text

Despite decades of policy innovation aimed at improving school performance, the number of public schools defined as low performing in the U.S. continues to grow. Yet, most explanations of low performance do not consider the fact that many of the country’s lowest performing schools share high rates of turnover among both staff and students, or organizational instability. The purpose of this dissertation was to develop the theoretical underpinnings of both the concept of organizational stability and its relationship with school contextual factors and performance, and to assess the relationship quantitatively. I hypothesized that teacher turnover, principal turnover, and student mobility partially mediate the impact of a school’s socioeconomic context on its academic performance. In order to test the proposed partial mediation model, I conducted a quantitative analysis using path analysis and data from North Carolina public schools. I constructed several samples, including one that included all schools, and five others that focused in on high instability, high poverty, and urban schools. The results of the analyses depended in large part on the type of school under investigation. Specifically, I found that the relationships between the context variables varied according to the sample being examined. Similarly, the presence of direct and mediating effects between the organizational stability variables was contingent on the kind of school. The results of these analyses support previous findings and contribute a new understanding of the role of instability in helping to explain low school performance. This dissertation engages the ongoing debate about the effects of teacher and principal turnover on school performance, suggesting that both do indeed have a deleterious effect on performance. Finally, I make several methodological contributions to the literature on both turnover and school performance by utilizing path analysis, which allows for the prioritization of effects and the testing of indirect effects. / text

Increasing demand on freeway facilities is a major challenge facing urban areas in the United States and throughout the world. Active Traffic Management (ATM) strategies can be used to increase the performance of these facilities through improved operations without the significant expenditure associated with adding capacity. One ATM strategy that has been widely deployed in the current state of practice is ramp metering, which controls the traffic demand placed on a freeway. Merge control strategies are less prevalent and largely undeveloped. This study examines the recurrently congested northbound section of Interstate Highway 35 that approaches downtown Austin, Texas. Using the VISSIM microsimulation platform, a model of this segment was developed and calibrated to reflect current peak-hour congestion. Within this model, ramp metering and merge control technologies were implemented. The impacts on traffic throughput, speed and travel time for each of these proposed systems are evaluated. / text

Our nation is faced with a shortfall in funding desired transportation improvement programs. Texas is one of many states currently at crisis level as it is faced with an estimated annual funding shortfall of about $7 billion dollars between projected needs and transportation fund availability. This is a result of increased vehicle miles traveled and significant population growth that has outpaced the state’s transportation fund availability. Not only is the Texas transportation system declining in quality, but it is rapidly falling behind in the quantity needed to maintain current quality of life. As the Texas Mobility Fund balance approaches zero, the state needs to look toward non-traditional financing mechanisms to address its revenue shortfall. This report examines the problems associated with increased transportation demands in Texas and financing the expansion and maintenance of the state’s transportation system. / text

Student mobility is a common phenomenon in longitudinal data in educational research. The characteristics of education longitudinal data create a problem for the conventional multilevel model. Grady and Beretvas (2010) introduced a cross-classified multiple membership growth curve (CCMM-GCM) model to handle Student mobility over time by capturing complex higher level clustering structure in the data. There are some limitations in the CCMM-GCM model. By creating dummy coded indicators for each measurement occasion, the new model can improve the accuracy and provides an easier and more flexible structure at the higher level. This study provides some support that the new model better fits a dataset than the CCMM-GCM model / text

The purpose of the simulation was to assess the impact of weight pattern assignment when using the multiple membership random effects model (MMREM). In contrast with most previous methodological research using the MMREM, mobility was not randomly assigned; rather the likelihood of student mobility was generated as a function of the student predictor. Two true weights patterns were used to generate the data (random equal and random unequal). For each set of generated data, the true correct weights and two incorrect fixed weight patterns (fixed equal and fixed unequal) that are similar to those used in practice by applied researchers were used to estimate the model. Several design factors were manipulated including the percent mobility, the ICC, and the true generating values of the level one and level two mobility predictors. To assess parameter recovery, relative parameter bias was calculated for the fixed effects and random effects variance components. Standard error (SE) bias was also calculated for the standard errors estimated for each fixed effect. Substantial relative parameter bias differences between weight patterns used were observed for the level two school mobility predictor across conditions as well as the level two random effects variance component, in some conditions. Substantial SE bias differences between weight patterns used were also found for the school mobility predictor in some conditions. Substantial SE and parameter bias was found for some parameters for which it was not anticipated. The results, discussion, future directions for research, and implications for applied researchers are discussed.

Not available / text