Browsing by Author "Li, Yan"
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Item Faculty perceptions about attributes and barriers impacting diffusion of web-based distance education (WBDE) at the China Agricultural University(Texas A&M University, 2004-11-15) Li, Yanhe purpose of this study was to examine faculty perceptions about attributes and barriers impacting diffusion of Web-based distance education (WBDE) at the China Agricultural University (CAU). Random and stratified sampling was used and 273 faculty participated in the study. About 70% of participants stayed in early stages in the innovation-decision process related to WBDE (no knowledge, knowledge, or persuasion) and about 30% were in later stages (decision or implementation). Faculty members' stage differed significantly by professional area, level of education, teaching experience, and distance education experience. Gender, age, and academic rank had no significant influence on faculty members' stage. CAU faculty tended to agree with the existence of the five attributes of WBDE (relative advantage, compatibility, complexity, trialability, and observability). Professional area, gender, age, level of education, and academic rank had no significant influence on the five perceived attributes. Teaching experience had no significant influence on the five perceived attributes, except compatibility. Distance education experience had no significant influence on the five perceived attributes, except compatibility and observability. CAU faculty perceived ten factors (concerns about time, concerns about incentives, WBDE program credibility, financial concerns, planning issues, conflict with traditional education, fear of technology, technical expertise, administrative support, and infrastructure) as moderate barriers to diffusion of WBDE. Age, level of education, academic rank, and teaching experience had no significant influence on faculty perception about the ten barriers. Professional area and gender had no significant influence on faculty perception about the ten barriers, except concerns about time. Distance education experience had no significant influence on faculty perception about the ten barriers, except conflict with traditional education. Faculty members' stage in the innovation-decision process had no significant influence on faculty perceptions about relative advantage of WBDE and nine of the ten barriers. Faculty members' stage in the innovation-decision process, however, did have a significant impact on faculty perception about compatibility, complexity, trialability, observability of WBDE, and WBDE program credibility as a perceived barrier. Relative advantage, compatibility, complexity, and trialability were correlated with at least one of the ten barriers. Observability was not related with any of the barriers.Item System and circuits investigation of wideband RF polar transmitters using envelope tracking for mobile WiMAX/WiBro applications(2009-08) Li, Yan; Lie, Donald Y.C.; Dallas, Timothy E. J.High efficiency radio frequency (RF) power amplifiers (PAs) are critical in portable battery-operated wireless communication systems because they can dominate the power consumption. Linear power amplifier is required for applications using non-constant envelope modulated signals (e.g., WiMAX) to minimize distortions. Traditionally, linear power amplifiers are implemented by “backing-off†the output power of PA. However, for a spectrally-efficient high peak-to-average (PAR) signal, the average PA efficiency is much lower than the peak efficiency, which is very undesirable. This thesis focuses on Envelope Elimination and Restoration (EER) and Envelope Tracking (ET) efficiency enhancement techniques to improve the PA average efficiency for high PAR signals. A modified bias-dependent Cann’s model has been developed to study the behavior of our monolithic class-E SiGe PAs. The time-mismatch sensitivity between the amplitude and RF paths is compared for ET vs. EER systems based on our modified model and with measurement data. Besides, RF+Analog+Digital system simulations were performed to study the static and group delays in ET/EER based polar TX systems. Both simulations and measurement show that an ET-based polar system has higher resilience against timing mismatches between the amplitude and the RF/phase paths. Our laboratory measurement also validates the bias dependence of our modified Cann’s model. A high efficiency wideband envelope amplifier is also designed for EER/ET based polar transmitters for WiBro/WiMAX applications. Several simulation and measurement results were discussed in terms of optimized efficiency and system linearity. Finally, detailed system co-design simulations including RF circuits and digital DSP blocks show that our class-E PA can be linearized by the open-loop ET technique, and the entire ET-based polar TX system meets the stringent 802.16e TX mask with ~33% overall average efficiency at output power of 18.5dBm.Item Upscaling methods for multi-phase flow and transport in heterogeneous porous media(2011-02-22) Li, YanIn this dissertation we discuss some upscaling methods for flow and transport in heterogeneous reservoirs. We studied realization-based multi-phase flow and transport upscaling and ensemble-level flow upscaling. Multi-phase upscaling is more accurate than single-phase upscaling and is often required for high level of coarsening. In multi-phase upscaling, the upscaled transport parameters are time-dependent functions and are challenging to compute. Due to the hyperbolic feature of the saturation equation, the nonlocal effects evolve in both space and time. Standard local two-phase upscaling gives significantly biased results with reference to fine-scale solutions. In this work, we proposed two types of multi-phase upscaling methods, TOF (time-offlight)- based two-phase upscaling and local-global two-phase upscaling. These two methods incorporate global flow information into local two-phase upscaling calculations. A linear function of time and time-of-flight and a global coarse-scale two-phase solution (time-dependent) are used respectively in these two approaches. The local boundary condition therefore captures the global flow effects both spatially and temporally. These two methods are applied to permeability distributions with various correlation lengths. Numerical results show that they consistently improve existing two-phase upscaling methods and provide accurate coarse-scale solutions for both flow and transport. We also studied ensemble level flow upscaling. Ensemble level upscaling is up scaling for multiple geological realizations and often required for uncertainty quantification. Solving the flow problem for all the realizations is time-consuming. In recent years, some stochastic procedures are combined with upscaling methods to efficiently compute the upscaled coefficients for a large set of realization. We proposed a fast perturbation approach in the ensemble level upscaling. By Karhunen-Lo`eve expansion (KLE), we proposed a correction scheme to fast compute the upscaled permeability for each realization. Then the sparse grid collocation and adaptive clustering are coupled with the correction scheme. When we solve the local problem, the solution can be represented by a product of Green's function and source term. Using collocation and clusering technique, one can avoid the computation of Green's function for all the realizations. We compute Green's function at the interpolation nodes, then for any realization, the Green's function can be obtained by interpolation. The above techniques allow us to compute the upscaled permeability rapidly for all realizations in stochastic space.