Browsing by Author "Chang, Yu-Jung"

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Cross-layer QoS analysis of opportunistic OFDM-TDMA and OFDMA networks
(2007-05-01)
Chang, Yu-Jung
;
Chien, Feng-Tsun
;
Kuo, C. -C. Jay
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Performance analysis of multiuser orthogonal frequency division multiplexing (OFDM-TDMA) and orthogonal frequency division multiple access (OFDMA) networks in support of multimedia transmission is conducted in this work. We take a cross-layer approach and analyze several quality-of-service (QoS) measures that include the bit rate and the bit error rate (BER) in the physical layer, and packet average throughput/delay and packet maximum delay in the link layer. We adopt a cross-layer QoS framework similar to that in IEEE 802.16, where service classification, flow control and opportunistic scheduling with different subcarrier/bit allocation schemes are implemented. In the analysis, the Rayleigh fading channel in the link layer is modeled by a finite-state Markov chain, and the channel state information (CSI) is assumed to be available at the base station. With the M/G/1 queueing model and flow control results, our analysis provides important insights into the performance difference of these two multiaccess systems. The derived analytical results are verified by extensive computer simulation. It is demonstrated by analysis and simulation that OFDMA outperforms OFDM-TDMA in QoS metrics of interest. Thus, OFDMA has higher potential than OFDM-TDMA in supporting multimedia services.
Delay Analysis and Comparison of OFDM-TDMA and OFDMA under IEEE 802.16 QoS Framework
(2006)
Chang, Yu-Jung
;
Chien, Feng-Tsun
;
Kuo, C. -C. Jay
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
The delay analysis and comparison of OFDM-TDMA and OFDMA using a flow control scheme under the QoS framework of IEEE 802.16 are conducted in this work. We investigate the maximum packet delay performance by deriving delay bounds for several multiaccess modes with different subcarrier/time-slot assignment and bit allocation schemes. The analytical delay bounds provide insights into the performance difference among different multiaccess schemes, and demonstrate each scheme\'s capability of supporting real-time, delay-sensitive multimedia applications. The derived analytical results are verified by computer simulation.
Is what I need what I want? Reconceptualising college students' needs in English courses for general and specific/academic purposes
(2011-12-01)
Liu, Jin-Yu
;
Chang, Yu-Jung
;
Yang, Fang-Ying
;
Sun, Yu-Chih
;
英語教學研究所
The purpose of the study is to explore English as a foreign language (EFL) college students' needs in English for General Purposes (EGP) and English for Specific/Academic Purposes (ESP/EAP) courses in terms of their perceptions of three subcategories of needs-necessities, wants, and lacks-and reasons for students' course enrollment. A questionnaire was administered to 972 EFL college students from six universities in Taiwan. The results showed that students had different perceptions of necessities, wants, and lacks in the different language skills taught in EGP and ESP/EAP courses. The findings also demonstrated discrepancies between the students' perceptions of needs and the actual courses they took, thereby highlighting the importance of understanding needs as a complex, multiple, and conflicting concept. Finally, factors that determined students' enrollment in EGP courses included necessities, short-term, and long-term goals while enrollment in ESP/EAP courses depended more on job relevancy. Implications for language instructors and curriculum designers are discussed. (C) 2011 Elsevier Ltd. All rights reserved.
Performance Comparison of OFDM-TDMA and OFDMA with Cross-Layer Consideration
(2006)
Chang, Yu-Jung
;
Chien, Feng-Tsun
;
Kuo, C. -C. Jay
;
電子工程學系及電子研究所
;
Department of Electronics Engineering and Institute of Electronics
Performance comparison of multiuser OFDM-TDMA and OFDMA systems by considering physical and link cross-layer behavior is conducted in this work. We provide a new framework for performance evaluation that centers on "scheduling" with several different subcarrier/bit allocation schemes. The performance metrics include: the bit rate, the bit error rate (BER), packet throughput and delay. The instantaneous channel conditions of users are assumed to be available at the base station, which can be achieved by channel estimation at the mobile terminal (MT), and then fed back to the base station through a control channel. To perform the link layer comparison, we model the Rayleigh fading channel with a finite-state Markov chain, and adopt the M/G/1 queueing model as the analytical tool. Computer simulation is performed to verify conducted analytical results.
When technology speaks language: an evaluation of course management systems used in a language learning context
(2010-09-01)
Yu, Wen-Kai
;
Sun, Yu-Chih
;
Chang, Yu-Jung
;
英語教學研究所
;
Institute of Teaching English to speakers of other Languages
In light of the growing popularity of the use of computer management systems (CMSs) in higher education today, this study critically evaluates CMS adoption through a content-specific lens. By employing a mixed-method approach, the study examines college teachers' and students' experiences and perceptions of CMS adoption for language learning and teaching purposes. The findings show that despite the users' perceived advantages of using CMSs in language courses, the systems' lack of content-area specificity undermines many of the potential benefits. The study calls for better-rounded professional training to assist language teachers in integrating CMS functions strategically into their disciplinary pedagogy and incorporating multimedia language resources selectively to maximize the benefit of CMSs. Furthermore, the findings demonstrate the importance of developing content-specific CMSs with functions tailored toward pedagogical needs in different contexts.