Browsing by Author "Tu, YM"

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A new and efficient hybrid model for estimating space diversity in indoor environment
(2000-03-01)
Tarng, JH
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Chang, RS
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Huang, JM
;
Tu, YM
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電信工程研究所
This study presents a novel hybrid model based on a two-dimensional (2-D) site-specific model and a statistical model to investigate space diversity in indoor environments. The statistical model describes the field scattered by rough surface boundaries and randomly positioned scatterers, which may be important when one or both of the receiving and transmitting antennas are close to the boundaries of the scatterers. Comparing the computed spatial correlation with the measured one with the transmitting frequency at 2.44 GHz at many different sites demonstrates the effectiveness of the hybrid model. In addition to accurately predicting field strength, the hybrid model can quantify the relative mean contribution of diffused scattering in an indoor environment with a factor r. The factor is equal to the ratio of ensemble average of a randomly scattered envelope to the spatially averaged envelope. Its optimum value is in a narrow range from 0.3 to 0.5 when the intensity fluctuation at the measurement sites is in or close to saturation regions, where a large number of micromultipaths are generated. This finding confirms the relative ease in applying the hybrid model. Experimental results suggest that choosing r = 0.4 in a saturation allows the hybrid model to yield a satisfactory performance, as confirmed by a blind test. Moreover, good diversity gains can be obtained with an antenna spacing greater than or equal to one wavelength. Our results further demonstrate that the diversity gain of horizontally spaced antennas exceeds that of vertically spaced antennas.
Constraint time buffer determination model
(1998-04-01)
Tu, YM
;
Li, RK
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工業工程與管理學系
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Department of Industrial Engineering and Management
In today's competitive environment, reducing Work in Process (WIP) to improve cycle time, delivery and product quality has become the key to maintaining profitability. However, due to conservative attitudes and conceptual mistakes, WIP has spread along the entire production pipe line. Although intended to maximize the utilization of resources, WIP can be caused to become over-sized. This maximum utilization of resources syndrome has been proved incorrect by the Theory of Constraint and WIP should be queued only in front of constraint machine, thereby ensuring the constraint machine not starvation. Although this concept has been proved correct and despite many reported examples of successful implementation in many companies, in practice, the WIP level is still determined by the trial and error approach. Therefore, a WIP computation model is deemed necessary. In this paper, a constraint time buffer determination model is proposed. The model first proposes a machine-view's bill of routing representing a structure that serves as a fundamental structure for formulating and computing the maximum time buffer. The machine-view's bill of routing is a tree structure, the constraint machine is the root node and its feeder machines are entry nodes (or subtrees) of the root node. With this tree structure, the behaviour (or stability) of the constraint machine and its feeder machines can then be studied and formulated. By incorporating the MTTR of each feeder machine, a mathematical relationship can then be formulated and the time buffer computed. Furthermore, to validate the proposed constraint time buffer determination model, an example is illustrated and a simulation model is also developed.