Browsing by Author "Chen, L"

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Fabrication and raman analysis of ZnSe quantum dots in glass matrix thin films by pulsed laser evaporation
(1998-07-01)
Yin, SB
;
Chen, L
;
Hsieh, WF
;
光電工程學系
;
Department of Photonics
We have successfully fabricated ZnSe-microcrystal-doped silica-glass thin films by pulsed laser evaporation from ZnSe powder embedded in sol-gel glass targets and demonstrated that the spatial cell-elation model call be used to analyze the LO Raman mode of ZnSe semiconductor doped glass thin films. The obtained correlation lengths differ from the real sizes of the particulates observed by scanning electron microscopy which may bz due to the fact that the particulates art polycrystalline, with different orientations separated by grain boundaries, and the correlation length just provides a measure of the separated average grain dimension. The photoluminescence spectrum indicates that the carriers may tunnel through the grain walls in the polycrystalline particulates. Low temperature annealing only causes aggregate of particulates without merging the grain boundaries.
Outage probability analysis of downlink power allocation mechanisms in CDMA systems during soft handoff
(2005)
Wang, LC
;
Chen, L
;
電信工程研究所
;
Institute of Communications Engineering
This paper presents an analytical approach to evaluate the downlink outage probability for a handoff user in code division multiple access (CDMA) systems under the log-normally shadowed and Rician fading channel subject to co-channel interference. Since soft handoff performance is related to the power allocation algorithms, we consider various downlink power allocation schemes, including the site selection diversity transmission (SSDT), the link proportional power allocation (LPPA), the equal power allocation (EPA), and the quality balancing power allocation (QBPA). From our analytical model, it is shown that SSDT is most sensitive to fast fading and co-channel interference. By contrast, LPPA can achieve better outage in the presence of fast fading at the cost of slightly sacrificing power efficiency. The proposed analytical framework can be applied to evaluate the effects of handoff thresholds, orthogonality factors of channelization codes, and Rician factors on the outage performance of soft handoff users subject to co-channel interference from other cells.
Performance analysis of link proportional power allocation for downlink soft handoff in CDMA systems
(2004)
Wang, LC
;
Chen, L
;
電信工程研究所
;
Institute of Communications Engineering
The growing demands of downlink traffic volume in mobile environments have posed a new challenge on wireless data networks. This paper investigates the performance of current downlink power allocation mechanisms for soft handoff in the code division multiple access (CDMA) systems, including the equal power allocation (EPA), quality balancing power allocation (QBPA), site selection diversity transmission (SSDT), and link proportional power allocation (LPPA). We evaluate the outage performance of these power allocation schemes from the perspectives of base station transmission power limit and power control error. Traditionally, the site selection diversity transmission (SSDT) scheme has been viewed as the optimal solution for downlink handoff from the standpoint of power efficiency. The SSDT scheme selects the best base station from the active set to transmit downlink signals to the user. However, we find that SSDT is very sensitive to power control error as the user is at cell boundary. Thus with consideration of both base station power shortage and transmission control errors, LPPA exhibits the lowest outage probability when the system load is not heavy, while SSDT is the best choice only in the case of heavy traffic load.
Supercooled vortex liquid and superheated vortex solid in type II superconductors
(2005-06-01)
Chen, L
;
Li, DP
;
Rosenstein, B
;
電子物理學系
;
Department of Electrophysics
metastable homogeneous state exists down to zero temperature in systems of repelling objects. Zero "fluctuation temperature" liquid state therefore serves as a (pseudo) "fixed point" controlling the properties of vortex liquid below and even around melting point. There exists Madelung constant for the liquid in the limit of zero temperature which is higher than that of the solid by an amount approximately equal to the latent heat of melting. On the other hand, the superheated solid exists until the spinodal line. This picture is supported by an exactly solvable large N Ginzburg - Landau model in magnetic field and it is confirmed by the recent experiment.