Browsing by Author "Chang, C"

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A combined channel assignment mechanism for hierarchical cellular systems
(1998-09-01)
Lo, KR
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Chang, CJ
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Chang, C
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Shung, CB
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電子工程學系及電子研究所
This paper proposes a combined channel assignment (CCA) mechanism for hierarchical cellular systems with overlaying macrocells and overlaid microcells. The proposed CCA mechanism combines overflow, underflow, and reversible schemes, where new or handoff calls having no available channel to use in the overlaid microcell can overflow to use free channels in the overlaying macrocell, handoff calls from a neighboring macrocell can underflow to use free channels in the overlaid microcell, and handoff attempts from a macrocell-only region to an overlaid microcell can be reversed to use free channels in the microcell. We apply the CCA mechanism in two different hierarchical cellular systems of Strip type and Manhattan type and compare the CCA with the overflow channel assignment (OCA) scheme. Simulation results show that the CCA mechanism outperforms the OCA scheme by once in forced termination probability, by several times in new call blocking probability, and by 4.7% in system utilization for a hierarchical cellular system, and the CCA mechanism is more suitable for the Manhattan type than for the Strip type. (C) 1998 Elsevier Science B.V.
A hierarchical cellular system with reneging and dropping for waiting new and handoff calls
(1997)
Chang, C
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Chang, CJ
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Lo, KR
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交大名義發表
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電信工程研究所
In this paper, we analyze a hierarchical cellular system with overflow scheme and finite queues for new and handoff calls. The effect of guard channel scheme and the effects due to reneging of waiting new calls because of the caller's impatience and dropping of queued handoff call as the callers move out of the handoff area are considered. We successfully solve the system by adopting the multi-dimensional Markovian chain and use the transition-probability matrix and the signal-how graph to obtain the average new-call blocking probability, the forced termination probability, and the average waiting time of queued new and handoff calls. We further investigate how the guard channel scheme and buffer provision scheme affect the hierarchical cellular system. The results show that it seems unnecessary to provide guard channel and waiting buffer for microcells in the hierarchical cellular system with over flow mechanism.
A QoS-guaranteed fuzzy channel allocation controller for hierarchical cellular systems
(2000-09-01)
Lo, KR
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Chang, CJ
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Chang, C
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Shung, CB
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電信工程研究所
This paper proposes a fuzzy channel allocation controller (FCAC) for hierarchical cellular systems. The FCAC mainly contains a fuzzy channel allocation processor (FCAP) which is designed to be in a two-layer architecture that consists of a fuzzy admission threshold estimator in the first layer and a fuzzy channel allocator in the second layer. The FCAP chooses the handoff failure probability, defined as quality-of-service (QoS) index, and the resource availability as input linguistic variables for the fuzzy admission threshold estimator, where the Sugeno's position gradient-type reasoning method is applied to adaptively adjust the admission threshold for the fuzzy channel allocator. And the FCAP takes the mobility of user, the channel utilization, and the resource availability as input variables for the fuzzy channel allocator so that the channel allocation is finally determined, further based upon the admission threshold. Simulation results show that FCAC can always guarantee the QoS requirement of handoff failure probability for all traffic loads, Also it improves the system utilization by 31.2% while it increases the handoff rate by 12.9% over the overflow channel allocation (OCA) scheme [7]; it enhances the system utilization by 6% and still reduces the handoff rate by 6.7% as compared to the combined channel allocation (CCA) scheme [10], under a defined QoS constraint.
A variable partition duplex scheme on packet reservation multiple access protocol (VPD-PRMA)
(1998)
Chang, C
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Chang, CJ
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電信工程研究所
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Institute of Communications Engineering
A variable partition duplex scheme on packet reservation multiple access protocol (VPD-PRMA) is proposed in this paper. Variable partition duplex scheme allows both uplink and downlink traffic to share a common communication resource and takes advantage of the nature of a conversation pair that only one party is talking in most of the conversation intervals. By adopting VPD-PRMA, wireless communication systems can get higher statistical multiplexing gain due to the trunking effect. We study performance of the VPD-PRMA system by computer simulation. The results show that the system capacity of VPD-PRMA system is 23 and that of traditional PRMA system is 19. By adopting VPD-PRMA on DECT frame structure, the total capacity of VPD-PRMA system is 230 voice channels, which is almost twice the capacity (120 channels) of the typical DECT system.
A variable partition duplex scheme with enlarged reservation duration on packet reservation multiple access protocol
(1999-05-01)
Chang, C
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Chang, CJ
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電信工程研究所
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Institute of Communications Engineering
A variable partition duplex scheme on packet reservation multiple access protocol (VPD-PRMA) is analyzed in this paper. We assume a four-state speech model for a conversational pair and successfully obtain performance measures by approximate Markovian analysis. Analytical results show that they quite fit simulation results; and VPD-PRMA can get higher statistical multiplexing gain than fixed partition duplex (FPD)-PRMA, due to the trunking effect. We further investigate the effect of design parameters of permission probability and enlarged reservation duration on system performance by computer simulation. Simulation results shows that it exists appropriate values for these two design parameters so that the packet dropping probability can be minimized. The adjustment of permission probability can greatly improve the performance of uplink traffic with slight deterioration of the performance of downlink traffic; the provision of enlarged reservation duration scheme can enhance the system performance.
Analysis of a hierarchical cellular system with reneging and dropping for waiting new and handoff calls
(1999-07-01)
Chang, C
;
Chang, CJ
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Lo, KR
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電信工程研究所
;
Institute of Communications Engineering
In this paper, we analyze a hierarchical cellular system with finite queues for new and handoff calls. Both the effect of the reneging of waiting new calls because of the callers' impatience and the effect of the dropping of queued handoff calls as the callers move out of the handoff area are considered, besides the effect of guard channel scheme. We successfully solve the system by adopting the multidimensional Markovian chain and using the transition-probability matrix and the signal-flow graph to obtain the average new-call blocking probability, the forced termination probability, and the average waiting time of queued new and handoff caps. We further investigate how the design parameters of buffer sizes and guard channel numbers in macrocell and microcells affect the performance of the hierarchical cellular system. The results show that provision of buffering scheme and guard channel scheme can effectively reduce the new-call blocking probability and the forced termination probability in the hierarchical cellular system, and the effectiveness is more significant in the macrocell than in the microcells.
Tablet PC as a mobile PACS terminal using wireless LAN
(2003)
Tsao, BH
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Ching, YT
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Lee, WC
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Chen, S
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Chang, C
A PACS mobile terminal has applications in ward round, emergency room and remote teleradiology consultation. Personal Digital Assistants (PDAs) have the highest mobility and are used for many medical applications. However, their roles are limited in the field of radiology due to small screen size. In this study, we built a wireless PACS terminal using a hand-held tablet-PC. A tablet PC (X-pilot, LEO systems, Taiwan) running the WinCE operating system was used as our mobile PACS terminal. This device is equipped with 800x600 resolution 10.4 inch TFT monitor. The network connection between the tablet PC and the server was linked via wireless LAN (IEEE 802.11b).