Browsing by Author "Chang, Chih-Wen"

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A Joint Subcarrier Power Allocation and Interference Avoidance Code Assignment Strategy for Multi-Rate MC-DS-CDMA with TF-Domain Spreading
(2006)
Chang, Chih-Wen
;
Wang, Li-Chun
;
電信工程研究所
;
Institute of Communications Engineering
In the multi-rate multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system, how to allocate subcarrier power to improve the received signal quality, control the multiple access interference (MAI) at an acceptable level, and simultaneously maintain high call admission is an interesting and challenging issue. In this paper, we propose a joint subcarrier power allocation and code assignment scheme for the synchronous multi-rate MC-DS-CDMA system with time-and frequency-domain spreading. The proposed scheme consists of a subcarrier power allocation method to optimize the received signal power as well as an interference avoidance code assignment strategy to eliminate MAI. For this purpose, we define a performance metric called the MAI coefficient to quantitatively predict the incurred MAI before assigning a spreading code. The simulation results show that the proposed joint subcarrier power allocation and code assignment strategy not only can reach a better received signal quality but also achieve high call admission rate.
An interference avoidance code assignment strategy for downlink multi-rate MC-DS-CDMA with TF-domain spreading
(2007-07-01)
Wang, Li-Chun
;
Chang, Chih-Wen
;
Huang, Howard
;
電信工程研究所
;
Institute of Communications Engineering
Frequency selective fading may affect the orthogonality of the spreading codes in the multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems. In this paper, we define a new performance metric called the multiple access interference (MAI) coefficient for the MC-DS-CDMA system to quantitatively predict the impact of inter-code interference with the time- and frequency-domain spreading in a frequency selective fading channel. With the help of MAI coefficient, a novel interference avoidance code assignment strategy is proposed. By jointly considering the incurred MAI effect and the blocking probability in the code tree structure, the proposed interference avoidance code assignment method can effectively reduce the MAI for the multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.
An Interference Avoidance Code Assignment Strategy for Multi-Rate MC-DS-CDMA with Time- and Frequency-Domain Spreading
(2006)
Wang, Li-Chun
;
Chang, Chih-Wen
;
Huang, Howard
;
電信工程研究所
;
Institute of Communications Engineering
Multi-carrier direct sequence code division multiple access (MC-DS-CDMA) systems can support multi-rate services with a great deal of flexibility by dynamically changing time- and frequency-domain spreading gains. However, in a frequency-selective fading channel, the multi-rate users using the time spreading codes with an ancestor-to-child relation, even with orthogonal frequency spreading codes, may still be affected by different quantities of multiple access interference (MAI). How to design an effective radio resource management strategy for the MC-DS-CDMA systems subject to different levels of MAIs becomes a new and important issue. In this paper, based on the error rate analysis in the physical layer, we observe and define a new performance metric, called the MAI coefficient, which can quantitatively predict the MAI effect imposed on a particular code in a multi-rate MC-DS-CDMA system. Furthermore, from the radio resource management perspective, we develop an interference avoidance code assignment scheme based on the MAI coefficient. By simulations and analysis, we demonstrate that the proposed interference avoidance code assignment method can effectively reduce the MAI in a multi-rate MC-DS-CDMA system, while maintaining very good call blocking rate performance.
Effects of subcarrier power allocation on an interference avoidance code assignment strategy for multirate MC-DS-CDMA systems
(2008-05-01)
Chang, Chih-Wen
;
Wang, Li-Chun
;
電信工程研究所
;
Institute of Communications Engineering
In this paper, we propose a joint subcarrier power allocation (SPA) and code assignment scheme for the synchronous multirate multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with time- and frequency-domain spreadings. Based on the newly defined metric multiple-access interference (MAI) coefficient, the proposed code assignment strategy can quantitatively predict the incurred MAI before assigning a spreading code. The SPA mechanism aims to maximize the received signal power. In addition to lowering the MAI, the proposed code assignment strategy further considers the compactness of the assigned codes in the entire 2-D tree structure. The simulation results show that the proposed joint SPA and code assignment strategy not only can reach a better received signal quality but can also achieve a high call admission rate.
Gap processing time analysis of stall avoidance schemes for high-speed downlink packet access with parallel HARQ mechanisms
(2006-11-01)
Wang, Li-Chun
;
Chang, Chih-Wen
;
電信工程研究所
;
Institute of Communications Engineering
The parallel multichannel stop-and-wait ( SAW) hybrid automatic repeat request (HARQ) mechanism is one of key technologies for high-speed downlink packet access in the wideband code division multiple access system. However, this parallel HARQ mechanism may encounter a serious stall problem, resulting from the error of the negative acknowledgement (NACK) changing to the acknowledgement (ACK) in the control channel. In the stall situation, the receiver waits for a packet that will be no longer be sent by the transmitter and stops delivering the medium access control ( MAC) layer packets to the upper layer. The stall issue seriously degrades the quality of service for the high-speed mobile terminal owing to the high probability of NACK-to-ACK errors. In this paper, we present an analytical approach to compare three stall avoidance schemes: the timer-based, the window-based, and the indicator-based schemes. To this end, we first propose a new performance metric-gap processing time, which is defined as the duration for a nonrecoverable gap appearing in the MAC layer reordering buffer until it is recognized. Second, we derive the probability mass functions and the closed-form expressions for the average gap processing time of these three stall avoidance schemes. It will be shown that our analytical results match the simulations well. Further, by analysis, we demonstrate that the indicator-based stall avoidance scheme outperforms the timer-based and the window-based schemes. The developed analytical approaches can help determine a proper number of processes for the parallel SAW HARQ mechanisms. We also show that the analytical formulas can be used to design the number of acceptable fully loaded users for an admission control policy subject to the gap processing time constraint. In the future, our analysis can facilitate the MAC/radio link control (RLC) cross-layer design because the gap processing time in the MAC layer is closely related to the RLC timeout mechanism and the window size in the RLC retransmission mechanism.
Real-time translucent rendering using GPU-based texture space importance sampling
(2008-04-01)
Chang, Chih-Wen
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Lin, Wen-Chieh
;
Ho, Tan-Chi
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Huang, Tsung-Shian
;
Chuang, Jung-Hong
We present a novel approach for real-time rendering of translucent surfaces. The computation of subsurface scattering is performed by first converting the integration over the 3D model surface into an integration over a 2D texture space and then applying importance sampling based on the irradiance stored in the texture. Such a conversion leads to a feasible GPU implementation and makes real-time frame rate possible. Our implementation shows that plausible images can be rendered in real time for complex translucent models with dynamic light and material properties. For objects with more apparent local effect, our approach generally requires more samples that may downgrade the frame rate. To deal with this case, we decompose the integration into two parts, one for local effect and the other for global effect, which are evaluated by the combination of available methods [DS03, MKB* 03a] and our texture space importance sampling, respectively. Such a hybrid scheme is able to steadily render the translucent effect in real time with a fixed amount of samples.