Browsing by Author "Chang, Dah-Chung"

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ACTIVE NOISE CANCELLATION WITH A NEW VARIABLE TAP LENGTH AND STEP SIZE FXLMS ALGORITHM
(2013)
Chang, Dah-Chung
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Chu, Fei-Tao
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電機工程學系
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Department of Electrical and Computer Engineering
The filtered-X least mean square (FxLMS) algorithm is widely used for active noise cancellation (ANC). Some variants of FxLMS algorithms have been studied to reduce computational complexity or to improve convergence rate. In general applications, a long tap length is usually required for the conventional FxLMS method which convergence rate is very slow though its structure is possibly very easy to implement. In this paper, a new ANC system is proposed with a variable tap length and step size FxLMS algorithm. Taking into account the effect of the lowpass filter in the secondary path of an ANC system, the impulse response of the control filter is modeled with a two-sided exponential decay envelope to develop our algorithm. Simulation results show that the proposed algorithm does provide a significant performance improvement on convergence rate and noise reduction ratio compared to the fixed tap FxLMS and previously proposed variable step size FxLMS algorithms.
Modified Decision Feedback Method for Mobile OFDM Channel Estimation
(2010)
Cheng, Chien-Chun
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Chang, Dah-Chung
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Chen, Yung-Fang
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傳播研究所
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Institute of Communication Studies
In some frame-based OFDM applications such as WLAN and WMAN, the channel estimation is accomplished through the aid of the preamble. By means of the preamble, the estimated channel is only well applied under the assumption of a static channel status within the frame. If we consider a time-varying channel status in those applications, the channel tracking has to be performed without the aid of pilots. A past idea for channel tracking was to use a decision-feedback loop with a smoothing method to reduce the drawback of decision-error propagation. In our work, we design the error detectors for detecting error propagation and propose some compensation methods for reducing the influence of error propagation. In simulation results, the performance of the proposed decision-error detectors and compensation methods is compared. We show that the smaller bit error rate can be achieved for the proposed time domain decision feedback algorithm and thus, bandwidth efficiency can remain without the pilots used for channel tracking.