在直接序列分碼多重存取(DS-CDMA)通訊系統的傳統功率控制設計中，史密斯預測濾波器(Smith prediction filter)已經被利用於補償固定的迴路延遲而追蹤指定的功率對干擾雜訊比(SINR)。然而史密斯預測濾波器對於時變的迴路延遲系統非常靈敏，因此在本篇論文中提出多模式史密斯預測濾波器(multiple-mode Smith prediction filter)來有效率地補償時變的迴路延遲系統問題，而且利用基於追蹤誤差的可能性函數(likelihood function)來計算每一個模式史密斯預測濾波器的權重。因此對於時變延遲的分碼多重存取微細胞通訊系統，多模式史密斯預測濾波器是一個非常強健的功率控制器。
所以在此系統中，如何在時變的迴路延遲、非確定的通道衰減、未知的干擾及非線性傳輸限制等環境下，提升更高的通訊品質及更大的系統使用容量是目前主要研究的重心。尤其在未來第三代行動通訊系統中，行動台所扮演的角色不再只是單純的傳送語音，而是應用在更多的娛樂及資料傳輸上，例如：多媒體等。因此在資源有限的行動台上，功率控制及傳送都會受到限制而造成延遲。為了克服時變的迴路延遲以及所有不確定的影響，我們提出了多模式史密斯預測濾波器來補償時變的迴路延遲和通道中不確定的影響，而且為了掌握時變延遲的狀態，我們提出了可能性函數來估測每一個延遲的可能性，進而決定每一個模式的史密斯預測濾波器的權重，以達到更準確的迴路延遲補償。

In conventional power control design of direct-sequence code division multiple access (DS-CDMA) systems, Smith prediction filter has been employed to compensate for the fixed round-trip delay to track a target signal to interference and noise ratio (SINR). However, Smith prediction filter is sensitive to the variation of round trip delay. In this study, a multiple-mode Smith predictor is proposed to cooperate with to treat efficiently the compensation problem of time-varying round-trip delay in the power control design of DS-CDMA systems. The likelihood function based on the probability density function of tracking errors is employed to compute the weighting of cooperation for each mode of Smith predictor. From simulation results, it is shown that the proposed multiple-mode power control method is robust to time-varying round-trip delay in CDMA cellular radio systems.

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