由於載波頻率和訊號頻寬兩者的大小比例很接近，因此水下音響通道它是一個寬帶的系統，進而引起了與頻率相關的都卜勒偏移。由此產生的都卜勒效應，造成每個子載波都會有頻率上的偏移，且不同的子載波偏移的量也不同，偏移的大小主要是決定於子載波本身的頻率。由於在水下所使用的載波頻率非常的小，因此就無法假設所有的子載波由於都卜勒效應所引起的頻率偏移量近似為一個常數，也就是說傳統在無線窄帶通信通道的都卜勒頻率偏移的估計技術是無法應用到水下音響的環境的。在這篇論文中，我們將此都卜勒的效應處理成水下音響環境在所有的傳播路徑裡都有一個相同的都卜勒縮放因子，並提出一個由前序引導封包輔助的都卜勒縮放因子估計方法，所使用的是補零前述碼正交分頻多工系統。這個都卜勒縮放因子估計方法的基礎概念是利用在接收端收到的兩個連續且相同的前序引導封包在一樣的子載波指標去計算出兩者之間的相位差，再利用此相位差去協助我們估計出都卜勒縮放因子。在我們的模擬中將我們所提出的方法與匹配濾波器法和多個相關器法做比較，比較的基準是使用均方誤差，從模擬結果可以得知，我們所提出的方法是明顯優於另外兩個方法的。

Underwater acoustic (UWA) channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, and introduce frequency-dependent Doppler shift. The resulting Doppler effect translates each frequency component by a different Doppler amount. Therefore, the conventional techniques for Doppler shift estimation in wireless narrowband communication channels cannot be applied to the UWA environments. In this thesis, we consider a UWA environment having a common Doppler scaling factor on all propagation paths and propose a preamble-assisted Doppler scaling factor estimation scheme for ZP-OFDM (zero padding orthogonal frequency division multiplexing) systems. The Doppler scaling factor is estimated based on the phase difference of the received signals on the same subcarrier in two consecutive OFDM preambles. In the simulations, we show that the proposed scheme outperforms the matched-filter and multiple correlator methods in terms of mean square error (MSE).

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