本文主要目的為建構一水下通訊系統，此系統能克服因多重路徑傳遞與傳遞介質不均勻所造成的訊號失真。研究中發射端應用雙相移位鍵控數位調變技術傳送數位訊號，而接收端應用時間反轉法還原波形並於解調變後探討接收訊號之錯誤率。利用時間反轉法聲波時間反轉傳回訊號之不變性，藉此改善調變訊號經由水下環境造成變化而產生訊號之失真使其恢復波形完整度，解調變後可降低位元錯誤率。吾人應用虛擬聲源法模擬一水下反射環境藉此探討調變訊號經由通道後之錯誤率並使用時間反轉法降低其位元錯誤率。實驗部分利用LabVIEW將調變之聲源訊號經由水下換能器發射、接收調變波形，接收回來之訊號並應用時間反轉法提高波形相關度。實驗地點位於8m×4m×175m之拖航水槽，結果證明時間反轉法應用於水下通訊可改善其通訊品質，克服多重路徑之問題，達到水下通訊之目的。

The purpose of this paper is to propose a new procedure for underwater communication system to compensate wave distortion due to multipath effect. Transmitted signal is digitalized and modulized by binary phase shift-keying (BPSK) technique. After signal propagating, received signal at receiver location is distorted and then restored to original source waveform via time reversal process. Both received signal and restored one are demodulated to compare the bit error rate (BER). In concept, the distortion due to underwater environment is solved based on the invariance of time reversal process. Therefore, the BER is greatly reduced after demodulating processed signal.
In simulation, reflective underwater environment is constructed using image method to simulate multipath effect by using Matlab. In experiment, modulated source signals are transmitted and received by underwater transducers in 8m × 4m × 175m towing tank. The signal is detected by using LabVIEW software. Then, the design, received signals are processed using demodulation, and time reversal process. All the BER evaluations are all performed and the results show that time reversal process is efficient to help overcome multipath effect and reduce BER to improve the quality of underwater communication.

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