摘 要
　　多輸入多輸出正交分頻多工(MIMO-OFDM)系統是一種結合了MIMO信號處理以及OFDM調變方法的技術，這種傳輸技術使用在無線通訊環境中可以增加通道容量，資料速率和連結可靠性。　　MIMO-OFDM系統相較於OFDM對於載波頻率偏移更加敏感，而且一般經常使用於OFDM系統的同步技術都不能夠直接拿來使用在MIMO-OFDM中。在本論文中，我們針對MIMO-OFDM系統提出一種高效能的同步技術，裡面包含了三個部份：粗調時間擷取(coarse time acquisition)，載波頻率估測(carrier frequency estimation)，細調時間擷取(fine time acquisition)，載波頻率估測又分成小數倍估測以及整數倍估測。接收天線針對某一個傳輸天線的所作的粗調時間擷取是去尋找接收信號與相對應週期前置(cyclic prefix)部分的訓練序列(training sequence)交叉相關性的最大值來完成。在接收端利用我們提出來的粗調時間擷取方法我們可以得到每一根傳輸天線大略的到達時間，因此可以估測出每一根傳送天線對接收天線的載波頻率並且更進一步的結合成一個更精確的值。細調時間擷取與粗調時間擷取基本上類似，但是細調時間擷取是計算整段的訓練序列與接收信號的交叉相關性(不包括週期前置部分)。這篇論文提出的同步技術在接收天線可以同時針對每一根傳輸天線的信號做好同步。電腦模擬結果將會表示我們提出的多輸入多輸出分頻多工系統之同步技術的效能比以前常用的技術好，當天線越多的時候，改善的效能越大。

Multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) is a transmission scheme that combines MIMO signal processing and OFDM techniques. This transmission scheme can increase data rate and link reliability in wireless environments. A MIMO-OFDM system is much more sensitive to the carrier frequency offset than an OFDM system, and the conventional synchronization techniques used for OFDM systems cannot be directly applied. In this thesis, we propose an effective synchronization scheme for MIMO-OFDM systems, where three stages are included: coarse time acquisition, carrier frequency offset estimation, and fine time acquisition. The proposed coarse time acquisition for a transmit-antenna signal at a receiver antenna is accomplished by finding the maximum value of the cross-correlation function between the received signal and the cyclic prefix part of the corresponding training sequence. Since we can obtain the approximate arrival time of each transmit-antenna’s signal at the receiver by using the proposed coarse time acquisition method, the carrier frequency offset of each transmit-antenna’s signal can be estimated and then properly combined to form a more accurate estimate. The fine time acquisition stage is similar to the coarse time acquisition stage, except that the cross-correlation function is computed based on the received signal and the whole training sequence (without the cyclic prefix part). The proposed scheme can simultaneously synchronize all the signals from the transmit antennas at the receiver. Computer simulation results show that the proposed synchronization scheme reaches better performance for MIMO-OFDM systems than a previously described method.

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