正交分頻多工系統於非地面網路的下鏈同步中，由於衛星的高速移動，使得傳送端與接收端之間產生較大的相對速度，造成顯著的都卜勒位移效應，進而在接收端引發嚴重的載波間干擾，影響用於隨機接入過程的主同步訊號之偵測。因此，本論文在第五代行動通訊新無線電的下鏈規格中，針對頻率同步與主同步訊號偵測進行研究，並將都卜勒位移效應產生的頻率偏移拆分成小數與整數兩個部分進行同步，小數部分頻率偏移在時域上由文獻中基於循環字首的演算法估測，整數部分頻率偏移與主同步訊號在頻域上使用文獻中基於主同步訊號良好自相關性的演算法偵測，並將上述文獻中所使用的單天線系統推廣至傳送端單天線接收端多天線系統，使其能夠在接收端使用接收端分集技術，探討以類似等增益合併方法分別在時域與頻域上應用，藉此提高接收訊號在多路徑衰減通道與受都卜勒位移效應影響下的可靠性。在多數模擬環境中，使用類似等增益合併方法所估測和偵測出的頻率偏移值更貼近真實頻率偏移值，進一步提高了頻率同步與主同步訊號偵測的效能。

The orthogonal frequency division multiplexing (OFDM) system in non-terrestrial networks (NTNs) faces significant challenges in downlink synchronization due to the high relative velocity between the transmitter and receiver caused by the rapid movement of satellites. This results in a pronounced Doppler shift effect, leading to severe inter-carrier interference (ICI) at the receiver and adversely affecting the detection of the primary synchronization signal (PSS) used for random access procedure. To address this, this thesis delivers a method within the downlink specifications of the fifth generation new radio (5G NR) to separate the frequency offset caused by the Doppler shift into fractional and integer parts for synchronization. The fractional part is estimated in the time domain using a cyclic prefix-based algorithm from the literature, while the integer part, along with the PSS, is detected in the frequency domain using an algorithm based on the good autocorrelation properties of the PSS as documented in literature. This thesis proposes to extend the algorithms from a single-antenna system to a single-transmit, multi-receive antenna system, enabling the use of receive diversity techniques to improve the synchronization performance. In the thesis, we apply a method similar to equal gain combining (EGC) in both the time and frequency domains, to improve the reliability of the received signal under multipath fading channel effects and Doppler shift effects. In most simulation scenarios, the frequency offset values estimated and detected using a method similar to EGC are closer to the actual frequency offset values, thereby further enhancing the performance of frequency synchronization and PSS detection.

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