窄頻物聯網是新的蜂窩式網路技術，是由第三代合作夥伴計畫所提出的通訊系統標準。窄頻物聯網下行鏈路使用正交分頻多工技術，然而正交分頻多工系統有對時間與頻率偏移敏感的缺點，例:載波偏移導致子載波之間失去正交性。細胞搜尋在下行鏈路中是非常重要的一個環節，包含了時間上的同步和頻域上的同步還有搜尋細胞基地台，窄頻物聯網系統利用兩組同步訊號作為細胞搜尋與同步使用，包含窄頻主同步訊號和窄頻輔同步訊號。由於窄頻物聯網的使用者裝置具有低複雜度的需求。我們在此篇論文針對細胞搜尋提出低複雜度的演算法。藉由同步序列的特性，提出分群的作法減少搜尋次數，因而減少運算複雜度，我們提供了不同複雜度底下的效能模擬結果，並透過參數的選擇，提供複雜度與效能間的取捨。

Narrowband internet-of-things (NB-IoT) is a new cellular technology introduced by the 3rd generation partnership project (3GPP). NB-IoT downlink uses orthogonal frequency division multiplexing (OFDM) technique. It is well-known that OFDM system is sensitive to the timing and frequency offset, e.g. the carrier offset causes loss of orthogonality between subcarriers. Hence, synchronization is a very important part of the downlink transmission. Synchronization procedure includes time synchronization and frequency synchronization as well as the cell search. The frequency offset is caused by the mismatch of oscillators between transmitter and receiver. In the 3GPP NB-IoT system. Two synchronization signals are used including narrowband primary synchronization signal (NPSS) and narrowband primary synchronization signal (NSSS). The user equipment (UE) of NB-IoT has to meet the requirement of low complexity. In this thesis, we propose a low-complexity algorithm for cell search according to the property of the synchronization sequences. The Grouping Method is proposed to reduce the number of search processes. Thus, it reduces the computational complexity. We provide performance evaluation for different complexity-reduction methods. By taking different values of parameters, we provide a trade-off between complexity and performance.

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