A New Time and Frequency Synchronization Scheme in 3GPP LTE Downlink Systems

本論文提出了一個在長期演進技術下行系統中的綜合時間與頻率同步演算法，目的是基於正交分頻多工存取系統下取得時間位置與頻率偏移的資訊。此演算法包含兩部份：一）綜合時間與頻率偏移小數部份的估測，二）頻率偏移整數部份的估測。在第一部份中，我們利用循環字首來同時估計時間位置與通道長度，而理論的分析也證明了我們所設計的成本函數可以在沒有雜訊的干擾下正確的估計出時間與通道長度，而頻率偏移的小數部份也能由於利用估計出的時間位置與通道長度來改善估計精準度。第二部份，我們利用在長期演進技術標準中所制定的主要同步信號-Zadoff Chu序列來進行頻率偏移整數部份的估測，藉由此數列的特殊性質，我們能在快速傅立葉轉換的運算之前進行估測，我們也會在文中介紹Zadoff Chu數列的特殊性質。最後，藉由電腦模擬結果我們可以藉由觀察估計的均方誤差以及偵測的錯誤率 來看出與前人的方法比較下所進步的地方。This thesis presents a joint time and frequency synchronization algorithm for 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) orthogonal frequency division multiplexing access (OFDMA) downlink systems. The proposed method is an initial synchronization in order to acquire the OFDM symbol timing and carrier frequency offset (CFO). The proposed method contains two parts: 1) A joint estimation of fractional CFO and OFDM symbol timing, and 2) the detection of integer CFO. In the first part, the joint timing and channel order estimation using cyclic prefix (CP) is presented. Theoretical derivation proves that, in absence of noise, the maximum peak of the cost-function appears at the correct timing and channel order. The accuracy of fractional CFO estimation might be improved by jointly utilizing the estimated timing and channel order. Second, the detection of integer CFO using the special property of Zadoff-Chu (ZC) sequences is presented before the fast Fourier transform (FFT) operation. The studies of ZC sequences are also introduced in the thesis. Comparing with the previously presented algorithms, simulation results show the improvements in mean squared error and the detection error rate performances of the proposed methods