LTE系統初始細胞搜尋程序之研究

Abstract

在LTE的系統下，基地台(eNodeB)會固定每隔5 ms廣播二個同步訊號，分別是主同步訊號(Primary Synchronization Signal, PSS)與輔同步訊號(Secondary Synchronization Signal, SSS)，用戶設備(User Equipment, UE)藉由偵測此同步訊號來達成初始細胞搜尋。本論文研究LTE系統下的初始細胞搜尋程序，針對此搜尋程序的不同階段來進行討論。在第一階段，我們選擇在時域上使用接收訊號做循環字首基礎最大似然(Cyclic Prefix-Based Maximum Likelihood, CP-Based ML)計算以估計出符元時間(Symbol Timing)與小數載波頻率偏移(Fractional Carrier Frequency Offset, FCFO)，來取代傳統在時域上直接使用接收訊號與用戶設備本地的主同步訊號做互相關(Cross Correlation, CC)的運算，以提升符元時間估計的效能。在第二階段，我們在頻域上做主同步訊號的偵測，可以選用差分相關(Differential Correlation, DC)或部份相關(Partial Correlation, PC)的運算方式來減低通道頻率選擇性衰減的影響，並且這兩種方法都可以同時估計整數載波頻率偏移(Integer Carrier Frequency Offset, ICFO)。在第三階段，我們在頻域上做輔同步訊號的偵測，根據兩種偵測架構，來分別進行討論。在進行完這三個階段後，用戶設備即完成了初始細胞搜尋程序。

In the LTE system, the base station (eNodeB) periodically broadcasts two synchronization signals, namely, the primary synchronization signal (PSS) and the secondary synchronization signal (SSS) every 5 ms. The user equipment (UE) makes use of the synchronization signals to accomplish initial cell search. In this thesis, the initial cell search procedure is discussed and is divided intothree different stages. In the first stage, we prefer to use cyclic prefix-based maximum likelihood (CP-Based ML) method in time domain to estimate the symbol timing and fractional carrier frequency offset (FCFO),instead of the conventional method which calculates the cross-correlation between the received signal and the UE local replicas of PSS in the time domain. The CP-Based ML method can improve the symbol timing estimation performance. In the second stage, we can use differential correlation (DC) method or partial correlation (PC) method to detect PSS in frequency domain for the mitigation ofchannel’sfrequency selective fading effect. Integer carrier frequency offset (ICFO) can also be jointly estimated by both methods. In the third stage, we discuss two structures to detect SSS in the frequency domain. The UE initial cell search procedure ends when the above three stages are completed.