3GPP LTE 初始下行同步主同步訊號偵測之探討

Abstract

在行動通訊系統中，一個基本的實體層程序是行動用戶設備(UE)的細胞搜 尋、時間和頻率同步程序，使UE 在細胞內取得同步並偵測出細胞識別碼。當 UE 試圖做初始網路存取時，就要作細胞搜尋。UE 在完成網路連結後，也會因 為移動而再執行細胞搜尋，目的在搜尋鄰近的細胞以備切換之用。 3GPP UTRA 行動通訊標準的長期演進(LTE) 版本， 即進化版UTRA (E-UTRA)，包括一個基於正交分頻多工(OFDM)的下行無線介面。其中定義了一 個專用的同步通道(SCH)，傳送兩種同步信號，即主同步訊號(PSS)和次同步訊號 (SSS)。在同步通道中，兩種同步序列係放在直流(DC)子載波旁邊對稱位置的62 個子載波上。它們傳送在第一個和第六個次訊框(標號0 和5)的最後兩個OFDM 符元中，即每5 毫秒傳送一次。 在本論文中，我們的焦點在研究主同步訊號的偵測以及時間和頻率同步。前 者目的在作細胞搜尋，而後者目的則在使能解調次同步訊號，以能完全偵測細胞 識別碼。UE係利用收到的主同步訊號與已知所傳送的訊號相關運算，找其峰值 所在，來完成時間上的同步。對於這部分的問題，我們探討一些在時域做部分互 相關值計算的方法，來求得細胞識別碼及正確的符元時間估計。正交分頻多重進 接(OFDMA)技術的缺點之一是它的傳輸效能很受載波頻率偏移(CFO)所影響。為 了克服這個問題，有一個基本的想法就是將載波頻率偏移拆成可分開估計的小數 頻率偏移(FFO)和整數頻率偏移(IFO)。我們用模擬所得的均方誤差(MSE)來評估 小數頻率偏移估計的效能，其中小數頻率偏移之估計係在時域使用基於循環字 首(cyclic prefix)的方法來達成。我們也對整數頻率偏移做估測。在此，我們研究了在頻域做差分相關計算的方式來得到較準確的整數頻率偏移估測。所有演算法都以加成性白高斯雜訊(AWGN)通道以及具有加成性白高斯雜訊干擾的多路 徑通道環境下進行評估。

In a cellular system, one of the fundamental physical layer procedures is the cell-search, time and frequency synchronization procedure at the mobile user equipment (UE) which serves to synchronize the UE to a cell and detect its cell ID. Cell search is executed when the UE seeks initial network access and also for mobility reasons after the network access has been established meaning when it searches for neighbor cells for possible handovers. The long term evolution (LTE) of 3GPP UTRA cellular standard, which results in the evolved UTRA (E-UTRA), includes an OFDM based downlink radio interface. A dedicated synchronization channel (SCH) is specified in LTE for transmitting two synchronization signals, the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). Within the SCH, both synchronization sequences are mapped on 62 subcarriers located symmetrically around the DC-subcarrier. They are transmitted within the last two OFDM symbols of the first and sixth sub-frames (indexed 0 and 5), i.e. every 5 ms. In this thesis, we focus on detection of PSS required for cell search as well as time and frequency synchronization necessary for demodulating the SSS to completely detect the cell ID. The UE achieves synchronization by correlating the receive PSS signal with a replica of the transmitted signal by identifying a correlation peak at the proper symbol timing. For this part of the problem, we study partial cross-correlation schemes in time-domain for obtaining the cell identity and correct symbol timing estimate. One of the drawbacks of OFDMA is its vulnerability to carrier frequency offset (CFO). The basic idea considered is to split the CFO into a fractional frequency offset (FFO) and an integer frequency offset (IFO), which can be estimated individually. The performance is evaluated in terms of simulated mean squared error (MSE) for FFO estimation. FFO is obtained in time domain using cyclic prefix based scheme. We also perform IFO estimation. We study the differential correlation scheme in frequency domain for obtaining the accurate IFO estimation. All the algorithms are evaluated under AWGN channel as well as multipath channel conditions with AWGN interference.