- 學位論文
正交分頻多工系統時頻同步之研究
A study of synchronization for OFDM frequency and timing
摘要
作為新一代無線通信調變技術，OFDM(正交分頻多工)能夠提供大容量的訊息傳送，特別適合在多徑傳播的無線移動信道中高速傳輸數據。基於OFDM技術的寬帶無線通信正處於快速發展中，為能達到實用程度上的OFDM系統，同步是一個關鍵的問題。因為在接收端，對於從發送機到接收機的傳播時延一般是未知的，為了對解調器輸出同步抽樣，必須從接收信號導出符號定時。OFDM中的時間同步包括符號同步和幀同步，即估計出幀和符號的起始時刻，主要解決定時的問題。當採用相干檢測時，接收端需要提供一個與發送端調變載波同頻同相的相干載波，這個相干載波的獲取就稱為載波同步。OFDM中的頻率同步主要指載波同步，即估計出由都卜勒頻移和接收端与發送端晶体振盪器頻率差所產生的固定頻偏。時間同步和頻率同步是OFDM系統中最基本也是最重要的同步問題。 正交分頻多工(OFDM)技術具有抵抗多徑傳輸時延、抵抗通道衰落、頻譜利用率高，硬體實現簡單等優點，近年來頗受關注。但OFDM技術存在兩個主要缺點：對非線性問題敏感、對定時偏差和頻率偏移敏感，如何克服這兩個缺點是OFDM技術研究的主要方向之一，本文主要是研究正交分頻多工技術中的定時偏差和頻率偏移敏感問題亦即同步問題。 本文先詳細闡述OFDM的發展、基本原理和同步技術，繼而完整敘述及推導出兩篇關於時間和頻率聯合同步的經典論文精要，及其性能模擬結果。在現有的定時同步技術上提出了幾種基於訓練序列的定時同步改進算法，並對不同訓練序列安排算法的模擬結果進行比較，證明改進算法優於以往算法。利用此算法，即使在低訊雜比(SNR)的情況下，也能夠實現精確的定時同步，獲得良好的系統性能。OFDM對載波頻偏十分敏感，接收機的頻偏估計能力是影響OFDM接收性能的重要因素。本論文亦分析推導了根據訓練序列進行的整數部份和小數部份的頻偏估計方法。
並列摘要
As a new generation modulation technique for wireless communication, OFDM (Orthogonal Frequency Division Multiplexing ) has the merit of huge message transport and suitable for high-data-rate transmission in the wireless mutlipath channel. Nowadays, the development of wideband wireless communication based on OFDM is fast growing. Therefore, the main probelems of OFDM, such as synchronization, must be considered. Owing to the unknown time delay from the transmitter to the receiver, it is necessary for us to estimate the symbol timing of the receiver to demodulate the received signal correctly. There are two kinds of timing probelems in OFDM: symbol synchronization and frame synchronization. With correct timing, we can find the starting of symbol and frame. For coherent detection, the phase and frequency of the carrier must be the same for both transmitter and receiver. To acheieve this goal, carrier synchronization is necessary. Frequency synchronization in OFDM mainly means carrier synchronization, whose purpose is to estimate frequency offset between the transmitter and receiver which affected by Doppler effect. Synchronization of frequency and timing in OFDM systems are the most fundmental and principal issues. OFDM technique has the advantages of insensitive to multipath delay spread, channel distortion resistence, high spectrual efficiency and simple hardware implementation, etc. Therefore, it has been widely applied to wireless communication systems in these years. However, it has two main disadvantages: high senstitivity to nonlinear problems and synchronization error. To overcome theses problems, in this thesis, we study the synchronization methods of OFDM technique. This thesis is organized as follows. First, we briefly introduces the development, basic principles and synchronization techniques of OFDM. Then, the detailed description and performance simulations for previous frequency and timing synchronization algorithms is discussed. The proposed algorithms based on training sequence for timing synchronization are described in the same chapter. Besides, the simulation results of different algorithms are compared. The results show that the new algorithms outperform the traditional ones in many situations. By proposed algorithms, more accurate timing synchronization and better performance can be achieved even in low SNR environments. High sensitivity to carreier frequency offset is one of main disadvantages in OFDM, the performance of the receiver for OFDM depends on frequency offset estimation. In his thesis, we also discuss the estimation of integer frequency offset (IFO) and fractional frequency offset (FFO) based on training sequences.