臺灣博碩士論文加值系統

正交分頻多工(Orthogonal Frequency Division Multiplexing OFDM)傳輸技術以高傳輸資料量與抵抗衰減通道的優異性能，使其在近幾年的通訊系統發展中，已被廣泛應用在各無線通訊系統、數位電視、數位廣播與無線網路等領域。正交分頻多工系統比一般的多載波傳輸系統更能有效的提高頻譜的使用率、解決通訊系統中的字符(Symbol)干擾現象和對抗多重路徑衰減等問題。但在正交分頻多工系統的通訊過程中，對於頻率偏移有很高的敏感度，容易造成正交分頻多工系統中各個子載波失去正交性，而喪失了原有的優異通訊性能。有鑑於頻率偏移問題對正交分頻多工系統的重要性，本篇論文比較及討論數種不同的非資料輔助型正交分頻多工系統之頻率偏移估計方法，所謂非資料輔助型即是不使用週期性導引信號或其他週期性的訓練信號來協助估計之方法。此外，我們也提出一個以虛擬子載波為基礎的ML估計方法。經由電腦模擬的結果可知，此方法在合理的訊雜比之情形下，得到令人滿意的結果。最後，提出對於正交分頻多工系統所使用之頻率偏移估計方法的建議。期望能對於將來想研究此一領域的同好有所助益。

Orthogonal Frequency Division Multiplexing (OFDM) has been widely applied to several mobile radio communication systems, DVB, DAB, and Wireless LAN recently during the development of communication system. OFDM technique is an effective transmission scheme to cope with many channel impairments compared to other multi-carrier communication systems. For example, it offers better performance under multi-path fading channels, yielding higher spectral efficiency and may reduce the effects of inter-symbol interference (ISI). However, OFDM is highly sensitive to carrier frequency offset, which may easily cause each sub-carrier in the system to lose orthogonality and therefore lose its original outstanding performance. Owing to the importance of carrier frequency offset to OFDM, this paper, several non-data-aided frequency synchronization algorithms are compared. In addition , we propose a maximum likelihood (ML) estimation of the CFO in the presence of virtual carriers. Simulation results show that the proposed algorithm performs well for reasonable SNR values. This paper will be ended in the suggestion of frequency offset estimation of OFDM. This research is done under the expectation to be beneficial to future studies for the researchers who have the same interests in this field.

摘要 I
Abstract II
目錄 IV
圖表目錄 vii

第一章緒論 1
第二章無線多重路徑衰減通道之特性與模型 4
2.1 無線通道特性 4
2.2 大範圍衰減 6
2.3 小範圍衰減 8
2.3.1 時間延遲擴散 9
2.3.2 頻率選擇性衰減模型 11
2.3.3 都卜勒擴散效應 12
2.4 多重路徑衰減通道的數學模型 14
2.5 Rayleigh 衰減分佈 15
第三章正交分頻多工系統簡介及架構 18
3.1 正交分頻多工系統簡介 18
3.2 正交分頻多工的基本概念 18
3.2.1 正交分頻多工的正交性 23
3.2.2 以快速傅立葉轉換為基礎的正交分頻多工系統 24
3.3 快速傅立葉轉換概述 25
3.3.1 快速傅立葉轉換簡介 25
3.3.2 離散傅立葉基本原理 26
3.3.2.1 離散傅立葉轉換 26
3.3.2.2 離散傅立葉轉換的對稱性 28
3.3.2.3 離散傅立葉轉換的週期性 28
3.4保護區間與循環前置 29
第四章正交分頻多工系統之同步 36
4.1 字符同步與取樣頻率同步 36
4.2 載波頻率同步 39
4.2.1 造成正交分頻多工系統載波頻率偏移因素 39
4.2.2 頻率偏移對正交分頻多工系統之影響 42
4.3 載波頻率演算法分類 46
第五章 非數據輔助正交分頻多工頻率偏移估計比較 48
5.1解耦合最大相似性估計方法(Decoupled Maximum Likelihood method)[13] 51
5.2 利用虛擬子載波正交性估計方法(Virtual Carrier orthogonality method)[17] 56
5.3 運用多次取樣的頻率偏移之非資料輔助估計方法(Oversampling method)[18] 60
5.4利用循環前置的最大相似性估計方法(CP ML method)[14] 65
5.5 以虛擬子載波演算法為基礎的頻率偏移ML估計 71
5.6頻率偏移估計方法之比較 78
第六章 結論 80
參考文獻 81
附錄 88

圖表目錄
圖 1： 多重路徑傳輸環境...........................................................4
圖 2： 自由空間路徑損失簡單模型...........................................5
圖 3： 不同相位造成包線衰減...................................................8
圖 4： 時間延遲擴散造成之相鄰字符相互干擾(ISI)效應.....9
圖 5: 都卜勒效應示意圖.........................................................12
圖 6: 衰落通道的分類.............................................................13
圖 7： 時變通道脈衝響應.........................................................14
圖 8： Rayleigh 衰減信號包封機率密度函數.......................16
圖 9： Rayleigh 衰減分佈模擬基本模型...............................16
圖10: 將一串高傳輸速率的資料串，分成多個傳輸速率較低
的資料串.......................................................................18
圖11： 分頻多工FDM 與正交分頻多工OFDM 比較圖.............19
圖12： 基本的正交分頻多工傳輸端方塊圖............................20
圖13： 單一載波和多載波頻譜................................................23
圖14： 以快速傅立葉轉換FFT 為基礎的正交分頻多工系統架
構圖...............................................................................24
圖15: 保護區間不含訊號受到多重路徑干擾的影響............29
圖16: 保護區間加入循環前置的正交分頻多工symbol .......30
圖17： 正交分頻多工接收訊號之時序忍受示意圖................31
圖18： 多路徑傳輸加入保護區間示意圖................................32
圖19： 正交分頻多字符之時域頻譜........................................32
圖20： 正交分頻多字符之頻域頻譜........................................33
圖21: 失真振盪器對正交分頻多工訊號的影響....................39
圖22: 正交分頻多工字符頻譜的取樣圖................................40
圖23： 正交分頻多工載波頻率偏移說明圖............................42
圖24： 載波偏移對系統效能的影響........................................44
圖25: 正交分頻多工系統受到頻率偏移的離散時間基頻模型
.......................................................................................48
圖26： Decoupled ML method 的 Performance ....................53
圖27： Virtual Carrier orthogonality method 的
Performance .................................................................58
圖28： Oversampling method 的 Performance ....................63
圖31： CP ML method 的 Performance ..................................69
圖32： 使用1、2、3、及4 個資料區塊的Performance ......76
圖33： 多種盲目估計之NMSE 比較..........................................78
表一： 多種盲目估計之NMSE 比較...........................................78

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