臺灣博碩士論文加值系統

　　近年來正交分頻多工（orthogonal frequency division multiplexing，OFDM）技術被廣泛的應用在高速傳輸通訊系統上。OFDM技術普及的主要原因之一是透過保護區間（guard interval，GI）的應用，使得OFDM系統在多重路徑通道底下能比其他系統展現較佳的性能。由於使用保護區間，它可將符號原先會受到符際間干擾（inter-symbol interference，ISI）的狀態轉換成不受符際間干擾的狀態。但使用不夾帶任何資訊的保護區間將會造成頻道間干擾（inter-carrier interference，ICI）。為了解決這個問題，保護區間便使用循環字首（cyclic prefix，CP）來避免從相鄰符號所造成的符際間干擾與從其他子通道所造成的頻道間干擾。但由於使用過長的保護區間將會使傳輸速率下降、降低頻譜效率（spectral efficiency）與增加訊雜比的損失（SNR loss）。假如我們選擇較短的保護區間時，通道長度也許會大於循環字首的長度，由於循環字首長度不足所造成的干擾可能會使得OFDM系統的性能嚴重衰減。為了解決此問題，通常是在OFDM傳輸系統的接收端設計一時域等化器來縮減通道長度，以此來減少符際間干擾與頻道間干擾。然而，由於複雜度的關係，最佳化設計的時域等化器是較難以去實現的
　　因此我們使用時頻域交錯的重複式技術來減輕OFDM系統中由於循環字首長度不足所造成符際間干擾與頻道間干擾。並且在通道估測演算法中也加入我們的重複式技術來避免符際間干擾與頻道間干擾。重複式技術除了可以移除符際間干擾與維持循環摺積的特性。透過重複式技術的使用，我們可以改良通道估測的性能使得OFDM符號就像使用了足夠長度的循環字首。
　　模擬時，我們考量最糟的情況，就是當OFDM符號傳送時，不使用保護區間。模擬結果顯示我們提出的方法可以有效地消除符際間干擾。並且將我們所提出包含通道估測與資料解調的演算法和決策回授通道估測、避免符際間干擾訓練序列通道估測與殘餘符際間干擾消除演算法比較。最後我們模擬幾種不同的通道環境來比較這些演算法面對不同通道環境時所能提供的系統性能改善情形。

　　In recent year, orthogonal frequency division multiplexing （OFDM） technology has been widely used in high-speed communication systems. One primary reason for the popularity of OFDM is its ability to provide good performance in multi-path channels than the other systems through the use of Guard interval(GI). By using the Guard interval, it can convert these inter-symbol interference (ISI) channels into ISI-free channels. But Guard interval without any information will caused inter-channel interference. In order to solve this problem, a guard interval using cyclic prefix (CP) is inserted to avoid inter-symbol interference from the adjacent symbols and inter-channel interference from other sub-channels. However, using long cyclic prefix will decrease the transmission rate, reduce the spectral efficiency, and increase the signal-to-noise power ratio（SNR）loss. If we choose a shorter one, the channel length may be longer than the cyclic prefix. The interference caused by insufficient cyclic prefix can seriously degrade the performance of OFDM systems. In order to solve this problem, a time domain equalizer（TEQ）is usually used in the receiver to shorten the channel length of OFDM transmission system, and therefore minimize the ISI and ICI. However, because of its high complexity, the optimum design of TEQ is hard to realize.
　　So we use an iterative channel estimation technique between time domain and frequency domain to mitigate the ISI and ICI which is caused by insufficient cyclic prefix. The iterative technique can remove ISI and hold the circular convolution property. By utilizing the iterative technique we can improve the channel estimation performance as the OFDM symbol used sufficient cyclic prefix.
　　In the computer simulations, we consider the worst case that the OFDM symbol is transmitted without guard interval. The results show that our proposed method can effectively suppress residual ISI. The comparison between our proposed method including both proposed channel estimation（PCE）and proposed data demodulation（PDD）, decision feedback channel estimation（DFCE）, avoid ISI preamble channel estimation（AISIP）, and residual ISI cancellation（RISIC）algorithm are made in this thesis. Finally, the performance improvement of the proposed algorithm under several channel conditions is considered and compared with other algorithms.

摘要 i
Abstract ii
目錄 iv
圖索引 vi
表索引 xi
第一章　簡介 1
1.1　研究背景與動機 1
1.2　文獻探討 2
1.3　各章提要 4
第二章　OFDM系統概論 5
2.1　OFDM系統架構 5
2.1.1　OFDM系統基本架構 6
2.1.2　保護區間 9
2.2　通道估測與等化 13
2.2.1　導航符號排列與設計方式 13
2.2.2　通道估測 15
2.2.3　等化器 16
2.3　符際間干擾與頻道間干擾之分析 19
2.3.1　符際間干擾 19
2.3.2　頻道間干擾 22
2.3.3　減少符際間干擾與頻道間干擾之研究 27
第三章　消除符際間干擾與頻道間干擾之演算法設計 29
3.1　系統架構 30
3.2　現有文獻探討 32
3.2.1　通道估測 32
3.2.2　干擾消除與資料解調 37
3.3　提出之演算法 38
3.3.1　通道估測 38
3.3.2　干擾消除與資料解調 44
第四章　系統模擬 45
4.1　不同程度之符際間干擾比較 45
4.1.1　觀測區間之決策 54
4.1.2　郊區通道模型 85
4.1.3　典型都會通道模型 90
4.1.4　丘陵地形通道模型 99
4.1.5　廣播通道模型 103
4.2　改變資料符號的調變方式 118
4.2.1　郊區通道模型 119
4.2.2　典型都會通道模型 122
4.2.3　丘陵地形通道模型 124
4.2.4 廣播通道模型 126
4.3　 改變訓練序列與資料符號傳送比例 127
4.3.1　郊區地形通道模型 127
4.3.2　典型都會通道模型 128
4.3.3　丘陵地形通道模型 130
4.3.4　廣播通道模型 131
4.4　接收機移動速率的影響 131
4.4.1　郊區通道模型 134
4.4.2　典型都會通道模型 142
4.4.3　丘陵地形通道模型 149
4.4.4 廣播通道模型 160
第五章 結論 166
參考文獻 167

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