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博碩士論文 etd-0508106-164152 詳細資訊
Title page for etd-0508106-164152
論文名稱
Title
多輸入輸出正交分頻多工系統之通道估計研究
Study on channel estimation of MIMO-OFDM systems
系所名稱
Department
畢業學年期
Year, semester
語文別
Language
學位類別
Degree
頁數
Number of pages
78
研究生
Author
指導教授
Advisor
召集委員
Convenor
口試委員
Advisory Committee
口試日期
Date of Exam
2005-06-30
繳交日期
Date of Submission
2006-05-08
關鍵字
Keywords
多輸入輸出、正交分頻多工、通道估測
OFDM, channel estimation, MIMO
統計
Statistics
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中文摘要
多輸入輸出(multiple-input multiple-output,MIMO)正交分頻多工(orthogonal frequency division multiplexing,OFDM)技術在現今的無線通訊領域上被廣泛的應用。由於無線通道的通道情況往往隨著時間而不斷的改變,導致信號在傳輸的過程中會受到無線通道的影響而失真,因此我們必須正確的估測出通道的脈衝響應(channel impulse response),並且利用此估測的響應來補償通道對信號所造成的失真效應。我們可以在發射信號中穿插已知的導航信號(pilot signal),透過這些已知的導航信號,接收端可以有效的估測出位於導航信號位置的通道響應,進而透過通道內插(channel interpolation)來得到其餘資料信號位置的通道響應。然而,通道內插所造成的通道估測誤差是無法避免的。在本篇論文中我們提出了一個適用於MIMO OFDM系統的導航信號輔助式重複最大相似(maximum likelihood,ML)通道估測演算法。首先,我們利用導航信號來得到初始的ML通道估測值,接收端可利用此通道估測值來作信號偵測及信號決策,其次我們將信號決策值視為已知的資訊,並且利用這些資訊來作下一步的通道估測。重複以上的通道估測和信號偵測與信號決策程序,我們可以逐漸的縮減因為通道內插而導致的通道估測誤差,而通道估測的精確度也會因為增加此重複程序的次數而逐漸提升。
模擬結果證實重複式ML通道估測演算法能夠提供比一般演算法更好的估測均方差(mean-square-error,MSE)及位元錯誤率(bit-error-rate,BER)性能。透過不同系統參數的模擬我們可以看到此演算法對於不同的系統改善的情況也會有所不同。在使用較少導航信號的系統中,利用重複式ML通道估測演算法所能得到的系統性能改善幅度較大;而在使用較多導航信號的系統中,利用此演算法所能得到的性能改善幅度較小。最後,我們模擬幾種不同的通道環境來比較重複式ML通道估測演算法在面對不同通道環境時所能提供的系統性能改善情形。
Abstract
Multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) technology has been used widely in many wireless communication systems. Signals will be distorted when they are transmitted in wireless channels. For the reason that wireless channel is time or location variant, we have to estimate the channel impulse response and use the channel state information to compensate the channel distortion. Pilot signals can be spaced separated in the transmitted symbols. In the receiver, the channel impulse response can be estimated at the positions of pilot signals. The other channel information at the data signals can be obtained by interpolating the estimated channel impulse response. However, error caused by channel interpolation can not be avoided. In this paper we propose a pilot-aided iterative maximum likelihood (ML) channel estimation algorithm for MIMO OFDM systems. At first, an ML channel estimate is obtained by using pilot tones. The receiver uses the estimated channel to help the detection/decision of data signals. And then the channel estimation treats the detected signals as known data to perform a next stage channel estimation iteratively. By utilizing the iterative channel estimation and signal detection process we can reduce the estimation error caused by channel interpolation between pilots. The accuracy of the channel estimation can be improved by increasing the number of iteration process.
Simulation results demonstrate the iterative ML channel estimation algorithm can provide better mean-square-error and bit-error-rate performance than conventional methods. By changing the system parameters we can see that the improvement provided by this algorithm is different. Systems with fewer pilots have more improvement from the iterative ML algorithm. On the other hand, systems with more pilots have less improvement from the iterative algorithm. Finally, several channel environments are also considered in this thesis to compare the performance improvement introduced by the iterative algorithm.
目次 Table of Contents
誌謝 i
摘要 ii
Abstract iii
目錄 iv
圖索引 v
表索引 vii
第一章 簡介 1
第二章 多輸入輸出正交分頻多工系統 4
2.1 正交分頻多工技術 4
2.1.1 正交分頻多工技術的基本概念 5
2.1.2 正交分頻多工系統信號模型 6
2.1.3 正交分頻多工系統基本架構 6
2.1.4 守護區間 9
2.2 多輸入輸出通道信號模型 13
第三章 重複式最大相似通道估測演算法 16
3.1 導航符號的排列方式 16
3.2 系統模型 18
3.3 最大相似估測器 20
3.4 導航符號的設計 22
3.5 信號偵測 24
3.6 重複式最大相似通道估測器 26
第四章 系統模擬 30
4.1 導航符號數量不同的比較 30
4.2 接收機移動速率的影響 52
4.3 符際間干擾的影響 59
第五章 結論 68
參考文獻 69
參考文獻 References
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