臺灣博碩士論文加值系統

空時編碼技術近年來廣泛地被討論，而它與OFDM系統也已成為第四代寬頻行動通訊及IEEE 802.11x/802.16/802.20等的標準技術之一。在高速資料傳輸系統中，由於多路徑延遲的影響，導致位元錯誤率的增加。OFDM可將多路徑通道效應簡化為平坦衰減通道，並可用快速傅立葉轉換來的設計與實作簡單的等化器。加上空時碼使用空間分集特性配合特殊的編碼，以提供衰落通道下的分集增益，來降低位元錯誤率。但是若是在多使用者共同存取相同通道的環境下，由於OFDM並沒有特殊的機制來區分使用者，因此大幅增加了多使用者的OFDM系統設計的困難度。
本研究主要討論在頻率選擇衰減通道下空時碼多輸入多輸出多使用者OFDM系統的設計與探討。首先我們將建立所有的使用者接存取同樣通道的條件下空時碼多輸入多輸出多使用者OFDM系統的訊號模型，繼而利用空時碼特性開發並證明新的盲目通道估測技術，並利用少許的引導信號找出真實通道與用雜訊子空間估測出來通道的角度差，並比較其均方誤差及探討此方法性能的極限。其次我們討論接收端濾波器的權重，使用理想的MMSE方法，並比較其位元錯誤率的性能與使用真實通道係數作為濾波器權重時的性能差距。我們也將利用Forward-backward averaging的技術來降低通道估測的均方誤差及位元錯誤率，最後我們將MMSE濾波器以實際的有限取樣環境來探討，並加入特徵空間的技術使用信號子空間進一步消除因有限取樣而造成的濾波器權重誤差，並比較其與理想的MMSE濾波器之間的位元錯誤率性能差異。

Space-time coding has been widely studied for wideband cellular communication. The combination of OFDM and space-time coding is considered as one of key technologies for 4G and 802.11x/802.16/802.20 standards. The high data rate transmission system often suffers from the multipath fading effect which causes the increasing of bit error rate. Using OFDM can simplify the frequency selective fading channel into flat fading channel and then can simply the design of the receiver by using FFT. To reduce the bit error rate, the space-time coding technique utilizes the spatial diversity (with multiple transmit/receive antenna) with specific form of coding. It provides the diversity gain to the receiver in the multipath fading channel. However, the system design becomes more difficult due to the more than one user transmitted simultaneously on the same frequency band because OFDM have no way to separate symbols from different users.
The thesis is mainly to study the design and analysis of the blind receiver of the space-time coded MIMO multiuser-OFDM systems in frequency-selective fading channels. First we will establish the signal model of space-time coded MIMO multiuser-OFDM system which apply to all users can transmitted simultaneously on the same frequency band. Using the property of the space-time block codes and singular value decomposition, we then develop and prove the blind channel estimation techniques.We will use a little number of pilots to find the ambiguity between real channel and the channel we found by EVD, and then analysis its MSE and find lower bound can EVD do. Secondly, we will analyzes the receive filter weights.We will use MMSE filter and compare BER performance between using real channel and our method.. The forward-backward averaging techniques are utilized to reduce the biasness of the correlation matrix and to enhance the system performance. Finally, we utilize the property of STC to simply the design of MMSE fikter to reduce system complexity.

Abstract (in Chinese)…………………………………………………………………….i
Abstract …………...………………………………………………………………….…ii
Acknowledgement………………………………………………………………………iii
Contents………………………………………………………………………………....iv
List of Figures……………………..……………………………………..………….... ..vi
1 Introduction……………………………………...……………………1
1.1. Background of this thesis……………………………………………………..1
1.2. Overview of OFDM …………………………………………………..…3
1.3. Space-Time Block Code………………………………………………………6
1.4. Outline of this thesis…………………………………………………………..8
2 Basic Fundamentals………………………...……..………..………...9
2.1. STC-MIMO-OFDM System Model………………...…………………...……9
2.2. Blind Channel Estimator…………………..……………………………...…13
2.3. Problem Formulation………………………...………………………………16
3 Blind Channel Estimation and Resolve Ambiguity…...…………...18
3.1. System Model……..……………………..…………………………………..18
3.2. Blind Channel Estimation……………………………………………………20
3.3. Identifiability of Channel Estimation………………………………………..22
3.4. Resolve Ambiguity………………………...………………………………...28
3.5. Computer Simulation Results…………...…………………………..……….32
4 Filter Design and Performance Enhancement……………..............40
4.1. Traditional Filter Design and Weight Analysis……………….…….………..40
4.2. Performance enhancement…………………………………………………...44
4.3. Computer Simulation Results…………...…………………………..……….49
5 Conclusion……………………………………………………………65
References………………………………………………………………..67
Appendix A……………………………………………………………….74

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