在這篇論文中，我們考慮一個雙向中繼系統，在此系統中兩個終端點會藉由一個放大轉送中繼點以雙向傳輸方式彼此交換訊息。然而，在雙向中繼協定下，兩個終端點的訊號將通過不同的串聯通道於不同的時間抵達接收端，導致在時間同步和通道估測方面相較於傳統單向中繼系統更為複雜。為了同時處理雙向放大中繼系統之同步與通道估測問題，我們提出一個基於在某種特殊訓練序列安排的聯合時間同步與通道估測技術。而每個終端點所採用的訓練序列是由一完美週期性序列(具有理想的自相關函數)、適當的循環字首以及循環字尾所構成。我們所提出的技術依賴通道首路徑的選擇，而其效能更是高度地依賴閾值的選擇。藉由分析相關器輸出的機率密度函數，我們得到最佳的閾值來區分只含有雜訊的輸出值以及包含通道首路徑增益的輸出值。根據這些閾值，在較低的訊雜比下，所提出技術可提供優於最大似然法則作法之效能，而隨著訊雜比的增加，兩種作法的效能則漸趨一致。透過分析，我們亦展示所提技術所需的計算複雜度遠低於基於最大似然法則的作法，實用價值高。

In this thesis, we consider a two-way relay system where two terminals exchange their information via an amplify-and-forward relay in a bi-directional manner. Due to the two-way relay protocol, signals from both terminals travel through different cascaded channels and this makes synchronization and channel estimation much more complicated than those in conventional one-way relay systems. To cope with these problems, we propose a joint time synchronization and channel estimation scheme based on a specific training sequence arrangement, where each terminal’s training sequence consists of a perfect sequence (with an ideal auto-correlation function) attached by an appropriate cyclic prefix and postfix. The proposed scheme relies on a first channel tap selection process, whose performance is highly dependent on the choice of a threshold to distinguish the signal outputs from noise. By analyzing some possible probability density functions of the correlator output, we derive optimal thresholds for selection of the first channel tap. With these thresholds, the proposed scheme provides better time synchronization performance than the maximum-likelihood approach for low signal-to-noise ratio (SNR) cases; both have similar performance for high SNR cases. It is also shown that the proposed scheme involves much less computational complexity than the maximum-likelihood approach.

ABSTRACT i
CONTENTS ii
LIST OF FIGURES iv
LIST OF TABLES vi
Chapter 1 Introduction 1
1.1 OFDM Basics and System Overview 1
1.2 Cooperative Communication System Overview 3
1.3 Outline of the Thesis 5
Chapter 2 Synchronization and Channel Estimation for Two-Way Amplify-and-Forward Relay Systems 6
2.1 Introduction 6
2.2 An Overview of Channel Estimation Methods for Two-Way Amplify-and-Forward Relay Systems 8
2.3 Joint CFO and Channel Estimation for Two-Way Amplify-and-Forward Relay Systems 9
2.4 Joint Time Synchronization and Channel Estimation for Two-Way Amplify-and-Forward Relay Systems 10
2.4.1 Bayesian Based Markov Chain Monte Carlo (MCMC) Algorithm 10
2.4.2 ML Algorithm Based on Training Sequences 11
Chapter 3 The Proposed Low-Complexity Joint Synchronization and Channel Estimation Scheme for Two-Way Amplify-and-Forward Relay Systems 17
3.1 Motivation 17
3.2 System Model 18
3.3 The Training Sequence Structure 20
3.4 Derivation of the CCF Output 22
3.5 The Proposed Joint Synchronization and Channel Estimation Scheme 24
3.6 Analysis of the PDFs of the CCF Output 25
3.6.1 The Formulation Associated with Noise Terms 25
3.6.2 The Formulation Associated with Signal Terms 29
3.7 Threshold Selection for the Proposed Joint Time Synchronization and Channel Estimation Scheme 31
3.8 Performance Analysis of the Proposed Joint Scheme 33
3.8.1 The probability of perfect time synchronization 33
3.8.2 Data Detection 33
3.8.3 Complexity Analysis 36
Chapter 4 Simulation Results 38
Chapter 5 Conclusion 50
APPENDIX A Derivation of the CCF Output 51
APPENDIX B Derivation of the PDFs of CCF Output Magnitude Associated with Noise Terms 52
REFERENCES 55

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