雖然許多探討基於中繼站的同調合作式通訊系統的研究已經顯示其可達到如多天線技術般使用空間分集之效益，稱之為合作分集；但是，這些研究分析主要是基於接收端可獲得完美的通道狀態資訊之假設。在實際應用情形下，通道狀態資訊必須經由估計取得且非完美通道狀態資訊對系統效能的影響也需要加以分析。在本篇論文中，我們探討採用放大轉傳模式的中繼站為基礎之合作式通訊系統並研究其通道估測問題。對於放大轉傳的傳輸機制而言，中繼站可以固定增益或變動增益來放大接收來自源端的訊號再進而傳送到終端。取決於中繼站的移動性，我們考慮固定端至移動端間通道之模型以及移動端至移動端間通道之模型。我們利用導引符號輔助調變(PSAM)來落實通道估測的工作，並把線性最小均方誤差(LMMSE)視為估測準則，再加以電腦模擬舉例說明非完美通道狀態資訊對採用放大轉傳模式的中繼站為基礎之合作式通訊系統所產生的影響。

Although many researches on relay-based cooperative coherent communication systems have shown that the benefits of spatial diversity, saying cooperative diversity, can be achieved as the multi-antenna technologies do, the analyses primarily rely on the assumption that the perfect channel state information (CSI) is available at the receiver. In practical scenarios, the CSI has to be estimated and the effect of imperfect CSI on system performance needs to be analyzed. In this thesis, we study the channel estimation problem for amplify-and-forward (AF) relay-based cooperative communication systems. For AF transmission scheme, the relay amplifies the received signal from the source by a fixed or variable gain and forwards it to the destination. Depending on the mobility of the relay, we consider both the fixed-to-mobile channel model and the mobile-to-mobile channel model. To facilitate the channel estimation task, we employ the pilot symbol assisted modulation (PSAM) and consider the linear minimum mean-squared error (LMMSE) as the estimation criterion. Computer simulations are conducted to illustrate the influences of the imperfect CSI on the performance of AF relay-based communication systems.

TABLE OF CONTENTS

ACKNOWLEDGENMENTS	I
中文摘要	II
ABSTRACT	III
TABLE OF CONTENTS	IV
LIST OF FIGURES	VI
CHAPTER 1  INTRODUCTION	1
CHAPTER 2  WIRELESS TRANSMISSION ENVIRONMENTS	5
2.1	Radio Propagation Phenomena	5
2.1.1	Large scale fading: pathloss and shadowing	5
2.1.2	Small scale fading: multipath scattering	7
2.1.3	Line-of-sight and non line-of-sight surroundings	9
2.2	Wireless Mobile Channel Characteristics	10
2.2.1	Multipath delay spread and coherence bandwidth	10
2.2.2	Doppler spread and coherence time	13
2.3	 Diversity Techniques for Multipath Fading Channel	14
2.3.1	Diversity types	14
2.3.2	Combining techniques	14
CHAPTER 3  WIRELESS MOBILE CHANNEL MODELS	15
3.1	Fixed-to-Mobile Channel Models	15
3.2	Mobile-to-Mobile Channel Models	19
3.3	Wireless Relay Channel Models	22
3.4	Simulation Results	27
CHAPTER 4  CHANNEL ESTIMATION AND ESTIMATOR CRITERIA	32
4.1	System Model	32
4.2	Channel Estimation Design	34
4.2.1	Pilot symbol assisted modulation	34
4.2.2	Estimator criteria	36
4.3	Estimation of the AF Relay Channel	41
4.3.1	Fixed gain case	41
4.3.1.1	Stationary relay	42
4.3.1.2	Mobile relay	43
4.3.2	Variable gain case	44
4.3.2.1	Stationary relay	45
4.3.2.2	Mobile relay	47
4.4	Maximal Ratio Combining	48
4.5	Numerical Results	49
4.5.1	Performance of the LMMSE estimator	49
4.5.2	Pilot insertion period	51
4.5.3	The number of pilots	54
4.5.4	Doppler spread	54
4.5.5	The effect of MRC	55
CHAPTER 5  CONCLUSIONS AND FUTURE WORK	57
5.1	Conclusions	57
5.2	Future Work	57
REFERENCES	58


LIST OF FIGURES

Figure 1.1   The three-node channel model of the cooperative system	3
Figure 3.1   The single-ring model	16
Figure 3.2   Sketch of the double-ring model	20
Figure 3.3   Sketches of the AoA and the AoD	21
Figure 3.4   AF relay channel model	23
Figure 3.5   Pdfs of the envelope of the fixed-to-mobile channel model	27
Figure 3.6   Correlation functions of the simulated fixed-to-mobile channel model	28
Figure 3.7   Correlation functions of the simulated mobile-to-mobile channel model	28
Figure 3.8   Pdfs of the envelope of the mobile-to-mobile channel model	29
Figure 3.9   Comparison of the envelopes’ pdfs between the fixed-gain and the variable-gain AF relay channel models	29
Figure 3.10  Correlation functions of a stationary relay with fixed gain	30
Figure 3.11  Correlation functions of a stationary relay with variable gain	30
Figure 3.12  Correlation functions of a mobile relay with fixed gain	31
Figure 3.13  Correlation functions of a mobile relay with variable gain	31
Figure 4.1   Sketch of the pilot symbol assisted modulation	35
Figure 4.2   LMMSE estimator performances for a stationary relay	50
Figure 4.3   LMMSE estimator performances for a mobile relay	50
Figure 4.4   The average MSE in estimating the composite AF relay channel	51
Figure 4.5   Influence of pilot spacing on estimator performances (fixed gain)	52
Figure 4.6   Influence of pilot spacing on estimator performances (variable gain)	52
Figure 4.7   Influence of pilot numbers on estimator performances (fixed gain)	53
Figure 4.8   Influence of pilot numbers on estimator performances (variable gain)	53
Figure 4.9   Effects of Doppler spreads on estimator performances (fixed gain)	54
Figure 4.10  Effects of Doppler spreads on estimator performances (variable gain)	55
Figure 4.11  LMMSE estimator performances when a direct link is available	56
Figure 4.12  LMMSE estimator performances when a weaker direct link is available	56

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