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研究生:柯鴻均
研究生(外文):Hung-Chung Ke
論文名稱:應用離散弦波轉換對正交分頻多工系統之通道估測研究
論文名稱(外文):Studies of Discrete Sinusoidal Transform in The Channel Estimation for OFDM System
指導教授:許超雲許超雲引用關係
指導教授(外文):Chau-Yun Hsu
學位類別:碩士
校院名稱:大同大學
系所名稱:通訊工程研究所
學門:工程學門
學類:電資工程學類
論文種類:學術論文
論文出版年:2008
畢業學年度:96
語文別:英文
論文頁數:65
中文關鍵詞:通道估測
外文關鍵詞:OFDMDFTDCTchannel estimation802.11a
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在無線通訊系統裡,衰減是一個影響訊號傳送的主要因素,在高速行動傳輸系統中衰減補償是首先要解決的問題。
將正交分頻多工系統的部份子載波中加上前導碼(pilot),再輔以內插方式可以有效地得到通道估測值,這個機制提供有效解決訊號衰減所造成的問題。它藉著這些通道估測值,有效的找回經過通道衰減的信號。目前常用的內插方法有DFT、DCT等方法,本論文模擬以802.11a所傳送的訊號,經不同之調變方式(BPSK、 QPSK、 16 QAM),及通過不同衰減參數(AWGN、Doppler frequency、Jakes model)下,在接收端可透過前導碼得到不同的頻率響應,我們使用所得到的頻率響應可以去除訊號衰減的影響並正確估測原始訊號。從上述中,使用不同的通道估測方法而得到誤碼結果,得知DFT-base比混合型 DCT-base 通道估測方法好。為何要使用混合型?因為當使用DCT 第二型做通道估測時,發現放大倍率(up-sampling factor)為偶數時,前導碼(pilot)的值會跑掉而奇數時則不會,因此吾人使用DCT第二型做正轉換(forward transform)後,再採用DCT第一型做逆轉換(inverse transform)以便解決放大倍率(up-sampling factor)為偶數所導致前導碼(pilot)資料不正確的問題。
In wireless communications system, fading is one of the main problems to affect the transmitting signal, hence the fading compensation is the most important issue to be solved especially in high-speed mobile communication.
We can place pilot in some sub-carriers of OFDM system to get estimation signals by using interpolation. It provides good effect to recover the destructed signals to the original ones. DFT-base and DCT-base methods are popular interpolation for channel estimation. In this thesis, we use different modulations (BPSK, QPSK, 16 QAM) to transmit OFDM symbol according specification of IEEE 802.11a. The OFDM symbol pass through non-ideal channel (with AWGN and Rayleigh fading) and we can get frequency response by extracting pilots. We use the frequency response to estimate the fading effect and remove the estimated fading effect then the signals will be recovered.
From above mention, we use different interpolation and get DFT-base method’s symbol error rate is better than hybrid DCT-base method. Why do we adopt hybrid DCT-base method? When we perform DCT type II to do interpolation, the even up-sampling factor would not recover the original pilot value but the odd up-sampling factor can recover it. We propose forward transform by DCT type II, inverse transform by DCT Type I to overcome this problem.
ACKNOWLEDGEMENTS III
ENGLISH ABSTRACT IV
CHINESE ABSTRACT V
TABLE OF CONTENTS VI
LIST OF FIGURES IX
CHAPTER 1 INTRODUCTION 1
1.1 Introduction 1
1.2 The Organization of The Thesis 2
CHAPTER 2 CHANNEL MODEL 3
2.1 Introduction 3
2.2 Additive White Gaussian Noise 3
2.3 Mobile Radio Propagation 6
2.3.1 Radio Wave Propagation 7
2.4 Small-Scale Fading 8
2.4.1 Small-scale Multi-path Propagation 9
2.4.2 Factors Influencing Small-scale Fading 11
2.4.3 Doppler Shift 12
2.5 Types of Small-Scale Fading 14
2.5.1 Fading Effects Due to Multi-path Time Delay Spread 15
2.5.1.1 Flat Fading 15
2.5.1.2 Frequency Selective Fading 17
2.5.2 Fading Effects Due to Doppler Spread 19
2.5.2.1 Fast Fading 19
2.5.2.2 Slow Fading 20
2.6 Ricean Fading Distribution 21
2.7 Rayleigh Fading Distribution 22
2.8 Simulation of Rayleigh Fading Channel 23
CHAPTER 3 CHANNEL ESTIMATION 26
3.1 Introduction 26
3.2 Digital Modulation Signals 26
3.3 Linear Modulation Technique 27
3.3.1 Binary Phase Shift Keying 28
3.4 Constant Envelope Modulations 28
3.4.1 Binary Frequency Shift Keying 29
3.5 Combined Linear and Constant Envelope Modulation 29
3.5.1 M-ary Quadrature Amplitude Modulation 30
3.6 Pilot Aided Modulation 31
3.7 Fading Compensation Method by Using LS 32
3.8 Interpolation via DFT and Hybrid DCT 33
3.8.1 Interpolation Using DFT 33
3.8.2 Interpolation Using Hybrid DCT 34
3.9 Channel Estimation System Block Diagram 36
CHAPTER 4 SIMULATIONS AND RESULTS 38
4.1 Simulation Model 38
4.2 Simulation Parameters 40
4.3 Data Change through Fading Channel 40
4.4 Symbol Error Rate of Propose Scheme in Different Rayleigh Fading Channel 45
CHAPTER 5 CONCLUSIONS 50
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