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研究生：	汪新堯 Wang, Hsin-Yao
論文名稱：	均勻時間-頻率展頻之正交分頻多工調變-分碼多重存取系統 OFDM-CDMA with Uniform Time-Frequency Spreading
指導教授：	張名先 Chang, Ming-Xian
學位類別：	碩士 Master
系所名稱：	電機資訊學院 - 電腦與通信工程研究所 Institute of Computer & Communication Engineering
論文出版年：	2006
畢業學年度：	94
語文別：	英文
論文頁數：	53
中文關鍵詞：	均勻時間-頻率展頻、正交分頻多工調變-分碼多重存取系統
外文關鍵詞：	OFDM-CDMA, Uniform Time-Frequency Spreading
相關次數：	點閱：146 下載：2
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分碼多工存取系統結合正交多載波調變被廣泛地認知為OFDM-CDMA系統，其在近代的無線通訊吸引了許多目光。而且有關展頻碼排列方式的不同將帶給整體系統性能有截然不同的性能表現，本論文將對展頻碼排列方式與系統性能彼此間的關聯做深入的討論。在早期的OFDM-CDMA系統中，展頻碼排列方式不外乎是在某固定子通道下，延著時間軸位置做排序;要不就是在某固定時間點上，延著子通道位置做排序。如此的展頻碼排列方式可視為一維空間上的展頻，換言之，即是展頻在時間軸上亦或是頻域軸上。最近有許多新的展頻方案被提出來，有別於過去的展頻碼排列方式，其將展頻碼同時排放於時間領域及頻率領域的二維空間上，如此的展頻碼排列方式可視為二維空間上的展頻。現今將展頻碼排放在時間和頻率領域的二維空間上，以其能同時獲得時間增益(time diversity)和頻率增益(frequency diversity)。就實例而言: Joint Frequency Time Spreading (JFTS) 與Random Frequency Time Spreading即是二維空間上展頻的例子。
在本篇論文中，我們提出了一個名為Uniform 2-D Spreading 方法並且研究在二維空間上展頻時，通道時間和頻率關聯性彼此間的影響。因此，通道關聯性函數 [10]在此篇論文中被引用來觀察在不同的展頻碼排列方式下，展頻碼所處位置彼此間的關聯性。除此之外，我們使用獨立複合高斯程序來獲得一個在時間及頻率衰減下，完全無關聯性的通道。其所對應的系統性能表現被用來當作二維展頻方法的一個下界(Lower bound)。我們進一步發現，我們所提出來的Uniform 2-D Spreading 方法在通道具有高都卜勒頻率 (Doppler shift) 下及較大的延遲傳遞 (delay spread) 時能趨近此下界。

Orthogonal multi-carrier modulation combined with CDMA is well known as Orthogonal Frequency Division Multiplexing – Code Division Multiple Access (OFDM-CDMA) and has drawn a lot of interest in modern wireless communications. On top of that, the permutation of spreading codes has impact on the system performance and is studied in this thesis. In early OFDM-CDMA systems, the spreading code is put along time slots on a fixed sub-channel, or along sub-channels at a fixed time slot. Such arrangement can be regarded as one-dimensional (1-D) spreading, i.e., spreading in either time domain or frequency domain. Recently, there are schemes proposed that allocate the spreading code in both time and frequency domains, which can be considered as two-dimensional (2-D) spreading. The 2-D spreading schemes exploit both time and frequency diversity. Examples of 2-D spreading schemes are Joint Frequency Time Spreading (JFTS) and Random Frequency Time Spreading.

In this thesis, we propose a Uniform 2-D Spreading Scheme and study the effects of channel time and frequency correlations on the 2-D spreading schemes. Hence, the channel correlation function [10] is cited in this thesis to see the correlation among the position of spreading code in different permutation method. Besides, we use the independent complex Gaussian process to obtain a channel with fully uncorrelated time and frequency fading, and the corresponding performance is served as a lower bound of 2-D spreading schemes. It is observed that the performance of the Uniform 2-D Spreading Scheme approaches the lower bound when the channel has higher Doppler shift and larger delay spread.

Contents

Chinese Abstract                                     Ⅰ
English Abstract                                     Ⅲ
Acknowledgements                                     Ⅴ
Contents                                             Ⅵ
List of Figures                                      Ⅷ                   
Chapter 1 Introduction                                1
Chapter 2 System Structure                            3
1 Orthogonal Frequency Division Multiplexing(OFDM) .3
2 FadingChannel....................................6
2.1 Rayleigh Fading Channel………………………….6
2.2 Multi-Path Fading Channel……………….7
2.3 Channel Estimation……………………….10
3 Code Division Multiple Access (CDMA)………………14
4 OFDM-CDMA Structure……………………………….....16
4.1 Multi-Carrier CDMA (MC-CDMA )………..16
4.2 Multi-Carrier Direct Sequence CDMA 				     (MC-DS-CDMA)………………………………..17
5 Detection Techniques in Down-link………………….19
5.1 System Structure…………………….....19
5.2 Zero Forcing (ZF) Equalizer…………..21
                  2.5.3 Minimum Mean Square Error (MMSE) 
                        Equalizer…………………………………..22
5.4 Simulation Result……………………....23
Chapter 3 Channel Correlation Functions In 2-D Spreading Schemes
1	Channel Correlation Functions………………..24
2	2-D Spreading Schemes……………………… ...26
         3.3 Simulation Result of Channel Response Correlation 
             in 2-D Spreading Schemes……………………………...31
Chapter 4 Simulation Results			       34
Chapter 5 Conclusion		         		       51




List of Figures

1  OFDM system structure	……………………………………..........4
2　Sampling diagram………………………………………..............5
3　Uncorrelated Rayleigh fading generator……………………......7
4　Illustration of cyclic prefix……………..…..………………...8
5　Rayleigh fading channel with AWGN……………..………........10
6　The illustration of pilot position……………...……………..11
7	Least-square fitting of coarse estimates………….………12
8　Channel estimation by least-square fitting……………….....13
9	The illustration of CDMA……….…………………………....14
10	Concept of Spreading and Despreading...…………………..15
11	System structure of MC-CDMA............................16
12	System structure of MC-DS-CDMA….......................18
13	System structure of MC-CDMA for multi-user in
         downlink……...........................................19
14	ZF and MMSE equalizer comparison….………..............23


1	The 2-D Spreading Scheme of MC-CDMA in downlink………..26
2	Spreading code permutation types(a,b,c,d)…………….….27
2	Spreading code permutation types(e)…………….………...28
3	Illustration of permutation in Distance Eight………....29
4	Simulation Parameters…………………………….……….....30
5	Channel Response Correlation of Theory.................31
6	Channel Response Correlation of simulated original       channel……….     .............................................32
7	Channel Response Correlation of Distance Eight permuted  method……......................................................32
8	Channel Response Correlation of JFTS permuted
     method.....................................................33
1	Least square fitting of channel estimation in OFDM system..........................................................38
2	The perfect channel of two-path Rayleigh fading…......38
3	Channel response correlation of MC-CDMA…..............39
4      Channel response correlation of MC-DS-CDMA…...........40

5	Channel response correlation of JFTS….................41
6	Channel response correlation of Random Spreading.......42
7	Channel response correlation of Uniform 2-D Spreading Scheme   (Distance Eight) ……………..……….............................43
8  Statistic distribution ( , )………………...................44
9  Statistic distribution ( , )….……………..................44
10  Statistic distribution ( , )………………..................45
11  Statistic distribution ( , )………………..................45
12  Statistic distribution ( , )………………..................46
13  Statistic distribution ( , ).……………...................46
14  OFDM-CDMA system comparison ( ,slow fading, ).............47
15  OFDM-CDMA system comparison ( ,slow fading, )….………....47
16  OFDM-CDMA system comparison ( ,fast fading, ).…..………..48
17  OFDM-CDMA system comparison ( ,fast fading, )……….…....48
18  OFDM-CDMA system comparison ( ,fast fading, )…...………..49
19  OFDM-CDMA system comparison ( ,fast fading, )…..….……..49
20  OFDM-CDMA system comparison ( ,slow fading, )…..…….....50
21  OFDM-CDMA system comparison ( ,fast fading, )….………....50





                                    

Bibliography

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[9] Mika Lasanen, Antti Anttonen and Mauri Nissila, “A Simulation Study on Time-Frequency Localized CDMA Using MMSE Per Carrier Equalization”, vol. 3, 6-9 Oct. 2003 Page: 1833-1837.
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校內：2008-06-28公開
校外：2008-06-28公開

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