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研究生:林欣儀
研究生(外文):Shin-Yi Lin
論文名稱:多用戶偵測技術及應用於光分碼多工系統之研究
論文名稱(外文):Multiuser Detection with Multistage Interference Cancellation for Asynchronous Optical CDMA Systems
指導教授:吳靜雄
指導教授(外文):Jingshown Wu
學位類別:碩士
校院名稱:國立臺灣大學
系所名稱:電信工程學研究所
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:2004
畢業學年度:92
語文別:英文
論文頁數:96
中文關鍵詞:平行化干擾消除光正交碼光分碼多工系統多用戶偵測技術
外文關鍵詞:Optical CDMA (OCDMA) SystemsMultiuser Detection
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分碼多工技術在無線通訊系統中發展已相當成熟。在近幾年來,釵h研究把注意力放在如何將分碼多工技術應用在光纖的傳輸上;光分碼多工本身有釵h優點,包括;非同步傳輸,提供保密機制,對於網路設計較有彈性,支援突增流量和多位元速率服務,以及抗干擾的能力等等。光分碼多工系統的效能表現與系統中同時存在漕洏峈抯ぁ埵雪奶j的相關性,當使用者數目越多時,彼此之間訊號的干擾就愈形嚴重,容易造成接收端偵測上的錯誤,使得傳輸錯誤率增加,因而導致通訊品質變差。

為了提升光分碼多工系統的效能,以往相關的研究,主要是致力於發展特性優良的展頻碼,讓不同展頻碼彼此的相關性降到最低,如此一來,用戶們可採用不同的展頻碼作為身分的識別,而且訊號相互間的干擾可以降到最低。由於光訊號能量恆正的特性,適用於光分碼多工系統的展頻碼和傳統無線系統中所使用的不同,乃是由零與一所組成的序列,且彼此間滿足特定自我相關以及交互相關的特性,一般稱之為光正交碼。然而,在多用戶的光系統中,為了達到好的效能表現,每個使用者必須採用極長的光正交碼,如此一來會降低頻寬的有效使用率。近年來,研究上發展出同時在波長和時間上展頻的二維光正交碼,也稱為多波長光正交碼,可提升系統效能且增加可容納的用戶數目。

此外,另一方面的研究則著重於接收器的設計,透過特殊演算法,藉由多用戶偵測技術消除使用者彼此間的干擾,來提高偵測的正確性。本論文主要是研究光分碼多工系統中的多用戶偵測技術與接收器設計,配合已經廣為發展的光正交碼,可以進一步提升光分碼多工系統之效能。同時,採用多用戶偵測技術來輔助接收,可以在使用者相互干擾存在的情況下,仍然正確的偵測到所需要的信號;如此一來,系統採用較短的展頻碼就可以達到好的效能表現,改善了原本光正交碼極長的缺點,提高頻寬的有效使用率也擴大系統可容納的用戶量。本論文中,吾人將多級數平行化干擾消除的演算法(multi-stage parallel interference cancellation)引用到光分碼多工系統中並分析之,由於光系統中要讓使用者的信號維持能量均等是可行的,因此吾人發現平行化干擾消除演算法提供了比循序化干擾消除演算法更加的效能。此外,吾人提出以多級數權重式干擾消除的演算法(multi-stage partial parallel interference cancellation)來實現適用於光分碼多工系統中的多用戶偵測技術,經由本論文的分析與模擬發現,此種光系統中新式的多用戶偵測器,提供了極佳的效能表現,趨近由干擾已知情況下所推導出的效能界限。此外,吾人提出一新式的接收器設計,稱之為正交碼權重分割接收器(code-weight-partition receiver),此接受器的結構簡單,且能提供比傳統光接受器較好的效能,結合前述的多級數平行化干擾消除演算法所產生的多用戶偵測器,經由模擬結果發現,亦能提供相當好的效能。簡言之,本論文針對光分碼多工系統所提出的多用戶偵測技術,可達到極佳的效能表現且複雜度只和用戶數目呈線性增長,和所謂最佳偵測器的指數性增長相比,實用價值極大,對於提升頻寬有效使用率,增加可容納的用戶數目均有助益,確實提升光分碼多工系統之效能表現。
For optical code division multiple access (OCDMA) communication systems, the optical orthogonal codes (OOCs) with ideal auto- and cross-correlation properties have been widely studied. Another approach to improve the performance is to employ multiuser detection techniques. In this thesis, we present several multiuser detection (MUD) schemes for OCDMA systems. Characterization of the intensity-modulation/ direct- detection OCDMA system with the avalanche photodiode is addressed. The parallel interference cancellation (PIC) algorithm is applied to OCDMA and results in a multi-stage PIC detector, which is shown to outperform the correlator. A modified partial parallel interference cancellation (PPIC) algorithm for OCDMA is proposed and analyzed. The main concept is to remove only a fractional of the multiuser interference (MUI) instead of complete cancellation when the estimation is poor, and raise this weighting factor as the fidelity of decision and estimation improved. The multistage PPIC detector is found to significantly reduce the degradation due to the MUI and offer excellent performance with complexity proportional linearly to the number of users. With appropriate choice of the weighting factor, OCDMA systems with the PPIC detector will achieve the known-interference lower bound as shown in the simulation and numerical results. Moreover, the code-weight- partition receiver, which can be considered as the modified version of the correlator, is proposed and can be combined with the PIC algorithm. The proposed multiuser detectors in this thesis are of excellent performance and acceptable complexity.
List of Figures
List of Tables
1 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . 2
2 Overview of OCDMA systems 3
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Asynchronous and SynchronousOCDMASystems . . . . . . . 7
2.2.1 AsynchronousOCDMASystem . . . . . . . . . . . . . 7
2.2.2 SynchronousOCDMASystem . . . . . . . . . . . . . . 11
2.2.3 Two-DimensionalWavelength/Time Codes . . . . . . . 12
2.3 Description of OCDMA Systems . . . . . . . . . . . . . . . . . 14
2.3.1 Encoder Structure . . . . . . . . . . . . . . . . . . . . 15
2.3.2 Receiver structure . . . . . . . . . . . . . . . . . . . . . 17
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Conventional Multiuser Detection Techniques 21
3.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2 Single-User CorrelationDetector . . . . . . . . . . . . . . . . . 24
3.3 Optimal Multiuser Detector . . . . . . . . . . . . . . . . . . . 25
3.4 Some SuboptimalMultiuserDetectors . . . . . . . . . . . . . 26
3.4.1 LinearMultiuserDetectors . . . . . . . . . . . . . . . . 27
3.4.2 NonlinearMultiuserDetectors . . . . . . . . . . . . . . 31
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4 Proposed Multiuser Detection Techniques for OCDMA systems
37
4.1 General System Model for Proposed Algorithms . . . . . . . . 37
4.1.1 Multiuser Photon Channel . . . . . . . . . . . . . . . . 37
4.1.2 Signal Model of Avalanche Photodiode (APD) Detector 41
4.2 Multistage Parallel Interference Cancellation (PIC) Multiuser
Detector forOCDMA. . . . . . . . . . . . . . . . . . . . . . . 48
4.3 Multistage Partial Parallel Interference Cancellation (PPIC)
Multiuser Detector for OCDMA . . . . . . . . . . . . . . . . . 50
4.4 Code-Weight-Partition Receiver . . . . . . . . . . . . . . . . . 59
4.5 Theoretical Analysis . . . . . . . . . . . . . . . . . . . . . . . 61
4.5.1 Single-User Lower Bound . . . . . . . . . . . . . . . . . 61
4.5.2 Known-Interference Receiver . . . . . . . . . . . . . . . 62
4.5.3 Saddlepoint Approximation for BER Computation . . 63
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5 Simulation and Numerical Results 69
5.1 Default Systemand Simulation Parameters . . . . . . . . . . . 69
5.2 Multistage PICMultiuserDetector forOCDMA . . . . . . . . 72
5.3 Multistage PPICMultiuserDetector forOCDMA . . . . . . . 74
5.4 Code-Weight-Partition Receiver forOCDMA . . . . . . . . . . 80
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
6 Conclusion 87
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