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研究生:張仲堯
研究生(外文):Chung-Yao Chang
論文名稱:正交頻率多工無線通訊系統之性能分析及天線陣列技術應用
論文名稱(外文):Performance of OFDM-Based Wireless Communication Systems and Its Applications with Antenna Arrays
指導教授:陳巽璋
指導教授(外文):Shiunn-Jang Chern
學位類別:博士
校院名稱:國立中山大學
系所名稱:電機工程學系研究所
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:166
中文關鍵詞:多載波分碼多工反向QR分解正交頻率多工智慧型天線適應性限制性演算法
外文關鍵詞:OFDMAdaptive constrained algorithmSmart antennaInverse QR-decompositionMC-CDMA
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為了滿足行動及個人寬頻通訊快速的成長需求,近年來已發展出許多革新性的技術並廣泛地應用在無線傳輸當中。在無線通訊系統中,儘管系統效能會受到通道的特性,如:多路徑衰落及背景雜訊,而有所衰退,但這些影響可透過利用分集及結合的觀念來做顯著地消除。然而,有些不同特性的干涉信號依然存在著,包含了在峰巢式通訊系統中因多重接取所造成的嚴重架構形式上的干擾,在此稱之為多用戶干擾,及在共同使用的重疊頻帶上所不可避免的干擾源,目前已成為最主要的困難點並進一步地破壞接收品質及降低系統容量。為了消除這些干擾源,有些前瞻性的訊號處理方法被建議使用,如智慧型天線、多用戶檢測、干擾消除、最佳化之適應性訊號處理、及頻率或時框的同步…等,此舉不僅在本質上減緩干擾,同時也可以提升訊號品質。
正交頻率多工是一種重要且值得注意的多載波技術,並已經廣泛地使用在許多商業化通訊上,如:數位廣播及無線區域網路。在一個寬頻系統的下鏈傳輸當中,它被認為是一種最有效的技術來對抗多路徑衰落及多用戶干擾。除此之外,空間性的處理為使用了由智慧型天線陣列所提供的分集概念,在此意指為適應性波束形成器的使用,而它也是一種不需要額外增加使用頻譜就能增進無線系統容量的效益及品質的方法。並且因為多根天線的使用,它允許系統充分利用空間的多樣性。
在此篇論文中,吾人考量了基於正交頻率多工技術的無線通訊系統架構及智慧型天線的應用,並且利用反相QR分解遞迴式最小平方和演算法,我們將著重於此種適應性線性限制性逼近法的推衍。此LC-IQRD-RLS演算法擁有許多好處,例如:數值穩定性、快速的收斂速度和有效率的執行方式,這些都比傳統適應性演算法更為優越。此外,結合微分式限制性條件,原窄頻波束形成器可改善其強韌性用以對抗寬頻及同調干擾。在此,擁有二階限制條件的疊代式二階最大相似性(IQML)演算法被用來估計干擾子空間。電腦模擬證實了利用窄頻波束形成器配合著適當的演算法,如LC-IQRD-RLS或IQML,可以達到消除干擾的預期效能。其次,在擁有頻率結合觀念的多載波分碼多工系統中,上述演算法的應用性將被完整地討論。事實上,透過以限制性最小輸出能量法則為基礎的最佳化逼近法,可達成此頻率分集的概念,但不幸的是,此法則對於因估計誤差所引起的訊號不匹配現象是相當敏感的。為了處理這種不匹配現象的問題,擁有不變特性的常模法與原本所提出的LC-IQRD-RLS演算法,將被進一步的整合與發展。模擬結果可看出利用頻率結合器配合著強韌性LCCM IQRD-RLS演算法能夠恢復傳送時的訊號而不受通道影響或有通道失真,即使在嚴重的遠近問題中,它也能夠有效率地消除多用戶干擾。為了進一步提升檢測性能及增加系統容量,我們提出了一種結合智慧型天線及多載波傳輸技術優點的時空多載波分碼多工接收機,此種直接全效地時空多載波分碼多工接收機可利用一種稱之為克維克乘法器的數學運算子來執行。為了進一步的探討,在某些假設前提下,我們可求得位元錯誤率的完整表示式來做理論分析,此舉將幫助我們進一步地瞭解因子載波數目及陣列感應器數目所受的影響。在最後一個章節,將以正交分頻多工為基底的無線區域網路中,採用IEEE 802.11 a/g的標準,我們探討了一個常見的問題即為載波頻率偏差。為此,我們提出了完整地頻率同步規劃並由三個部分所組成,分別為初步及精密的自動頻率控制及鎖相迴路。根據所提的頻率同步規劃裝置,即使在有時間不確定的因素情況之下,相對於目前的規格它仍節省了將近2分貝的功率消耗。
To satisfy the growing demands of the mobile and personal broadband communications, recently, many innovative technologies have been devised and extensively used for wireless transmission and reception. In the wireless communication systems, even though the performance would be degraded due to channel characteristics, such as multipath fading and background noise, those impacts can be eliminated dramatically through the utilization of diversity and combining. However, some different kinds of interfering sources, including the significant structure interference due to their operation as multiple access in the cellular communication systems, referred to as the multiple access interference (MAI), and inevitable jammers appeared in the overlapped frequency band for common utility, are still existing and now become the main difficulties to collapse the reception performance and system capacity. To suppress the interferences, some advanced signal processing methods, e.g., smart antenna (SA), multiuser detection, interference cancellation, adaptive optimization, and frequency/ frame synchronization, have been suggested to not only alleviate the effects fundamentally but also enhance the signal quality.
Orthogonal frequency division multiplexing (OFDM) is a significant multicarrier (MC) technology, and has been widely employed in some commercial communications, such as digital broadcasting and wireless local area network (WLAN). It is considered to be the one of the most promising techniques to combat multipath fading and MAI for the downlinks transmission of the broadband systems. Moreover, spatial processing exploits the diversity provided by SA or intelligent antenna arrays, in which the adaptive beamformer is utilized, and it is an alternative approach to increase the efficiency of wireless system capacity and performance without allocating additional frequency spectrum. It allows the system to make full use of spatial diversity due to multiple antennas.
In this dissertation, the wireless communications based on the OFDM technique and the applications of SA are considered. Also, an adaptive linearly constrained (LC) approach via inverse QR-decomposition (IQRD) recursive least-squares (RLS) algorithm is emphasized. The proposed LC-IQRD-RLS algorithm has the merits, such as numerical stability, fast convergence rate, and implementation efficiency, over the conventional adaptive algorithms. Furthermore, by incorporating with derivative constraint, the narrowband array could improve the robustness against to the wideband and coherent jammers. Here, the iterative quadratic maximum likelihood (IQML) algorithm with norm constraint set is utilized to estimate the jammer subspace. Computer simulations verify that the use of narrowband beamformer with an appropriate algorithm, e.g., LC-IQRD-RLS or IQML, could achieve the desired performance for jammer suppression. Next, their applications to the MC-CDMA system with frequency combining process will be fully addressed. In fact, the frequency diversity is achieved through the optimization approach, based on constrained minimum output energy (CMOE) criterion. Unfortunately, it is very sensitive to the signal mismatch due to channel estimation error. To deal with the mismatch problem, the invariant-property provided by constant modulus (CM) criterion along with the LC-IQRD-RLS algorithm is developed. Simulation results show that the frequency combiner with the robust LCCM IQRD-RLS algorithm could be used to recover the transmitted signal without channel mismatch or distortion, and mitigate the MAI efficiently even in the significant near-far effect environment. To further enhance the detection performance and increase system capacity, the space-time MC-CDMA receiver is proposed by combining the advantages of SA and multicarrier transmission technique. This direct fully space-time MC-CDMA receiver can be implemented via a mathematical operator, i.e., kronecker product. For further investigation, a theoretical analysis could be evaluated under certain assumptions to obtain a closed-form expression of bit error rate (BER). This will help us look more inside the impacts due to the numbers of subcarriers and array sensors. In the last chapter, the familiar problem of carrier frequency offset (CFO) is investigated following the standard of IEEE 802.11 a/g OFDM-based WLAN. The overall frequency synchronization scheme consists of three parts, viz., the coarse and fine automatic frequency control (AFC) circuits, and phase locked loop (PLL). With the proposed frequency synchronization scheme, it reserves 2dB power consumption compared with the current specification even some timing issues presented.
Acknowledgment i
Abstract ii
Chinese Abstract iv
Contents vi
List of Figures ix
List of Tables xiii
Acronyms and Abbreviations xiv
Chapter 1 Introduction 1
1.1 Overview of antenna arrays 2
1.2 Overview of MC-CDMA and OFDM systems 3
1.3 Problem statement and literature survey 7
1.4 Objective and Organization of this dissertation 11
Chapter 2 Adaptive Linearly Constrained Inverse QRD-RLS Algorithm for Jammers Suppression and LCMV Filtering 14
2.1 Introduction 14
2.2 Linearly constrained inverse QRD-RLS beamformer 15
2.2.1 Formulation of antenna array 15
2.2.2 Optimal solution of LC-IQRD-RLS beamformer 17
2.2.3 Recursive implementation of LC-IQRD-RLS algorithm 18
2.3 Geometrical interpretation of constraint drift 22
2.4 Systolic array implementation and complexity analysis 23
2.4.1 Systolic array implementation 25
2.4.2 Complexities of adaptation gain for RLS-based algorithms 26
2.5 Simulation results 28
2.5.1 Spatial beamformer for jammers suppression 28
2.5.2 Temporal LCMV filtering 31
2.6 Summary 33
Chapter 3 Derivative Constraint Narrowband Beamformer for Wideband and Coherent Jammers Suppression 43
3.1 Introduction 43
3.2 Derivative constraint narrowband beamformer 44
3.3 New IQML beamforming algorithm 47
3.4 Implementation issues and remarks 51
3.4.1 Skew-symmetric matrix 51
3.4.2 Alternative approach of IQML algorithm with linear constraint 52
3.5 Simulation results 53
3.5.1 Wideband jammer 54
3.5.2 Coherent jammer 56
3.6 Summary 58
Chapter 4 Adaptive MC-CDMA Receiver with Constrained Constant Modulus IQRD-RLS Algorithm 63
4.1 Introduction 63
4.2 System model 65
4.3 LCCM IQRD-RLS algorithm for MC-CDMA detector 68
4.3.1 Unconstrained CM with IQRD-RLS algorithm 68
4.3.2 Direct linearly constrained CM with IQRD-RLS algorithm 70
4.3.3 Robust LCCM IQRD-RLS algorithm 72
4.4 Performance analysis of output SINR 76
4.5 Simulation results 80
4.5.1 Case without channel mismatch 81
4.5.2 Case with channel mismatch 83
4.6 Summary 85
Chapter 5 Performance of Space-Time MC-CDMA Receiver with Adaptive Linearly Constrained Constant Modulus Algorithm 92
5.1 Introduction 92
5.2 Problem formulation and space-time system model 93
5.3 Adaptive implementation of normalized LCCM-gradient algorithm 98
5.4 Performance analysis of BER 100
5.5 Simulation results 105
5.6 Summary 109
Chapter 6 Frequency Synchronization for IEEE 802.11 a/g OFDM-based WLAN 114
6.1 Introduction 114
6.2 ML estimation for carrier frequency offset 115
6.3 Phase locked loop 122
6.3.1 Phase detector 123
6.3.2 Design of loop filter 125
6.4 Simulation results 128
6.5 Summary 133
Chapter 7 Concluding Remarks 139
Appendix A Derivation of recursive equation and implementation of Givens rotation for inverse QRD-RLS based algorithm 143
Appendix B Proof of mirror effect in blind adaptation algorithm 146
Appendix C Proof of correlation ergodic in mean-squared error sense 149
Appendix D Derivation of MAI suppression capability for LCCM detector 152
Appendix E Optimal weights of LCCM detector 156
Bibliography 157
Publication List 164
Vita 166
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