本篇論文研究了如何使用子空間(Subspace)分解的方法對正交分頻多重存取(Orthogonal Frequency-Division Multiple Access, OFDMA)系統中上傳端(Uplink)進行半盲式通道估測(Semi-Blind Channel Estimation)。我們利用空載波(Virtual Carriers, VCs)與循環字首(Cyclic Prefix, CP)造成的雜訊子空間與訊號子空間正交以及不同用戶使用不同子載波的特性來估測與鑑別出每個用戶所經過的通道。在正交分頻多重存取系統中，不一定所有用戶都會同時傳輸，我們也利用此特性，將屬於沒有傳輸的用戶的子載波當成空載波來幫助通道估測。此外，本篇論文也證明了子空間半盲式通道估測演算法用於正交分頻多重存取系統的充分條件。
另外我們提出了一種新的架構來提升半盲式通道估測演算法的效能，此架構稱為空載波回覆(Virtual Carriers Recovery, VCR)系統。主要是在接收端將受到雜訊干擾的空載波回復為零來抑制雜訊對通道估測的影響。由分析及模擬結果可以看出在系統不是滿載(Full-Loaded)或低訊雜比(Signal-to-Noise Ration, SNR)的情況下，空載波回覆系統改善的效果會特別明顯。此外空載波回覆系統也有助於減少半盲式通道估測所需要收集的資料，降低估測的複雜度。

This thesis investigates the semi-blind channel estimation in uplink (UL) of Orthogonal Frequency Division Multiple Access (OFDMA) systems based on subspace decomposition. We exploit the orthogonality between signal subspace and noise subspace induced by virtual carriers (VCs) and cyclic prefix (CP) and the property of that the exclusive sub-carriers set is assigned to each user to estimate and identify the channels for each user individually. In OFDMA systems, when some users don’t communicate with base station, the sub-carriers of non-active user provide extra redundancy for channel estimate to enhance the accuracy of channel estimation. Furthermore, the sufficient channel identifiability condition is developed.
Furthermore, a novel scheme, called as virtual carriers recovery (VCR) scheme, is proposed to improve the performance of the subspace-based channel estimation method. It suppresses the noise interference by recovering the VCs to zeros at receiver. The simulation results illustrate that the enhancement of VCR scheme is particularly apparent for the partially loaded OFDMA system at low signal to noise ratio (SNR). In addition, the VCR scheme increases the convergence rate of the subspace-base semi-blind channel estimation.

Chapter 1 Introduction 1
1.1 Introduction of OFDM 1
1.2 Introduction of OFDMA 1
1.3 Literature Review 2
1.4 Introduction of Proposed Method 3
1.5 The Structure of Thesis 4
1.6 Notation 4
Chapter 2 System Model 6
2.1 OFDM System Model 6
2.2 OFDMA System Model 10
Chapter 3 Introduction of Subspace Decomposition Approach 16
3.1 The Subspace Principle 16
3.2 Subspace Semi-Blind Channel Estimation in OFDM System 18
Chapter 4 The Proposed Subspace Semi-Blind Channel Estimator Used in OFDMA UL 24
4.1 The Conditions for Identifiability 24
4.2 The Subspace Semi-Blind Channel Estimation in OFDMA UL 25
4.3 Resolving the Scalar Ambiguity 27
Chapter 5 Proposed Virtual Carrier Recovery Scheme 29
5.1 The Structure of Virtual Carrier Recovery Scheme 29
5.2 Analysis of the Noise Reduction of VCR scheme 31
Chapter 6 Simulation Results 33
6.1 The NMSE Performance of the Proposed Channel Estimator in OFDMA UL 34
6.2 The Performance Improvement by using VCR Scheme 35
6.3 Estimator’s NMSE Dependence on Data 38
Chapter 7 Conclusions and Future Work 40
7.1 Conclusion 40
7.2 Future Work 40
Abbreviation 42
References 45

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