臺灣博碩士論文加值系統

在正交分頻多工(OFDM)系統中，由於信號存在著非常高的平均必v峰值比(PAPR)，使得它對必v放大器的非線性特性有著高敏感度。預失真技術可以用來處理非線性的問題，如此一來其在頻帶內所造成的失真及頻譜增生效應便可減輕。在一般的技巧中，不論是估測必v放大器的特性或是設計預失真器通常是利用無記憶性的多項式模型，並從信號在時域上的特性，利用最小平方估測或是適應性演算法來找出多項式之係數。然而，迴授電路中濾波器引起的延遲時間若不是取樣時間的整數倍，則從時域補償延遲的效應就變得很複雜。在此論文中，我們從頻域上探討這個問題，並提出五種演算法。其中，使用了兩種估計的準則，一項是「必v大放器反函數特性估測之平方誤差最小化」，另一項為「介於預失真器輸入及必v放大器輸出平方誤差之最小化」。此外，我們也提出一個補償延遲效應的簡單方法。最後，所提出的方法會運用到一個64-QAM的OFDM系統，其模擬結果也會呈現出來。

OFDM signal is very sensitive to the nonlinear distortion mainly caused by a power amplifier (PA) as a result of its high peak-to-average power ratio (PAPR). Predistortion, an effective countermeasure for balancing off the nonlinearity of a PA, is usually necessary for the sake of mitigating in-band distortion and spectrum regrowth. In the conventional schemes, a memoryless polynomial is generally used in modelling the PA characteristic or designing the predistorter. The polynomial coefficients are solved by least square (LS) estimation or adaptive identification algorithms in the time domain.
However, most of the time domain schemes are not easy to implement owing to the
practical difficulty in compensating the delay effect caused by the transmission filter and the receiving filter in the feedback path. In this thesis we examine this issue from
frequency domain and propose five predistortion schemes. Two different criteria are used in the proposed algorithms. The first one is based on the minimization of the square error of the PA input, termed PA-Input-LS criterion. The second one is based on the minimization of the square error between the input of the predistorter and the output of the PA, termed PA-output-LS criterion. We also propose an easy method to cope with the delay effect. Finally, the simulation of application of the proposed schemes to a 64-QAM OFDM system is presented.

Abstract iii
Contents v
List of Figures vii
1 Introduction 1
2 System Model 4
2.1 Introduction to OFDM . . . . . . . . . . . . . . . . . . . . . . . . . .4
2.1.1 OFDM Signals . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 OFDM system Architecture . . . . . . . . . . . . . . . . . . . . . . .7
2.1.3 PAPR Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.2 Power Amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1 Description of Memoryless Nonlinearity . . . . . . . . . . . . . . . .15
2.2.2 Introduction to PA Model . . . . . . . . . . . . . . . . . . . . . . .17
2.2.3 Effect of Nonlinearity . . . . . . . . . . . . . . . . . . . . . . . .21
2.3 Predistorter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
2.3.1 The Concept of Predistortion . . . . . . . . . . . . . . . . . . . . .24
2.3.2 LUT-Based Predistorter . . . . . . . . . . . . . . . . . . . . . . . .26
2.3.3 Polynomial-Based Predistorter . . . . . . . . . . . . . . . . . . . . 28
3 Proposed Schemes 32
3.1 Zero-Forcing Algorithm . . . . . . . . . . . . . . . . . . . . . . . . .32
3.2 PA-Input-LS Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.3 Adaptive PA-Input-LS Algorithm . . . . . . . . . . . . . . . . . . . . .36
3.4 Adaptive Finite Difference Algorithm . . . . . . . . . . . . . . . . . .37
3.5 Adaptive Secant Algorithm . . . . . . . . . . . . . . . . . . . . . . . 40
3.6 Timing Estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4 Simulation Results 45
4.1 Zero-Forcing Algorithm and PA-Input-LS Algorithm . . . . . . . . . . . .46
4.2 Adaptive PA-Input-LS Algorithm . . . . . . . . . . . . . . . . . . . . .49
4.3 Adaptive Finite Difference Algorithm and Adaptive Secant Algorithm . . .50
4.4 Performance Evaluation . . . . . . . . . . . . . . . . . . . . . . . . .54
5 Conclusion 56

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