臺灣博碩士論文加值系統

正交分頻多工系統最具有吸引力多載波調變機制方法之一，且具有很高頻譜效率及強力對抗多路徑衰減，即使有正交分頻多工系統的很多優勢，它有兩個主要缺點︰峰均值功率比和頻率的偏移問題。為了克服這些峰均值功率比問題，不同的方法已經被提出降低正交分頻多工系統的峰均值功率比問題。這些技術可被分成兩個種類︰信號攪拌和信號失真技術。在論文裡我們透過結合使用WHT和壓縮方法來降低OFDM傳送信號系統的峰均值功率比。首先WHT用來分解OFDM輸入序列的相互間關係。另一方面，既然正交分頻多工系統信號類似語音信號特性；發生大的信號機率是非常小，壓縮方法適用於這些信號可以用來改進OFDM傳輸性能。最後我們透過使用Monte Carlo方法模擬證實我們的分析的結果。

The orthogonal frequency division multiplexing (OFDM) technique is one of the most attractive multicarrier modulation schemes for its high bandwidth efficiency and strong immunity to multipath fading. Even though there are many advantages of OFDM, it has two main drawbacks: high Peak to Average Power Ratio (PAPR) and frequency offset. To overcome these PAPR probblems, different methods have been proposed to mitigate the PAPR problem of OFDM. These techniques can be divided into two categories: Signal scrambling and Signal distortion technique. In the thesis, we study the PAPR reduction of OFDM transmission system by jointly using both Walsh-Hadamard transform and companding technique. First, Hadamard transform is used to decompose the correlation relationship of OFDM input sequence. On the other hand, since OFDM signal is similar to speech signals in the sense that large signals occur very infrequently, the companding technique usually applying to these signals might be used to improve OFDM transmission performance. Finally, we verify our analytical results by using Monte Carlo simulation.

Contents

Abstract in Chinese i
Abstract in English ii
Acknowledgment iv
Contents v
List of Figures and Tables viii
Chapter 1 Introduction 1
Chapter 2 Basic Principles 4
2.1 OFDM Systems 4
2.2 Formulation of PAPR Problems 10
2.3 The Peak Power Problem in OFDM systems 12
2.3.1 Linearity for Power Amplifies 13
2.3.2 Clipping and Filtering 13
2.3.3 The Block Coding Scheme 15
2.3.4 The Selective Scrambling Scheme 15
2.3.5 The M sequence Scheme 16
2.3.6 The Peak Windowing Scheme 16
Chapter 3 Walsh-Hadamard Transform & Companding Technique 18
3.1 Walsh-Hadamard Transform (WHT) for PAPR Reduction in OFDM Transmission 20
3.1.1 Autocorrelation Characteristic for IFFT Inputs 21
3.1.2 The Probability Distribution of IFFT Outputs 22
3.1.3 The Effects of the Walsh-Hadamard Transform 23
3.2 PAPR Reduction in the OFDM Transmission using A Companding Technique 24
3.2.1 The Companding technique ( -law Companding ) 24
3.2.2 A Theoretical Analysis 24
3.3 PAPR Reduction in OFDM Transmission by Using WHT & Companding Techniques 32
Chapter 4 Simulation Result 34
4.1 OFDM Waveform and Distribution of Signal Amplitude 34
4.2 PAPR Reduction 37
4.2.1 PAPR Reduction by Using WHT 37
4.2.2 PAPR Reduction by Using the Companding Technique 39
4.2.3 PAPR Reduction by Using the Joint Scheme of WHT & Companding Techniques 41
4.3 BER Performance 44
Chapter 5 Conclusions 47
References 48
List of Figures and Tables

Fig.2.1 Block diagram of typical OFDM system 5
Fig.2.2 Discrete-time signals at the output of the P/S convert 7
Fig.2.3 Continuous-time signals at the output of the D/A convert 7
Fig.2.4 In-phase and quardrature-phase modulator 8
Fig.2.5 In-phase and quardrature-phase modulator detector 9
Fig.2.6 The curve of a non-ideal PA and the 1-dB compression point 13
Fig.3.1 Block Diagram of OFDM system with Walsh-Hadamard Transform 19
Fig.3.2 The autocorrelation function 23
Fig.3.3 -law companding curve 24
Fig.3.4 Block Diagram of OFDM system with companding technique 25
Fig.3.5 Block Diagram of OFDM system with WHT & companding 33
Fig.4.1 QPSK-OFDM signals with for various PAPR reduction scheme 35
Fig.4.2 Amplitude histograms of QPSK-OFDM Signals with N=64 36
Fig.4.3 PAPR distribution of OFDM with and without Walsh-Hadamard transform 39
Fig.4.4 PAPR distribution of OFDM with and without -law companding 40
Fig.4.5 PAPR distribution of OFDM with companding & WHT 43
Fig.4.6 Bit error rates performance on an AWGN channel 46

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