臺灣博碩士論文加值系統

摘要：
正交分頻多工系統(OFDM)將傳送的資料分配於正交的子載波上，達到高速率無線傳輸，並能有效降低經無線多路徑傳輸通道所造成ISI效應。然而OFDM有個主要的缺點為其峰值對平均功率的比值(PAPR)較高。所以當傳輸訊號經由非線性裝置時會造成嚴重失真，如傳輸端的功率放大器。
本研究將探討SLM和PTS等降低PAPR的方法。雖然這些方法可以有效的減低PAPR，但所付出的代價就是增加系統的複雜度，這是因為它們使用過多的IFFT所造成的運算複雜度太高。因此我們提出一個新的方法用IFFT Conversion來解決複雜度的問題，也就是用IFFT Conversion來取代IFFT。由於IFFT Conversion是利用IFFT輸入和輸出之間的轉換關係在特殊的情形下所求得，因此複雜度比IFFT少很多，如此一來將使得SLM和PTS等方法的複雜度大為降低，同時也能有效的改善PAPR。
第一章簡介：
正交分頻多工(OFDM)是一種多載波的傳輸技術，且具有高速傳輸的能力，在服務要求日漸多樣化的無線傳輸中是絕佳的選擇之一。它的方法主要是將欲傳送的訊息以頻率分割多工(FDM)的方式平均分配於許多次載波上。然後每個次載波彼此相互重曡，再經由FFT使其正交；因為每個次載波的中心頻率點的位置上，其它次載波的強度均為零，而不會互相干擾，如此其頻譜可達最佳的使用效率。
然而，OFDM有個很大問題為峰值對平均功率的比值(PAPR)過高，如此會使訊號對於非線性裝置的失真。因此本篇論文主要的目的就是討論幾種可以降低PAPR的方法，並針對其中的MSR方法缺點，提出新的技巧加以改良。最後我們提出的IFFT Conversion這個方法可以有效的降低系統的複雜度。
第二章OFDM基本原理：
在本章中，我們介紹OFDM系統的原理及其歷史的發展，並討論連續時間和離散時間下的系統模型。接下來也研究OFDM系統的優點，也就是在每個OFDM符元的前端加上一段保護區間，其保護區間主要是由複製一段OFDM訊號而來，如此一來就可避免符際干擾(ISI)和子通道干擾(ICI)，使得OFDM能克服多重路徑衰減。
第三章峰值功率的問題：
由於OFDM訊號是由很多獨立的次載波線性組合而成，當N個同相位的訊號被加在一起，則產生的峰值功率為平均功率的N倍。因此，一般OFDM系統具有很高的PAPR，這對類比/數位、數位/類比轉換器來說，必須有足夠的位元來計算峰值振幅，而且發射端功率放大器必須在整個振幅範圍保持線性放大，這將導致成本提高及高功率消耗。因此在本章中我們討論這些問題，及PAPR的計算。
第四章降低PAPR的方法：
在本章中，我們介紹幾個降低PAPR的方法，同時對於其中的MSR方法進行模擬，最後並了解其中的優劣。降低PAPR的方法主要可分為三種：signal distortion，coding，multiple signal representation (MSR)。Clipping是一個簡單的方法，但會造成訊號的失真；Coding可以有效的降低PAPR，但卻受限於次載波的個數；MSR的技巧可以提供很好的性能，但系統的複雜度卻太高。因此如何改善其中的缺點，將在下一章討論。
第五章以MSR和IFFT Conversion來降低PAPR：
在本章中，我們主要針對其中一個降低PAPR的方法為MSR，由於MSR雖可有效的降低PAPR，但卻因為過多的IFFT而增加系統的複雜度。因此我們提出一個新方法為IFFT Conversion來改善上述的缺點，也就是利用IFFT Conversion低複雜度的優點來取代IFFT，並藉由此方法來結合SLM和PTS而達成更好的性能，如此一個可以有效降低PAPR，且系統複雜度低的方法得以實現。
第六章結論：
正交分頻多工是一種多載波傳輸技術，具有高速傳輸的能力，在服務要求日漸多樣化的線傳輸中，OFDM傳輸技術是絕佳的選擇之一。然而它有一個嚴重缺點為有很高的峰值對平均功率的比值(PAPR)。本篇論文主要是利用SLM和PTS的技巧來降低PAPR，由於SLM和PTS的方法使用許多IFFT造成系統複雜度太高，因此我們使用低複雜度的IFFT Conversion來取代IFFT這個技巧而改善系統的複雜度，使得MSR的方法得以完善。

Abstract
Orthogonal frequency division multiplexing (OFDM) systems support high data rate wireless transmission using orthogonal frequency channels, and can effectively take care of the inter-symbol interference induced by multipath propagation over radio channels. One of the major drawbacks associated with OFDM is the high peak-to-average power ratio (PAPR), which can result in significant distortion when the signal is transmitted through a nonlinear device, such as a transmitter power amplifier.
In this thesis, some main PAPR reduction methods are reviewed, including the selective mapping (SLM) approach and the partial transmit sequences (PTS) approach. Although the SLM and PTS techniques are very efficient in reducing PAPR, they would involve high system complexity, where input data sequence and several parallel IFFTs are required. To alleviate this problem, we propose an IFFT conversion method that can generate a set of different IFFTs from a computed IFFT. Since the proposed IFFT conversion method doest not involve any multiplication, it has much lower complexity than the IFFT compotation. With this IFFT conversion method, the complexity of the SLM and PTS techniques can be reduced significantly.

Contents
Abstract i
Contents iiii
List of Figures v
List of Tables vii
1 Introduction 1
2 OFDM Basis 5
2.1 History of OFDM………………………………………………………….7
2.2 OFDM Contiuous-Time Model 8
2.3 OFDM Discrete-Time Model 11
2.4 Guard Interval and Cyclic Prefix 12
3 The Peak Power Problem in OFDM 16
3.1 Introduction 16
3.2 PAPR in OFDM Systems 17
3.3 Distribution of the PAPR 20
4 PAPR Reduction Techniques 24
4.1 Introduction………………………………………………………………24
4.2 Singal Distortion Techniques……………………….……………………25
4.3 Coding Techniques…………………………………………………........31
4.4 Multiple Signal Representation Techniques……………………………..33
4.4.1 Selected Mapping……………………………………………………………35
4.4.2 Partial Transmite Sequence………………………………………………….38
4.4.3 Random Interleaving………………………………………………………...42
4.5 Summary……………………………………………………………..…..44
5 PAPR Reduction Using MSR and IFFT Conversion 45
5.1 Introduction………………………………………………………………45
5.2 PAPR Reduction Using MSR and IFFT Conversion.……………………46
5.2.1 IFFT Conversion .…...……………………………………………………….46
5.2.2 SLM and IFFT Conversion .…………………………………………………48
5.2.3 Memory ………………...……………………………………………………52
5.2.4 PTS and IFFT Conversion ..…………………………………………………53
5.3 Summary....………………………………………………………………57
6 Conclusions 59
References 61

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