臺灣博碩士論文加值系統

正交多頻分工（OFDM）係利用多載波來傳送資料，資料被分散到許多的子載波，就像平行傳輸，而非單一載波傳輸。每一個子載波都以低傳輸速率傳送小量資料，因此，OFDM就像是平行資料傳輸及分頻多工等技術的結合。OFDM可以增強對抗頻率選擇性衰減及多重路徑傳播干擾的能力。
OFDM在歐洲、日本及台灣已廣泛應用在無線數位電視地面廣播，此外，OFDM也應用在高速率之電話線路（例如ADSL）、無線區域網路(例如IEEE802.11a/g)等。
OFDM擁有許多優點，但存在峰值對平均功率比（PAPR）很高的問題。這是由於多載波訊號的合成會產生比平均訊號位準高出許多的峰值訊號，導致高PAPR的嚴重缺點。
高PAPR將導致訊號非線性失真、接收資料錯誤率增加及鄰頻干擾等嚴重問題，因此必須使用高階之發射機、高解析之類比數位（A/D）及數位類比（D/A）轉換器等方法來克服這些問題，相對導致設備及營運成本的增加。
本研究目的在於探討如何利用離散餘弦轉換（DCT）降低PAPR，以改善OFDM應用成本。

OFDM (Orthogonal Frequency Division Multiplexing) is using multi-carries to transmit data. The transmitted data are separated into many sub-carriers as parallel transmission, instead of one carrier. Only a small amount of the data is transported on each sub-carrier, and by this lowering of the bit rate per sub-carrier. OFDM can be seen as a parallel data transmission and frequency multiplexing techniques (FDM). The approach can increase robustness against frequency selective fading and multipath immunity.
OFDM is used for Terrestrial Digital TV broadcasting in Europe, Japan and Taiwan. In addition, OFDM is used in high-speed telephone line communications such as ADSL, and wireless LAN such as IEEE802.11a/g.
Although OFDM has many advantages, it has a problem which is high Peak-to-average power ratio (PAPR). OFDM has high peak more than the average signal level because OFDM is composed of thousands of orthogonal waves. It is a serious defect.
The defect results in these serious problems such as nonlinear distortion and increases bit error rate (BER) of receiver, next-channel Interference, etc. We must use a high-level transmitter, an high-resolution A/D and D/A converter, etc. to overcome these defects. It will increase equipment and business capital.
The main purpose of this thesis is to discuss how to use discrete cosine transform (DCT) to reduce PAPR and improve application capital of OFDM.

ACKNOWLEDGMENT i
ABSTRACT ii
中文摘要 iii
TABLES OF CONTENTS iv
LIST OF FIGURES vi
Chapter 1 Introduction 1
1.1 Motivation 1
1.2 Literature Review 1
1.3 Scope of the Present Study 2
Chapter 2 Overview of OFDM 3
2.1 History of OFDM 3
2.2 Description of OFDM System 4
2.3 Characteristics of OFDM 7
2.4 PAPR problem 8
Chapter 3 PAPR Reduction Techniques 12
3.1 Signal Distortion Techniques 12
3.1.1 Clipping 12
3.1.2 Peak Windowing 13
3.1.3 Peak Cancellation 13
3.2 Coding Techniques 14
3.2.1 Block Coding 14
3.2.2 M Sequences 14
3.2.3 Golay Complementary Codes 15
3.3 Multiple Signal Representation 16
3.3.1 Selected Mapping (SLM) 16
3.3.2 Partial Transmit Sequence (PTS) 18
3.3.3 Random Interleaving (RI) 20
3.4 Discrete Cosine Transform (DCT) 22
Chapter 4 Results and Discussion 27
4.1 PAPR Reduction using DCT by QPSK 27
4.2 PAPR Reduction using DCT by 16-QAM 29
4.3 PAPR Reduction using DCT by 64-QAM 32
4.4 The PAPR Reduction Performance of DCT Compare with MSR 34
Chapter 5 Conclusions 35
Chapter 6 Recommendation for Future Studies 36
REFERENCES 37




LIST OF FIGURES

Fig. 2.1 Block diagram of an OFDM system 5
Fig. 2.2 The QPSK, 16-QAM and 64-QAM mappings 6
Fig. 2.3 The spectrum of OFDM 7
Fig. 2.4 OFDM Cyclic Prefix 8
Fig. 2.5 The reason of causing peak 9
Fig. 2.6 A large-PAPR signal 9
Fig. 2.7 A small-PAPR signal 10
Fig. 3.1 Block diagram of Selected Mapping 17
Fig. 3.2 PAPR reduction performance of using SLM 17
Fig. 3.3 Block diagram of Partial Transmit Sequence 18
Fig. 3.4 PAPR reduction performance of using PTS 19
Fig. 3.5 Block diagram of Random Interleaving 21
Fig. 3.6 PAPR reduction performance of using RI 21
Fig. 3.7 The OFDM system by using DCT 23
Fig. 3.8 The energy compaction property of DCT by using 16 the same 16-QAM symbols 24
Fig. 3.9 The energy compaction property of DCT by using 16 different 16-QAM symbols 26
Fig. 4.1 PAPR reduction performance (QPSK, 2K mode) 28
Fig. 4.2 Mapping symbols Error Rate (QPSK, 2K mode) 28
Fig. 4.3 PAPR reduction performance (QPSK, 8K mode) 29
Fig. 4.4 Mapping symbols Error Rate (QPSK, 8K mode) 29
Fig. 4.5 PAPR reduction performance (16-QAM, 2K mode) 30
Fig. 4.6 Mapping symbols Error Rate (16-QAM, 2K mode) 30
Fig. 4.7 PAPR reduction performance (16-QAM, 8K mode) 31
Fig. 4.8 Mapping symbols Error Rate (16-QAM, 8K mode) 31
Fig. 4.9 PAPR reduction performance (64-QAM, 2K mode) 32
Fig. 4.10 Mapping symbols Error Rate (64-QAM, 2K mode) 32
Fig. 4.11 PAPR reduction performance (64-QAM, 8K mode) 33
Fig. 4.12 Mapping symbols Error Rate (64-QAM, 8K mode) 33
Fig. 4.13 The comparison of PAPR reduction performance between DCT and MSR 34

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