臺灣博碩士論文加值系統

正交分頻多工調變技術(Orthogonal Frequency Division Multiplexing, OFDM)是一種相當可靠且適合在高速傳輸的調變技術。在OFDM系統上，我們探討的是一種在時變(time variance)的快速衰減通道上，使每個載波之間失去其正交性，而造成載波間的相互干擾(Inter-Carrier Interference, ICI)。
在論文中，我們使用了一種新的可以抑制ICI方法，也就是利用我們所推導的演算式子，在頻域上設計訊號，再經由反相快速傅利葉轉換(Inverse Fast Fourier Transform, IFFT)後，使其在時域上讓訊號成為對稱方式。在接收端，則利用本身的訊號對稱的性質，加上原本對稱訊號的反向訊號，即可降低在時變的快速衰減通道的影響。其優點：沒有繁雜的演算，較容易實現；缺點：浪費一半的頻寬做為對稱訊號。此OFDM系統的模擬，除了已用C、C++程式語言實現；最後，我們將用Verilog硬體描述語言(Verilog Hardware Description Language, VHDL)去實現OFDM系統架構。

Orthogonal frequency division multiplexing (OFDM) system is a modulation technique which is reliable and suitable for high data rate transmission. However, in fast-fading channels, the time variance of the channel leads to the loss of orthogonality among subcarriers of the OFDM system, and this causes the inter-carrier interference (ICI).
In this thesis, we first study the fundamental functions of the OFDM system. Then we propose a new method of ICI suppression that designs the signal in frequency domain such that the resulted time-domain signal is symmetric. At the receiver, the received signal and its reversal version are added to reduce time-varying effect of the channel. The proposed algorithm is of much low complexity and the performance shows it is effective in ICI cancellation. In addition, we can easily implement the system. The price of our algorithm is that it wastes a half of the bandwidth due to the design of the symmetric signal. We also implement the proposed method with Verilog hardware description language (VHDL) in the OFDM system.

Chinese Abstract I
English Abstract III
Acknowledgement IV
Contents V
List of Figure VII
Chapter 1 Introduction 1
Chapter 2 OFDM System and Channel Model 3
2.1 OFDM Overview 3
2.2 OFDM System Specification 4
2.2.1 Convolution Encoder/Viterbi Decoder 5
2.2.2 Interleaver/De-interleaver 6
2.2.3 OFDM Transmitter 7
2.2.4 Guard Interval (GI) Insertion 8
2.2.5 OFDM Receiver 9
2.3 Channel Model 10
2.3.1 AWGN 10
2.3.2 Rayleigh Fading 10
2.3.3 Inter-carrier Interference (ICI) 11
2.4 Channel Estimation and Equalizer 12
2.5 Simulation Results 16
Chapter 3 ICI Cancellation for OFDM Systems 20
3.1 Intercarrier Interference Self-Cancellation 21
3.2 ICI Cancellation by the Designed Symmetric Signal 26
Chapter 4 OFDM System with VerilogHDL Implementation 34
4.1 Implementation with VerilogHDL 35
4.1.1 Modulation 35
4.1.2 Designed Symmetric Signal 36
4.1.3 FFT/IFFT 38
4.1.4 Cyclic Prefix 40
4.1.5 AWGN 41
4.1.6 Interface 46
4.2 Simulation Results 46
Chapter 5 Conclusion 51

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