臺灣博碩士論文加值系統

正交性頻率分割調變 (OFDM) 是一種能有效地減輕由無線通道所造成的
損害的一種大有可為的多載波調變技術。而它已經被很多無線寬頻通訊
標準所採用，例如 IEEE 802.11a 和HIPERLAN/2。在某些標準中 (如 IEEE 802.11a) ，框化被使用來減輕對頻率偏移和相位雜訊的敏感度，
同時也能保存頻寬。然而，框化卻導致副載波之間正交性的喪失。除此
之外，假設一個OFDM的系統沒有適當地設計好的話，OFDM方塊之間的干擾 (IBI) 所引發的正交性喪失也會發生。
在這篇論文中，我們在接收機端提出了一個決策迴授正交回復濾波器 (DFORF) 來回復副載波之間的正交性。一個新的通道內插演算法也會被
提出。使用 IEEE 802.11a參數的模擬結果證明利用所提出的演算法，
明顯的成果將能夠被達成。

Orthogonal Frequency Division Multiplexing (OFDM) is a promising multi-carrier modulation technique that can effectively mitigate impairments caused by wireless communication channels. OFDM has been adopted in many wireless broadband communication standards, such as IEEE 802.11a and HIPERLAN/2. In some OFDM standards (e.g., IEEE 802.11a), windowing is employed to reduce sensitivity to frequency offset and phase noise, and at the same time conserve bandwidth. However, windowing causes the loss of orthogonality between subcarriers. Furthermore, loss of orthogonality also happens due to inter-OFDM-block interference (IBI) if an OFDM system is not designed properly.
In this thesis, we propose a decision feedback orthogonality restoration filter (DFORF) for restoring subcarrier orthogonality at the receiver. A novel channel interpolation algorithm is also proposed. Simulation results using IEEE 802.11a parameters show that with the proposed algorithm, significant performance gain can be achieved.

Chapter 1 Introduction……………………………………………………...1
Chapter 2 OFDM Basics……………………………………………………5
2.1 Modulation by IFFT/FFT……………………………………………………...5
2.2 Cyclic Prefix…………………………………………………………………...8
2.3 Windowing…………………………………………………………………...10
2.4 Equalization………………………………………………………………..14
2.5 Summary……………………………………………………………………..15
Chapter 3 IEEE 802.11a Physical Layer Specifications……..17
3.1 Physical Layer Convergence Procedure Frame Format……………………...17
3.2 PLCP preamble (SYNC)……………………………………………………..19
3.3 Signal field (SIGNAL)……………………………………………………….21
3.4 DATA field…………………………………………………………………..22
3.4.1 Service field, Tail, and Pad bits………………………………………..22
3.4.2 Scrambler and convolutional encoder………………………………….23
3.4.3 Data interleaving and subcarrier mapping……………………………...25
3.4.4 Pilot subcarriers and OFDM modulation………………………………26
3.5 Summary……………………………………………………………………..28
Chapter 4 System Model and Problem Formulation………….31
4.1 System Model………………………………………………………………...31
4.1.1 Choice of OFDM Parameters…………………………………………..31
4.1.2 Wireless Channel Model……………………………………………….34
4.2 Problem Formulation…………………………………………………………42
4.2.1 Orthogonality Loss Due to Windowing………………………………..42
4.2.2 Orthogonality Loss Due to Inter-Block Interference…………………..45
4.3 Summary……………………………………………………………………..47
Chapter 5 Proposed Receiver…………………………………………...49
5.1 Orthogonality Restoration Filter……………………………………………..49
5.2 Decision Feedback Orthogonality Restoration Filter………………………...53
5.3 Channel Estimation and Interpolation………………………………………..58
5.3.1 Channel Estimation…………………………………………………….59
5.3.2 Channel Interpolation…………………………………………………..61
5.4 Channel Equalization………………………………………………………...63
5.5 Simulation Results……………………………………………………………63
5.5.1 Time-Invariant Channel With Perfect Channel Estimation……………64
5.5.2 Time-Varying Channel With Channel Estimation And Interpolation….66
5.6 Summary……………………………………………………………………..76
Chapter 6 Conclusion………………………………………………………77
Bibliography……………………………………………………………....79

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[2] ETSI Web URL: http://www.etsi.org
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