臺灣博碩士論文加值系統

為了無線數據通訊快速增加的需求，正交分頻多工（OFDM）技術在近幾年中致力地被發展。1998年七月，電子電機工程師協會（IEEE）標準化小組決定選用正交分頻多工技術來作為新5-GHz頻帶的無線區域網路標準（WLAN）。這個新標準是首次使用正交分頻多工的技術以作為封包形式的通訊。在此之前，OFDM技術的使用侷限於連續傳輸系統。
本論文中將描述OFDM基本原理、同步錯誤的影響和IEEE 802.11a標準中的重要參數。此外，提出一個符合WLAN環境的preamble接收機。它提供平滑的演算法抵抗時域散亂通道，並且利用short preamble以獲取載波頻率偏移。Short preamble較短的週期有利於對抗多路徑環境，也因此幫助系統在long preamble中作通道預估。 儘管在802.11a標準中提議short preamble作為初步的頻率偏移預估。

For the rapidly increasing requirements of wireless data communications, the orthogonal frequency division multiplexing (OFDM) techniques have been heavily developed in recent years. In July 1998, the IEEE standardization group decided to select OFDM as the basic multiplexing technique for their new 5-GHz wireless local area network (WLAN) standard, and this new standard is the first one to use OFDM in packet-based communications, while the use of OFDM was limited to continuous transmission systems before.
In this thesis, basic OFDM principles, the effects of synchronization errors, and important parameters of IEEE 802.11a are described. Besides, a preamble receiver is proposed for the wireless LAN. It provides the smoothing algorithm to resist time-dispersive channels and uses the short preamble for the carrier frequency offset acquisition. The short preamble format, which has a shorter period, is more robust to the delay spread and helpful for the channel estimation in long preamble, even though the short preamble is used for a coarse frequency offset estimation in IEEE 802.11a proposal.

1 Introduction
1.1 OFDM history……………………………………………………1
1.2 The Standard IEEE 802.11a……………………………………5
1.3 Content of the thesis…………………………………………7
2 OFDM Fundamentals
2.1 OFDM Continuous-time Model…………………………………9
2.2 OFDM Discrete-time Model……………………………………10
2.3 Guard Interval and Cyclic Prefix…………………………11
2.4 Typical Baseband Model of OFDM Systems…………………13
2.4.1 Baseband OFDM transmitter structure………………13
2.4.2 Baseband OFDM receiver structure……………………16
3 Physical Layer Convergence Procedure of 802.11a
3.1 Overview of the PPDU Encoding Process……………………18
3.2 PLCP Preamble……………………………………………………22
3.3 Signal Field……………………………………………………23
3.4 DATA Field………………………………………………………25
3.4.1 PLCP DATA scrambler and descrambler………………26
3.4.2 Convolutional encoder…………………………………26
3.4.3 Data interleaving………………………………………28
3.4.4 Subcarrier modulation mapping………………………28
3.4.5 Pilot subcarriers………………………………………29
3.5 Other Important PMD Regulation……………………………31
4 Synchronization & Channel Equalization
4.1 Effect of the Frequency and Timing Offset………………33
4.2 Synchronization Scheme………………………………………37
4.2.1 Synchronization in time domain………………………38
4.2.2 Synchronization in frequency domain…………………41
4.3 Channel Equalization……………………………………………45
5 OFDM Preamble Receiver Design
5.1 Clipping and Filter………………………………………………47
5.2 Symbol Timing Recovery……………………………………………51
5.3 Carrier Frequency Offset Acquisition…………………………58
5.4 Conclusion……………………………………………………………63
Appendix A…………………………………………………………………64

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