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研究生:石韻怡
研究生(外文):Yun-Yi Shih
論文名稱:IEEE802.11a無線區域網路之通道估測技術
論文名稱(外文):Channel Estimation Techniques for the IEEE 802.11a Wireless Local Area Network
指導教授:王晉良
指導教授(外文):Chin-Liang Wang
學位類別:碩士
校院名稱:國立清華大學
系所名稱:通訊工程研究所
學門:工程學門
學類:電資工程學類
論文種類:學術論文
論文出版年:2002
畢業學年度:90
語文別:英文
論文頁數:63
中文關鍵詞:無線區域網路通道估測
外文關鍵詞:IEEE 802.11aChannel estimation
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摘 要
無線區域網路是目前的一個重點發展領域,它擺脫了實體傳輸線的束縛,用戶在一定的距離以內,可以建立獨立的無線網路,不需移動網路線或尋找網路阜插座,擴大整個區域網路的資料傳輸範圍。
截至目前為止,已經有一些無線區域網路的規格被提出,其中,由IEEE規格制定協會所提出的802.11a 規格正受到大家的矚目。802.11a 的系統使用正交多工分頻技術,在5GHz 的頻帶上傳輸資料,有多種傳輸速率可以提供給用戶選擇,最大可以高達每秒54 M bits,大大的提昇了無線區域網路的傳輸速率。
無線傳輸的環境因為受到的干擾因素較多,所以傳輸通道往往比有線的傳輸通道來得難以讓人掌握,傳輸品質也相對的較差,因此,如何使通道干擾的效應減少,就成了一個提高傳輸品質的重要考量。通常我們都是先估測通道,再將通道的效應補償回來,利用這樣的方式來提高傳輸速率。在這篇論文中,我們針對802.11a 的規格,提出了兩個可實現的通道估測方法。一種是利用在頻域取平均的觀念,另一種則是利用已經解調的訊號,當作額外的已知訊號,增加在時域上取平均的數目,這兩種方法可以有效減少因高斯雜訊所產生的通道估測誤差,而估測出較正確的通道特性,進而提昇通訊品質。藉由模擬一些不同的通道環境,我們也比較了不同的通道估測技術在不同環境下的效能,做出分析與討論。

Abstract
Wireless local area networks (wireless LANs) are flexible data communication systems, which make us access the internet more unconstraint. Using radio frequency (RF) waves, wireless LANs transmit and receive data over the open space, minimizing the need for arranging wired cabling. With wireless LANs, users can acquire shared information without looking for a place to plug in, and network managers can set up or arrange networks without installing or moving wires.
Up to present, many institutes have created their standards of wireless LANs and one of the most popular is standardized by the IEEE. IEEE Societies and the Standards Coordinating Committees of the IEEE standards Association (IEEE-SA) Standards Board had developed a standard 802.11a for one of the implementations of wireless LANs. The 802.11a system is an OFDM-based wireless LAN, which can provide a data rate up to 54 Mbps in 5 GHz bands.
When implementing the 802.11a wireless LAN, one of the important components is the estimation of wireless channels. There exist many effects (multipath fading, time delay spread, and Doppler spread) in a wireless transmission environment where the channel is worse than a wired transmission environment. In order to achieve a higher data transmission rate within an acceptable bit error rate, a good channel estimator is necessary. In this thesis, we propose two practically achievable channel estimation algorithms for the 802.11a wireless LAN. One is called the average-in-frequency-domain (AIF) estimator, which uses the average concept in the frequency domain. The other is called the decision-aided-average-in-time-domain (DA-AIT) estimator, which uses the detected signals as “virtual” pilots and averages more individual pilot-estimated channel responses in the time domain. As compared to a widely used channel estimation method for the 802.11a wireless LAN, the proposed approaches have better performance in both additive white Gaussian noise channels and slowly time-variant channels with no significant frequency selective effects. However, each of these two proposed methods has its own benefits and limitations. The AIF estimator has less complexity but is not suitable for frequency selective channels. In contrast, the DA-AIT estimator is applicable to frequency selective channels, but it needs more computational complexity.

Contents
Abstract i
Contents iii
List of Figures v
List of Tables vii
Chapter 1 Introduction 1
1.1 Introduction to Wireless LAN 1
1.2 Wireless LAN Technology 2
1.3 Wireless LAN Configurations 3
1.4 Development of the IEEE 802.11 Specifications 4
1.5 Thesis Outline 4
Chapter 2 OFDM Basics 6
2.1 System Models 7
2.2.1 Continuous-Time Model 7
2.2.2 Discrete-Time Model 10
2.2 Guard Time and Cyclic Extension 11
2.3 Timing Errors 12
2.4 The Peak-to-Average Power Ratio 12
2.5 OFDM Advantages and Disadvantages 14
Chapter 3 Introduction to the IEEE 802.11a Standard 19
3.1 OFDM PHY Outline Description for the 802.11a 20
3.2 Mathematical Conventions in the Signal Descriptions21
3.3 The PLCP Preamble 23
3.4 The Convolutional Encoder 24
3.5 Data Interleaving 24
3.6 Subcarrier Modulation Mapping 26
3.7 OFDM Modulation 26
3.8 Pilot Subcarriers 27
Chapter 4 Channel Estimation 34
4.1 MMSE Estimation 35
4.2 Decision-Directed Channel Estimation 37
4.3 The Frequency-Domain Adaptive Filter (FDAF) 38
4.4 The Approach of Average in Time Domain (AIT) 39
4.5 The Approach of Average in Frequency Domain (AIF) 43
4.6 The Approach of Decision-Aided AIT (DA-AIT) 44
Chapter 5 Performance Evaluation 49
5.1 A Multipath Channel Model 50
5.2 Simulation Results 52
5.2.1 The Case of a Fixed Multipath Channel 52
5.2.2 The Case of a Time-Variant Multipath Channel 53
Chapter 6 Conclusions 59
References 61

References
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[4] The Institute of Electrical and Electronics Engineers (IEEE), Inc. “Part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications: high-speed physical layer in the 5 GHz Band”, LAN/MAN Standards Committee of the IEEE computer Society, Sep. 16, 1999.
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