Performanc Simulation of DS-CDMA and OFDM Combined in Radio Fading Channel 楊嘉豪、李金椿
E-mail: [email protected]
ABSTRACT
A DS-CDMA-OFDM system, which is a combination of DS-CDMA and OFDM technologies, is proposed to reduce the effect of Inter-symbol interference (ISI) caused by frequency selective fading radio channel and Multi-access interference (MAI). The energy efficiency of proposed system is investigated based on simulation, and the results is compared with a pure DS-CDMA having a RAKE receiver. In the proposed DS-CDMA-OFDM system, the message is spread by a spreading code, and follows by OFDM modulation. To keep the bandwidth of the OFDM sub-carrier smaller than the channel coherence bandwidth in high data
transmission rate, the number of the OFDM sub-carriers must be greater than the spreading factor (SF). From the simulation results, for single user situation, we find that the energy efficiency of DS-CDMA-OFDM is approximately the same as the pure DS-CDMA system with RAKE receivers. For the multiple-user situation, the performance of the DS-CDMA-OFDM system is better than the pure DS-CDMA system irrespective of the spread factor. In a BER of 10-2 and an SF of 32, the energy efficiency of the
DS-CDMA-OFDM system is at least 3 dB better than that of the pure DS-CDMA system, while at least 5.4 dB better for a BER of 10-3 and an SF of 64. We find that the MAI rejection capability of the DS-CDMA-OFDM system is proportional to the spread factor. In general, the proposed DS-CDMA-OFDM system is not only able to reduce the effect of ISI ability, but also to reject the MAI.
Keywords : OFDM、DS-CDMA、RAKE receiver、ISI、MAI
Table of Contents
封面內頁 簽名頁 授權書．．．．．．．．．．．．．．．．．．．．．．．．．iii 中文摘要．．．．．．．．．．．．
．．．．．．．．．．．．iv 英文摘要．．．．．．．．．．．．．．．．．．．．．．．．v 誌謝．．．．．．．．．
．．．．．．．．．．．．．．．．．vi 目錄．．．．．．．．．．．．．．．．．．．．．．．．．．vii 圖目錄．．．
．．．．．．．．．．．．．．．．．．．．．．ix 表目錄．．．．．．．．．．．．．．．．．．．．．．．．．xii 第 一章 緒論．．．．．．．．．．．．．．．．．．．．．．1 第二章 無線衰變通道．．．．．．．．．．．．．．．．．
．．4 2.1無線通道衰減形式．．．．．．．．．．．．．．．5 2.1.1路徑損失效應．．．．．．．．．．．．．． 6 2.1.2 遮蔽效應．．．．．．．．．．．．．．．． 7 2.1.3多重路徑效應．．．．．．．．．．．．．． 8 2.2無線通道衰變特性
．．．．．．．．．．．．．．．9 2.3多路徑通道之數學模型．．．．．．．．．．．． 12 2.4分集技術．．．．．．．
．．．．．．．．．．． 16 第三章 DS-CDMA與OFDM原理．．．．．．．．．．．18 3.1展頻技術原理．．．．．．．
．．．．．．．．． 18 3.1.1 DS-CDMA原理 ．．．．．．．．．．．． 22 3.1.2耙式接收器．．．．．．．．．．．．．
．． 24 3.2 OFDM原理．．．．．．．．．．．．．． ．． 28 第四章 DS-CDMA-OFDM系統．．．．．．．．．．．．
． ．41 4.1 DS-CDMA-OFDM系統．．．．．．．．．．．． 41 4.1.1 DS-CDMA-OFDM發射模型．．．．．．．．42 4.1.2 DS-CDMA-OFDM接收模型．．．．．．．．46 4.2 DS-CDMA-OFDM傳輸頻寬．．．．．．．．．． 48 第五章 系 統效能模擬 ．．．．．．．．．．．．．．．．．． 51 5.1搭配耙式接收機之CDMA系統模擬架構 ．．．． 51 5.2 DS-CDMA-OFDM系統模擬架構．．．．．．．． 52 5.3系統參數．．．．．．．．．．．．．．．．．． 53 5.4單用戶 之效能模擬結果．．．．．．．．．．．． 55 5.5多用戶干擾下之效能模擬結果．．．．．．．．．． 56 5.6不同載波數 目之效能模擬結果．．．．．．．．．． 59 第六章 結論．．．．．．．．．．．．．．．．．．．．．60 參考文獻．．
．．．．．．．．．．．．．．．．．．．．．61 圖 目 錄 圖 2.1 整體行動通訊簡略架構圖．．．．．．．．．．．．．
．4 圖 2.2 多路徑所造成的ISI現象示意圖．．．．．．．．．．．5 圖2.3 路徑損失示意圖．．．．．．．．．．．．．．
．．．6 圖 2.4 遮蔽效應簡易示意圖．．．．．．．．．．．．．．．7 圖2.5 多重路徑示意圖．．．．．．．．．．．．
．．．．．8 圖2.6 建設性和破壞性示意圖．．．．．．．．．．．．．．9 圖2.7 道衰變特性示意圖．．．．．．．．．
．．．．．．．10 圖2.8 頻率選擇性衰落．．．．．．．．．．．．．．．．．12 圖2.9 非頻率選擇性衰落．．．．．．
．．．．．．．．．．12 圖2.10簡單電波路徑的行進示意圖．．．．．．．．．．．．14 圖2.11 多路徑通道模型．．．．
．．．．．．．．．．．．．15 圖3.1 訊息經展頻技術所得效應：(a)未展頻信號頻譜 (b)展頻後信號頻譜(包含其他用戶) (c) 針對原用戶進行解展頻信號頻譜．．．．．．．．．．．．．．．．．．．．．．20 圖3.2 直接序列式展頻技術示意圖
：(a)調變 (b)解調變．．．21 圖3.3 單一載波DS-CDMA發射機結構．．．．．．．．．22 圖3.4 單一載波DS-CDMA接收機 結構．．．．．．．．．23 圖3.5 衰落呈現頻率選擇性衰落．．．．．．．．．．．．．24 圖3.6 耙式接收器架構．．．
．．．．．．．．．．．．．．25 圖3.7 無重疊(un-overlap spectral)性．．．．．．．．．．．29 圖3.8 重疊(overlap spectral)性．．．．．．．．．．．．．29 圖3.9 多載波調變結構．．．．．．．．．．．．．．．．．30 圖3.10多載波解 調變結構．．．．．．．．．．．．．．．31 圖3.11傳統FDM傳輸方式．．．．．．．．．．．．．．32 圖3.12接收方訊 息式示意圖．．．．．．．．．．．．．．32 圖3.13 OFDM傳輸頻譜圖．．．．．．．．．．．．．．．34 圖3.14利 用IDFT/DFT取代多OFDM調變/解調．．．．35 圖3.15 OFDM系統發射架構．．．．．．．．．．．．．．36 圖3.16每 一路傳輸頻譜．．．．．．．．．．．．．．．．．37 圖3.17 OFDM信號傳輸頻譜．．．．．．．．．．．．．．37 圖3.18通道對傳輸信號呈現選擇性頻率衰落示意圖．．．．．38 圖3.19 OFDM系統接收架構．．．．．．．．．．．．．
．39 圖4.1所提DS-CDMA-OFDM系統發射架構．．．．．．．43 圖4.2展頻後的頻譜．．．．．．．．．．．．．．．．
．．44 圖4.3每一個子載波信號所佔用的頻帶．．．．．．．．．．45 圖4.4 所提DS-CDMA-OFDM系統接收架構．．．
．．．．47 圖4.5 DS-CDMA-OFDM每一路傳輸頻譜．．．．．．．．48 圖4.6 DS-CDMA-OFDM信號傳輸頻譜．．．．
．．．．．49 圖4.7 通道對DS-CDMA-OFDM信號呈現平坦衰落．．．．49 圖5.1搭配耙式接收機之CDMA系統流程．．
．．．．．．52 圖5.2 所提出的DS-CDMA-OFDM系統流程．．．．．．．53 圖5.3單一用戶、展頻增益為32．．．．．
．．．．．．．．55 圖5.4單一用戶、展頻增益為64．．．．．．．．．．．．．56 圖5.5五個用戶、展頻增益為32．．．
．．．．．．．．．．57 圖5.6十個用戶、展頻增益為32．．．．．．．．．．．．．57 圖5.7五個用戶、展頻增益為64．
．．．．．．．．．．．．58 圖5.8十個用戶、展頻增益為64．．．．．．．．．．．．．58 圖 5.9針對不同展頻增益對應 使用不同載波數目時之效能模擬．．．．．．．．．．．．．．．．．．．．．．．59 表 目 錄 表5.1在DS-CDMA系統下
，多路徑通道參數．．．．．．．．54 表5.2在DS-CDMA-OFDM系統下，多路徑通道參數．．．．．54 REFERENCES
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