在多用戶的上傳系統中，用戶間的時間同步是個長久以來被討論的議題，在傳統PON上傳系統中，多個ONUs傳輸至OLT以分時多重存取(TDMA)的方式讓不同用戶在不同時間槽傳輸，然而此種方式需要精準的時間同步與時間分配，才能避免用戶間彼此干擾。而由於OFDM技術的發展進步，人們希望藉由此更高的頻譜效益的技術進行傳輸，然而，在多用戶的OFDM系統中，不同用戶之間的時間同步比起其他傳統上傳系統更為重要，若用戶訊息到達基地台的時間不一致，將無法使OFDM訊號正確的解調回來，而在實際生活中，大多數情況使用者皆在不同時間地點上傳訊號到基地台。
此篇論文中，我們將把傳統用於CDMA系統中的非同步碼，金氏碼，運用在OFDM系統中，經過時域展頻後，使使用者彼此間時間不同步的情況下，依然可以因為其高的自相關與不論任何時間延遲的低互相關將使用者訊號獨立解調出來。這裡，我們將以兩路使用者分別傳輸不同的光纖長度後，接收端將會在不同時間接收訊號，而使用金氏碼編碼後的OFDM訊號依然可以解回兩路不同使用者的訊號，並且BER表現皆在FEC threshold下。

In multiuser uplink system, the timing synchronization for users has been discussed for a long time. In traditional PON uplink system, TDMA is a way for users transmitting signals from multiple ONUs to OLT in different time slot. However, in order to avoid interference among different users, timing synchronization and arrangement are employed. With OFDM technology getting improved, its target is to transmit signal with high spectral efficiency. However, in multi-user OFDM system, the synchronization among different users is more important than other traditional uplink systems. If the information of users arrive at the baseband at different time, the OFDM signal may not be demodulated correctly. In our real life, users transmit information to the central office at different time and place in most cases.
In this work, we employed one of asynchronous code which is used in traditional CDMA system, Gold code, to OFDM system. After time spreading, users’ signals can be individually demodulated based on their high auto-correlation and low cross-correlations in time asynchronous scenario. Here, we make two users transmit signals via different lengths of fiber. Even the receiver may receive the signal at different time, the OFDM signals coded by Gold sequence from different users can still be demodulated, and their bit-error-rate (BER) are all below FEC threshold.

中文摘要................................................i
ABSTRACT...............................................ii
致謝...................................................iii
目錄 ...................................................iv
圖目錄 .................................................vi
表目錄..................................................ix
第1章 緒論............................................1
1.1 前言..............................................1
1.2 研究目的與動機.....................................3
1.3 論文架構..........................................5
第2章 正交分頻多工與多重存取(Multiple Access)系統.......6
2.1 正交分頻多工系統...................................6
2.2 多重存取傳輸系統....................................9
2.3 分碼多重存取系統....................................11
2.3.1 直接序列分碼多重存取技術及原理....................12
第3章 展頻碼分析與比較..................................15
3.1 華氏碼.............................................16
3.2 線性回授移位暫存器和虛擬隨機碼.......................17
3.3 最大長度序列.......................................19
3.4 金氏碼.............................................20
3.5 展頻碼之相關性比較..................................24
3.5.1 華氏碼之相關性................................... 27
3.5.2 金氏碼之相關性....................................30
第4章 OFDM-CDMA系統原理.................................37
4.1 OFDM-CDMA 系統.....................................37
4.2 OFDM-CDMA 解碼原理..................................40
第5章 模擬與實驗.........................................43
5.1 MATLAB 模擬.........................................44
5.1.1 使用者同步接收.....................................44
5.1.2 使用者非同步接收...................................51
5.2 實驗架構與參數.......................................56
5.3 實驗結果.............................................59
第6章 結論...............................................68
參考文獻....................................................69

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