光分碼多重擷取 (Optical Code-Division Multiple-Access, OCDMA) 技術提供彈性與安全性的網路傳輸，使用者可以同時地非同步擷取網路系統，因此適合應用於區域網路的架構下。在早期的一維編碼 (One-Dimensional Code) 系統中，若要將系統可容納的使用者增大，則必須大幅的增加碼的長度。如此一來，非線性效應將會產生，而且也不易於硬體的實現。二維編碼 (Two-Dimensional Code) 的出現則是為了改善一維編碼的缺點，並保有原先在一維編碼上良好的相關特性。
在本篇論文中，我們提出一個新的二維編碼架構，這種編碼是將原本的一維光正交碼 (Optical Orthogonal Codes, OOCs) 摺成二維的形式，我們稱為摺型光正交碼 (Folded OOCs, FOOCs)。此摺型光正交碼系統利用波長與時間兩種維度來傳輸信號。經過分析後證實，摺型光正交碼能保有良好的相關特性，並且有更多的碼能供系統使用。
針對以上的二維編碼系統，我們提出一個新穎且簡易的方法來偵測即時的使用者數量，即在每位使用者的傳輸碼字中都加入一個特定的波長來傳輸。由此，我們在接收端可依據此波長的功率大小來判定即時使用者的數量，進而調整系統的臨限決策值。在此系統中，每個特定的即時使用者數都將會對應到一個最佳臨限決策值，只要即時設定最佳的臨限決策值，便可達到系統的最佳效能。

Optical code-division multiple-access (OCDMA) techniques not only provide a flexible and secure transmission but also allow multiple users access the network asynchronously and simultaneously. Therefore, they are the most suitable solution in local area network (LAN). In early one-dimensional (1-D) coding systems, the code length should be increased rapidly in order to accommodate large active users. Hence, the nonlinear effect will come into existence and the systems are hard to realize. Two-dimensional (2-D) coding systems have been introduced to improve the defect and they also maintain good correlation properties.
In this thesis, we propose a new 2-D code family for OCDMA system called folded optical orthogonal codes (FOOCs). We fold the original 1-D optical orthogonal codes (OOCs) into the matrix form to produce FOOCs. FOOCs use two dimensions, i.e. time and wavelength, for transmission. According to our analysis, FOOCs have good correlation properties and can supply large number of codes.
In the OCDMA system using FOOCs, we propose a novel and simple method to detect the active users. A special wavelength called pilot wavelength is added into the original transmission patterns of every user. At the receiver, therefore, we can acquire the number of the active users from the power of the pilot wavelength. Consequently, if we adjust the threshold value to the optimum threshold value according to the active users, we can decrease the bit error rate (BER) and improve the system performance.

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