前向糾錯技術已日漸受到廣泛的重視，並被認為是達成高可靠及高品質傳輸之有效方法。其主要的原理是藉著有系統地加入額外之資訊，以用來偵測和儘可能的更正因通道或接受器所導致的錯誤。迄今為止，前向糾錯技術已成功地應用於有線通訊、無線通訊與許多的儲存媒體系統中。

近幾年來，前向糾錯碼中的低密度位元檢查(low-density parity-check, LDPC)碼因其優越的改錯能力而越來越受到重視。若依建構方式來區分，低密度位元檢查碼可分為兩類:(1)亂數式(random)建構；(2)結構式(structured)建構。一般而言，在長碼長時，亂數建構的低密度位元檢查碼的改錯效能比其等效的結構式低密度位元檢查碼更趨近Shannon limit，但亂數建構低密度位元檢查碼在編碼上比較不容易實現。相反地，結構式低密度位元檢查碼在編碼實現上比較簡單，特別是循環(cyclic)或類循環(quasi-cyclic)低密度位元檢查碼。因此，在本論文中，我們提出一個簡單的方法來設計基於一次碰撞序列碼(One-Coincidence Sequences)的類循環低密度位元檢查碼。由模擬結果得知，這些碼的效能表現可和現有文獻中的低密度位元檢查碼相匹敵。

此外，隨著寬頻傳輸(Broadband Transmission)的需求量日益遽增，光通訊系統因其可提供大頻寬和低傳輸損耗，而越來越受到重視。其中，光分碼多重擷取(Optical Code-Division Multiple-Access, OCDMA)技術被視為是區域網路的最佳選擇，因為它可在一個具bursty traffic的環境下提供高的統計多工增益。早期的非同調光分碼多重擷取網路是在時域上使用類正交序碼對信號編碼，但這些碼不僅長度很長，而且多重擷取干擾(multiple access interference)限制了網路的同時使用者數。因此有人提出頻域振幅編碼(spectral-amplitude-coding)光分碼多重擷取網路來解決多重擷取干擾的問題。

然而，在頻域振幅編碼光分碼多重擷取網路中，當接收信號功率較高時，相位強度雜訊(phase-induced intensity noise, PIIN)為主要的雜訊。已經有人證明當相位強度雜訊為主要的雜訊時，系統的效能是無法藉由加大傳送功率來改善。為了改善這個問題，我們提出了一個可運用在頻域振幅編碼光分碼多重擷取網路的modified Welch-Costas (MWC)碼。藉由MWC碼的特性，可利用陣列波導光柵路由/布雷格光纖光柵(fiber Bragg gratings)來建構一小巧且簡單的編/解碼器。除此之外，由於MWC碼為一具有低互相關值的類正交序碼，相位強度雜訊是可以被有效的抑制。因此和之前傳統的頻域振幅編碼光分碼多重擷取網路比較起來，這個以MWC碼來編碼的光分碼多重擷取網路擁有較為優越的的效能表現。

此外，為了滿足高速光傳輸系統的需求，如何建構一個低成本且高效能的光分碼多重擷取網路是一個重要的議題。因此，我們亦評估將所提出的類循環低密度位元檢查碼，當作另一種用來改善光分碼多重擷取網路效能的方法。從模擬結果得知，藉由類循環低密度位元檢查碼的加入，可以有效的提升以MWC碼來編碼的光分碼多重擷取網路在高速傳輸時的效能。而且隨著光電元件的發展，特別是在製造費用方面，我們的模擬結果將提供網路設計者一個最具成本效益的方案，藉由一高效率之光電技術的混合，可有效地增加光分碼多重擷取網路的容量。

Forward error correction (FEC) technology has received more and more attentions and is considered as an efficient method to reach high-reliability and high-quality transmission. The principle is to systematically add extra information, which can be used to detect and possibly correct errors caused by corruption from the channel and the receiver. So far, it has been successfully applied to cable, wireless communications and mass storage devices.

In recent years, among the FEC schemes, low-density parity-check (LDPC) codes have recently received more attentions due to the superior error correction ability. Based on methods of construction, LDPC codes can be divided into two categories: 1) random LDPC codes and 2) structured LDPC codes. In general, long random LDPC codes perform closer to the Shannon limit than their equivalent structured LDPC codes, but the lack of structure makes the encoding process difficult to realize. On the contrary, structured LDPC codes have encoding advantage over random ones, especially cyclic or quasi-cyclic (QC) LDPC codes. Thus, in this dissertation, we propose a simple method to design the QC-LDPC codes from the one-coincidence sequences (OCSs). Simulation results show that these QC-LDPC codes can perform as well as previous LDPC codes in the literature.

In addition, with the increasing demands for broadband transmission, the optical fiber communication systems are getting more attractive since they offer a large bandwidth and low attenuation. Among them, optical code division multiple-access (OCDMA) offers high statistical multiplexing gain in a bursty traffic environment and is thought to be a more suitable solution in local-area network. Early incoherent optical CDMA networks used pseudo-orthogonal sequences to encode signals in the time domain, but the codes were long and multiple access interference (MAI) limited the number of simultaneous users. Thus, spectral-amplitude-coding (SAC) optical CDMA networks were proposed to eliminate the influence of MAI.

Nevertheless, for SAC-OCDMA networks, the phase-induced intensity noise (PIIN) is the dominant noise when the signal power is relatively high. It has been shown that the system performance can not be improved by increasing the received optical power when PIIN is the major noise. In order to alleviate this problem, we propose one modified Welch-Costas (MWC) code family for the SAC-OCDMA networks. On utilizing the property of the MWC codes, more compact coder/decoders can be realized via waveguide and fiber gratings. In addition, the influence of PIIN can be efficiently suppressed due to the low cross correlation of the MWC codes. Thus, the MWC-coded OCDMA network has a superior performance as compared to traditional SAC-OCDMA networks.

Furthermore, to satisfy the high-speed optical transmission, how to construct a low-cost OCDMA network with high performance is an important topic. Therefore, we evaluate the OCS-LDPC codes as another method to improve the performance of the OCDMA networks. Simulation results show that the high speed MWC-coded OCDMA networks with superior performance can be achieved by the use of LDPC codes. Moreover, as optical and electronic devices evolve, their fabrication costs will decrease. Our results allow the network designer to increase the capacity of OCDMA networks with low cost by an efficient mix of technologies.

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