考慮編碼方式及頻道使用效率之實用分波多工編碼系統

Abstract

在光纖通訊系統中，波散限制(dispersion-limited)的現象會使得位元誤 碼率出現下限(floor)的情況。由於使用單一錯誤更正碼，可以輕易地將 此位元誤碼率的下限由一階(O(Pe))降至二階(O(Pe^2))，故在此波散限制 的光纖系統下，向前式錯誤更正碼(forward error correction code)將 是降低此位元誤碼率下限的一種可行的方法。在本論文中，我們提出了一 個有效且簡易的分波多工系統，稱之為生成碼分波多工系統(product coded WDM coding system,PC-WDMC)。在此系統中，我們於行及列的方向 上皆使用同位元檢查碼，且使用平行的輸方式，因此不論在編碼、傳輸或 解碼，它都可以輕易達成。且由於只須兩個同位元檢查頻道，使它顯得極 有效率。其次，我們也提出了另一改良系統，在此改良系統中，我們利用 同步光纖之訊框同步信號(SONET framing signal)取代前一系統中的訊框 對齊字元(frame alignment word,FAW)作編碼訊框的同步，如此可以減少 編碼字元的長度。若再改用夏合(Hsiao)碼，則可將原來所須要的二個位 元檢查頻道降到只剩一個，有效地簡化編/解碼器的設計。由於頻道效率 的提昇以及編碼方式的減化，此二新系統將成為分波多工系統中引人注意 的編碼方式。 Forward error correction is a feasible approach to decline the bit error floor of a dispersion-limited lightwave system, arising from the fact that a simple single-error-correcting code can reduce the error floor from O(Pe) to O(Pe^2). Here we prosose two novel WDM coding schemes to realize efficient and simple coding systems for high-capacity lightwave systems. First, a WDM system using product codes is proposed. The product coded WDM coding system applies simple parity check codes in rows and columns of the array codeword of the product code for single-error correction. The product codes can be implemented simply since the encoding, transmssion, and decoding can be executed all in parallel. It is an efficient WDM coding system, merely requiring two parity-check channels. In this system synchronization is needed in the decoder to distinguish successive coding blocks. A frame alighment word (FAW) is used for this purpose whose function is similar to the frame synchronization in the TDM systems. Second, we propose another WDM coding system using the SONET framing signal for coding block synchronization. In SONET systems, there are A1,A2 bytes in the STS-N frame to specify the frame boundary. This signal is adapted for coding block sychronization to eliminate the FAW in the product coded WDM coding system. In addition, Hisao codes are applied to reduce the number of additional channels to one. The resulted system has a small coding block, leading to a simple encoder/decoder. The proposed WDM coding systems just need one and two additional channels, respectively, and could correct single errors in all the data channels with one encoder/decoder only. Moreover, the encoder/decoder is very simple. They just needs a single system clock and provide an infinite coding gain for a dispersion-limited WDM system. It is superior to usual serial coding approaches, being an attractive coding schemes for high- speed WDM systems.