在過去的光纖通訊領域當中，做通道補償的方式大部分以光學的角度來出發，
但是光學補償的缺點就是價格昂貴，架構複雜以及動態補償效果差，所以近年慢
慢的演變成數位訊號處理扮演著主要補償的角色，其原因是數位訊號處理可以因
應不同的需求，來改變其中的補償機制，並且將其中的演算法轉換成積體化電路
時，所需要的成本將會遠低於在光學上面所做的補償。
在本篇論文中，我們首先會提到 coherent OFDM 完整的系統架構，接著會提
到傳送端跟接受端的數位訊號處理，而本篇論文的重心會放在相位偵測上面，其
原因是在光纖通訊上面主要可以分為兩種調變模式，一種為 direct detection
OFDM 另一種就是本篇論文所提及的 coherent OFDM，而 coherent OFDM 系統是
在接收端使用 local laser 來做 beating down 的，相對於 direct detection OFDM 是
在傳輸的時候帶著一根載波，而在接收端的時候用這根載波來做 beating down 的，
所以 coherent OFDM 系統必須要比 direct detection OFDM 系統多做相位偵測，其
中相位偵測的方法在本論文裡面所提到有三種方法，(一) data-aided、(二)
pilot-aided、(三)phase noise suppression algorithm，在這幾種方法中我們可以發現
到 phase noise suppression algorithm 這種方法最好，而我們會在論文裡面看到三
種不同演算法在不同的傳輸距離中比較以及經過 800 公里的單模光纖(SMF)傳輸
下，我們所使用的 phase noise suppression algorithm 可以比 pilot-aided 演算法提升
2dB 的光訊雜比(OSNR)，同時也不需要任何色散補償光纖，完全只需要數位訊
號處理的部分，而在正交分頻多工系統下的等化器(equalizer)也比 single carrier
系統的等化器複雜度要來的低。

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