近年來，OFDM系統已經漸漸取代了傳統的OOK(on-of keying)調變方式。由於OFDM系統其正交的特性，使訊號頻率彼此間可以重疊，因此頻率使用率可以大幅提升。而目前在光OFDM系統的架構主要分成直接檢測(direct detection)和同調檢測(coherent detection)兩種。在直接檢測方面，接受後的訊號受雜訊影響較大，也易受載波功率的影響；而同調檢測方面，由於它需要較好的雷射品質和較複雜的偏振穩定和相位的評估等，也就是說它需要較完善的補償機制。相較於同調檢測，直接檢測的架構較為簡單只需要一個photodiode就能進行解調，所以目前都是以直接檢測為實驗上的基礎架構。
由於直接檢測在經過photodiode之後會產生Signal-Signal Beating Interference (SSBI)影響我們的訊號接收，所以一般情況下，直接檢測會保留一定的頻寬間隔來避免SSBI影響到我們要接收的訊號，但由於增加頻寬間隔的方法會降低頻譜的使用率，因此目前學術界也有些關於減少頻寬間隔的討論。
關於減少頻寬間隔的方法目前主要有兩種：(1)載波放大機制(carrier boosting)[1][2]，此種作法是先將接收到的載波經過放大器放大之後再和接收的訊號合併變成直接檢測做解調，(2)訊號處理機制(iterative estimation & cancellation technique)[3]，有別於上一個做法，此種方式是運用軟體(ex. Matlab)來估測並解決SSBI的問題而相較於上個方法則是利用硬體的方式(ex. 光學元件)間接解決SSBI的問題，兩者比起來，訊號處理機制更能有效的處理SSBI的問題。而本篇提到的Beat Interference Cancellation Receiver, 簡稱BIC，則屬於硬體上消除SSBI，相較於載波放大機制，BIC解調也能以簡單的接收端架構達到減少頻寬間隔的效益。
因此，在本篇中接著會針對BIC架構做各種參數影響的討論，例如filter order和保護頻帶等的探討，並且更進一步分析載波功率和訊號功率之間的取捨來改善解調後的表現。

OFDM system has recently replaced the traditional OOK (on-of keying) modulation.
Due to orthogonal is characteristic of OFDM, so that each carrier can overlap with the other, therefore, we can enabling higher spectral efficiency. And the fundamentals of optical OFDM are using coherent detection and direct detection, respectively. Compared with the coherent detection, the lower-priced and less-complex direct detection would be more suitable to base on experimental architecture.
However, Due to square-law nature of the photodiode, the Signal-Signal Beating Interference (SSBI) would be mix with our detected signal and thus severely influence the system performance. To avoiding this problem is inserted a frequency gap with the same width as the detected signal. Nevertheless, the drawback in a direct detection OFDM system is the low spectral efficiency. Hence, there have been some proposals to the gap issue if the gap width could be reduced or even be discarded.
So far, it can be mainly two proposals at this subject：(1) Carrier-boosting, in which the optical carrier is filtered out and amplified with an EDFA. (2) Signal processing, in which estimates and removes the SSBI by using a digital iterative equalizer. Therefore, our method is also using optical component to solved this gap issue just like Carrier-boosting, but compared with Carrier-boosting, Beat Interference Cancellation (BIC) system can also reduce the frequency gap with least complexity receive architecture.
In this paper, we discuss BIC system with different parameter, ex. Filter order and frequency gap. Furthermore, acquired the better performance we analysis between carrier power and signal power.

1 正交分頻多工系統簡. . . . . . . . . . . . . . . . . . .1
2 光學正交分頻多工系統
2-1 OFDM系統傳送端和接收端. . . . . . . . . . . . .. . . . 4
2-2 光學OFDM調變和解調的原 . . . . . . . . . . . . . . . .10
3 使用拍頻干擾消除接收端架構
3-1 拍頻干擾消除架構簡介 . . . . . . . . . . . . . . . . . 16
3-2 不同保護頻帶的頻譜分析. . . . . . . . . . . . . . . . . 18
3-3 調整filter order的效能分 . . . . . . . . . . . . . . .22
3-4 B_gap和filter order的綜合分. . . . . . . . . . . . . .26
4 拍頻干擾消除架構的實驗操作
4-1 實驗架構. . . . . . . . . . . . . . . . . . . . . . . .32
4-2 實驗流程. . . . . . . . . . . . . . . . . . . . . . . .36
4-3 利用拍頻干擾消除架構消除SSBI 的實. . . . . .. . . . . . .40
4-4 實驗結果. . . . . . . . . . . . . . . . . . . . . . . .43
5 調整CSPR對於拍頻干擾消除架構的影響
5-1 分析CSPR對於拍頻干擾消除架構的影響 . . . . . . . . . . 52
5-2 CSPR在不同B_gap下的表現. . . . . . . . . . . . . . . . 54
6 結論. . . . . . . . . . . . . . . . . . . . . . . . . 57

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