Optical OFDM系統近年來已成為熱門的研究領域，OFDM系統由於其正交的特性，使子載波可以互相重疊，因此頻譜使用效益比傳統OOK(on-off keying)調變方式大幅上升，然而在大多數optical OFDM研究中都會使用較好的實驗環境，而為了能使OFDM系統能夠廣泛運用，因此optical OFDM傳輸系統的實現就變得相當重要。本篇論文針對DDO-OFDM系統設計了一16路平行化運算處理之接收器，並將其實現到FPGA上，其接收器包含I/Q down-conversion、frame detection、CP removal、FFT與channel estimation and equalization，而其throughput可達到0.632Gb/s。
在實驗測試方面，使用了Optical up-conversion optical OFDM system與electrical up-conversion optical OFDM system兩種光學系統，由實驗結果可得知，electrical up-conversion scheme的OSNR sensitivity會比Optical up-conversion好1dB。而為了得知FPGA的benchmark，我們使用了電對解架構來模擬，並得到了其FPGA在電對解的OSNR sensitivity為15dB。我們也在Optical up-conversion optical OFDM system中會產生carrier frequency offset的影響做討論，我們使用16QAM來測試FFT size從512降到128、symbol length從100降到32的實驗，而由實驗結果可得知，如要避開CFO的影響可以降低FFT size或是減少symbol length的長度，而在electrical up-conversion optical OFDM system中，因為不受CFO影響，因此其16QAM的結果會比Optical up-conversion scheme好上許多。

In recent years, optical orthogonal frequency-division multiplexing (OFDM) system has become a popular topic for research. Because of the orthogonal characteristics in OFDM signal, the subcarriers will be arranged in an overlapping scenario. Therefore, the spectrum efficiency is much higher than traditional modulation techniques, such as OOK (on-off keying). However, most of research results for optical OFDM in laboratories are still far from realization in various fields. To apply OFDM system extensively, it’s important to implement real-time optical OFDM system. In this thesis, we design a direct detection receiving optical OFDM system with realizing through a FPGA of 16 parallelism. The baseband OFDM receiver including I/Q down-conversion, frame detection, CP removal, FFT, channel estimation and equalization. The throughput can achieve 0.632 Gb/s.
In the experiment, we use two kinds of optical OFDM system respectively, the optical up-conversion optical OFDM system and electrical up-conversion optical OFDM system. For the experimental results, the electrical up-conversion optical OFDM system has better performance which OSNR sensitivity is 1-dB lower than the optical up-conversion scheme. To evaluate the benchmark of the employed FPGA, we use an electrical experiment to simulate optical system. We find that the OSNR sensitivity of the FPGA is 15 dB. We also discuss the effects of carrier frequency offset in optical up-conversion optical OFDM system. In the 16QAM experiment, we test the FFT size from 512 to 128 and symbol length from 100 to 32. As a result, we can reduce the FFT size and the symbol length to avoid the effect with carrier frequency offset. Finally, we successfully demonstrate a real-time receiver via FPGA for an optical OFDM system. In addition, it’s worthy to note that the optical up-conversion scheme needs reduce FFT size and symbol length to achieve a similar performance as the electrical up-conversion scheme.

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