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.

第一章 緒論 1
1-1 前言 1
1-2 研究目的與動機 3
1-3 論文架構 4
第二章 光學正交分頻多工系統 5
2-1 OFDM基本原理 5
2-1.1 OFDM系統調變與解調 5
2-1.2 離散傅立葉轉換 7
2-1.3 循環字首 (Cyclic Prefix) 8
2-2 光學調變器 12
2-2.1 Mach-Zehnder Modulator 12
2-2.2 IQ Optical Modulator 14
2-3 直接偵測光學OFDM系統 15
2-4 光學OFDM系統中的Carrier frequency offset and sampling clock offset 17
第三章 基頻訊號處理及接收端設計與實現 19
3-1 訊號參數設定 19
3-2 OFDM 接收器 21
3-2.1 IQ Down Conversion 22
3-2.2 Frame Detection 23
3-2.3 Channel Estimation and Equalization 25
3-3 Circuit Design for Baseband Receiver 27
3-3.1 IQ Mixer 27
3-3.2 Frame Detection 29
3-3.3 通道估測與補償 31
第四章 直接偵測光學OFDM系統實驗與電對解實驗 34
4-1 電對解實驗結果與比較 34
4-2 Optical Up-Conversion Optical OFDM System 42
4-3 Electrical Up-Conversion Optical OFDM System 47
4-4 實驗結果 49
第五章 光學OFDM系統中的載波頻率偏移 54
5-1 頻率載波偏移實驗 54
5-2 實驗結果與討論 55
第六章 結論 61
參考文獻 62

[1] K. C. Kao and G. A. Hockham, “Dielectric-fibre surface waveguides for optical frequencies”, Proceedings of the Institution of Electrical Engineers, July 1966, vol.113, pp.1151-1158.
[2] R. W. Chang, “Synthesis of band-limited orthogonal signals for multi-channel data transmission”, Journal of Bell syst.Tech.J., Dec. 1966, vol.45, pp.1775-1796.
[3] R. Chang and R. Gibby, “A Theoretical Study of Performance of an Orthogonal Multiplexing Data Transmission Scheme”, IEEE Transactions on Communication Technology, August 1968, vol.16, pp.529-540.
[4] S. Weinstein and P. Ebert, “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform”, IEEE Transactions on Communication Technology, October 1971, vol.19 , pp.628-634.
[5] James W. Cooley and John W. Tukey, “An Algorithm for the Machine Calculation of Complex Fourier Series”, Mathematics of Computation, April 1965, vol. 19, pp. 297-301.
[6] Richard van Nee and Ramhee Prasad, OFDM Wireless Multimedia Communication, (Artech House, Boston/London, 1999).
[7] W. Shieh and C. Athaudage, “Coherent optical orthogonal frequency division multiplexing”, Electronics Letters, 11 May 2006, vol.42, pp. 587- 589.
[8] M. Sieben, J. Conradi, and D.E. Dodds, “Optical single sideband transmission at 10 Gb/s using only electrical dispersion compensation”, Journal of Lightwave Technology, Oct 1999, vol.17, pp.1742-1749.

[9] S. Chen, Y. Ma, and W. Shieh, “110-gb/s multi-band real-time coherent optical OFDM reception after 600-km transmission over ssmf fiber”, in Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), March, paper OMS2.
[10] A. Lowery, “ Amplified-spontaneous noise limit of optical OFDM lightwave systems”, Optical Society of America, 2008, Vol. 16, Issue 2, pp. 860-865.
[11] W. Shieh, “PMD-supported coherent optical ofdm systems”, Photonics Technology Letters, IEEE, feb.1, 2007, vol. 19, pp. 134 –136.
[12] B. Schmidt, A. Lowery, and J. Armstrong, “Experimental demonstrations of electronic dispersion compensation for long-haul transmission using direct-detection optical OFDM”, Lightwave Technology, Journal of, jan.1, 2008, vol. 26, pp. 196 –203.
[13] ] W.-R. Peng, B. Zhang, K.-M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques”, Lightwave Technology, Journal of, dec.15, 2009, vol. 27, pp. 5723 –5735.
[14] S. Chen, Q. Yang, and W. Shieh, “Demonstration of 12.1-gb/s single-band real-time coherent optical OFDM reception”, in OptoeElectronics and Communications Conference (OECC), 2010 15th, July 2010, pp. 472 –473.
[15] Y. Hong, E. Viterbo, and A. Lowery, “Improving the sensitivity of direct-detection optical OFDM systems by pairing of the optical subcarriers”, in Optical Communication (ECOC), 2011 37th European Conference and Exhibition on, Sept., paper OTHB11.
[16] A. Lowery, “Improving sensitivity and spectral efficiency in direct-detection optical OFDM systems”, in Optical Fiber communication/National Fiber Optic Engineers Conference, OFC/NFOEC 2008. Conference on, Feb., paper OMN.
[17] Huang-Ren Shih, ”Design and Implementation of Direct Detection Optical OFDM System”, Institute of Communication engineering in NTHU, master thesis, 2012
[18] J. Lee, D. Toumpakaris, H.-L. Lou, and J. Cioffi, “Effect of carrier frequency offseton time-domain differential demodulation in ofdm systems”, in Vehicular Technology Conference, 2004. VTC2004-Fall. 2004 IEEE 60th, vol. 1, sept. 2004, pp. 568– 572 Vol. 1.
[19] X. Jin and J. Tang, “Optical ofdm synchronization with symbol timing offset and sampling clock offset compensation in real-time imdd systems,” Photonics Journal, IEEE, april 2011, vol. 3, no. 2, pp. 187 –196.
[20] ] C.-H. Peng, K.-T. Shr, M.-H. Lin, and Y.-H. Huang, “A baseband receiver for optical ofdm systems,” in VLSI Design, Automation and Test (VLSI-DAT), 2011 International Symposium on, april 2011, pp. 1 –4.