在本篇論文中，我們提出了一種基於直流偏置光正交分頻多工(DCO-OFDM)的可見光通訊(VLC)系統，並且設計了自適應通道分配於系統中。根據通道狀態動態分配各個子載波的比特和功率，來改善系統性能。此外，提出的DCO-OFDM收發機通過採用單個紅色發光二極體(LED)，在15公分的通信距離下可以實現24百萬位元/秒的傳輸速率且位元錯誤率(BER)無誤。
由於每個燈具的調變頻寬不同且各個子載波的通道狀態也不同，因此可以通過自適應通道分配方案動態的為每個子載波分配比特和功率。在實際傳輸前先傳送訓練序列，接收端即可獲得通道狀態資訊(CSI)。比特及功率分配採用Levin-Campello演算法，分配的目標為在總功率限制下最大化數據速率。如果子通道的訊號雜訊比(SNR)高，就可以利用高階的調變來增加傳輸速率。反之，如果子通道的訊號雜訊比(SNR)低，就利用低階的調變來減少錯誤率。我們模擬了在三種通道狀態下自適應通道分配方案的結果，並且與固定16-正交振幅調變(QAM)的調變方案比較。採用適應性通道分配方案可使數據速率從68.6百萬位元/秒提高至84百萬位元/秒，且位元錯誤率(BER)低於2×10^(-3)。
此外，提出的DCO-OFDM收發機在40百萬赫茲的頻寬下，採用16-正交振幅調變(QAM)的調變技術。利用最小均方(LMS)演算法估測通道增益並補償，在訊號雜訊比(SNR)為21分貝的情況下，可達到位元錯誤率(BER)無誤。將我們設計的DCO-OFDM系統跟工研院的模組做整合後，可以使用VLC多媒體播放器播放720p高清視頻並在斷開連接後立即重新連接。

In this thesis, we present a visible light communication (VLC) system based on DC-based orthogonal frequency division multiplexing (DCO-OFDM), and design adaptive channel allocation in the system. The bit and power of each subcarrier are allocated dynamically according to the channel state to improve the system performance. Moreover, the proposed DCO-OFDM transceiver can achieve the transmission rate of 24 Mbps and the bit error rate (BER) is 0 at the communication distance of 15 cm through a red light-emitting diode (LED).

Since each luminaire has different modulation bandwidth and different channel state of each subcarrier, bit and power can be dynamically allocated for each subcarrier through an adaptive channel allocation scheme.
The training sequence is transmitted before the actual transmission, so that the receiver can obtain channel state information (CSI). The bit and power allocation uses the Levin-Campello algorithm, and the goal is to maximize the data rate under the total power constraints. If the SNR of the sub-channel is high, then high-order modulation can be used to increase the transmission rate. On the contrary, if the SNR of the sub-channel is low, low-order modulation is used to reduce the error rate. We simulate the results of adaptive channel allocation scheme in three channel states and compared it with the fixed 16-orthogonal amplitude modulation (QAM) scheme. The data rate can be increased from 68.6 Mbps to 84 Mbps, and the BER is below 2×10^(-3)

In addition, the proposed DCO-OFDM transceiver employs 16-QAM at the bandwidth of 40 MHz. The least mean square (LMS) algorithm is used to estimate and compensate the channel gain. Based on a signal-to-noise ratio (SNR) of 21 dB, the BER can be achieved without errors. After integration with the Industrial Technology Research Institute (ITRI) module, the VLC multimedia player can be used to play 720p high definition video and reconnect immediately after disconnection.

Abstract i
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Main Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Visible Light Communications Technologies 7
2.1 Related Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Basics of VLC System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Optical OFDM for VLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.1 DC-biased Optical OFDM Systems . . . . . . . . . . . . . . . . . . 13
2.3.2 Asymmetrically Clipped Optical OFDM Systems . . . . . . . . . . . 14
2.3.3 Flip OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.4 Unipolar OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3.5 Performance Comparison between Different Optical OFDM . . . . . 17
3 DCO-OFDM System Design 19
3.1 System Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2 Design flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3 DCO-OFDM system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.4 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.4.2 Subcarrier Allocation (Hermitian Symmetry) . . . . . . . . . . . . . 23
3.4.3 Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) 26
3.4.4 Cyclic Prefix Insertion . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4.5 Preamble Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.5 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.5.1 Timing Synchronization . . . . . . . . . . . . . . . . . . . . . . . . 29
3.5.2 Sampling Clock Offset Estimation and Compensation . . . . . . . . 31
3.6 Channel Estimation and Compensation . . . . . . . . . . . . . . . . . . . . . 31
3.6.1 Least Square (LS) . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.6.2 Least Mean Square (LMS) . . . . . . . . . . . . . . . . . . . . . . . 33
3.6.3 Linear Interpolation . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 Adaptive Channel Allocation Scheme . . . . . . . . . . . . . . . . . . . . . 37
3.7.1 Bit and Power Allocation . . . . . . . . . . . . . . . . . . . . . . . . 40
4 Hardware Architecture Design 45
4.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 Subcarrier Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.3 Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) . . 48
4.4 Cyclic Prefix Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.5 Timing Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.6 Sampling Clock Offset Estimation and Compensation . . . . . . . . . . . . . 50
4.7 Channel Estimation and Compensation . . . . . . . . . . . . . . . . . . . . . 52
5 Implementation Results and Discussion 55
5.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
5.1.1 Peak to Average Power Ratio Reduction . . . . . . . . . . . . . . . . 55
5.1.2 Channel Estimation and Compensation . . . . . . . . . . . . . . . . 56
5.1.3 Adaptive Channel Allocation . . . . . . . . . . . . . . . . . . . . . . 58
5.2 Circuit Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.3 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.5 Device Utilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.6 Performance Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.7 Comparison with Related Works . . . . . . . . . . . . . . . . . . . . . . . . 73
6 Conclusions and Future Works 75
6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6.2 Future Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Bibliography 77

[1] T. Komine and M. Nakagawa, “Fundamental analysis for visible light communication system using LED lights,” IEEE Transactions on Consumer Electronics, vol. 50, no. 1, pp. 100–107, 2004.
[2] S. Singh, G. Kakamanshadi, and S. Gupta, “Visible light communication-an emerging wireless communication technology,” in 2015 2nd International Conference on Recent Advances in Engineering & Computational Sciences (RAECS), 2015, pp. 1–3.
[3] L.-Y. Wei, C.-W. Hsu, C.-W. Chow, and C.-H. Yeh, “40-Gbit/s visible light communication using polarization-multiplexed R/G/B laser diodes with 2 m free space transmission,” in 2019 Optical Fiber Communications Conference and Exhibition (OFC), 2019.
[4] S. Rajagopal, R. D. Roberts, and S.-K. Lim, “IEEE 802.15.7 visible light communication: modulation schemes and dimming support,” IEEE Communications Magazine, vol. 50, no. 3, pp. 72–82, 2012.
[5] H. L. Minh, D. O’Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung, Y. Oh, and E. T. Won, “100 Mb/s NRZ visible light communications using a postequalized white LED,” IEEE Photonics Technology Letters, vol. 21, no. 15, pp. 1063–1065, 2009.
[6] B. Yu, H. Zhang, and H. Dong, “Optimized 481 Mb/s visible light communication system using phosphorescent white LED,” Chinese Optics Letters, vol. 12, no. 11, p. 110606, 2014.
[7] Y. Wang, Y. Wang, and N. Chi, “Experimental verification of performance improvement for a gigabit wavelength division multiplexing visible light communication system utilizing asymmetrically clipped optical orthogonal frequency division multiplexing,”
Photonics Research, vol. 2, no. 5, pp. 138–142, 2014.
[8] P. Manousiadis, H. Chun, S. Rajbhandari, R. Mulyawan, D. A. Vithanage, G. Faulkner, D. Tsonev, J. J. D. McKendry, M. Ijaz, E. Xie, E. Gu, M. D. Dawson, H. Haas, G. A. Turnbull, I. D. W. Samuel, and D. O’Brien, “Demonstration of 2.3 Gb/s RGB whitelight VLC using polymer based colour-converters and GaN micro-LEDs,” in 2015 IEEE
Summer Topicals Meeting Series (SUM), 2015, pp. 222–223.
[9] D. Tsonev, H. Chun, S. Rajbhandari, J. J. D. McKendry, S. Videv, E. Gu, M. Haji, S. Watson, A. E. Kelly, G. Faulkner, M. D. Dawson, H. Haas, and D. O’Brien, “A 3-Gb/s single-LED OFDM-based wireless VLC link using a gallium nitride LED,” IEEE Photonics Technology Letters, vol. 26, no. 7, pp. 637–640, 2014.
[10] Y. Hong, T.Wu, and L.-K. Chen, “On the performance of adaptive MIMO-OFDM indoor visible light communications,” IEEE Photonics Technology Letters, vol. 28, no. 9, pp. 907–910, 2016.
[11] G. Zhang, J. Zhang, X. Hong, and S. He, “Low-complexity frequency domain nonlinear compensation for OFDM based high-speed visible light communication systems with light emitting diodes,” Optics Express, vol. 25, no. 4, pp. 3780–3794, 2017.
[12] T. Lang, Z. Li, F. Lu, Z. Dong, L. Wang, C. Li, F. Zhang, G. Chen, and A. Wang, “LED based visible light communication and positioning technology and SoCs,” in 2018 14th IEEE International Conference on Solid-State and Integrated Circuit Technology
(ICSICT), 2018, pp. 1–4.
[13] R. C. Kizilirmak and Y. H. Kho, “Mitigation of illumination interference caused by PWM dimming in OFDM based visible light communication systems,” in 2015 International Conference on Computer, Communications, and Control Technology (I4CT), 2015, pp. 489–492.
[14] W.-Y. Wang, Y.-J. Zhu, Y.-Y. Zhang, and J.-K. Zhang, “An optimal power allocation for multi-LED phase shifted based MISO VLC systems,” IEEE Photonics Technology Letters, vol. 27, no. 22, pp. 2391–2394, 2015.
[15] H.-C. Kim, J.-H. Yoo, S.-Y. Jung, and J.-P. Jeon, “Design of multi-spectral coded VPPM for optical wireless LED communications,” in 2013 Fifth International Conference on Ubiquitous and Future Networks (ICUFN), 2013, pp. 375–379.
[16] S. D. Dissanayake and J. Armstrong, “Comparison of ACO-OFDM DCO-OFDM and ADO-OFDM in IM/DD systems,” Journal of Lightwave Technology, vol. 31, no. 7, pp. 1063–1072, 2013.
[17] R. Islam, P. Choudhury, and M. A. Islam, “Analysis of DCO-OFDM and flip-OFDM for IM/DD optical wireless system,” in 8th International Conference on Electrical and Computer Engineering, 2014, pp. 32–35.
[18] E. Shawky, M. A. El-Shimy, H. M. H. Shalaby, A. Mokhtar, and E.-S. A. El-Badawy, “Kalman filtering for VLC channel estimation of ACO-OFDM systems,” in 2018 Asia Communications and Photonics Conference (ACP), 2018, pp. 1–3.
[19] J. Armstrong and A. Lowery, “Power efficient optical OFDM,” Electronics Letters, vol. 42, no. 6, pp. 370–372, 2006.
[20] R. S. J. Godwin, K. Veena, and D. S. Kumar, “Performance analysis of direct detection flip-OFDM for VLC system,” in 2016 International Conference on Emerging Trends in Engineering, Technology and Science (ICETETS), 2016, pp. 1–5.
[21] B. Aly, “Performance analysis of adaptive channel estimation for U-OFDM indoor visible light communication,” in 2016 33rd National Radio Science Conference (NRSC), 2016, pp. 217–222.
[22] N. Fernando, Y. Hong, and E. Viterbo, “Flip-OFDM for optical wireless communications,” in 2011 IEEE Information Theory Workshop, 2011, pp. 5–9.
[23] Z. Wang, T. Mao, and Q. Wang, “Optical OFDM for visible light communications,” in 2017 13th International Wireless Communications and Mobile Computing Conference (IWCMC), 2017, pp. 1190–1194.
[24] L. U. Khan, M. I. Babar, and Z. Sabir, “Robust modified MMSE estimator for combtype channel estimation in OFDM systems,” in 2013 15th International Conference on Advanced Communications Technology (ICACT), 2013, pp. 924–928.
[25] L. U. Khan, N. Khan, M. I. Khattak, and M. Shafi, “LS estimator: performance analysis for block-type and comb-type channel estimation in OFDM system,” in Proceedings of 2014 11th International Bhurban Conference on Applied Sciences & Technology (IBCAST) Islamabad, Pakistan, 2014, pp. 420–424.
[26] P.-C. Wang, “Design and implementation of variable-length fast fourier transform processor in OFDM systems,” Master’s thesis, National Central University, 2007.
[27] C. Zhang and K. Pang, “Synchronization sequence generated by modified Park algorithm for NC-OFDM Transmission,” IEEE Signal Processing Letters, vol. 22, no. 4, pp. 385– 389, 2015.
[28] S. Coleri, M. Ergen, A. Puri, and A. Bahai, “Channel estimation techniques based on pilot arrangement in OFDM systems,” IEEE Transactions on Broadcasting, vol. 48, no. 3, pp. 223–229, 2002.
[29] P. S. Diniz, Adaptive filtering. Springe INDIA, 2012.
[30] M. T. Alresheedi, A. T. Hussein, and J. M. Elmirghani, “Uplink design in VLC systems with IR sources and beam steering,” IET Communications, vol. 11, no. 3, pp. 311–317, 2017.
[31] D. Hughes-Hartogs, “Ensemble modem structure for imperfect transmission media,” 1989.
[32] P. Chow, J. Cioffi, and J. Bingham, “A practical discrete multitone transceiver loading algorithm for data transmission over spectrally shaped channels,” IEEE Transactions on Communications, vol. 43, no. 2/3/4, pp. 773–775, 1995.
[33] H. Levin, “A complete and optimal data allocation method for practical discrete multitone systems,” in Proc. IEEE Global Telecommunications Conference (GLOBECOM), 2001, pp. 369–374.
[34] J. Campello, “Optimal discrete bit loading for multicarrier modulation systems,” in IEEE International Symposium on Information Theory, 1998, p. 193.
[35] L. Jiancai, Y. Wanwan, Z. Weili, W. Yafu, Y. Zhihui, X. Jiakai, and N. Xinbao, “The improvement of adaptive bit and power loading algorithm with low complexity in MIMO-OFDM systems,” in 2009 IEEE International Conference on Internet Multimedia Services Architecture and Applications, 2009, pp. 1–5.
[36] R. Bhakthavatchalu, M. Sinith, P. Nair, and K. Jismi, “A comparison of pipelined parallel and iterative CORDIC design on FPGA,” in 2010 5th International Conference on Industrial and Information Systems, 2010.
[37] D. Xie, “Equalizer design and implementation for ACO-OFDM VLC system with real-valued Hartley transform,” Master’s thesis, National Central University, 2015.
[38] E. O. Garcia, R. Cumplido, and M. Arias, “Pipelined CORDIC design on FPGA for a digital sine and cosine waves generator,” in International Conference on Electrical and Electronics Engineering, 2006, pp. 1–4.
[39] Q. Lai, “Design and implementation of channel estimation for LTE downlink system in time-varying channel environment with FPGA,” Ph.D. dissertation, National Central University, 2013.
[40] C. W. Chow, C. H. Yeh, Y. F. Liu, and P. Y. Huang, “Background Optical Noises Circumvention in LED Optical Wireless Systems Using OFDM,” IEEE Photonics Journal, vol. 5, no. 2, p. 7900709, 2013.