室內可見光通訊系統(visible light communication, VLC)使用發光二極體(light emitting diode, LED)，不僅作為室內照明，同時當作傳輸資料之訊號源。一顆發光二極體(以下簡稱LED)可視為一個燈具。在一個室內空間中，會擺放多個燈具，以達到基本照明需求。
多燈具傳輸方式，大致可分為分時多址(time division multiple access, TDMA)、分頻多址(frequency division multiple access, FDMA)以及分碼多址(code division multiple access, CDMA)三大類。在本可見光通訊系統中，我們將空間內各個燈具載上相同的訊號，並且使用相同頻段同時進行傳輸。接收器將收到來自各個燈具傳輸的訊號，每個燈具到接收器的距離不同，訊號抵達接收端的時間無法一致，導致訊號之間彼此會有干擾的現象產生。為解決干擾問題，本論文使用多載波分碼多址(multi-carrier CDMA)的技術，配合金氏碼(Gold code)的特性，使得接收端不同步的混合訊號能夠還原，得到原始訊號，整個室內空間都可獲得良好的通訊品質。

In visible light communication (VLC) system, LEDs are used for illumination as well as the transmitted sources of communication. In an indoor space, there are multiple LEDs set to supply basic lighting requirements.
The methods of transmission for multiple LEDs are usually divided into three ways: time division multiple access(TDMA), frequency division multiple access (FDMA), and code division multiple access(CDMA). In this thesis, we mainly use CDMA with orthogonal frequency division multiplexing(OFDM) technique to transmit signals from each LED at the same time and same frequency band. At the receiver end, we will detect all signals with multipath interference. Due to the feature of CDMA, we can separate each signal from each LED. On the other hand, since the distances from each LED to photon detector(PD) are not the same, we use asynchronous CDMA instead of synchronous CDMA. Thus, we don’t need to consider time synchronization between LEDs to extract the original signal. Therefore, we can get well signal performance in overall space.

[1] Dambul, K.D., D.C.O. Brien, and G. Faulkner, Indoor Optical Wireless MIMO System With an Imaging Receiver. IEEE Photonics Technology Letters, 2011. 23(2): p. 97-99.
[2] Hsu, C.W., et al., High Speed Imaging 3 × 3 MIMO Phosphor White-Light LED Based Visible Light Communication System. IEEE Photonics Journal, 2016. 8(6): p. 1-6.
[3] Indoor VLC using existing light sources Available from: https://www.researchgate.net/figure/216680021_fig3_Figure-21-Indoor-VLC-using-existing-light-sources.
[4] Kalikulov, N. and R.C. Kizilirmak. Adaptive OFDM for VLC systems with multiple transmitters. in 2016 IEEE MTT-S International Wireless Symposium (IWS). 2016.
[5] Komine, T. and M. Nakagawa, Fundamental analysis for visible-light communication system using LED lights. IEEE Transactions on Consumer Electronics, 2004. 50(1): p. 100-107.
[6] Global Lifi Tech. Available from: http://globallifitech.com/en/LiFi-Technology.php.
[7] Mulyawan, R., et al., MIMO Visible Light Communications Using a Wide Field-of-View Fluorescent Concentrator. IEEE Photonics Technology Letters, 2017. 29(3): p. 306-309.
[8] Chang, R.W., Synthesis of band-limited orthogonal signals for multichannel data transmission. The Bell System Technical Journal, 1966. 45(10): p. 1775-1796.
[9] OFDM. Available from: http://www.ist.hokudai.ac.jp/labo/w-icl/study/ofdm.html.
[10] FDMA versus TDMA versus CDMA. Available from: http://www.rfwireless-world.com/Terminology/FDMA-versus-TDMA-versus-CDMA.html.
[11] CDMA Basics. Available from: http://www.dolcera.com/wiki/index.php?title=CDMA_Basics.
[12] Schotten, H.D. and H. Hadinejad-Mahram. Analysis of a CDMA downlink with non-orthogonal spreading sequences for fading channels. in VTC2000-Spring. 2000 IEEE 51st Vehicular Technology Conference Proceedings (Cat. No.00CH37026). 2000.
[13] Walsh Transforms and Signal Detection. Available from: https://ipnpr.jpl.nasa.gov/progress_report2/42-37/37Q.PDF.
[14] Xu Duan, L. and C. Kyung Hi, Optimal PN sequence design for quasisynchronous CDMA communication systems. IEEE Transactions on Communications, 1997. 45(2): p. 221-226.
[15] Gold code. Available from: https://en.wikipedia.org/wiki/Gold_code.
[16] Gold Sequences. Available from: http://www.wirelesscommunication.nl/reference/chaptr05/cdma/codes/gold.htm.
[17] Mollah, M.B. and M.R. Islam. Comparative analysis of Gold Codes with PN codes using correlation property in CDMA technology. in 2012 International Conference on Computer Communication and Informatics. 2012.
[18] 各種場所照度標準. Available from: http://www.tmc.com/ninshow.asp?RecordNo=37.
[19] Stefan, I. and H. Haas. Analysis of Optimal Placement of LED Arrays for Visible Light Communication. in 2013 IEEE 77th Vehicular Technology Conference (VTC Spring). 2013.
[20] Lin, C.-N. Applications and Studies on Visible Light Communication Systems Using Multiple Light Sources. 2014; Available from: http://handle.ncl.edu.tw/11296/ndltd/14980217392044603689.
[21] 照明常用術語的定義. Available from: http://www.twepc.com.tw/led/k2.htm.
[22] 流明. Available from: https://zh.wikipedia.org/wiki/%E6%B5%81%E6%98%8E.
[23] The Human Eye's Response to Light. Available from: https://www.nde-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/lightresponse.htm.
[24] Barry, J.R., et al., Simulation of multipath impulse response for indoor wireless optical channels. IEEE Journal on Selected Areas in Communications, 1993. 11(3): p. 367-379.
[25] Huynh, V.V., et al. Inter-cell interference mitigation using soft frequency reuse with two FOVs in visible light communication. in 2012 International Conference on ICT Convergence (ICTC). 2012.
[26] Mondal, R.K., et al. Interference-aware optical resource allocation in visible light communication. in 2012 International Conference on ICT Convergence (ICTC). 2012.
[27] Quadrature amplitude modulation. Available from: http://www.wikiwand.com/en/Quadrature_amplitude_modulation.
[28] what is the difference between QPSK and 4QAM? ; Available from: https://dsp.stackexchange.com/questions/36515/what-is-the-difference-between-qpsk-and-4qam.