Research On DFT Spread Spectrum Optical OFDM Transmission System

Over the past decades, with the rapid development of the Internet, the userdemands for the large amount of data greatly increase. Optical communicationnetwork, as backbone networks of data transmission has been rapidly developed withthe continuous emergence of new technologiesin various aspects of optical devices,optical transmission systems and receiviers. New modulation techniques in wirelesscommunication gradually migrate to the applications in a variety of opticalcommunication systems. Orthogonal frequency division multiplexing has becomeone of the hot spot, due to efficient spcctrum efficiency, flexible digital signalprocessing for transceivers, etc.,.Optical OFDM technology has the advantages insuppression the ifber dispersion, low system cost and t ift for long-distancetransmission, etc.. It thus has become one of alternative technologies for the nextgeneration long-distance transmission systems with ultra high-speed.In this thesis, optical OFDM transmission systems based on optical DFT-spreadare mainly studied. The main works are as follows:(1)We study the development of OFDM technology, the realization of OFDMsignal by discrete Fourier transform. Research status at home and abroad aresummarized and the crucial problems in optical OFDM technologies are studied,patricularly, different algorithms in optical OFDM systems to reduce the value ofPA PR.(2)Based on the platform of Optisystem software, we establish CO-OFDMsimulation system and study the principle and implementation of the optical OFDMsignal，and the basic principles of coherent optical OFDM transmission system anddirect detection optical OFDM transmission system. We analyze the major influentialfactors to the oplical OKDM systems, such as line width of lasers, opto-electric converters, dispersion of ifber, and cyclic preifx of coherent light.(3)We study the causes of high PAPR values in optical OFDM systems and theperformance impact to the optical transmission systems. We establish systemsimulation on the platform of Matlab，and comparatively analyze four differentmethods to reduce the PAPR values, and test the impacts to optical OFDMtransmission systems based on the optical DFT-spread.(4)Based on (3) we compare the inflecnces of different sub-carrier spreadspectrum approaches to systems. Combined with the characteristics of the dispersionmapping, a selective mapping method based on the interpolation of subcarirers isproposed and validated in established simulation system. The simulation resultsshow that the value of PAPR in optical OFDM system based on DFT-spread withscheme of selectivity mapping in the subcarirer interpolation is lower than that withthe scheme of conventional optical transmission OFDM0.5-2dB.