| Recently, with the rapidly progress of HD, ultra high HD, the broadband demand of user has been rapidly increased. Optical fiber has been applied into the backbone network because of the advantages such as:good anti-intererence ability, wide frequency band, low loss, light weight and low cost."Copper back into the light"has already been the general trend. It is much easier to do the digital signal processing in the receiver by coherent detection, because of this, dispersion and polarization mode dispersion are no longer the key restrict of transmission rate, but the loss and fiber nonlinearity. Losses can be solved by developing a large effective area of low-loss optical fiber, but for fiber nonlinearity, as a worldwide problem, there is still not a precise theory to estimate or compensate it. So the analysis of the theory of nonlinearity model is worthy of in-depth study. Only when we know how exactly the system coefficient affect the fiber nonlinearity, we can in further propose assumptions for nonlinear compensation algorithm.Orthogonal frequency division multiplexing (OFDM) is a special kind of multi-carrier modulation. It has high spectrum effiency, because of the overlap between the orthogonal subcarrier, which has been the key technology in wireless communication. It has arrived in time as the key technology for the ultra high-speed long-haul optical transmission of the next generation due to the high spectrum efficiency, high receiver sensitivity, and convenient digital signal processing technology. In this paper, the mechanism of nonlinear effects of multi-carrier coherent optical OFDM (CO-OFDM) communication system is in-depth studied. We started from the setup of the simulation platform to channel estimation, and then fiber nonlinearity and we obtained the following innovative research results:1. An improved timing synchronization algorithm is proposed in multi-carrier CO-OFDM systems, which can effectively eliminate the smoothing platform phenomenon in the timing metric curve caused by cyclic prefix. The simulation results show that the algorithm can accurately estimate the symbol timing moments and has better performance than the classical Schimdl method.2. An improved channel estimation algorithm is proposed in112Gbps polarization division multiplexing CO-OFDM system. This method combines the advantages of the time domain average and the frequency domain average, can achieve more accurate channel estimation. Simulation results show that even at relatively rapid changed environment, the system performance remains stable. When the differential group delay (DGD) equals to1000ps, our combined channel estimation performance outperforms the ISFA algorithm2.2dB. When considering the linewidth and frequency offset, there is more than0.5dB advantage to ISFA method.3. The Bell Labs air space timing coding (BLAST) algorithm is applied to improve the nonlinear tolerance in112Gbps polarization division multiplexing CO-OFDM system. We simulated system performance under different dispersion compensation ratio. Simulation results show that under the absence of dispersion compensation condition, aftering transmitting800km, the system performance has at least5dB advantage compared with ISFA algorithm.4. The autocorrelation analysis method is proposed to extract the phase noise and the polarization scattering induced by XPM to analyze the characteristics of the XPM in wavelength multiplexing CO-OFDM system. Through this analysis method, we can clearly see how the system parameters affect XPM. In addition, we verified the OFDM system phase noise is the dominative impairment while there is no phase recovery algorithm in the receiver.5. The limitation of the theory about considering all the fiber nonlineratiy noise to be four wave mixing (FWM) in densely spaced wavelength division multiplexing CO-OFDM system is proposed which can not be applied in other modulation format because of the major assumptions in the reference. Our results show that the theoretical value can be affected by the parameter in estimation formula. Under different fiber condition and different system parameter, we can make the theoretical value indentical with the simulation value by adjusting the parameter, however it exist a big difference in different conditions. So this estimation method is not precise enough and not completes enough.In all, this paper has done a comprehensive study about fiber nonlinearity in coherent optical OFDM system, which was from the setup of the single channel simulation platform to the channel estimation, and then the synchronization estimation, phase estimation and then the analysis of the fiber nonlinearity in WDM channel. |
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