Research On Nonlinear Effects And Palliation Methods In Optical OFDM System

Optical orthogonal frequency division multiplexing(OOFDM)technology is an effective scheme of realizing high data rate transmission thanks to its robustness to multipath fading,high frequency efficiency,elimination of inter-symbol interference(ISI)and inter-carrier interference(ICI),and the ability to combat both chromatic dispersion(CD)and polarization mode dispersion(PMD)in optical transmission.However,a major drawback of optical OFDM signals is their high peaks which will lead to the signals work in the nonlinear range of the nonlinear electrical/optical components.And it will cause signal distortion,spectrum spread and system performance degradation.Meanwhile,high peaks will result in nonlinear phase noises induced by fiber self-phase modulation(SPM).The objective of this thesis is to research the techniques for nonlinear effects and palliation methods based on reducing the peak-average-power ratio(PAPR)or nonlinear distortion metrics in optical OFDM system.The main content of the paper and innovation are as follows:First,The intensity modulation/direct detection(IM/DD)OOFDM system with Companding Transform(CT)technique is proposed.CT technique is simple,easy to realize,with low complexity,and suitable for high speed digital signal processing(DSP)in optical communication.As traditional ? law companding transform will enhance the power of transmitted signal,? law companding transform is used at first,and then the linear transform is used for the sake of keeping the signals of equal average power.In order to reduce the influence of expanded noises,the nonlinear CT technique without decompanding operation at the receiver is used.Simulation results show that as a function of ?,BER performance increases at first,and then decreases.As PAPR reduction increases with the increasing of ?,we should choose an optical? according to the tradeoff between PAPR reduction and BER performance.And we choose ? of 2 for experimental investigation.The PAPR of OFDM signal is decreased apparently with CT technique.Theoretical investigation and experimental analysis have shown that PAPR reduction could mitigate not only the nonlinearities of nonlinear components,but also the nonlinear phase noises in the fiber link when the optical power into fiber was high.Second,A novel null subcarrier shifting(NSS)scheme is proposed in an IM/DD OOFDM system for PAPR reduction.Null subcarrier shifting technique is a distortion-less method to reduce the PAPR without BER loss.However,it needs side information,and the computational complexity is high.A constant subcarrier is proposed to be as the embedded side information to implement demodulation accurately at the receiver.The position of the shifted null subcarriers can be detected by checking the power levels of null subcarriers and the sign of constant subcarriers.Meanwhile,the shifting possibility is degraded apparently by shifting in turn scheme without sacrificing PAPR reduction capability and BER performance.Considering about the tradeoff among the length of the guard band,spectrum utilization and PAPR reduction,the value of p should be set to be low,and we experimentally investigate the system performance when P is 2.Experimental results have shown that the shifted null subcarriers could be detected accurately at the receiver,and the computational load was degraded significantly.Compared to the original OFDM system,the proposed novel null shifting scheme can not only decrease the PAPR of OFDM signals,but also increase the receiver sensitivity a lot.Moreover,the improvement of receiver sensitivity is larger when the optical launch power is higher.It is because that PAPR reduction can not only minimize the nonlinear effects of nonlinear devices,but also significantly reduce fiber nonlinearities.Third,PTS technique is induced to a 60 GHz radio over fiber(ROF)system for PAPR reduction of OFDM signals.PTS technique is a distortion-less method to reduce the PAPR based on looking for the optimal phase factor.A 60 GHz ROF system is built in this paper,and the 60 GHz optical millimeter wave is gotten by frequency quadrupling scheme which is achieved by using one dual-arm Mach-Zehnder modulator(MZM)that is biased at the minimum transmission point to suppress the odd-order sidebands and an inverted rectangle optical filter which is used to filter the central carrier.In the ROF system,5Gbps and lOGbps 64QAM-OFDM signals with PTS technique to decrease the PAPR are transmitted.Simulation results have shown that inducing PTS technique in ROF system could decrease the PAPR of OFDM signals,and increase the receiver sensitivity.Fourth,A new method of calculating the nonlinear distortions of MZM is proposed,and two techniques are used to reduce the distortion to signal ratio(DSR)of MZM.The DSR is a measurement of nonlinear distortions induced by MZM.Firstly,an equivalent mathematical polynomial model of MZM is built to calculate the DSR of MZM.Then the DSR metric can be minimized with different techniques,and the nonlinear noise of MZM is minimized.According to the MZM mathematical model,an IM/DD OOFDM system with PTS technique and null shifting technique to reduce the DSR of MZM is proposed.Theoretical investigation,simulation analysis and experimental results have shown that compared to the PAPR reduction scheme with PTS technique,the computational complexity of the proposed scheme was decreased a lot,and that the system BER performance was better as well.Four kinds of palliation methods of nonlinear effects are proposed in this paper.The first three schemes are based on PAPR reduction,and the fourth scheme is based on distortion metrics reduction of MZM.CT technique which is very simple and with low complexity is suitable for systems with high DSP rate requirement.Both of the null shifting technique and PTS technique have an effective performance improvement of PAPR reduction without any signal distortion,and they are suitable for systems with high accuracy requirement.Both of the BER performance and computational complexity of the DSR reduction scheme are better than those of the PAPR reduction scheme.However,the fourth scheme is limited to the optical systems with MZM.In real systems,we should choose proper palliation method of nonlinear effects according to the system requirement and device level.