Compensation For The IQ Imbalance In OFDM Optical Fiber Communication System

With the rapid growth of the gross information content, the demandsfor high-capacity, high-speed communications technology is more and moreurgent. As the main communication technology of backbone network,optical fiber communication technology faces enormous challenges. Inorder to obtain a higher transfer bit rate and higher spectral efficiency,a more advanced modulation formats is bound to adopt, in this case, OFDMoptical fiber communication technology is generated. OFDM has someunique advantages, such as the high spectrum efficiency and the abilityto fight against the fiber dispersion. Therefore, the study of opticalfiber communication with OFDM technology is hot off the press in recentyears.The subcarriers of OFDM are orthogonal, which reduces the spacebetween the subcarriers and improve the spectrum efficiency, but alsomakes OFDM more sensitive to phase noise, frequency offset thantraditional modulation format. At the sending end or receiving end, theimperfect technology and undesirable production environment or theimbalance structure, will engender the IQ imbalance between the I/Q twoways of the OFDM signal. IQ imbalance will cause the subcarriers toproduce a mirror effect, in the process of up-conversion ordown-conversion OFDM spectrum. OFDM subcarriers are distributedsymmetrically, which causes lethal mirror interference between thesymmetric subcarriers. Therefore, the elimination of IQ imbalance inOFDM optical fiber communication system is imperative. In order to avoidthe system performance falling sharply caused by IQ imbalance, this makesa higher demand to analog components used in transceiver, which is bound to cause a sharp rise in costs. Accordingly, in order to release therequirements for the hardware device, it becomes a trend to exploreappropriate compensation package to combat the effects of IQ imbalanceusing the electric DSP at the receiving end, lower IQ imbalance impacton the system.Main work of this paper is as follow:Firstly, we used OFDM optical fiber communication system as the basicmodel, present details of the OFDM optical fiber communication system.Discuss the causes of IQ imbalance causes from the transmitter andreceiver separately, derivate the transfer matrix of the system in thepresence of IQ imbalance in theory. Verify the feasibility of the ICAfor the IQ imbalance compensation in OFDM optical fiber communicationsystem, with the analysis on the characteristics of imbalance matrix andthe basic theory of ICA algorithm.Secondly, on the study of the ICA algorithm, we used a fixed-pointFast-ICA algorithm based on negative entropy maximization to achieve IQimbalance compensation, and give the flow of the IQ imbalancecompensation method based on the Fast-ICA algorithm. This IQ imbalancecompensation method is a blind IQ imbalance compensation method, whichachieves IQ imbalance compensation with no prior knowledge, only basedon containing information in the mixed signal. Compared with the existingIQ imbalance compensation method based on the training sequence, thisIQ imbalance compensation method improves the spectrum efficiency andmeets the needs of real-time online compensation.Finally, we use VPI Transmission Maker8.7software to build thesimulation platform, analog the communication scene in the presence ofIQ imbalance, analysis of the IQ imbalance OFDM optical fibercommunication system, verify the compensation performance of the IQ imbalance compensation method based on the Fast-ICA algorithm. After IQimbalance compensation, the OSNR requirement is reduced by nearly7dBat theBER1.086103, when IQ imbalance are1d B50.