| As one of the most important technology in optical communication,Coherent optical orthogonal frequency division multiplexing(CO-OFDM)has the advantage of a excellent performance in high speed and long distance communication environment.Therefore,it is widely concerned and researched in recent years.Traditional optical communication is easily affected by dispersion,while the OFDM technology has a strong ability to resist dispersion.Coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology combining OFDM technology and coherent optical communication system,possesses both advantages of these two technologies.With the rapid development of communication network and the unceasingly upgrading of communication systems,the CO-OFDM system is widely used in practice is the future development trend of optical communication.Although many researchers have already proved the feasibility of this model through the results,it still faces some problems that need to be solved.The transmitter and receiver hardware devices are not ideal and other factors will cause IQ(in phase and quadrature)imbalance problem,and IQ imbalance leads to the presence of inter-carrier interference at the receiver.At the same time,the frequency deviation of the CO-OFDM system due to the center frequency of the transmitter and receiver will lead to carrier frequency deviation.IQ imbalance problem and carrier frequency offset problem will interfere with each other,seriously affect the system performance,resulting in signal constellation point of the divergence,rotation and other issues,resulting in the received data in the demodulation process can not accurately restore the source signal,serious deterioration System performance.In this paper,the problem of IQ imbalance in CO-OFDM system and the problem of carrier frequency offset and IQ imbalance simultaneously exist are discussed respectively.Through the mathematical theory,not only established the solution model,but also proposed the corresponding improved algorithm.The simulation platform is established by Matlab and Optisystem,and the algorithm is verified by simulation,specifically divided into the following two points:(1)In order to solve the inter-carrier interference problem caused by IQ imbalance in CO-OFDM system and to against the data-helper class based on three subsets proposed by Won Soo Chung * et al existing the problem of increasing the cost of the system.Based on the structure of the IQ imbalance estimation model,the pilot structure of the traditional data assistant algorithm is improved by mathematical derivation,which the improved algorithm for transforming the training sequence of three subsets of W algorithm into two subsets(TS,Two subsets),without increasing the overhead of the system,the accuracy of the IQimbalance factor estimation is improved and the IQ compensation is performed by the improved algorithm.When using the same number of OFDM symbols and the system optical signal to noise ratio(OSNR)is the same,the TS algorithm reduces the bit error rate of the systems more than the W algorithm,improving the reliability of the system.This conclusion is verified by the simulation of the bit error rate variation of the system with the OSNR variation of the W algorithm and the TS algorithm.In the case of the transmitter IQ amplitude imbalance 0.7d B,phase imbalance 25 degrees of the system,the transmission distance of240 km,and to ensure that the same bit error rate case,the TS algorithm requires a lower OSNR than the W algorithm.In the case of bit error rate of-410,the TS algorithm requires about 1d B of OSNR lower than the W algorithm.(2)In order to solve the problem of IQ imbalance and carrier frequency offset existing simultaneously in CO-OFDM system,an improved TS algorithm is proposed for CO-OFDM system based on the existing classical method of carrying out carrier synchronization using repetitive sequence structure,combing the IQ imbalance and carrier frequency offset compensation algorithm in wireless OFDM system with the TS compensation algorithm proposed in Chapter 4 of this paper.The improved TS algorithm not only transforms the three-level structure of IQ imbalance and carrier frequency offset compensation algorithm in wireless OFDM system into two-layer structure,but also inherits the high validity of TS algorithm.Although the TS algorithm increases some overhead of the system,it can effectively balance the system with carrier imbalance and IQ imbalance.This conclusion is verified by the simulation of the bit error rate variation of the system with the OSNR variation of the W algorithm,TS algorithm and the improved TS algorithm.In the case of the transmitter IQ amplitude imbalance 0.7d B,phase imbalance 25 degrees of the system,the normalized frequency deviation is 0.05,and to ensure that the same bit error rate case,the improved TS algorithm requires a lower OSNR than not only the W algorithm but also the TS algorithm.In the case of bit error rate of-410,the improved TS algorithm requires about5 d B of OSNR lower than the W algorithm.,which improves the reliability of the system greatly.The result is because the W algorithm and the TS algorithm only consider the IQ imbalance of the system,which not balance the carrier frequency offset.It can be shown that the improved TS algorithm not only inherits the advantages of the TS algorithm which can effectively reduce the system IQ imbalance,but also can reduce the influence of carrier frequency offset on system performance,which has a greater tolerance for both IQ imbalance and carrier frequency offset. |
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