Frequency-domain Compensation Of IQ Imbalance For CO-OFDM Communication System

With the growth of the demand for information, backbone is to work toward theaim of the ultra-high speed, large capacity, long distance. The single channel rate isat an advanced stage of400Gb/s even to1Tb/s, so the bandwidth it required isbeyond the ITU-T limit of a WDM network grid and channel spacing. In order tobreak the concept, the OFDM technique emerged. According to the mode ofdetection, OFDM fiber communication system can be divided into DDO-OFDM andCO-OFDM communication system. Compared with DDO-OFDM，The CO-OFDMcommunication system combines the advantages of coherent detection and opticalcommunications, and has attracted much attention for high receiving sensitivity, noprotection sideband, easy to equalization. However, there is also some drawbackssuch as high PAPR, sensitivity to frequency offset, phase noise, IQ imbalances, etc.at the same time. In this article, the influence and compensation methods of IQimbalance in CO-OFDM system is reseached.In actual CO-OFDM system, the amplitude of signal for branch I and Q is notcompletely equal, and the phase is not strictly orthogonal, which can create mirrorinterference between subcarriers and impact the performance of the system seriously.However, it is inevitable for drawback of manufacturing technique for analoguedevice. Thus, the key to solve the problem is to explore suitable IQ imbalancecompensation method at the receiver. While, the technical sheme is complex and thecompensate efficiency is unsatisfactory in the few existing report, and lacks the jointcompensation method for IQ imbalance at transmitter and receiver. Therefore, basedon the theory of IQ imbalance, this article carried out the following research workcombined with DSP technology:First, studied the two kinds of structure of a transmitter and a receiver for CO-OFDM communication system, analyzed the reason of IQ imbalance, derived thetheory model of IQ imbalance for the transmitter and the receiver and showed theimpact of IQ imbalance in the system.Then, analyzed the common IQ imbalance distributed compensation methods,i.e. GSOP method for the IQ imbalance at receiver and the common pilot aided method for IQ imbalance at transmitter. Then, this article proposed a jointcompensation method based on corrected pilot aided and a frequency domainmethod based on complexd-ICA, which can realize the transmitter and the receiverIQ imbalance compensation simultaneously, and has the characteristic of robustnessfor inherent damage in the system.Finally, the single mode optical fiber communication system with bandwidth of10Gb/s was built by the co-simulation of VPI Transmission Maker8.7and Matlab2010b, and made the comparison of compensating performance for the abovecompensation methods.The simulation results showed that the two methods can achieve the effectivecompensation, when adopted16QAM in this system and10-3as the target BER,compared with the common method, the OSNR penalty of joint compensationmethod based on corrected pilot aided and the frequency domain method based oncomplexd-ICA respectively improved0.13dB、0.3dB in the case of6dB amplitudeimbalance and0.14dB、0.27dB in the case of15°phase imbalance. Among them,joint compensation method based on corrected pilot aided has the advantage ofsimple structure and strong real-time performance, while, the frequency domainmethod based on complexd-ICA adopts the batch mode of signal and has higheraccuracy.