Research On IQ Mismatch Compensation Algorithm In Coherent Optical OFDM Systems Considering The Laser Phase Noise

Coherent optical orthogonal frequency division multiplexing(CO-OFDM) technology not only has advantages of coherent detection technology, such as high sensitivity, long distance transmission, but also advantages of OFDM, like large capacity, high spectrum efficiency, its robustness against dispersion(CD) and polarization mode dispersion(PMD), etc. Unfortunately, CO-OFDM is proved to be extremely sensitive to non-idealities in the transmitter(Tx) and receiver(Rx), e.g., in-phase/quadrature(IQ) mismatch and laser phase noise. Both effects will cause inter-carrier interference(ICI) and degrade the system performance seriously. In this thesis, the IQ mismatch and laser phase noise compensation methods are studied and simulated systematically. The details are described as follows:(1) A novel compensation method for Tx IQ mismatch is proposed in coherent optical orthogonal frequency division multiplexing(CO-OFDM) system considering laser phase noise, the laser phase noise and channel distortion are combined as a new channel transfer factor. The conventional compensation method is that the mismatch factor and channel distortion are compensated first, then the phase noise is compensated based on the decision statistics from the first step. However, the estimated mismatch factor and channel distortion will be affected by the laser phase noise, which will lead to inaccurate decision statistics, especially for larger linewidth. In our thesis, the new channel transfer factor can be updated on a symbol-by-symbol basis which retrieves an estimation of the phase noise value by extracting and averaging the phase drift of all OFDM sub-channels, and both the distortions can be eliminated by the equalization process, then the IQ mismatch distortion is compensated by using the equalized symbol. Simulation results show that when transmisssion rate is 20Gb/s, the phase and amplitude mismatch are 15 o and 3d B respectively, a 2.4d B optical signal-to signal ratio is improved for 60 k Hz of the laser line-width, and the tolerable laser linewidth can be improved by 98 k Hz. Furthermore, the complexity of the proposed method is reduced by 43% compared with the DA in terms of the number of required complex multiplications per bit.(2) A novel compensation scheme for Rx IQ mismatch is proposed in CO-OFDM system considering laser phase noise. The channel distortion with laser phase noise and Rx IQ mismatch can be estimated separately. Conventional compensation methods, such as using Gram-Schmidt orthogonalization procedure(GSOP) or two same training symbols, the laser phase noise has to be compensated based on pilot-aided. In the proposed scheme, laser phase noise and channel distortion were combined as a new channel transfer factor, the IQ mismatch factor and initial channel transfer factor can be estimated independently based on the relationship of IQ mismatch factors. And the channel transfer factor can be updated on a symbol-by-symbol basis which retrieves an estimation of the phase noise value by extracting and averaging the phase drift of all OFDM sub-channels. Numerical results indicate that when transmisssion rate is 20Gb/s, the phase and amplitude mismatch are 10 o and 2d B respectively, a 1.6d B optical signal to noise ratio is improved for 60 k Hz of the laser line-width. Furthermore, the complexity of the proposed method is analyzed in terms of the number of required complex multiplications per bit.(3) A joint scheme to compensate for transceiver IQ mismatch and laser phase noise simultaneously is proposed. Conventional method compensation for Tx IQ mismatch laser phase noise and Rx IQ mismatch requires multiple continuous processes. The estimation and compensation noise accumulation continues until the last procedure is completed. The influence parameters are defined including IQ mismatch information and laser phase noise which makes them estimated together via analyzing the signal model. Simulation results show that when transmisssion rate is 20Gb/s, the Tx phase and amplitude mismatch are 10 o and 2d B, Rx phase and amplitude mismatch are 5o and 2d B, a 1.2d B optical signal-to noise ratio is improved for 60 k Hz of the laser linewidth. Furthermore, the complexity of the proposed method is reduced by 46.15% compared with the hybrid method in terms of the number of required complex multiplications per bit.