Phase Estimation Method For Coherent Optical OFDM System

Nowadays,with the requirement of the increasing demand for communication capacity, it is an inevitable trend that the new technologies will continue to emerge and develop. After decades of development, the coherent optical communication has become a mainstream of optical communication technology. And Orthogonal Frequency Division Multiplexing(OFDM) has been extensively studied as a new generation multiplexing technique. Coherent optical orthogonal frequency division multiplexing(CO-OFDM) technology is now an important part of the study of optical communication technology because of the advantages, such as high spectral efficiency, its robustness against dispersion(CD) and polarization mode dispersion(PMD). But, there is two dis-advantages of CO-OFDM,the peak to average power ratio of CO-OFDM is high and CO-OFDM is extremely sensitive to the frequency shift and the phase noise in the system. To estimate and compensate the phase noise in the CO-OFDM system, our lab has prevented a pilot-based blind(PBB) phase estimation method, which has combine the advantages of the blind method and the pilot based(PA) non-blind method. The unipolar amplitude modulation(AM) signal is loaded on the pilot subcarriers to estimate the phase noise. This paper proposes two improved methods of phase noise estimation based on the PBB method, including the mixed-polar-amplitude-modulation pilot-based blind(MPAM-PBB) method and the data-aided linear fitting pilot-based blind(DALF-PBB) method. The performance of the proposed phase noise estimation methods including the conventional pilot-aided(PA) estimation method, PBB method, MPAM-PBB method and DALF-PBB method are compared in a CO-OFDM transmission simulation. We analyze and demonstrate the feasibility and the performance of the proposed two methods. The main contents of this paper are as follows:(1) In the mixed-polar-amplitude-modulation pilot-based blind(MPAM-PBB) method, half of the signals loaded on the pilot subcarriers are the unipolar AM signals. Meanwhile, the other half are the bipolar AM signals, of which the order of the AM signals are higher than unipolar AM signal. And the data capacity laded on the pilot subcarriers are improved. The main idea of improving is that, since in the PBB method, the pilot subcarriers are loaded with the unipolar AM signal, which would lead to a waste of resources, is only signaled a unipolar, so the bipolar AM signals are added to improve the order of the AM signals and it is effective to reduce the DC component of the signal so as to further improve the system spectral efficiency. The simulation system simulation results indicate that this method could improve the spectral efficiency while also ensure the system bit error rate(BER) performance.(2) In the data-aided linear fitting pilot-based blind(DALF-PBB) method, the data is used to assist the estimation of the phase noise, so the number of the selected pilot subcarriers could be halved. The unipolar half of the pilot subcarriers in the PMAM-PBB method are reserved and the bipolar half of the pilot subcarriers are no more needed. The main idea of improving is that, by the aided of the data, the number of the pilot subcarriers could be decreased and the spectral efficiency could be improved. Through the simulation, 0.4d B OSNR improvement and good tolerance to the line-width can be obtained. The simulation results show that the performance of the DALF-based method is better than the conventional PA-based method.