Research On Phase Noise Compensation Algorithm In Coherent Optical OFDM Systems

Coherent optical orthogonal frequency division multiplexing（CO-OFDM） technology combining OFDM technology and coherent optical communication system, not only possesses the high sensitivity, long distance transmission of coherent optical communication technology, also has the OFDM technology resistance dispersion, polarization mode dispersion optical fiber channel, high spectrum efficiency and other advantages.CO-OFDM technology is one of the future development directions of optical communications, got the attention of the all society.Because of the non-ideal of laser line width on CO-OFDM system, the sending and receiving of laser optical carrier can’t be strictly relevant that leading to received signal exists phase noise. Phase noise will not only lead to CPE（Common Phase Error）, caused by the rotation of the signal constellation points, also can destroy the orthogonality between the signal carrier, caused ICI（Inter Carrier Interference）, deteriorating performance of system. The paper mainly studied the phase noise existing in the OFDM system, aiming at the effects of phase noise for receiving signals as well as the problems of the existing algorithm, put forward the corresponding improved algorithm respectively, and through the Matlab simulation platform, and jointly set up Optisystem simulation have been carried out to verify the algorithm.This article firstly focuses on the CPE compensation algorithms for OFDM system. To against existing problem of pilot method and algorithm of APNC that needs to insert pilots and increase the cost of the system, the article put forward the phase noise compensation with reduced（ROPC） method. The method based on the least-squares algorithm gets more accurate channel transfer function to estimate channel.by adding windows to channel transfer function, filtering out noise. Then by the quasi-pilot-aid mehod, the ROPC merhod makes the use of the relationship between the pilots and transmit data to compensate the phase noise of the signal. The required number of pilots can be reduced on the premise of the system’s reliability. The simulation results of system show when the error rate of ROPC method and APNC method are about 10^-3, the number of pilots required by ROPC method is about half of APNC method. The ROPC method reduces the cost needed by the system. With the linewidth of 0.1MHz and four pilots inserted,when the bit error rate is 10^-4,the ROPC decreased 2 d B of OSNR than the APNC merhod.The ROPC method could improve the reliability of the system.Phase noise can also lead to intercarrier interference which would cause constellation point cloud of divergence,increase the system’s bit error rate and reduce the reliability of the system.The ROPC algorithm has weakly inhibitory effect on ICI. To solve this problem,this paper presents an improved ROPC algorithm. Several subcarriers of cyclic prefix which are not subjected to intersymbol interference and subcarriers carryied the same information data are weighted average by improved ROPC algorithm to realize the intercarrier interference suppression. On the basis of the ROPC algorithm,the improved algorithm reduces the system bit error rate and improves the reliability of the system.At the same time, the improved algorithm which taks advantage of the effective subcarrier in cyclic prefix doesn’t add extra overhead and retains the advantages of high effectiveness without significant additional complexity.The simulation results show when the bit error rate of 10^-4,the signal-to-noiseratio of the improved algorithmis is improved about 1d B compared with the ROPC algorithm.