Research On Algorithms Of Carrier Phase Recovery In Wide-linewidth Coherent Optical Systems

With the rapid growth of data traffic,optical fiber communication systems are facing severe pressure of capacity expansion.Compared with Intensity Modulation/Direct Detection(IM/DD)systems,as coherent optical communication systems could simultaneously modulate multiple dimensions of the optical carrier,it has higher spectral efficiency.So,coherent optical communication systems could meet the increasing demand for optical transmission networks' traffic.However,coherent optical communication systems have the disadvantages of high cost and large power consumption.Traditional coherent optical communication systems require narrow-linewidth lasers at the transmitter and receiver,which occupies a large proportion of the cost in the system.If wide-linewidth lasers could be applied in coherent optical communication systems,the cost of the coherent optical systems would be reduced.Therefore,studying wide-linewidth coherent optical communication has important research significance and practical value.With the increase of lasers' linewidth,the phase noise of received signal becomes serious,and the probability of cycle slip increases.To solve the above problems,in this paper,we study the carrier phase recovery algorithms with large linewidth tolerance for low-cost and wide-linewidth coherent optical communication systems.The details of the research contents are as follows:(1)Carrier phase recovery algorithm based on Extended Kalman Filter.The Extended Kalman Filter(EKF)algorithm is used for carrier phase recovery.Using the characteristics of estimating random state vectors in EKF,the phase noise could be predicted and estimated.We analyze the optimal parameters and corresponding optimal performance of the carrier phase recovery algorithm based on the EKF under the QPSK and 16QAM modulation formats through the simulation system,and we compare this performance with the optimal performance under the optimal parameters of the Viterbi-Viterbi(VV)algorithm and the Blind Phase Search(BPS)algorithm.Simulation results show that the performance of the carrier phase recovery algorithm based on EKF is better than the performances of the VV algorithm and BPS algorithm under QPSK and 16QAM modulation formats,when the linewidth of lasers is larger than 5-MHz and 1-MHz,respectively.(2)Data-assisted based parallelized extended Kalman carrier phase recovery algorithm.To solve the serialized signal processing problem of the EKF algorithm,a parallelized-EKF carrier phase recovery algorithm based on the assisted data is proposed.By periodically inserting assisted data into the payload data block,the initial phase information could be provided to the EKF module.In addition,the reference phase information is calculated by the assisted data to effectively eliminate the cycle slip.The feasibility of parallelized-EKF algorithm and the linewidth tolerance of the proposed algorithm are verified by simulation.The simulation results show that under the 7%FEC threshold,when the OSNR Penalty is 1-dB,the normalized linewidth tolerance could reach 2.5×10-4.Furthermore,the parallelized-EKF algorithm was used to realize the transmission under 50-km standard single-mode optical fiber,with 20-MHz linewidth lasers.The results verify that the parallelized-EKF algorithm has great potential for the future coherent optical transmission with low-cost and wide-linewidth lasers.To sum up,in this paper,carrier phase recovery algorithms with large linewidth tolerance are studied for low-cost and wide-linewidth coherent optical communication systems,including the carrier phase recovery algorithm based on EKF and the parallelized-EKF carrier phase recovery algorithm based on assisted data.We verify the performance of the algorithms by simulation.The results in this paper provide a feasible solution for the large-capacity coherent optical communication systems based on the low-cost and wide-linewidth lasers.