Research On Algorithms Of Carrier Phase Recovery Based On Unscented Kalman Filter In Short-range Coherent Optical Communication Systems

With the rapid development of big data,internet of things,cloud computing and other technologies,data traffic is growing rapidly.The optical interconnection network of short-range data centers is facing severe expansion pressure.Compared with the intensity modulation/direct detection system,the coherent optical communication system provides a feasible expansion scheme for data center optical interconnection with the advantages of high spectral efficiency and high receiving sensitivity.However,coherent optical communication systems are difficult to be directly applied to the cost and power sensitive short-range communication systems with their high-cost and high-power consumption.To reduce the cost of coherent optical communication systems,on the one hand,low-cost electro absorption modulation laser(EML)can be used to replace the expensive narrow linewidth laser(Hundreds of kHz order of magnitude)commonly used in coherent systems.However,the low-cost EML has wide linewidth(MHz order of magnitude),resulting in serious phase noise.On the other hand,small optoelectronic devices can reduce the cost of the the system with their low-cost and low-power consumption.Meanwhile the bandwidth limitation of small optoelectronic devices can be reduced by increasing the order of the modulation formats.However,with the increase of the order of the modulation formats,the signals become more sensitive to phase noise.Therefore,the carrier phase recovery algorithm with high linewidth tolerance and the low-complexity carrier phase recovery algorithm suitable for high-order modulation formats are deeply studied in this paper for short-range coherent optical communication systems.The specific research contents are as follows:(1)The carrier phase recovery algorithm based on pilot-symbols-aided unscented Kalman filter(UKF)is proposed for the low-cost wide-linewidth coherent optical communication systems.The phase noise of pilot-symbols-aided estimation is used as the initial value of the UKF estimation,and the phase noise estimation is completed by using the carrier phase recovery algorithm based on UKF.This scheme improves the convergence speed of the UKF algorithm,eliminates the cycle slip caused by cumulative phase noise,and prevents the propagation of error codes.In this paper,a 56 GBaud dual-polarization 16QAM coherent optical communication system which transmits 20 km standard single-mode fiber is demonstrated to verify the algorithm performance.The results show that the pilot-symbols-aided UKF algorithm has good phase noise tracking capability.The proposed algorithm has the linewidth tolerance of 6.44 MHz,which is better than the carrier phase recovery algorithm based on extended Kalman filter and the no-pilot-symbols-aided UKF algorithm,under 7%forward error correction threshold with 1 dB optical signal noise ratio penalty.(2)For the coherent optical communication systems with high-order modulation formats,the observation equation of UKF is modified,and a carrier phase recovery algorithm based on enhanced UKF is proposed.To solve the problem that the radius-based observation equation is not applicable to some signals with high-order modulation formats,the observation equation based on constellation points is proposed to make the UKF algorithm suitable for carrier phase recovery with high-order modulation formats.In this paper,coherent optical communication systems with 28 GBaud dual polarization of 16QAM and 64QAM are demonstrated to verify the performance of the proposed algorithm.The results show that the proposed algorithm has lower computational complexity while keeping the same performance as the blind phase search algorithm,and the computational complexity is reduced by 61%.To sum up,this paper aims at the problems of linewidth tolerance limitation and high complexity faced by the application of coherent optical communication systems to optical interconnection of short-range data center,the carrier phase recovery algorithm based on pilot-symbols-aided UKF with high linewidth tolerance and the enhanced UFK based carrier phase recovery algorithm with low complexity are proposed,and the effectiveness of the algorithms is verified by simulation.The results provide a feasible scheme for carrier phase recovery algorithm in short-range coherent optical communication systems.