Research On Signal Equalization Algorithms For High-speed Coherent Optical Communication Systems

With the development of people's requirements,more and more network services need to be supported by high speed transmission rates.Hence,the transmission rate is the key breakthrough target of modern communication systems.The coherent optical polarization division multiplexing(PDM)technology is a communication technology which is widely used to improve the transmission rate in optical communication systems.Based on the polarization characteristics of light,the PDM technology uses two orthogonal optical carrier polarization states to simultaneously propagate two signals,which can achieve the goal of doubling the transmission rate.However,PDM technology also brings the corresponding polarization loss to the transmission signal,especially the rotation state of polarization(RSOP)will conduce two orthogonal optical signals to alias and interfere with each other,which brings great trouble to the demodulation of the receiver.In extreme weather with strong flashing lightning,the speed of RSOP can reach the level of 10⁶ rad/s,which makes the bit error rate(BER)of the communication systems rise rapidly to above 10^-3.Therefore,this paper focuses on the high-speed RSOP equalization algorithms to improve the performance of coherent optical communication systems.To realize the equalization of RSOP,adaptive equalization algorithms are widely used in coherent optical communication systems.The most classic of traditional adaptive equalization algorithms is the constant modulus algorithm(CMA).However,the CMA cannot cope with the phase loss existing in the channel and high-order modulation format signals.To overcome the above issue,a multi-modulus algorithm(MMA)is developed for coherent optical communication systems.When the speed of RSOP is high,the MMA cannot keep the BER of the communication system within an acceptable range.In this paper,the problem of the multi-mode algorithm is investigated and than a new algorithm is proposed to improve the performance of coherent optical communication systems.First,the hysteresis effect of the MMA equalization matrix is investigated.The hysteresis effect shows that the historical input contains the information of the equalization matrix in the future,so this information can be extracted to improve the equalization matrix at the current moment.Based on this idea,a new algorithm is proposed,which is called time reverse multi-modulus algorithm(TR-MMA).Compared with the MMA,the average BER and steady state error(SSE)of the TR-MMA are reduced.Moreover,the average tolerance of RSOP and convergence speed of the TR-MMA are increased.The advantages of the TR-MMA are more obvious when the speed of RSOP is higher.Furthermore,the three-parameter equalization matrix model and adaptive moment estimation are used to reduce the amount of calculation and output error,and speed up the convergence of TR-MMA.The simulation results show that,compared with the TR-MMA,the average BER and SSE of the improved TR-MMA are reduced.Moreover,the average tolerance of RSOP and convergence speed of the improved TR-MMA are increased.