Research On The Performance Of Carrier Phase Recovery Algorithms In Probabilistically-shaped Coherent Optical Systems

With the emergence of cloud computing,big data and various intelligent applications,data traffic presents an explosive growth trend,and the optical fiber communication systems are facing continuous capacity expansion demand.In order to improve the capacity,on the one hand,coherent optical systems can realize the high-order modulation format,and improve the spectral efficiency effectively.At present,coherent optical systems are the best solution to realize the capacity expansion.On the other hand,probabilistic shaping(PS)sends the constellation points with different probabilities to match the channel condition,so that the channel capacity can approache the Shannon limit.However,in the PS coherent optical communication systems,the probabilities of signals are not equal,which will affect the performance of carrier phase recovery(CPR)algorithm.Therefore,it is of great significance and practical value to study the carrier phase recovery algorithm in the probabilistically-shaped coherent optical communication systems.Due to the different transmission probabilities of PS signals,the performance of conventional carrier phase recovery algorithm designed for uniform distribution will be affected.In order to explore this problem,this paper studies the performance of carrier phase recovery algorithm and its improvement scheme for PS coherent optical communication systems.The details of the research contents are as follows:(1)Performance and improvement of QPSK Partition algorithm in PS coherent optical communication systems.The performance of conventional QPSK Partition algorithm in PS coherent optical communication systems is investigated,and a modified QPSK Partition algorithm based on maximum a posteriori probability(MAP)decision is proposed.By analyzing the statistical characteristics of the received signals,MAP decision is used to minimize the erroneous decisions.Meanwhile,the filter weight of the signals with different amplitudes is optimized to improve the performance effectively.The simulation results of PS 16-QAM system prove that the conventional QPSK Partition algorithm has serious performance damage in the PS systems.The modified QPSK Partition algorithm alleviates this damage,and realizes the mutual information(MI)improvement of more than 0.1 bit/symbol.The results verify the validity of the modified QPSK Partition algorithm based on MAP,and provide an improved idea for PS signals in carrier phase recovery.(2)Performance and improvement of Blind Phase Search(BPS)algorithm in PS coherent optical communication systems.The performance of BPS algorithm in PS coherent optical communication systems is investigated.The simulation results show that there is serious performance degradation of BPS in PS systems,and hundreds of symbols are needed for the filter to achieve the performance close to the theoretical value.To solve this problem,a recursive structure BPS algorithm is adopted.By introducing the forgetting factor,the filter structure in the conventional BPS is replaced with the recursive filter structure,and overlong filters are avoided by optimizing the forgetting factor.Simulation results of PS 64-QAM system show that the recursive BPS can effectively reduce the phase noise estimation error and alleviate the performance damage of conventional BPS in PS systems.The optical signal-to-noise ratio(OSNR)gain of more than 1 dB can be achieved when transmitting the same channel MI.The results show that the recursive BPS has important research significance and application value in PS coherent optical communication systems.To sum up,in this paper,the performance of carrier phase recovery algorithms in PS coherent optical communication systems are investigated.Aiming at the performance damage of conventional carrier phase recovery algorithm in PS system,a modified QPSK Partition algorithm based on MAP decision and a recursive BPS algorithm are proposed.We verify the performance of the algorithms by simulation.The results provide a feasible solution for carrier phase recovery algorithm in PS coherent optical communication systems.