Research On Nonlinear Transmission And Phase Noise Tolerance Of The Circular Quadrature Amplitude Modulation Format

With the explosive growth of smart phones and computer applications,the demand for communication capacity and bandwidth is increasing,and multi-order quadrature amplitude modulation(QAM)is widely used because of its high spectral efficiency.The traditional square QAM modulation scheme has deficiencies in many aspects,such as low resistance to phase noise and nonlinear effect,also,cannot use simple and efficient carrier phase noise recovery algorithm to complete the phase noise estimation and compensation,these shortcomings greatly limits the communication capacity and transmission distance.Therefore,it is very necessary to find a new modulation format with higher phase noise and nonlinear tolerance.In this paper,the influence of phase noise and nonlinearity on the modulation format is discussed.Based on this,the design of constellation of circular QAM,the improvement of carrier phase estimation algorithm,the amplitude optimization of circular QAM,nonlinear transmission performance comparison are studied deeply.The constellation design algorithm is used to filter out the modulation format with high tolerance to Gaussian white noise and phase noise.The algorithm of carrier phase noise estimation is modified and amplitude is optimized for different circular QAM constellation structures.Then,phase noise and nonlinear transmission performance of constellation structures are discussed.In the aspect of constellation design algorithm,constellation structure design of low-order 8QAM and high-order 16 QAM modulation formats has been completed.Through the constellation design algorithm,the constellation pattern combination is traversed to filter out the constellation morphology with the highest tolerance to white Gaussian noise.The fixed deflection angles are selected to simulate the existence of small phase noise and large phase noise to filter out the constellation structure with high phase noise tolerance.For phase noise estimation algorithm,blind phase search and Viterbi-Viterbi phase estimation(VVPE)algorithm are applied to circular QAM modulation formats,and the phase noise recovery algorithm is modified according to different constellation structures.The improved VVPE algorithm can reduce the complexity of the algorithm while improving the estimation accuracy of Gaussian white noise,and estimate the carrier phase noise of different circular QAM and rectangular QAM.Also,it verify that the phase noise tolerance of circular QAM is better than that of square QAM.In the aspect of constellation amplitude optimization,the symbol error rate is used as the criterion to scan the radius by setting a suitable scene,and the radius of theconstellation is optimized by selecting the appropriate radius to obtain a higher tolerance of Gaussian white noise.In this paper,the common linewidth is taken as an example to scan the radius of the constellation in the presence of Gaussian white noise and phase noise,so that the system's symbol error rate is further improved.In order to compare the performance of each constellation structure with that of nonlinear transmission system,a coherent optical communication system is constructed and the optical fiber transmission links are simulated for each modulation format.The tolerance of each modulation format to nonlinear effects is analyzed and compared.The simulation results show that the digital signal processing technique introduced in this paper can effectively compensate the signal damage,and the nonlinearity tolerance of partial circular QAM is higher than that of square QAM modulation.The result of this paper is of great significance to the design and optimization of the modulated signal.In a practical application scenario,a circular QAM can be designed according to different scenarios,which not only improves the tolerance of phase noise but also increases the signal transmission distance.For the optical fiber communication system provides a more flexible design.