Research On Nonlinear Frequency Division Multiplexing Optical Fiber Communication Technology

As the backbone of the information industry,optical fiber communication takes responsi-bility of most of the information transmission tasks of the global communication network.With the development of the fifth generation mobile communication technology and the use of higher-order modulation formats,optical fiber communication has played an increasingly important role in information transmission.However,an increasing number of network information resources and communication application devices ask higher demands on the transmission performance and spectrum resource utilization of optical fiber communication systems.There are four main factors affecting the transmission performance of optical fiber com-munication systems,namely attenuation,dispersion,noise and nonlinearity.Attenuation can be compensated by placing erbium-doped fiber amplifiers in the optical fiber link.Dispersion can be eliminated by using dispersion-compensating fibers.Noise makes signal-to-noise ratio an important factor in measuring transmission performance.Therefore,nonlinearity becomes a major factor affecting the transmission performance of optical fiber communication systems.In recent years,various nonlinear compensation methods have emerged,but it is difficult to implement an efficient compensation scheme.As a result,it is of great significance to study feasible and efficient nonlinear compensation methods.In this paper,considering the nonlinear characteristics of optical fiber and the difference between fiber channel and the linear channel such as wireless communication,the nonlinear Schr?dinger equation is established as the theoretical model for the propagation of light waves in the fiber channel.Based on the normalized nonlinear Schr?dinger equation,the equation is described by the Lax pair which contains L operator and M operator that can remain unchanged before and after transmission in the fiber-optic communication system.The spectrum analysis of the L operator leads to the concept of nonlinear Fourier transform and inverse nonlinear Fourier transform.Signal can be transformed from the time domain into nonlinear frequency domain through the nonlinear Fourier transform.The effect of fiber nonlinearity and dispersion on the signal in a nonlinear channel can be expressed as a linear phase rotation in the nonlinear frequency domain.Then the signal can be transformed from a nonlinear frequency domain to a time domain through the inverse nonlinear Fourier transform.Using nonlinear Fourier transform to design nonlinear frequency division multiplexing transmission scheme has obvious advantages over traditional linear multiplexing technology.At the transmitter,the information to be transmitted is encoded on the discrete spectrum or continuous spectrum,and then the signal is converted into a time domain signal by the inverse nonlinear Fourier transform and loaded into the optical fiber for transmission,and finally non-linear at the receiver.The Fourier transform transforms the time domain signal into a nonlinear frequency domain and eliminates the nonlinear effects of dispersion by phase compensation.Three schemes of soliton transmission,discrete spectrum frequency division multiplexing trans-mission and continuous spectrum frequency division multiplexing transmission are realized by simulation experiments,and the effectiveness and feasibility of the scheme are verified.In order to improve the system transmission efficiency,the polarization-multiplexed non-linear frequency division multiplexing transmission system is studied based on the continuous spectrum transmission scheme.The channel analysis is carried out on the propagation of two orthogonal polarization states of light waves in the fiber,the coupled nonlinear Schr?dinger equation is established,the nonlinear Fourier transform and its inverse transform algorithm are programmed,and finally the simulation is implemented to achieve polarization.When using128 nonlinear subcarriers and 64QAM modulation format,the simulation system transmission rate can reach 153.6Gb/s.Finally,the detection method in the digital signal processing module of the receiving end is optimized,and after the nonlinear Fourier transform algorithm is called to convert the received time domain signal into the nonlinear frequency domain,the constellation point of each transmission symbol and the corresponding code are calculated.The Euclidean distance between the constellation points and the received spectral constellation is rotated by an angle such that the sum of the corresponding Euclidean distances of all symbols is minimized.The simulation results show that the Euclidean distance detection method can increase Q~2by 4.4dB compared with direct detection,and analyze its robustness based on noise influence.