Research On Core DSP Algorithms Of FTN-NOFDM Coherent Optical Transmission System

With the development of mobile Internet,big data,cloud computing,online game applications and other technologies,people's demand for high data rate and large capacity optical transmission system is growing,increasing system capacity by improving spectrum efficiency has become one of the feasible technologies to solve the problem of spectrum resource limitation in optical transmission systems.Faster than Nyquist non orthogonal frequency division multiplexing(FTN-NOFDM)technology breaks the orthogonality of traditional orthogonal frequency division multiplexing(OFDM)technology,it compresses the bandwidth by compressing the subcarrier spacing,so as to obtain higher spectral efficiency than OFDM.Polarization multiplexing coherent faster than Nyquist non orthogonal frequency division multiplexing system(PM-CO-FTN-NOFDM)not only has the advantages of resisting chromatic dispersion as multicarrier modulation,but also mantains the advantages of high spectral efficiency and receiver sensitivity as polarization diversity coherent reception,in addition,it achieves high frequency spectral efficiency through subcarrier spacing compression.It can meet the requirements of large capacity,high speed,high spectral efficiency and high receiver sensitivity in future optical tansmission system,which has important research significance and application prospect.This paper focuses on the key technology of PM-CO-FTN-NOFDM optical transmission system-digital signal processing(DSP)algorithm.The main contents of the research are as follows.1.Severe inter-carrier interference(ICI)is introduced into FTN-NOFDM system,the DSP algorithm of traditional PM-CO-OFDM is no longer applicable.It is necessary to improve and design the DSP algorithm scheme of PM-CO-FTN-NOFDM system according to the damage characteristics of NOFDM system.This paper chooses to add zero to replace cyclic prefix in the transmitter according to the characteristics of FTN-NOFDM,and designs the frame structure accordingly.In the receiver,aiming at the influence of serious ICI,this paper designs algorithm processing flow and DSP algorithms such as frequency offset estimation,phase offset estimation and channel estimation.After compensating transmission damage,this paper uses signal detection algorithm to suppress inherent ICI.The feasibility of the proposed DSP scheme is verified by simulation under transmission distance of 300km in single channel 112Gbps PM-CO-QPSK-FTN-NOFDM system,and the results show that the spectrum utilization can be improved by 14.3%at the cost of 0.4dB optical signal to noise ratio.2.Aiming at the problem that non-orthogonal pilots are superimposed with other subcarriers in FTN-NOFDM system,a joint frequency offset and phase offset estimation algorithm based on time-domain radio frequency pilot(RF-Pilot)is proposed.The proposed algorithm adds direct current in the time domain and turns it into RF-Pilot after upconversion,it can estimate the frequency offset and phase offset by using its high power and zero phase characteristics without phase damage,so as to effectively reduce the impact of inherent ICI on the system performance.The simulation results of single channel 112Gbps PM-CO-QPSK-FTN-NOFDM optical transmission system show that when the spectrum efficiency is increased by 14.3%,the frequency offset tolerance is greater than 2GHz,the optimal pilot to signal ratio is-17?-13dB and the maximum overhead of guard subcarrier is 8.9%.3.Due to the poor performance of the traditional data-assisted channel estimation method in frequency domain and the high complexity of the blind channel estimation algorithm in FTN-NOFDM system,this paper proposes a frequency domain channel estimation algorithm without interpolation based on Jones matrix according to the characteristics of PM-CO-FTN-NOFDM transmission channel,which is not affected by the inherent ICI in FTN-NOFDM and has a low complexity.The simulation results of single channel 112 Gbps PM-CO-QPSK-FTN-NOFDM optical transmission system show that when spectrum utilization is increased by 14.3%,the proposed algorithm can compensate the polarization mode dispersion of at least 30 ps adaptively,and the maximum overhead of training symbol is 2.9%.