Research On Digital Signal Processing Algorithm Of Large-capacity Optical Comb Communication System

With the rapid increase of transmission capacity in optical transmission systems,coherent optical wavelength division multiplexing(WDM)system is a key technology to satisfy the continuous growth demand of capacity and speed for long-haul transmission systems.The accuracy and complexity of digital signal processing(DSP)algorithms also directly affect the quality and speed of signal recovery.However,further improvement of the system capacity and rate will cause problems such as the cost of receiving DSP.The WDM systems based on optical frequency comb with high stability and phase coherence can reduce the computational complexity of DSP and have attracted more attention in recent years.This thesis investigates the DSP algorithms for long-haul transmission of the optical comb system,comparing with those of the traditional WDM system.The main contents and innovations are is as follows:1.The application of master-slave carrier phase estimation(MS-CPE)algorithm is posssibe due to the phase correlation of multi-wavelength optical frequency comb.However,through numberical simulation of the effect of long-haul optical signal transmission,it is verified that the fiber dispersion-induced walk-off effect causes the phase coherence between different channels be destroyed,and the performance of the slave channels is worse than the master channel.In order to solve this problem,two improved methods are proposed.One is to use a two-stage CPE algorithm and increase the number of master channels;the other is to add optical delay modules to compensate for the dispersion walk-off.The performance of the above methods is analyzed.Simulation shows that compared with the traditional WDM system,the complexity of CPE module is reduced by 65.3% and 93.5% respectively,and the average SNR loss is within 0.79 d B and 0.74 d B when the transmission distance is 2800 km.Although subjected by the influence of the nonlinearity effects,the two methods can also maintain great performance.2.Joint equalization algorithm is adopted to reduce the influence of inter-channel interference caused by compressing protection bandwidth in high-capacity and highspectrum efficiency system.The simulation results show that the joint equalization performance based on LMS and RLS is better,and the performance based on CMA is slightly worse.In addition,when the system transmits with high spectrum efficiency,the influence of laser wavelength drift will cause frequency offset,and the mixed influence of the frequency offset and inter-channel interference will cause a sharp decrease in performance.In the optical comb system,the master-slave frequency offset estimation is used to achieve multi-wavelength carrier recovery.Compared with the performance of the system without frequency offset,the SNR loss of comb system is less than 1d B resulting from the optical comb system has the same offset of each channel,while the SNR loss of the traditional WDM system is more than 3d B.Both theoretical analysis and simulation results show that the optical comb system has obvious advantages in largecapacity and long-haul transmission.