The Research On Optical Performance Monitoring Technique Based On Fractional Fourier Transform

With the advent and booming of cloud computation,internet of everything,high-definition video and 5th generation network,the global IP traffic has seen high-speed and sustained growth.However,the capacity of fiber is approaching its maximum limit,so it is difficult to meet the insatiable demand only by increasing the bit rate on the single fiber.In order to make the most out of scarce network spectrum resources,optical networks require adequate operation and management.Therefore,it is vital importance to monitor various network performance parameter and acquire real-time information about the physical state of the network and health of the signal,and this is collectively referred to as optical performance monitoring(OPM).Fractional Fourier transform(FrFT)is a powerful digital signal processing tool,which can comprehensively analyze the signal characteristics compared to traditional time domain and frequency domain method.We take advantage of FrFT to achieve chromatic dispersion(CD)estimation,time/frequency synchronization,and modulation format identification(MFI)in OPM.The main research contents and novelties are summarized as follows:(1)We propose a superimposed FrFT training sequence(TS)to achieve CD estimation as well as time/frequency synchronization simultaneously in the coherent system.The principle is derived and the feasibility is verified on the simulation.Compared to the traditional blind estimation method,the estimation accuracy is the same and the superimposed TS does not sacrifice the spectrum efficiency and save 91.8% computational complexity.(2)We design a multifunctional FrFT TS to achieve time/frequency synchronization,CD estimation and MFI simultaneously.Under the condition where optical signal-to-noise ratio(OSNR)ranges from 8 dB to 28 dB,CD ranges from 1600 ps/nm to 32000 ps/nm,the TS identifies up to 6 kinds of modulation formats with 100% accuracy.Compared to traditional blind CD estimation method,the proposed CD estimation is robust to polarization mode dispersion(PMD).Even under the condition where differential group delay is 90 ps,the maximum CD estimation error is below 400 ps/nm.(3)We propose a multifunctional FrFT TS to achieve CD estimation and MFI simultaneously.4 different orders of FrFT is encoded to 4-bit modulation formats,and the maximum total number of MFI is 16.When OSNR is 10 dB and CD is 32000 ps/nm,the maximum CD estimation is less than 400 ps/nm,and the MFI accuracy is 100%.Moreover,we design a novel synchronization TS,which can synchronize with no error where signal to noise ratio is low and CD is uncompensated.The algorithms above have the advantages of multiple parameter monitoring,low complexity,strong robustness to impairments,which are expected to support the coherent receiver technically in the flexible optical network.