Research On Equalization Of Signal Impairments In KK Receivers Of Short Range Optical Communication Direct Detection System

At present,optical communication technology is developing in the direction of low cost and high speed.The optical long transmissions usually choose coherent receivers to receive signals in the long distance.However,the short reach systems prefer direct detection receivers due to the cost and equipment.In order to improve the transmission rate of the system,we can improve the application of high-order Modulation format in the direct detection system to improve the spectrum efficiency of the system,such as QPSK and QAM.At the same time,we can also use multiplexing technology to improve the transmission rate of the system.However,in the direct detection system,the square detection of photodetector will cause SSBI,which will affect the recovery of signals.Kramers-Kronig receivers was proposed to compensate the SSBI.At the KK receiver system,the signal phase can be obtained from the Hilbert transform of the current intensity of the signal,thereby reconstructing the signal,which can effectively eliminate the influence of SSBI.The spectral efficiency of the PDM KK receivers is the same as that of the dual-polarization coherent receiver system,and the equipment used in the KK receivers is simpler.Optical signals will be damaged in the optical transmission because of the frequency and other natures of the signal.It mainly includes power attenuation,CD,and Polarization effect damage.The Polarization effect damage usually includes PMD,PDL and RSOP.At the same time,the CFO and CPN will be introduced when the local oscillator laser is used at the receiving end.These damages will change the amplitude and phase of the signal and affect the decision and decoding of the signal.With the development of DSP technology,these damages can be compensated by the DSP technology at the receiver.Due to the short distance of direct detection system,the damage caused by PMD and PDL to the signal can be ignored.Therefore,in the KK receiver system,CD and RSOP are the main damages to be compensated.In the algorithm of CD compensation,the dispersion coefficient is generally regarded as a constant,so static compensation can be carried out in the frequency domain or time domain.However,due to the manufacturing process of the fiber and the temperature during transmission,the dispersion coefficient will change.In this case,static CD compensation will cause RCD.CMA and MMA are the traditional RSOP equalization algorithms,which include the MIMO equalization structure,while they suffer from the problem of singularity and slow convergence speed.Moreover,with the presence of RCD,they could not work well in the KK optical communication systems.Therefore,it is necessary to find an algorithm that can balance the faster RSOP damage and has a tolerance to RCD in the KK receiver system.So that the RCD will not affect the recovery of the signal.This thesis focuses on the research for the equalization of the damage in KK receiver system and analyzes the mechanism of generation and mathematical model of the impairments.Moreover,this thesis proposed the algorithms to compensate CD and RSOP jointly under the background of the mechanism of reconstruction signal of the KK receivers.The main work content and innovation points of this thesis are as follows:1.The structure of the KK receiver system and the transmission and reception of the high-order modulation format in the KK receiver system areintroduced in detail?The simulation of the PDM KK receivers is realized in the simulation platform.2.Aiming at the damage in the KK receivers in the DD optical communication,we modeled the damage mathematically.The algorithm canaccurately track the CD coefficient and compensate,without the traditional dispersion compensation module.3.By selecting the right elements of Kalman filter and combining with the steps of damage compensation in the KK receiver,the algorithm based onKalman filter was proposed to realize the joint compensation of CD and RSOP in the PDM DD system.The feasibility of the proposed algorithm was verified in a simulation platform.The result shows that,the proposed Kalman scheme exhibits better performance for ultra-fast RSOP tracking(up to speed of 2.5Mrad/s)and large accumulated CD equalization(more than 2040ps/nm)when OSNR is 23 dB and CSPR is 9dB.