Research On Key Techniques In QAM Optical Coherent Communication Systems

In recent years,with the rapid development of broadband information services such as Internet,video on demand,big data and cloud computing,the demand for bandwidth of various communication services has increased exponentially.In addition,optical fiber trasmission network resources have been exhausted and the traditional 10/40Gb/s intensity modulation/direct detection(IM/DD)communication system has been unable to meet the needs of modern communication services.Although the capacity of optical communication systems can be improved by erbium-doped fiber amplifier(EDFA),wavelength division multiplexing(WDM)and dense wavelength division multiplexing(DWDM),the development space has been exhausted,while the transmission network redeployment requires a high cost.Therefore,in order to meet the requirement of transmission capacity and spectrum efficiency for the explosive growth of broadband information service in today's society,optical coherent communication systems based on high-order modulation formats,coherent detection and digital signal processing(DSP)has been a hot research topic of optical communication network with ultra high-speed and ultra-large capacity.In the paper,the key technologies of optical coherent communication system such as high order QAM modulation formats,coherent detection and signal impairment compensation in electrical domain are studied.The main research work and achievements are as follows:1)The signal generation schemes for star,square and circular 16QAM are studied.In addition,a circular 16QAM signal generation scheme is proposed and the performance of the optical coherent communication system based on circular 16QAM modulation is verified.Finally,the advantages and disadvantages of several signal generation schemes are compared and analyzed.2)The principle of coherent detection is analyzed and the damage of optical signal in optical coherent communication system is studied.The optical coherent communication system based on different high-order modulation formats(QPSK,16QAM,64QAM)is built,and the influence of loss on different systems is simulated.3)The digital signal processing algorithms such as dispersion compensation,polarization de-multiplexing,nonlinear compensation and carrier phase noise estimation are studied,and several carrier phase noise estimation algorithms are studied in detail.The transmission system is set up and the effective compensation of the received signal is realized by the simulation.4)112Gbit/s,224Gbit/s,336Gbit/s optical coherent communication system based on square DP-16QAM are set up and the performance of the system is studied.The transmission of 112Gbit/s and 224Gbit/s DP-16QAM signal in 3520 km and 3100 km is simulated respectively by DSP compensation technology when the FEC threshold is.The result shows that the optimal fiber input power of 112Gbit/s DP-16QAM system is-1dBm.Considering in the case of fiber dispersion damage and compensation only,the transmission distances of 112Gbit/s,224Gbit/s and 336Gbit/s DP-16QAM system are 2400km,1700km and 1150km respectively when the bit error rate tolerance is.For 112Gbit/s DP-16QAM system,the bit error rate of system with non-linear compensation is reduced by at least an order of magnitude compared with system without non-linear compensation.Considering dispersion and nonlinearity only,the non-linear optimal weighting coefficient is 1.9 and the optimal compensation step is 60km.When the FEC threshold is,the OSNR cost of the system is only 3.2dB after the carrier phase noise estimation compared with theoretical value.