Research Of Modulation And Coherent Receiving DSP Technology For Super-Nyquist WDM Systems

The deeper digging and more effective using of optical transmission spectrum is one of the means to improve optical communication system transmission capacity.For wavelength division multiplexing(WDM)optical transmission system,more intensive channel multiplexing means that the transmission spectrum efficiency is enhanced when the modulation order is unchanged.Therefore,super-Nyquist WDM system,which has smaller channel spacing than the symbol rate and faster transmission rate than the Nyquist rate,is put forward and becomes a research hotspot.When the channel spacing is smaller than the symbol rate,traditional modulation methods used in super-Nyquist WDM system will bring serious inter-channel interference.Due to the advantages on narrow spectrum bandwidth of modulated signals,the dual-polarization quadrature duo-binary(DP-QDB)modulation is used in super-Nyquist WDM system to reduce inter-channel interference.As to transmission impairment and its own inter-symbol interference problems,a feasible solution is to use coherent reception and digital signal processing(DSP)technology.It is necessary to study DSP methods by which the DP-QDB coherent receiver can repair damage and eliminate the inter-symbol interference as far as possible.Besides,in order to combat transmission impairment and reduce the bit error rate,the coding technology such as low-density parity-check(LDPC)code is introduced to DP-QDB system.When the receiver adopts soft-input-soft-output(SISO)decoding,the prior bit detection module needs to provide soft decision information of bits.The soft-input-hard-output(SIHO)detection algorithm like Viterbi-based maximum likelihood sequence detection(VA-based MLSD)cannot meet the requirements,and SISO bit-detection methods of DP-QDB signals should be used in DP-QDB system.Dual-binary shaping process,which can be treated as Class ? partial-response-pulse shaping filtering,is added to the I-and Q-branches in the DP-QDB modulation.The main research at present is confined to combine Class ? partial-response-pulse shaping technology only with dual-polarization quadrature phase shift keying(DP-QPSK)in super-Nyquist WDM transmission.In addition,the research of modulation methods in super-Nyquist WDM system mainly concentrates on the quaternary modulation and lacks application of higher order modulation methods,which limits the improvement of spectrum efficiency.To solve the problems above,this thesis focuses on research of the DSP technology used in the coherent reception of the existing modulation DP-QDB and the new modulation methods for super-Nyquist WDM system.The front-end DSP modules in DP-QDB coherent receiver are used to repair the transmission impairment of DP-QDB signal,and the detailed DSP methods for each module are given in this thesis.In the adaptive equalization part,the reason for transforming DP-QDB signals to DP-QPSK signals before constant modulus algorithm(CMA)equalization is analyzed.In the frequency estimation part,two different execution methods of frequency estimation algorithm are compared and analyzed.The joint frequency offset estimation method,which combines the linear z transformation(CZT)with the estimation method based on four power and fast Fourier transform,is applied on DP-QDB signals.It can improve the estimation method and reduces estimation error by one order of magnitude when CZT number is 1024.In the case where bit soft decision information is required,the bit detection module in DP-QDB systems needs to adopt SISO detection methods.Traditional maximum logarithmic-domain maximum a posterior probability(Max-Log-MAP)algorithm and soft-output Viterbi algorithm(SOVA)algorithm are applied in DP-QDB signal detection,and a new SISO iterative detection method for DP-QDB called bit soft-information output belief propagation(BSO-BP)is proposed.These three SISO detection methods are put in un-coded and low-density-parity-check-code(LDPC)coded 32G-Baud DP-QDB systems with channel spacing of 30GHz,and compared by simulation analysis.The simulation results indicate that BSO-BP detection and SOVA detection do not need the returning-to-initial-state(RTIS)process like Max-Log-MAP detection even though Max-Log-MAP detection has the best performance.In LDPC-coded DP-QDB system,the scheme using BSO-BP detection reduces the OSNR requirement at bit error rate of 10-5 by 2.61dB relative to SOVA detection when the block length is set to be 100.The spectral efficiency of super-Nyquist WDM system can be improved by application of higher order modulation.A new hexadecimal modulation format called DP-QDQ,which is based on combination of Class I partial-response-pulse shaping technology and dual-polarization hexadecimal quadrature amplitude modulation(DP-16QAM),is proposed and applied in super-Nyquist WDM system.In this thesis,the modulation principle of DP-QDQ is presented,and the DP-QDQ signal spectrum is deduced.The performance of DP-QDQ modulation and its application in super-Nyquist WDM system are analyzed by simulations.The simulation shows that the DP-QDQ modulation can be used to realize super-Nyquist WDM transmission.Compared with Nyquist-pulse-shaped DP-16QAM,DP-QDB has 3dB spectrum bandwidth of 46%less,and exhibits the performance advantage on dispersion tolerance and sensitivity to the sampling time error.