Research On The DSP Technology In High Spectral Efficiency Optical Communication System

Optical fiber communication systems are the national information infrastructure,and the fiber transmission rate is increasing continuously.However,traditional wavelength-division-multiplexing（WDM）systems based on intensity modulation/direct detect（IM/DD）have been developed over 20 years.Their spectral efficiency is not sufficient for today’s high-speed communications.Coherent optical communication technology,as a research hotspot in late 1980’s,has come back to people’s perspective.The last generation of coherent optical systems has not employed analog-to-digital converter（ADC）and digital signal processing（DSP）technology,so the synchronization of transceiver lases’ frequency and phase has to be implement by optical methods.Nowadays,due to the availability of broadband ADC,coherent optical receiver can implement massive chromatic dispersion（CD）compensation,massive polarization mode dispersion（PMD）compensation,timing synchronization,carrier recovery,etc.at.To compensate the linear and nonlinear damages in coherent optical fiber communication system,some relevant algorithms are necessary.DSP technology is very important for today’s coherent optical communication system,so the technology is worthy of study.The thesis focuses on the application of DSP technology in high spectral efficiency optical communication system,including coherent optical communication system and short-reach IM/DD optical communication system.The thesis is based on theoretical and experimental proof.The main contributions of the thesis are as follows:Firstly,the performance of digital double-carrier signal and single-carrier signal are compared on the same test bed.It is experimentally demonstrated for the first time that,single-carrier signal has better performance than that of double-carrier signal.A digital double-subcarrier system based on orthogonal single-side band（OSSB）modulation is proposed.In common with 2-IFFT OFDM,when quadrature phase shift keying（QPSK）is applied to the subcarriers,OSSB modulation is presented as 9-quadrature amplitude modulation（9-QAM）.Therefore,the adaptive equalization by CMMA,frequency offset estimation（FOE）and carrier phase recovery（CPR）can be implemented without any overhead.Since the two modulation schemes have the same spectral-efficiency（4 bit/s/Hz）as polarization multiplexing single-carrier QPSK withNyquist pulses,their performances are experimentally compared.The experimental results demonstrate that,the single-carrier system is robust due to its better performance and less complexity than that of the double-subcarrier system.Secondly,a kind of blind polarization demultiplexing algorithms based on low-complexity and fast-converging independent component analysis（ICA）for QAM coherent optical communication systems is proposed.The polarization demultiplexing is achieved by maximizing the signal’s non-Gaussianity measured by the information theoretic quantity of negentropy.We demonstrate that some approximate nonlinear functions can be substitutes for the negentropy and this greatly reduces the computational complexity.An adaptive gradient optimization algorithm and a fast-converging Quasi-Newton algorithm are employed to maximize the negentropy.The numerical simulation and experimental results for polarization division multiplexing（PDM）-QPSK/16QAM without neglecting PMD reveal that the proposed ICA demultiplexing algorithms are feasible and effective for coherent optical receivers and ICA shows its own virtues of immunity against singularity and modulation format independence.Thirdly,a simple spectral-efficiency IM/DD system based on half cycle sub-carrier modulation（SCM）signal is proposed for short reach fiber communications in this paper.The signal impairment of frequency selective fading due to fiber CD is mathematically analyzed.The peak to average power ratio（PAPR）of the signal is also discussed in comparison with that of orthogonal frequency division multiplexing（OFDM）.The transmission of 16-QAM half cycle SCM signal with a sub-carrier frequency of half the symbol rate and Nyquist pulse shaping is experimentally demonstrated.The bit-error rate（BER）of 48-Gb/s polarization multiplexing division（PDM）16-QAM half cycle SCM signal is less than 7%forward-error-correction（FEC）threshold of 3.8x10^-3 after transmission over 83-km standard single-mode fiber（SSMF）.Without utilizing Erbium doped fiber amplifier（EDFA）,we have demonstrated the transmission of single-polarization 64/128-QAM half-cycle SCM using cost-effective directly modulated laser（DML）and also transmission of dual-polarization signal using a pair of external modulators.The experiment is enabled by spectral-efficient overlap frequency-domain-equalization（overlap FDE）and PDM channel estimation based on a pair of time-interleaved training sequences（TSs）.Utilizing an electrical bandwidth of only 3 GHz,the bit-error rate（BER）of the 32.5 Gb/s dual-polarization 64-QAM format after 20-km fiber transmission is under 3.8x10^-3and BER of the 38 Gb/s dual-polarization 128-QAM format is under 2.4x10^-2.