Electronic Dispersion Compensation In High Speed Optical Communication Systems

Nowadays, massive amount of data demands have aroused a myriad of industrial and academic researches to develop cost-effective optical communication systems with the rapid development of wide-bandwidth networks. The data rates have gone from 10G to 40G even 100G. The transmission distance has steadily increased as well. However, as the capacity grows, many signal degradation effects become the prominent and seriously limit factors to further upgrade for the system. Dispersion, one of them, is playing a much important role to impact on high speed transmission system's performance. Dispersion can lead to considerable pulse broadening and distortion. As a consequent, these impairments cause intersymbol interference (ISI) at the receiver side result in significant penalties. Compared to the traditional optical compensation techniques for combating ISI, the applications of Electronic Dispersion Compensation (EDC) have recently gained attractive attention in optical communications. EDC offers adaptive compensation which will be required in future dynamic optical networks avoiding the expensive cost and high optical loss.There are two challenges comes with the realization of high speed optical communication systems:how to transmit and receive the signals effectively? How to deal with the severe signal distortions caused by the dispersion? EDC can provide the optimum solutions to face the challenges.The Passive EDC techniques are intended to relief distortions at the transmitter side. As the called "passive" EDC techniques, many novel modulation formats have been raised and applied recently. In the traditional transmission system, such as the intensive modulation/direct diction (IM/DD) system, No return to zero (NRZ) is the main modulation format in the current 10G transmission systems. These kinds of amplitude-based modulation cannot be suitable for the next generation transmission systems because of which require the high optical-signal-to-noise ratio (OSNR), high dispersion and nonlinearities tolerances. Compared to NRZ, some new different duty circle RZ formats and phase shift keying (PSK) formats are widely used for the high speed transmission systems becase they have the characteristics mentioned above. Moreover, frequence and polarization of light are used to carry information besides amplitude and phase. Polarization Division Multiplexing (PDM) and Orthogonal Frequency Division Multiplexing (OFDM) are come out in recent years.However, as precautions against those effects passive schemes cannot solve the challenges perfectly, the active EDC techniques are necessary. There are some typical equalizers as active EDC techniques, such as feed forward equalizer (FFE), decision feedback equalizer (DFE) and maximum likelihood sequence estimation (MLSE) equalizer. These equalizers have the corresponding modules to compensate the chromatic dispersion (CD) and polarization mode dispersion (PMD) effectively. Additionally, with the rapid development of electronics and signal processing techniques, coherent detection is come back. The DSP at the receiver side can be implemented by the most of equalizers..In this thesis investigates the novel modulation formats, EDC in IM/DD and coherent optical communication systems. It will be make further research on simulation and design realization of DSP scheme. The work can be divided into four parts.1. Theoretical analysis of dispersion in fiber including CD and PMD and elaborates the techniques of dispersion compensation in optical domain and electrical domain briefly.2. Firstly, the different duty circles of RZ, DPSK and DQPSK are introduced and analyzed. Secondly, the simulation systems in Optisystem are set up and carried out. The simulation results show that CSRZ formant has the best performance in the intensity modulations. In phase modulation formats, the most outstanding format is DPSK. DQPSK modulation is also a very promising choice if the noise disturbance could be resolved.3. The structures of FFE, DFE and MLSE equalizer are discussed. The LMS theory based on MMSE (Minimum Mean Square Error) is explained. A new channel estimation method called "histogram" probability density function (PDF) and Viterbi algorithm based on transitions are proposed. The simulation systems in IM/DD are established by Optisystem and Matlab. The co-simulations via different equalizers and different modulation formats are carried out. The simulation results show that equalizers of EDC have improved the systems performance in some intensity formats. However, FFE and DFE show a slight better performance in CSRZ and phase shift keying formats, where as MLSE plays a much better role in all kinds of modulation formats. 4. A PDM-QPSK coherent optical communication system is introduced. The structure of every module is analyzed. The singularity problem is discussed. A point of view in Poincare sphere to explain the singularity problem is proposed. In the presence of polarization dependent loss (PDL), a constrained self-reconfiguring CMA and complex-valued QAM (C-QAM) ICA algorithm are brought forward and expounded. A 100Gb/s PDM-QPSK coherent optical communication system is simulated in Optisystem and Matlab. The simulation results show constrained self-reconfiguring CMA and C-QAM ICA algorithm are singularity-free and robust against source loss to polarization demultiplexing correctly.