Application And Research On Technology Related To OFDM Technology In Optical Communication System

With the rapid development of data service and video traffic, the demand of large transmission bandwidth increases rapidly with amazing speed. In order to meet the demands of network bandwidth and the highly continuous growth of single channel rate, high-speed optical communication systems are abandoning the conventional Intensity-Modulation/Direct-Detection (IM/DD) scheme. Applying the mature OFDM technology of wireless communication into the optical field which can be used to realize the high-speed optical transmission has become one of research focuses.Optical OFDM system, which has the advantages of high transmission rate, high spectrum efficiency, high immunity to chromatic dispersion and lower cost, combines the advantages of wireless OFDM technology and optical communications. The optical OFDM system is able to construct the optical networks with high-speed, lower-cost and long-distance transmission. Furthermore, the optical OFDM system has been considered to be one of potential technologies for next generation high-speed and long-distance transmission optical networks.This paper illustrates the related fundamental theories and systemic concepts of Coherent Optical OFDM (CO-OFDM) system. Then, the performance and some key technologies of optical OFDM were deeply investigated by constructing the CO-OFDM simulation platforms. Some major research contents and achievements are summarized as follow:(1) Based on the study of basic principles and key technologies about OFDM technology, this paper investigates the principles of optical OFDM system. From the formula derivation of basic theory, combined with the current status of domestic and foreign research, the paper deeply studies the theory of each component part in CO-OFDM system and analyzes advantages, disadvantages and potential applications of CO-OFDM in detail.(2) Based on the CO-OFDM simulation platform, which is constructed by VPI and Matlab simulation software, the paper verifies the correctness of CO-OFDM simulation platform. Meanwhile, the paper simulates and analyzes the influence of MZ modulator, DAC and ADC on the performance of CO-OFDM system.(3) Based on the CO-OFDM simulation platform, which is constructed by VPI and MATLAB, this paper investigates the theory and influence of phase noise, CD and PMD in CO-OFDM system. For compensating for impair caused by linewidth, CD and PMD, there are three algorithms presented in this paper:1.Pilot aided phase compensation algorithm (PAPC for linewidth); 2.Time domain average channel estimation algorithm based on training symbols (TSTA for CD); 3.Intra-symbol frequency domain averaging algorithm with time domain averaging (ISFA with TA for PMD). Simulation results indicate that PAPC can compensate for phase noise caused by linewidth very well, even when frequency offset exists and OSNR is low TSTA can estimate the phase shift aroused by CD accurately and ISFA with TA, with the advantages of time domain averaging and frequency domain averaging, can totally compensate for the influence caused by PMD in CO-OFDM system.(4) Based on the characteristic of CO-OFDM and the influence of frequency offset on OFDM system, this paper comprehensively analyzes the model of action and influence of frequency offset in CO-OFDM system. Blind frequency offset estimation based on cyclic prefix and virtual subcarrier is presented in CO-OFDM system. From the formula derivation, the paper comprehensively analyzes the influence of the fractional part, the integral part and the residual part frequency offset on CO-OFDM system. Meanwhile, for compensating for the damage of frequency offset for CO-OFDM system, a novel structure of OFDM symbol is provided in this paper. Simulation is done to evaluate the performance of frequency offset estimation algorithm. Simulation results show that even when OSNR is low, blind frequency offset estimation based on cyclic prefix and virtual subcarriers algorithm can totally compensate for the influence of frequency offset.(5) A novel joint frequency offset and channel estimation method utilizes training symbols for compensating for phase offset caused by the residual frequency offset. It not only improves the estimation accuracy, but also increases the system spectrum effectiveness through avoiding pilot. Simulation results show that the novel joint frequency offset and channel estimation method can nearly totally compensate for the system impairments caused by CD and frequency offset and increase the accuracy of frequency offset estimation.This paper comprehensively investigates the key technologies in CO-OFDM system, such as channel estimation and frequency offset estimation. At the same time, the corresponding compensation algorithms are presented. MZ modulator, the linewidth of laser, DAC and ADC are also included in the research domain of the paper.