| In recent years, with the rapid development of video sites, three-dimensional video,telemedicine, cloud computing,and cloud storage,the spectrum of optical network has become more and more valuable. Traditional optical networks, which has adopted ‘one size fits all’ approach for the bandwidth allocation and performance management, are characterized with a fixed signal rate, channel spacing and signal formats, as a results, it is with low spectrum utilization efficiency and difficult in the evolution and expansibility of network. Flexible bandwidth optical networks, which can dynamically adjust features of the each channel(such as signal rate, modulation formats, etc) so as to achieve greater system flexibility and improve spectrum utilization efficiency according to the special requirements from the physical state of the link, traffic demand, is an effective method to overcome the disadvantage of the traditional optical network, upgrade its performance, and thereby to meet growing application demands.Orthogonal frequency division multiplexing(OFDM) optical network, which is characterized with high spectral efficiency, and high tolerance of fiber chromatic dispersion(CD) and polarization mode dispersion(PMD), is also an effective method to enlarge its transmission capability and improve its performance.In view of the unique advantage of flexible bandwidth and OFDM, thus combining the two technologies so as to realize the flexible bandwidth in OFDM optical networks is significant for the spectrum utilization, capacity expansion, and performance inprovement. However, in order to realize the combination many technical problems need to be solved, one of them is to realize the flexible bandwidth optical transmitter used in OFDM optical networks. Theoretical analysis shows the signals bandwidth in channel can be dynamically adjusted by changing the modulation format of OFDM sub-carriers signals of optical transmitter used in OFDM optical networks, with this method, the flexible bandwidth optical transmitter used in OFDM optical network can be realized. Therefore, a modulation module which has a fixed structure but can produce variety of modulation format is required. In this dissertation, aiming at the demand, a thorough investigationfor the theoretical basis, research background, and research status at home and abroad are presented firstly, then the following research work are carried out.1. The modulation formats of single carriers are researched and the methods to generate a fixed modulation format(such as 16QAM) by using various modulated module structure are investigated. Starting from the constellation of 16 QAM modulation signals, the nine kinds of scheme to realize the two different constellations(star constellation and square constellation) has been given in detail. And the design of sub-carrier modulation module of optical transmitter has been done for the eight schemes.2. A fixed sub-carrier modulation module structure which can generate many modulation formats is introduced. And the processes of generating three kinds(8PSK, 8QAM, 16QAM) modulation signal based on the structure are described. By applying this structure into flexible bandwidth optical transmitter, a dynamic selection of sub-carrier modulation format can be achieved.3. A detailed design for 16 QAM optical transmitter has been presented, and based on the design a simulation platform has been set up using Opti System 7. Then, the simulation for the nine kinds of the optical signal modulation schemes has been carried out with the simulation platform. At last, aiming at the complexity of the transmitter, a comparative analysis for the seven kinds of 16 QAM optical transmitter module structures are presented. The results show that with the development of integrated optical modules, the proposed schemes in this dissertation, which utilize a fixed modulation module structures to generate 16 QAM signal, will greatly reduce the complexity of system implementation, and therefore, can meet the needs of future flexible bandwidth.4. A flexible bandwidth optical transmitter model based on OFDM optical networks is proposed. If the fixed sub-carrier modulation structure is used as a sub-carrier modulation module of optical transmitter, further, combined with the optical OFDM technology and multi-carrier generation technology, the flexible bandwidth optical transmitter can be realized... |
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