| Along with the rapid development of Internet applications, conventional optical communication systems which use direct-detection and wavelength division multiplexing are now unable to fulfill the increasing demands on capacity. As a result, how to increase the system capacity within the existing limited bandwidth, thus to realize high spectral efficiency transmission, is the core need of current optical communication systems.Transmission with high spectral efficiency could be achieved in electrical domain or optical domain according to the number of optical carriers. On one hand, if only one optical carrier is occupied and several orthogonal and overlapped subcarriers in electrical domain are used, this generates the main research topic of this dissertation-coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. CO-OFDM, which combines the advantages of coherent optical communication systems, orthogonal frequency division multiplexing techniques and digital signal processing, has become a key solution for future high-speed and long-haul optical communication systems. However, due to the high peak-to-average power ratio (PAPR) of OFDM signal, the system performance is limited by fiber nonlinearity. There are some solutions which either change the system structure, or occupy extra spectrum, or increase the system cost. In this dissertation, novel modulation formats based CO-OFDM system to improve the system performance is proposed and analyzed without changing any part of the system. Furthermore, differential encoding is introduced to novel modulation formats based CO-OFDM system. This could not only increase the nonlinear tolerance and spectral efficiency of the system, but also decrease the complexity of the system.On the other hand, superchannel which utilizes several optical carriers to realize multi-carrier transmission is introduced. Superchannel could be realized flexibly without being fixed in the International Telecommunication Union (ITU)50GHz grid. Two approaches to realize superchannel have been fully investigated:all optical orthogonal frequency division multiplexing (AO-OFDM) and Nyquist wavelength division multiplexing (Nyquist WDM, N-WDM). However, AO-OFDM could not support advanced modulation formats and N-WDM needs strict requirements of optical filtering and signal processing. Another approach named coherent wavelength division multiplexing (CoWDM) is mainly discussed in this dissertation, including its principles and advantages.The main research work and innovations in this dissertation are listed as follows:1. Novel modulation formats based CO-OFDM systemsa) The basic principles and the mathematical model of CO-OFDM system are studied. The performance of different modulation formats based CO-OFDM systems under transmission penalties including ASE noise, CD, fiber nonlinearity and laser linewidth are investigated over a simulation setup based on VPItransmissionMakerTM8.6and MATLAB.b) Due to the coherent overlapping of subcarriers, the PAPR (peak-to-average power ratio) of OFDM signal could be very high. Thus the fiber nonlinearity becomes the most major limitation to increase the transmission length of CO-OFDM system. Two novel modulation formats, which are amplitude phase shift keying (APSK) and star L QAM (L represents the number of constellation points), are introduced to CO-OFDM system. Quadrature amplitude modulation (QAM) format is the most commonly used modulation formats in CO-OFDM system due to its optimized average Euclidean distance (AED) in the additive white Gaussian noise dominated condition. However, compared with conventional QAM based OFDM signal, PAPR of APSK and SLQAM based OFDM signal is comparatively lower, which could in turn improve the system tolerance towards fiber nonlinearity and increase the transmission distance. However, the AED of APSK and SLQAM is smaller than conventional QAM format, so the tradeoff between the advantage of low PAPR and small AED should be made carefully for practical situations. Without changing the architecture, these methods could increase the transmission distance after balance fiber nonlinearity and ASE noise. And as almost all the algorithms to improve the tolerance towards fiber nonlinearity are transparent to modulation formats, they could be used to the novel modulation formats modulated CO-OFDM system for further improvement.c) To reduce the demands on apriori knowledge like training symbols and pilot subcarriers, differential relation is induced to CO-OFDM system in this dissertation. As the unique of OFDM signal, the differential relation could be induced between the adjacent subcarriers in one OFDM symbol, which could be called differential-relation-in-frequency-domain; and the differential relation could be induced between the adjacent symbols carried on one subcarrier, which is called differential-relation-in-time-domain. These two methods perform differently under different transmission penalties. The first symbol of differential-relation-in-time-domain signal could be regarded as a training symbol, so it performs better than differential-relation-in-frequency-domain signal under the influence of CD effect. While the fist data located on the first subcarrier works like a pilot subcarrier for differential-relation-in-frequency-domain signal, the performance is better than differential-relation-in-time-domain when lase linewidth is the dominated effect. As these two methods inherit the advantage of the low PAPR of novel modulation formats, and they don’t need training symbols and pilot subcarriers, the performance towards fiber nonlinearity, the spectral efficiency and the effective data rate are increased.2. offset-QAM based CoWDM systemsa) Theoretical models, principle and comparisons with N-WDM and AO-OFDM of offset-QAM based CoWDM system are analyzed.b) Performance under different filter bandwidths, ASE noise and fiber nonlinearity of these three techniques are investigated. The results show that the overlapped spectra of AO-OFDM make the channel crosstalk serious, so it could not support higher modulation formats than4QAM and performs worst. And although pre-filter helps to improve the performance of N-WDM, the memory length of pulse-shaping filters in N-WDM should be sufficiently large to create the temporal sinc-function pulse. The required signal spectral profiles to achieve channel orthogonality in offset-QAM based CoWDM system could be greatly relaxed and are not limited to sinc or rectangular functions. Meanwhile, the tolerance towards the bandwidth is increased. Compared to N-WDM, CoWDM could increase the transmission distance more than30%in single-polarization9channels system with symbol rate of25GS/s and channel interval of25GHz. While in polarization division multiplexing system, CoWDM still performs well towards different filter bandwidth and the FIR filters in the receiver require shorter memory length. Besides, the transmission performance is much better than N-WDM. In conclusion, offset-QAM based CoWDM is a major solution for next-generation optical communication system. |
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