| With the evolving of optical transport network driven by increasing traffic of cloud computing, mobile network and etc., the metro transport networks also have ushered rapid development. Meanwhile, to better support the bursty, diversification and high-capacity of network traffic,the metro transport networks need more optimal architecture and physical layer techniques.Optical transmission system employing orthogonal frequency division multiplexing (OFDM) have gained considerable research interest because of its inherent and unique advantages including, high transmission bit rate, potential for cost-effective implementation, high spectral efficiency, excellent immunity to both chromatic dispersion and polarization-mode dispersion, system scalability and flexibility, as well as dynamic provision of hybrid bandwidth allocation in both frequency and time domains. Considering the demands of future metro transport network, this paper mainly researches on metro transport network architecture design based on direct detection orthogonal frequency division multiplexing (DD-OFDM) and mitigation of physical layer impairments,and gains a number of innovative research achievements.The main work and innovative contributions are listed as follows.1. For the demands of metro transport networks, such as implementation cost,high-capacity transmission and flexibility, a metro ring network and node architectures based on direct-detection OFDM(DD-OFDM-MTN) was designed. The DD-OFDM-MTN supports protection switching operation and dynamic bandwidth allocation (DBA).In addition, a centralized scheduling mechanism and a best-effort allocation- and variable priority-based DBA algorithm is proposed. The simulation results imply that, comparing to static bandwidth allocation,the bandwidth utilization in the DD-OFDM-MTN was improved by?10%, the packet average delay and packet delay variation are decreased by ?8% and ?20% respectively.2. In DD-OFDM, the system performance is mainly restricted by beating interference between subcarriers after square-law detection. For field modulation single-sideband DD-OFDM (SSB-DD-OFDM) systems,iterative and cancellation technique for the signal-to-signal beat interference (SSBI) is typically employed, and the other mechanisms include inserting a blank guard-band between optical carrier and OFDM signals or loading data only in the odd subcarriers to keep the received signals immune from SSBI contamination. However, the spectral efficiency will be half reduced with guard-band insertion or even subcarriers being null. Based on the half-cycled SSB-DD-OFDM with interleaved subcarrier loading (HSSB-DD-OFDM), a balanced detection based SSBI cancellation receiver and the accompanied guard interval structure are proposed to increase the spectral efficiency, to lower the demand for optical carrier to signal power ratio (CSPR) and to improve the receiver sensitivity. The simulation results of the 40 Gbps HSSB-DD-OFDM signal show that the proposed scheme retains the immunity to SSBI interference while the spectral efficiency can be increased by 41%.3. In intensity-modulation direct-detection OFDM (IMDD-OFDM)systems, except for beating interference between subcarriers, dispersion-and chirp-induced power fading which occurs after photodetection due to the inherent double sideband spectrum also seriously degrades the received signal. Dynamic multiband scheme and bit loading can be deployed to overcome the power fading while sufficiently utilizing all the available passbands in electro-absorption modulated lasers (EAM) based IMDD-OFDM systems. However, the beating interference between subcarriers will rapidly strengthen with the increasing of transmission capacity and distance. This thesis analyses the cause of the beating interference between subcarriers and its relationship between modulator nonlinearity, chirp and chromatic dispersion; a baseband predistortion algorithm is proposed to mitigate the beating interference. The experimental results show that a 30 Gbps EAM-IMDD-OFDM system with the proposed algorithm could improve ?3dB receiver sensitivity compared with the system without the algorithm over 100 km long single mode fiber transmission and only decreases ?1.0dB in comparison with the back-to-back situation.4. High peak-to-average power ratio (PAPR) of the transmitted signal is one of the major drawback of OFDM systems. When using Mach Zehnder modulator or EAM to realize optical OFDM signal modulation, small optical modulation index is required to minimize modulator nonlinearity distortion due to high PAPR of the OFDM signal,thus resulting in decreased effective optical signal to noise ratio (OSNR).On the other hand, high PAPR could intensify the nonlinear effect of fiber,such as four-wave mixing (FWM), and then degrade the transmission performance. Considering the deep attenuated frequency zone due to power fading in the EAM-IMDD-OFDM systems, one way to combat high PAPR is to adopt Tone Reservation (TR) technique by inserting peak-cancelling signal in these forbidden bands to reduce the PAPR of the transmitted signal, but the inserted peak-cancelling signal inevitably aggravates beating interference between subcarriers and so negates the benefits promised from lower PAPR. In this thesis, a modified curve fitting based TR technique co-operated with subcarrier interleaving is proposed to mitigate the PAPR and realize beating interference cancellation in multiband EAM-IMDD-OFDM Systems with adaptive modulation formats. In the experiment, we successfully achieve a 20 Gbps OFDM signal over 100-km single-mode fiber (SMF) transmission by using economical EAM modulation and direct detection. The experimental results show that a 20 Gbps EAM-IMDD-OFDM system with the proposed scheme could improve ?1.8dB receiver sensitivity over 100 km long single mode fiber transmission and the PAPR reduction of the transmitted signal is ?3dB.5. The IMDD-OFDM systems usually use low-priced distributed feedback laser or vertical-cavity surface-emitting laser with a MHz-level linewidth. Optical OFDM signal is very sensitive to phase noise which will cause not only phase rotation on each subcarrier but also intercarrier interference (ICI) among subcarriers to degrade the performance after fiber transmission. This thesis shows a detailed analysis of the phase noise-induced phase rotation, ICI and beating interference in EAM-IMDD-OFDM systems; a pilot tone based phase noise and beating interference mitigation algorithm is proposed to reduce the requirement of laser linewidth and/or to extend the transmission distance. The simulation results show that, employing the algorithm, receiver sensitivity is improved by ?1.7dB using the laser with 3 MHz linewidth in a 40 Gbps multiband EAM-IMDD-OFDM system over 100 km long single mode fiber transmission. |
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