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Title: Design and analysis of optical orthogonal frequency division multiplexing in optical fiber transmission systems
Authors: Li, Xiang
Keywords: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
Issue Date: 2016
Abstract: To meet the ever increasing demand of Internet traffic, a higher channel capacity (maybe 10 times of the current channel capacity) is highly demanded in the optical network. Optical orthogonal frequency division multiplexing (OFDM) is considered as a promising solution for the future high capacity optical fiber transmission systems thanks to its high spectral efficiency, strong dispersion tolerance and ease of channel compensation. Generally, optical OFDM systems are classified into two categories according to detection schemes: direct detection optical OFDM (DDO-OFDM) and coherent optical OFDM (CO-OFDM) systems. Both CO-OFDM and DDO-OFDM systems have shown their potential in optical networks, ranging from wide area network (WAN), to metropolitan area network (MAN) and to local area network (LAN). However, a number of challenging issues need to be addressed before the massive deployment of optical OFDM systems in future optical networks. These issues include power fading due to limited bandwidth, laser phase noise (LPN), excessive overhead for channel compensation and fiber nonlinearity effects. These challenging issues correspond to the optical network ranging from tens of kilometers to more than 10000 kilometers. In order to enable optical OFDM technique in future optical networks at every level, this thesis is devoted to developing some enabling solutions for these challenging issues in optical OFDM systems. First of all, a novel optical OFDM scheme, namely zero padding OFDM (ZP-OFDM) is proposed to compensate the polarization mode dispersion (PMD) induced power fading effects in DDO-OFDM systems. The channel model of ZP-OFDM is developed and analyzed in detail. An equalization method based on sorted QR decomposition is derived for ZP-OFDM in DDO-OFDM system. Numerical simulation results show that ZP-OFDM has a significantly higher tolerance to PMD than conventional cyclic prefix OFDM (CP-OFDM) in DDO-OFDM systems when the channel spectral nulls occur at certain differential group delay (DGD) values. Furthermore, the QR decomposition method is applied to the discrete Fourier transform spreading OFDM (DFTS-OFDM) in DDO-OFDM systems. The effectiveness of the proposed method is experimentally demonstrated on a reflective semiconductor optical amplifier (RSOA)-based optical access networks. Secondly, a new pilot-aided channel equalizer (PACE) is proposed for polarization division multiplexing CO-OFDM (PDM-CO-OFDM) systems to compensate the PMD and laser phase noise simultaneously. Subsequently, adaptive PACE (APACE) and boosted PACE (BPACE) are proposed to further improve the performance of PACE after long-haul transmission. APACE and BPACE are demonstrated to achieve better laser phase noise tolerance than other channel equalizers (CEs) in 100-Gb/s PDM-CO-OFDM systems. An adaptive decision-directed channel equalizer (ADDCE) is also proposed for PDM-CO-OFDM system to estimate the channel state information (CSI) in time-domain adaptively. The TD-ADDCE shows better performance than other CEs for a simulated 100-Gb/s PDM-CO-OFDM long-haul transmission system. Furthermore, it is shown that no matrix inversion processing is required for the update of channel coefficients in TD-ADDCE, which significantly reduces the complexity. Thirdly, training symbols (TSs)-free channel equalizer based on independent component analysis (ICA) is proposed and analyzed in both DDO-OFDM and PDM-CO-OFDM transmission systems. The ICA models of DDO-OFDM and PDM-CO-OFDM are studied in detail. Several ICA algorithms are applied in DDO-OFDM and PDM-CO-OFDM transmission systems. Through both simulation and experiment, it is shown that the ICA-based channel equalizers achieve similar performances as TSs-based channel equalizers in both DDO-OFDM and PDM-CO-OFDM systems, but with higher spectral efficiency since training symbols are not required. The convergence speed of ICA algorithms in optical OFDM transmission systems is also examined in detail. To mitigate the fiber nonlinearity in long-haul transmission systems, amplitude phase shift keying (APSK) modulation in CO-OFDM system with DFT spreading (DFTS-CO-OFDM) is studied in detail. Simulations are carried out to investigate and compare the transmission performances of 16-APSK and 16-QAM formats after long-haul transmission. The results show that 16-APSK modulation can effectively improve the fiber nonlinear performance. The effects of constellation mapping scheme, radius ratio and phase offset are also investigated in detail for 16-APSK. Golay sequences coded DFTS-CO-OFDM are also proposed. New encoding and decoding algorithms are presented for a large number of subcarriers. The simulation results show that the performance of Golay sequences coded DFTS-CO-OFDM with quadrature phase shift keying (QPSK) modulation is better than that of the binary phase shift keying (BPSK) modulation in DFTS-CO-OFDM system after long-haul transmission under the same spectral efficiency
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