Reseach On Co-OFDM Systems Based On Digital Coherent Superposition

The advantages of OFDM are widely known, including high spectrum efficiency, channel filtering effect(CD, PMD and optical filtering) resilience, frequency domain flexibility and so on. However, the sensitivities to the fiber Kerr nonlinear and laser phase noise are two intrinsic problems to OFDM.In this dissertation, both two questions are intensely studied, using a DSP-based digital coherent superposition coherent optical OFDM system(DCS-CO-OFDM), in the scenarios of long-distance backbone network and access network application. The main points and innovations are summarized as follows:1. The first two chapters are optical OFDM(OOFDM) research background introductions, emphasizing on the research progress of optical OFDM, nonlinear suppression in long-distance optical transmission and the phase noise. The second chapter explains the simulation platform and the OOFDM related theories, like basic nonlinear measurements. In addition, the perturbation theory of nonlinear Schrodinger equation is expounded in detail, as well as the influence of nonlinear transfer function on nonlinearity.2. In the awake of the single carrier PCTW, we propose a phase-conjugated wave twin OFDM(PCTW-OFDM). Unlike traditional scheme which shrinks the distribution of nonlinear transfer function?(?), PCTWs use symmetrical dispersion mapping to approximate a real-valued?(?). Therefore, according to the nonlinear Schrodinger equation perturbation theory, PCTW nonlinear distortions are mutually anti-correlated, leading to first order cancellation upon coherent superposition. After 3200-km fiber transmission, a 6-d B ESNR improvement is verified at the optimal launch power point compared with the traditional PDM-OFDM system.3. Due to the mechanical movement and the diurnal temperature fluctuation, the channel dispersion is time-varying, thus the accurate dispersion might be not provided. In the case of dispersive fluctuation, we verify the PCTW-OFDM significantly relaxes the accuracy of electrical dispersion compensation. What’s more, the longer OFDM subcarrier symbol period performs a more balanced nonlinear variance cancellation. OFDM-PCTW also shows an insignificant nonlinear variance reduction in the presence of polarization mode dispersion(PMD), indicating the compatibility between OFDM and PCTW.4. Modeling the phase noise as a Wiener process, the ICI noise power of PCTWOFDM system is derived. Furthermore, the ICI power reduction after DCS is quantified as well as the remained fourth order ICI power’s dependence on the ratio of laser linewidth to OFDM subcarrier symbol rate. The laser linewidth tolerance for a PCTW-OFDM system raises an order of magnitude around FEC threshold after coherent superposition.5. Observing the curve of ESNR vs. Linewidth, we verify the theoretical ICI power formula from two aspects: the first is the linear relationship between ESNR and the linewidth. The second is ICI cancellation degradation with higher laser linewidth. The final analysis is a 16-QAM-DCS-OFDM simulation, verifying that the phase noise tolerance for higher-order modulation DCS system is superior to the traditional 4-QAM OFDM in certain scenario.