Circular Modulation Formats and Carrier Phase Estimation for Coherent Optical Systems
Abstract: Digital coherent receivers stand today as a promising technology for the next generation of high-capacity optical systems. Coherent systems enable the use of multilevel modulation formats which increase the spectral efficiency of a system. Key challenges of multilevel coherent systems are the strict laser linewidth requirements and receiver complexity which prevent a cost-effective implementation. The goal of this thesis is to address these challenges by investigating a novel approach to implement phase noise tolerant optical systems. The performance of a phase recovery scheme, normalized Viterbi-Viterbi carrier phase estimation (V-V CPE), is investigated for circular m-level quadrature amplitude modulation (C-mQAM) signals. C-mQAM provides inherent characteristics for phase noise mitigation, while V-V CPE enables an efficient hardware implementation in a blind feed-forward receiver. A coherent C-mQAM system was simulated in VPItransmissionMaker with phase recovery implemented with MATLAB. Phase noise tolerance was analyzed for C-16QAM and C-64QAM signals. Results show an enhanced phase noise tolerance at a low sensitivity penalty. The achieved linewidth tolerance shows an enhanced performance over available CPE schemes for square mQAM signals, and enables the use of cost-effective lasers. C-mQAM signals allow a straightforward employment of V-V CPE, which can be easily upgraded for higher order circular modulations without adding significant complexity. By combining the power of normalized V-V CPE with C-mQAM inherent characteristics, the phase noise tolerance is enhanced with an efficient implementation. These results show that C-mQAM implemented with V-V CPE is a viable and promising alternative for phase noise tolerant high-speed optical coherent systems.
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