title: Advanced digital signal processing for next-generation flexible optical networks
Creators: name: Lin, Xiang
Publisher: name: Memorial University of Newfoundland
Year: 2020

description: To keep pace with the rapid expansion in data-exchange traffic around the world, optical
 networks are anticipated to provide flexibility to maximize utilization of the deployed optical
 fiber resources. On the other hand, digital signal processing (DSP) has been employed in
 coherent optical systems to enable 100G and beyond optical fiber networks. The goal of
 the thesis is to develop advanced DSP techniques for the flexible optical networks.
 With the reconfigured modulation formats in the systems, modulation classification
 (MC) is essential in the DSP to facilitate the sequential compensation modules which are
 modulation format-dependent. Based on the cumulative distribution function (CDF) of
 received signal's amplitude, an MC algorithm for M-ary quadrature amplitude modulation
 (M-QAM) formats with M = 4, 8, 16, 32, and 64 is proposed. Results show that the proposed
 algorithm achieves accurate classification at optical signal-to-noise ratio (OSNR) of
 interest and is robust to frequency offset and laser phase noise. Relying on the CDF of
 received signal's amplitude, a non-data-aided (NDA) OSNR estimation algorithm is developed
 for coherent optical systems employing multilevel constellations. It outperforms the
 state-of-the-art NDA algorithm in terms of performance and complexity. Furthermore, a
 joint OSNR estimation and MC algorithm enabled by support vector machine is designed.
 Compared to deep neural network-based joint estimation approach, the proposed algorithm
 achieves better performance with comparable complexity.
 In addition, a low-complexity two-stage carrier phase estimation algorithm is proposed
 for coherent optical systems with 16-QAM format. The proposed algorithm exploits the
 second power operation instead of the conventional fourth power to remove the modulation
 phase, which is enabled by constellation partition and rotation. Optical back-to-back
 experiments and numerical simulations are carried out to evaluate the performance of the
 algorithm. Results show that, compared with the conventional fourth power-based CPE algorithm, the proposed algorithm provides comparable tolerance to the carrier phase noise,
 with reduced complexity.
 Lastly, a novel transmission scheme is investigated for the open and disaggregated metro
 coherent optical networks, which impose the requirements for multiple user connectivities
 on the limited orthogonal frequency resources. Thus, it is desirable to provide connections
 simultaneously to various users in a non-orthogonal way. A transmission scheme based
 on the non-orthogonal sparse code multiple access in a digital subcarrier multiplexing is
 proposed. Compared to power domain-based counterpart, the proposed scheme supports
 more than 2 users without user pairing and clustering. The feasibility of the proposed
 scheme is verified through numerical simulations. Three scenarios with 2, 4, and 6 users
 over 1, 2, and 4 subcarriers, respectively, are considered. Performance evaluations show
 that in all scenarios, the proposed scheme attains bit error ratio lower than the forward
 error correction limits with the transmission ranges of interest in metro applications.