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Title: Advanced optical fibre communication via nonlinear Fourier transform
Author: Tavakkolnia, Iman
ISNI:       0000 0004 7229 3906
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2018
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Optical fibre communication using the Nonlinear Fourier transform (NFT) is one of the potential solutions to tackle the so-called capacity crunch problem in long-haul optical fibre networks. The NFT transforms the nonlinear propagation of temporal signal, governed by the nonlinear Schr ̈odinger equation (NLSE), into simple linear evolutions of continuous and discrete spectra in the so-called nonlinear spectral domain. These spectra and the corresponding nonlinear spectral domain, defined by the NFT, are the generalized counterparts of the linear spectrum and frequency domain defined by the ordinary Fourier transform. Using the NFT, the optical fibre channel is effectively linearised, and the basic idea is to utilize degrees of freedom in the nonlinear spectral domain for data transmission. However, many aspects of this concept require rigorous investigation due to complexity and infancy of the approach. In this thesis, the aim is to provide a comprehensive investigation of data transmission over mainly the continues spectrum (CS) and partly over of the discrete spectrum (DS) of nonlinear optical fibres. First, an optical fibre communication system is defined, in which solely the CS carries the information. A noise model in the nonlinear spectral domain is derived for such a system by asymptotic analysis as well as extensive simulations for different scenarios of practical interest. It is demonstrated that the noise added to the signal in CS is severely signal-dependent such that the effective signalling space is limited. The variance normalizing transform (VNT) is used to mathematically verify the limits of signalling spaces and also estimate the channel capacity. The numerical results predict a remarkable capacity for signalling only on the CS (e.g., 6 bits/symbol for a 2000-km link), yet it is demonstrated that the capacity saturates at high power. Next, the broadening effect of chromatic dispersion is analysed, and it is confirmed that some system parameters, such as symbol rate in the nonlinear spectral domain, can be optimized so that the required temporal guard interval between the subsequently transmitted data packets is minimized, and thus the effective data rate is significantly enhanced. Furthermore, three modified signalling techniques are proposed and analysed based on the particular statistics of the noise added to the CS. All proposed methods display improved performance in terms of error rate and reach distance. For instance, using one of the proposed techniques and optimized parameters, a 7100-km distance can be reached by signalling on the CS at a rate of 9.6 Gbps. Furthermore, the impact of polarization mode dispersion (PMD) is examined for the first time, as an inevitable impairment in long-haul optical fibre links. By semi-analytical and numerical investigation, it is demonstrated that the PMD affects the CS by causing signal-dependent phase shift and noise-like errors. It is also verified that the noise is still the dominant cause of performance degradation, yet the effect of PMD should not be neglected in the analysis of NFT-based systems. Finally, the capacity of soliton communication with amplitude modulation (part of the degrees of freedom of DS) is also estimated using VNT. For the first time, the practical constraints, such as the restricted signalling space due to limited bandwidth, are included in this capacity analysis. Furthermore, the achievable data rates are estimated by considering an appropriately defined guard time between soliton pulses. Moreover, the possibility of transmitting data on DS accompanied by an independent CS signalling is also validated, which confirms the potentials of the NFT approach for combating the capacity crunch.
Supervisor: Safari, Majid ; Haas, Harald Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: optical fibre ; nonlinear Fourier transform ; NFT ; NFT-based communication ; continues spectrum ; discrete spectrum