3 research outputs found
A lower bound on the per soliton capacity of the nonlinear optical fibre channel
A closed-form expression for a lower bound on the per soliton capacity of the nonlinear optical fibre channel in the presence of (optical) amplifier spontaneous emission (ASE) noise is derived. This bound is based on a non-Gaussian conditional probability density function for the soliton amplitude jitter induced by the ASE noise and is proven to grow logarithmically as the signal-to-noise ratio increases
Advanced optical fibre communication via nonlinear Fourier transform
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