Ericsson’s in-house modelling tool suite for optical system simulation and link engineering33 is used throughout the research presented in this thesis. This research programme contributed to the development of these tools, in particular the combined wideband power variation model and narrowband nonlinear simulations. The author specified the functionality and carried out the testing of these components of the modelling tool suite. Link Design Tool (LDT) performs link engineering, OSNR calculations and Q margin estimation using simplified analytical models. Link
Simulation Tool (LST), performs full nonlinear system simulations, wideband OSNR
calculations and combined nonlinear simulations with wideband power variations. A commercially available network planning tool OPNET SP Guru Transport Planner is used for higher-level network planning research presented in Chapter 5. Figure 42 shows the three main modelling tools including the main data flows between each tool. The following sections give further details on the LST and LDT; the Network Planning Tool is described in section 5.4.3.
Figure 42 Overview of modelling tool suite and main data flows between each tool.
2.11.1
L
INKE
NGINEERINGLDT is developed primarily for use by systems engineers in designing networks for
customer deployment, and for potential customers when responding to RFPs and
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The algorithms within the tools were jointly developed by Ericsson’s Photonics Modelling Team of which the author was a member. Software implementation is the work of Dr Jeroen Nijhof and Gianmarco Bruno.
Network Planning
OPNET SP Guru Transport Planner
Link Engineering
Link Design Tool (LDT)
Full Nonlinear Simulation
Link Simulation Tool (LST)
Link Configuration Equipment Rules Wavelength Routing and Assignment (WRA) Network Topology
Engineering Rules System Performance Bill of Materials (BoM) Impairment Awareness (not implemented)
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RFQs. It is also used by expert system designers and network architects where extra features are available to carry out more detailed design and analysis, particularly for non-standard network applications. It also allows modification of the underlying component specifications and models which is useful when developing new system features (e.g. 100Gb/s) and analysing performance trade-offs due to component specification changes.
LDT allocates transponders, amplifiers and DCF to individual network elements
within a user-defined network topology, based on user-defined per span fibre characteristics and traffic matrix.
LDT also calculates performance metrics including OSNR, various transmission
impairment penalties (CD, PDL, PMD, nonlinearity, and filtering penalties) using analytical / semi-analytical models. Following subtraction of penalties and conversion of OSNR to Q via a suitable optical interface model and addition of FEC gain, a Q margin is calculated from which the BER can be estimated. This approach is computationally more efficient than full numerical simulation and more appropriate for multi-reach applications where nonlinear effects are not always a limiting factor as is the case for ULH applications.
For the research presented in this chapter LDT is only used to allocate network elements, amplifiers and DCF based on the fibre span data producing a configuration file. Optical performance along a link defined by LDT in a configuration file is accurately evaluated by running full nonlinear simulations using LST.
2.11.2
L
INKN
ONLINEARS
IMULATIONLST is a C++ based library for numerically solving the NLSE using the Split-Step
Fourier method as described in section 2.9.15 to 2.9.17. The library includes models of selected system elements: transmitters, amplifiers, fibres, filters, multiplexers and receivers. Narrowband nonlinear simulations can be carried out either in isolation or combined with wideband OSNR calculations. It is also possible to carry out wideband OSNR calculations in isolation.
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VPI Transmission Maker,34 a commercial physical layer modelling tool [67], was used to test and validate results during the development of the in-house modelling tools.
A high performance computing cluster (HPC) was used to run lengthy nonlinear simulation jobs in parallel. The cluster consists of a number of blades, some with dual processor, dual core and some with dual processor, quad core (AMD Opteron 2.4GHz) with a grand total of 132 cores. Simulation jobs are launched and processed for analysis by executing Perl scripts.
LST reads a link configuration file produced by LDT and simulates propagation of
the optical field through the defined link, writing out Q versus distance for all the channels, and the evolution of power and spectral width.
A limitation of nonlinear simulations is that they are usually carried out narrowband, in that at most 6 channels can be simulated due to computation time constraints and fairly limited errors. Spectral variations in signal gains and losses are not normally accounted for. To investigate performance across the C-band, simulations are done in 3 sets of 6 channels at a time at 1530, 1545 and 1560nm wavelengths.
Dispersion Map Optimisation
Dispersion map optimisation is carried out by reading the raw output, i.e. the field which is stored after the nonlinear compensation, and then calculating Q after applying various amounts of linear post-compensation to that field and selecting the best value which gives optimum performance for the 18 channels simulated across the C-band (i.e. the 3 groups of 6 channels near to 1530, 1545, 1560nm).
Combined Nonlinear Simulation and Model of Wideband Power Variations
As part of this research programme LST was extended35 with models to account for wideband power variations due to EDFA gain and noise figure spectrum, fibre attenuation spectrum and SRS induced gain tilt. Nonlinear simulations are carried out with amplifier gain and noise figure calculations driven by the wideband EDFA black-box model of spectral gain and noise. Wideband power variations are mitigated by input signal power pre-emphasis and periodic signal re-emphasis or levelling.
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