ULTIMATE CAPACITY UPGRADE WITH SPECTRUM ENGINEERING ON NEW AND LEGACY SYSTEMS – CURRENT AND FUTURE GENERATIONS OF SLTE TECHNOLOGY BY ALICE SHELTON
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endors are currently deploying third generation coherent technology enabling upgrades on new and legacy systems at bit rates up to 400Gb/s boosting available capacity and lowering the cost per bit. ‘Legacy’ systems (those designed with in-line chromatic dispersion compensation) have benefitted enormously from coherent technology in increasing capacity tenfold compared to their original design capacities. With future technology these capacities will increase but not in the same scale and with diminishing returns as these systems reach their economic capacity limit. For these systems the objective is to provide maximum flexibility in the SLTE to fully optimise capacity versus reach over the full spectrum bandwidth.
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‘New’ systems designed from the start for coherent technology (without in-line chromatic dispersion compensation) are upgradeable with each new generation of coherent technology. They will be able to take full benefit from the new technology in development today and will reach a point in the next couple of years where the ultimate capacities achievable approach the Shannon limit. For these systems we consider the new technologies in development that will take their capacities to a theoretical design limit.
INTRODUCTION
The first generation of coherent transponders was introduced to the submarine market in 2010 for the
upgrade to 40Gb/s per wavelength of systems previously running at 10Gb/s. Two options for modulation were available Polarisation Division Multiplexed Quaternary Phase Shift Keying (PDM QPSK) which transports 4 bits per symbol (2 along each polarisation) and Polarisation Division Multiplexed Binary Phase Shift Keying (PDM BPSK) which transports one bit per polarisation and displays a much-improved robustness to nonlinear effects compared to PDM QPSK. PDM BPSK could be transmitted over undersea distances and was also more robust to interactions with any existing intensity modulated signals at 10Gb/s. A second generation of coherent technology was introduced more adapted to undersea systems with soft decision FEC and higher chro-