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Flexible Optical Networks – Time Domain Hybrid QAM: DSP and Physical Layer Modelling

Periodic Reporting for period 1 - Flex-ON (Flexible Optical Networks – Time Domain Hybrid QAM: DSP and Physical Layer Modelling)

Reporting period: 2015-10-16 to 2017-10-15

A sustainable growth of data speeds and the generalized access to internet in Europe is nowadays seen as a key enabler for a highly connected and informed society, guaranteeing inclusion, equality of opportunities, and higher quality of life to all European citizens. However, due to the fast growth of internet data traffic (>25% per year) triggered by a plethora of emerging bandwidth-consuming services (such as HD-TV on demand, cloud computing, massive social networking, tele-working, …), the currently installed optical fiber system is starting to run out of capacity.

In order properly face these challenges and support the internet of the future, there is an urgent need for a strong investment in the research and development of future-proof optical broadband infrastructures, thus avoiding the upcoming capacity crunch on transport optical networks. The Flex-ON project has investigated novel technological paradigms in terms of signal generation, digital processing and control-plane management for flexible and high-capacity transport optical networks. The project has led to the development of a flexible transceiver prototype with intelligent reconfigurability and arbitrarily low bit-rate granularity, enabling to increase the network capacity and the spectral/energy efficiency, while providing a future-proof flexible solution for an increasingly heterogeneous global network.
Main technical/scientific results:
- Development of a comprehensive simulation tool based on MATLAB for the simulation of coherent optical communication systems with support to state-of-the-art technologies such as high-order QAM modulation and processing, electronic subcarrier multiplexing, probabilistic shaping and wavelength selective switch (WSS) optical filtering;
- Theoretical development and numerical simulation of the concept of flexible pulse amplitude modulation (FlexPAM) formats for improved data-rate granularity in single-carrier optical transmission systems;
- Experimental demonstration of benefits of symbol-rate optimization (SRO) for electronic subcarrier multiplexing systems, allowing for 10%-20% extended signal reach due to enhanced tolerance against nonlinear propagation impairments;
- Development and experimental validation of a novel advanced modulation technique based on the concepts of subcarrier-multiplexing and frequency hybrid modulation formats (FDHMF) to enhance the data-rate flexibility and nonlinear tolerance in long-haul optical fiber transmission;
- Experimental demonstration of the concept of hybrid quadrature amplitude modulation between optical carriers of a 400G superchannel, enabling to design optimized spectrally efficient solutions for the recently approved ITU-T grid with 12.5 GHz of granularity;
- Theoretical and experimental demonstration of the incoherent accumulation of nonlinear interference (NLI) generated during the propagation of electronic subcarrier multiplexing signals operating at the optimum baud-rate per subcarrier;
- Experimental comparison of performance and data-rate flexibility provided by the FDHMF technique against the recently proposed probabilistic shaping technique. The obtained results have shown that FDHMF can provide fine data-rate granularity without the need for complex digital signal processing at the transmitter and receiver sides;
- Development of a low-complexity digital pre-emphasis technique for subcarrier multiplexing signals based on power-ratio optimization per subcarrier;
- Experimental demonstration of the benefits of combining nonlinear tolerant FDHMF with capacity achieving probabilistic shaping for ultra-high performance long-haul optical transmission.

Main societal results:
- Presentation and demonstration of series of small experiments on basic properties of light given to students of Carmagnola’s high-school, Torino, Italy, December 2015.
- Co-funded OSA’s Student Chapter of Torino, a student organization co-financed by the Optical Society of America aimed at disseminating optics and photonics to the general public, with special incidence to younger audiences (mainly students from primary, middle and high-schools);
- Participation in the ICT Day 2016, promoted by Politecnico di Torino, in which the OSA Student Chapter activities were disseminated to other students and researchers, effectively attracting new members to our student society (;
- Participation on the European Researcher’s Night, at Piazza Castello, Torino, September 2016. Demonstration of several photonics-related experiments to the general public, including high school students and teachers, university students and other researchers, young kids and families.
- Presentation “Fotonica e le sue applicazioni: esperimenti e hands-on” given to middle and high school students in the region of Carmagnola, Torino, Italy, February 2017.
The Flex-ON project has made significant progress beyond the state-of-the-art, contributing to the development and optimization of key enabling technologies for future ultra-high-speed coherent optical communications. The most important scientific contributions include:
- Extension of the symbol-rate optimization (SRO) concept to electronic subcarrier multiplexing signals (SCM) composed of spectrally efficient polarization-multiplexed 16QAM subcarriers. The enhanced nonlinear robustness of these was demonstrated through a comprehensive experimental campaign in which we have tested the propagation performance of a plethora of SCM configurations. The optimum symbol-rate in these systems was identified to be in the region of 2-4 GBaud per subcarrier, enabling maximum reach increases of >10% without requiring any signal processing overhead for nonlinear compensation. It was also demonstrated that the benefits of SRO in these systems can also be combined with those provided by traditional digital nonlinear mitigation techniques, further extending the signal maximum reach;
- Demonstration of a novel flexible modulation technique based on frequency-hybrid modulation formats (FDHMF) that enable to achieve a spectral efficiency granularity of <0.25 bits/symbol, while also effectively combating nonlinearity through the SRO effect. The FDHMF concept has been experimentally demonstrated by laboratorial comparison against other state-of-the-art flexible modulation technique: probabilistic shaping. The obtained results show similar benefits in terms of data-rate flexibility without the need for complex DSP subsystems at the transmitter and receiver side. These results provide an important step forward on the feasibility and practical implementation of flexible modulation schemes for next-generation coherent optical transceivers;
- Development and experimental demonstration of a low-complexity power-ratio adaptation technique to implement transmitter-side digital pre-emphasis for subcarrier-multiplexing systems. This low-complexity technique enables to achieve similar performance as the most traditional pre-emphasis based on finite impulse response (FIR) filtering, while substantially reducing the requirements in terms of chip area and power consumption. This technique can therefore contribute to the practical implementation of next-generation transceivers with reduced form factor, power consumption and cost;
Logo of the Flex-ON project