Advanced Digital Signal Processing Techniques for High-Speed Optical Communications Links
Dario Pilori


















Logo-ScuDo-blu 




Electrical, Electronics and Communications Engineering



31th

















2



    Prof. Gabriella Bosco, Supervisor

    Prof. Roberto Gaudino, Co-supervisor






Prof. Magnus Karlsson, Referee, Chalmers Tekniska Högskola AB

Dr. Robert Killey, Referee, University College of London

Prof. Maurizio Magarini, Politecnico di Milano

Dr. Marco Secondini, Scuola Universitaria Superiore Sant'Anna Pisa

Prof. Renato Orta, Politecnico di Torino




2019




This dissertation has been submitted in partial fulfillment of the requirements for the degree of “Dottore di Ricerca” (Ph.D.)	at the Politecnico di Torino.




I hereby declare that, the contents and organization of this dissertation constitute my own original work and does not compromise in any way the rights of third parties, including those relating to the security of personal data.	





0.5Dario Pilori

Torino, 22 March 2019





	This thesis is licensed under a Creative Commons Attribution 4.0 International License; see <https://creativecommons.org/licenses/by/4.0/>.
	The text may be reproduced, provided that credit is given to the original author.


























Advanced Digital Signal Processing Techniques for High-Speed Optical Communications Links.
 



The main topic of this work is the application of advanced Digital Signal Processing (DSP) techniques to high data-rate optical links.

The first part of the thesis is devoted to direct-detection systems, with a specific focus on data-center (DC) communications. These links, until recently, used
very simple On-Off Keying modulation. However, the strong push towards higher data-rates introduced the use of some DSP algorithms in the DC. 
First, this thesis introduces a novel architecture for 400-Gbit/s Intra-DC (shorter than 2 km) applications. This architecture uses each fiber pair in both directions, doubling
the per-laser and per-fiber capacity. Crosstalk is reduced by using a short feed-forward adaptive equalizer and a small frequency shift between lasers transmitting
over the same fiber. This system, after an initial theoretical evaluation, was successfully tested with an experiment. Then,
Intra-DC links were analyzed, i.e. links interconnecting different DCs in the same region. The target distance was 80 km. For this purpose, Single Side-Band (SSB) modulation
was compared with Intensity Modulation/Direct Detection schemes over dispersion-uncompensated links with single-photodiode receivers. Using, as an application example,
the DMT modulation format, the comparison was carried over different span lengths. It was found that, for distances longer than 10 km, SSB has a significant advantage over IM/DD systems.

The second part of the thesis focuses on coherent systems. In particular, it focused on constellation shaping techniques, which have been recently
proposed for optical communications. To perform a fair analysis, shaped constellations were compared, at the same net data rates, with standard lower-cardinality
QAM constellations. It was found that, over a linear channel, the gain of constellation shaping is approximately 25%. Afterwards, this thesis discusses the
non-linear properties of the optical channel, which are known to be enhanced by constellation shaping. In particular, it focuses on the generation of
Non-Linear Phase Noise (NLPN) at different system conditions, using both standard QAM and shaped constellations. It was found that, in some scenarios, such as low symbol rates
or low dispersion fibers, NLPN significantly degrades the performance of shaped constellations. Then, this thesis proposes some compensation techniques, which
are able to partially mitigate the effect of NLPN in those conditions.




The development of any thesis, especially a PhD thesis, cannot be done without the support of many people, in any stage of my research career.
First of all, I would not be even close to getting a PhD without the constant support of my family.

For my PhD path, I would first like to thank my advisors Gabriella Bosco and Roberto Gaudino, for their invaluable teachings and support throughout the entire path. Then, I would
like to thank the entire OptCom research group, for the numerous discussions that allowed me learning many things. In particular, I would like to thank
Andrea Carena, Vittorio Curri and Pierluigi Poggiolini. Then, a big thank to all the PhD students and post-docs that were (and are) part of the group. From every discussion
with them I learned many small (and big) new ideas. In particular, I thank Mattia Cantono, Alessio Ferrari, Fernando P. Guiomar, Marianna 
Hovsepyan and Emanuele Virgillito.
Outside the OptCom group, I would like to thank the technical staff of the Politecnico. By collaborating with them, I got to learn a different (but very useful) aspect
of the work inside a University. In particular, I thank Marco Bertino and Paolo Squillari.

During my PhD, I had precious collaborations with the industry and the academia. 
In particular, I would like to thank Fabrizio Forghieri from Cisco Photonics for the discussions during the weekly conference calls. I would also want to thank the 
IPQ institute at the Karlsruhe Institute of Technology for hosting me during my three-month abroad research period. In particular, I thank 
M.M.H. Adib, Christoph Füllner and Sebastian Randel.

Going backwards, I would not have started a PhD without the precious teachings of the Nokia Bell Labs staff in Crawford Hill, NJ, during my two internships. In particular, I would
like to thank my advisor, Sebastian Randel, and Damiano Badini, Sergi Caelles, Junho Cho, S. Chandrasekhar, René Essiambre, Bob Jopson, Greg Raybon, Peter Winzer.
Then, last but not the least, Maurizio Magarini from Politecnico di Milano for giving me the opportunity to do my Master thesis in that amazing place.

And, of course, the biggest thank of all goes to Serena.






















PART: 
Direct-Detection Systems





PART: 
Coherent Systems

























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