Advanced mitigation techniques for band-limited channels applied to digital multi-subcarriers optical systems

The study and the analysis presented in this thesis consider a fiber-optic transmission system in which the signal is routed from the source to the destination using devices called ROADMs. These devices act as multiplexers, being able to select a certain wavelength and decide whether to let it pass or block it. However, they rely on Wavelength Selective Switches (WSS) that act like filters on the signal, impairing it on the frequencies at the edges. In particular, the larger the number of cascaded WSS, the greater the impact on the signal. To mitigate the impact of filtering on transmission, a promising solution consists in dividing the channel into subcarriers, each with the same spectral bandwidth , and adopt modulation techniques able to improve efficiency. Moreover, the usage of multi-subcarriers (MSC) systems is also beneficial to adjust the data rate with finer granularity, which is becoming important in recent years since data traffic is more heterogeneous. This can be obtained by exploiting Frequency Domain Hybrid Modulation Formats (FDHMF), that allows to assign a different modulation format to the subcarriers. In this thesis, standard single carrier transmission is compared to MSC systems. Specifically, two modulation format approaches, uniform 2^M-QAM, where M is the constellation cardinality, and Probabilistic Constellation Shaping (PCS), in which modulations are no longer uniform but rather shaped according to a probability distribution of the 2^M-QAM constellation points, are investigated. On top of this, further optimization strategies based on bit and/or power loading (in case of uniform-QAM) and on entropy loading with/without water-filling (in case of PCS) are studied to improve the system performance. The results retrieved from both theory and time-domain simulations will be compared and commented on, organizing them according to different criteria, such as the number of subcarriers, the type of technique, the number of cascaded WSS, or the overall Bit Rate.