Analysis of Multi-Carrier Modulation Techniques for Optical Communication Systems

The main goal in the development of telecommunication systems is to meet an always growing data demand. Research is focused on finding new solutions for more flexible networks able to sustain high data rates maintaining a good quality of the transmitted signal. In the area of optical networks, the development passed from the introduction of WDM transmission to the current use of Reconfigurable Add-Drop Multiplexers (ROADMs) in network nodes, enabling the possibility to implement dynamic wavelength routing. As a drawback, the introduction this flexibility also introduced distortions due to the filtering effect caused by cascading several ROADMs along the optical path of channel: this is becoming more and more relevant so a detailed study is necessary in order to find a good solution for reliable transmissions. The transmitted signal, passing through several ROADMs, is filtered with the consequent narrowing on the edges of the signal spectrum. A solution proposed recently is based on the used of multi-subcarrier transmission with the optimization of the modulation format and of the allocated power applied to each subcarrier: Frequency Division Hybrid Modulation Format. In this work, we have analyzed using analytical and simulative tools several scenarios considering different bit rates, symbol rates and modulation formats.The main novelty of this study with respect to previous work is the analysis of a realistic model fro ROADMs taking into account tuning uncertainty: the central frequency of the ROADMs is considered as random variable. We have shown the existence of a unique relationship between the overall transfer function properties (central frequency and -3 dB bandwidth) and system penalty. Based on this we propose a method for automatic monitoring of performance to be applied in practical receiver in real-time.