## Abstract

Due to the high potential of multi-band transmission (MBT) systems as a short- to medium-term solution to the ever-increasing demand for fiber capacity, one of the recent areas of research concerns the development of fast but still accurate models to estimate the quality of transmission (QoT) in these systems, since this capability will be paramount for efficient and cost-effective network planning and operation. However, accurately evaluating the QoT of MBT systems with moderate computational complexity is challenging, mainly because stimulated Raman scattering (SRS) becomes a major transmission impairment. Although several models have been proposed in the literature, to the best of the authors’ knowledge, there is a lack of a comprehensive comparison of the advantages and main limitations of the available methodologies. Therefore, this work analyzes the computational time and accuracy of several QoT estimation methods suitable for MBT systems [closed-form interchannel SRS Gaussian noise (ISRS-GN) and four-wave mixing (FWM) models], focusing on transmission systems comprising a total transmission bandwidth of up to 20 THz, which includes not only the C- and L-bands but also a part of the complete S-band. We also propose a modification to the FWM model to take into account the SRS effect in the calculation of the nonlinear interference, referred to as the enhanced FWM (eFWM) model. We show that the closed-form ISRS-GN model has the best trade-off between computational complexity (speed) and accuracy for launch powers per channel up to 4 dBm in a system with 10 spans and with span lengths varying from 50 km to 100 km. However, this method shows generalized signal-to-noise ratio estimation errors as high as 1.5 dB for higher launch powers. Similar conclusions were drawn when using the models to optimize the launch powers in the reference Italian backbone network, i.e., using the closed-form ISRS-GN model leads to more accurate optimized launch powers. However, in this case, the impact on the network-wide average channel capacity of using the launch power optimized using the different approaches is negligible.

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