###### Abstract  
Multiband (MB) optical transmission targets increasing the capacity of operators' optical transport networks. However, nonlinear impairments (NLI) affect each optical channel in the C+L+S bands differently, and, therefore, the routing and spectrum assignment (RSA) problem needs to be complemented with fast and accurate tools to consider the quality of transmission (QoT) within the provisioning process. This paper proposes a digital twin-assisted approach for lightpath provisioning to provide a complete solution for the RSA problem that ensures the required QoT in MB optical networks. The OCATA time domain digital twin is proposed, not only to estimate the QoT of a selected path but also to support the QoT-based channel assignment process. OCATA is based on a Deep Neural Network (DNN) to model the propagation of the optical signal. However, because of the different impacts of nonlinear noise on each channel and the large number of channels that need to be considered in C+L+S MB scenarios, OCATA needs to be adapted to make it scalable, while keeping its high accuracy and fast QoT estimation characteristics. In consequence, a complete methodology is proposed in this work that limits the number of channels being modeled to just a few. Moreover, OCATA-MB helps to mitigate NLI noise by programming the receiver at the provisioning time and thus with very little complexity compared to its equivalent implemented during the operation. NLI noise mitigation can be applied in the case when a lightpath cannot be provisioned because none of the available channels can provide the required QoT, making it an advantageous tool for reducing connection blocking. Exhaustive simulation results demonstrate the remarkable accuracy of OCATA-MB in estimating the QoT for any channel. Interestingly, by utilizing the proposed OCATA-MB-assisted lightpath provisioning approach, a reduction of the blocking ratio exceeding 50% when compared to traditional approaches is shown when NLI noise mitigation is not applied. If NLI mitigation is implemented, an additional over 50% blocking reduction is achieved. multiband optical transmission; digital twin; quality of transmission; service provisioning; nonlinear mitigation +
Footnote †: This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))). (MB) optical networks are seen as a solution to extend legacy optical networks' capacity [2]. The authors in [3] analyzed the expected traffic demand to be supported as a result of combining well-known mass market services with foreseen B5G services scenarios and showed that MB transmission will be required for all network segments. However, the adoption of MB optical transmission requires dealing with a phenomenon that increases nonlinear impairments (NLI)--Inter-channel Stimulated Raman Scattering (ISRS) [4]. Several studies have analyzed and assessed the increment of capacity achievable by using other bands, e.g., the S-band, in addition to the traditional C and L ones, from the perspective of the line system [5]. However, in addition to pure optical transmission, the provisioning of optical connections needs to be reviewed also to actually achieve the expected capacity increase. Traditionally, the provisioning of optical connections entails solving the Routing and Spectrum Allocation (RSA) problem [6], which can be complemented by considering the quality of transmission (QoT) of the computed route and the spectral allocation before accepting the connection request (see, e.g., [7; 8; 9; 10]). Estimation of the QoT has been thoroughly investigated, and several techniques have been proposed, e.g., based on the Gaussian Noise model [11; 12].