In order to effectively keep pace with the global IP traffic growth forecasted in the years to come, Flex-Grid over Multi-Core Fiber (MCF) networks can bring superior spectrum utilization flexibility, as well as bandwidth scalability far beyond the non-linear Shannon's limit. In such a network scenario, however, full node switching reconfigurability will require an enormous node complexity, pushing the limits of current optical device technologies at expenses of prohibitive capital expenditures. Therefore, cost-effective node solutions will most probably be the key enablers of Flex-Grid/MCF networks, at least in the shortand mid-term future. In this context, this paper proposes a cost-effective Reconfigurable Optical Add/Drop Multiplexer (ROADM) architecture for Flex-Grid/MCF networks, called CCC-ROADM, which reduces technological requirements (and associated costs) in exchange of demanding core continuity along the end-to-end communication. To assess the performance of the proposed CCC-ROADM in comparison with a fully-flexible ROADM (i.e., a Fully Non-Blocking ROADM, called FNB-ROADM in this work) in large-scale network scenarios, a novel lightweight heuristic to solve the route, modulation, core and spectrum assignment (RMCSA) problem in Flex-Grid/MCF networks is presented in this work, whose goodness is successfully validated against optimal ILP formulations previously proposed for the same goal. The obtained numerical results in a significant number of representative network topologies with different MCF configurations of 7, 12 and 19 cores show almost identical network performance in terms of maximum network throughput when deploying CCC-ROADMs vs. FNB-ROADMs, while decreasing network capital expenditures to a large extent.

Space Division Multiplexing (SDM) appears as a promising solution to overcome the capacity limits of single-mode optical fibers. In Flex-Grid/SDM optical networks, nodes offering full interconnection between input/output fiber ports and spatial channels, typical SDM-Reconfigurable Optical Add/Drop Multiplexer (SDM-ROADM) referred to as independent switching with lane support (InS with LC support), require very complex and expensive node architectures. Alternative designs have been proposed to relax their requirements, such as those realizing Joint-switching (JoS) by switching one spectrum slice across all spatial channels at once. In this work, we evaluate the benefits of a cost-effective SDM-ROADM architecture that makes a trade-off between (i) performance in terms of network throughput and (ii) architectural complexity by forcing the Space Continuity Constraint (SCC) end-to-end, that is, along the connection physical path. The performance and architectural complexity of such a SDM-ROADM solution are compared in dynamic Flex-Grid/SDM scenarios against benchmark networks based on InS with LC support and JoS SDM-ROADMs, under both spatial and spectral super-channels. We quantify the network throughput when scaling the spatial multiplicity from 7 to 30 spatial channels, considering Multi-Fiber (MF) as well as Multi-Core Fiber (MCF) SDM solutions. The obtained results reveal that differences in terms of network throughput employing InS without LC support SDM-ROADMs is merely up to 14% lower than InS with LC support SDM-ROADMs, while the network CAPEX can be dramatically reduced by 86%. In contrast, networks employing InS without LC support SDM-ROADMs carry up to 40% higher throughput than JoS ones, whereas the network CAPEX can be raised up to 3x. This paper also analyses the spatial multiplicity impact on both network metrics (throughput and CAPEX).

The majority of the research studies on Flex-Grid over multi-core fiber (Flex-Grid/MCF) networks are built on the assumption of fully non-blocking ROADMs (FNB-ROADMs), able to switch any portion of the spectrum from any input core of any input fiber to any output core of any output fiber. Such flexibility comes at an enormous extra hardware cost. In this paper, we explore the trade-off of using ROADMs that impose the so-called core continuity constraint (CCC). Namely, a CCC-ROADM can switch spectrum from a core on an input fiber to a chosen output fiber, but cannot choose the specific output core. For instance, if all fibers have the same number of cores, the i-th core in the input fibers can be just switched to the i-th core in the output fibers. To evaluate the performance vs. cost trade-off of using CCC-ROADMs, we present two Integer Linear Programming (ILP) formulations for optimally allocating incoming demands in Flex-Grid/MCF networks, where the CCC constraint is imposed or not, respectively. A set of results are extracted applying both schemes in two different backbone networks. Transmission reach estimations are conducted accounting for the fiber's linear and non-linear effects, as well as the inter-core crosstalk (ICXT) impairment introduced by laboratory MCF prototypes of 7, 12 and 19 cores. Our numerical evaluations show that the performance penalty of CCC is minimal, i.e., below 1% for 7 and 12-core MCF and up to 10% for 19-core MCF, while the cost reduction is large. In addition, results reveal that the ICXT effect can be significant when the number of cores per MCF is high, up to a point that equipping the network with 12-core MCFs can yield superior effective capacity than with 19-core MCFs.

Space Division Multiplexing (SDM) is a key technology to cope with the bandwidth limitations of single mode fibers. Multi-Core Fibers (MCFs) are considered as a promising candidate technology to implement SDM, due to their low inter-core crosstalk (ICXT), experimentally proven in laboratory prototypes. Among the different channel allocation options making use of the newly enabled space dimension, the so-called spatial super-channel (Spa-SCh) is the most likely solution to be implemented, given the inherent cost reduction of the joint-switching operation (i.e., jointly switching a spectrum portion in all MCF cores at once). This work targets the cost-effective Spa-SCh allocation over MCF-enabled Flex-Grid optical core networks. To this goal, state-of-the-art 22-core MCFs are assumed, although the proposed solutions are applicable to any MCF type. In particular, we propose and evaluate a partial-core assignment as a cost-effective strategy to improve spectrum utilization and save Capital Expenditure (CapEx) costs by minimizing the number of optical transceivers used per Spa-SCh. Numerical results reveal that reductions up to 44% and 33% in the number of active transceivers in the network can be obtained in national- and continental-wide backbone networks, respectively, without affecting the network Grade-of-Service (GoS), measured in terms of Bandwidth Blocking Probability (BBP). To evaluate the impact of the ICXT, we also compare the performance of the MCF scenarios under study against equivalent Multi-Fiber (MF) ones. From the obtained results, ICXT in MCF scenarios requires the utilization of less efficient modulation formats, which reduces the admissible offered network load by up to 17% for a 1% BBP target. Furthermore, this lower spectral efficiency also demands an increase of the symbol rate per sub-channel up to a 26%, a key indicator of the modulator electronic complexity.Peer ReviewedPostprint (author's final draft

We quantify the network capacity scaling from 7 to 30 spatial channels. While multifiber provides a 5x capacity increase, MCF limits it to 4x and 2x in national and continental backbone networks, respectively.

In this paper, we target an optimal Multiple-Input Multiple-Output Digital Signal Processing (MIMO-DSP) assignment to super-channels affected by inter-core crosstalk (ICXT) in Multi-Core Fiber (MCF)-enabled transparent optical core networks. MIMO-DSP undoes ICXT effects, but can be costly with high core density MCFs. Hence, its implementation in the network must be carefully decided. We address our objective as a joint Route, Modulation format, MIMO and Spectrum Assignment (RMMSA) problem, for which Integer Linear Programming (ILP) formulations are provided to optimally solve it in small network scenarios. Moreover, several heuristic approaches are also proposed to solve large-scale problem instances with good accuracy. Their goal is to minimize both network spectral requirements and the amount of MIMO equalized super-channels, taking a crosstalk-free Space Division Multiplexing (SDM) solution as a reference, for example, based on parallel single mode fibers (i.e., a Multi-Fiber -MF-scenario). For our evaluation, we consider several state-of-the-art MCF prototypes and different network topologies. The obtained results, with the considered MCFs, disclose that in national backbone networks, the desirable percentage of super-channels with MIMO equalization to match the performance of an equivalent crosstalk-free SDM solution ranges from 0-36%, while in continental-wide networks this range raises up to 0-56%. In addition, in the case of a non-ideal MIMO (with a 3 dB/km of crosstalk compensation), such percentages range from 0-28% and 0-45% in national and continental-wide backbone networks, respectively, experimenting a performance gap up to 12% with respect to the MF reference scenario.

We focus on a migration planning from elastic to spectrally-spatially flexible optical networks by upgrading regular with SDM-capable ROADMs. Our most promising strategy reveals that by migrating 30% of the regular ROADMs a 2x carried traffic increase can be achieved.

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