翻译： As mentioned in previous sections, MCFs are the most popular SDM fiber structures and there is a lot of ongoing research on these. In fact, the principles behind the MCF design are completely distinctive of the SMF ones. Researchers have been looking into fundamental design aspects, like efficient placement of the cores for reduced inter-core crosstalk, bending losses and cladding thickness [15], [23], [25]–[28]. Obviously, the challenge here is to pack as many cores as possible into a single fiber structure, while avoiding large penalties from inter-core interference. Bearing this in mind it is important to investigate how many cores could fit in a MCF and then link those MCFs with different kinds of networks and use-cases. This is what we will show in the following part, by simulating crosstalk (XT) for two popular MCF implementations, the first one with 7 cores [15] and the second with 19 cores [23]. Intercore crosstalk interference in MCFs is defined as the ratio of the optical power inserted from adjacent cores to the one under study, divided by the power of the signal already in that core and it is measured in dB. The threshold, beyond of which the signal integrity is altered, can vary between −18 dB and −32 dB, depending in the modulation format that is used [134]. In our simulations, we use a threshold of around −24 dB, which is in the middle of this range and also represents a signal modulated with 16-QAM. To calculate the statistical mean XT of a homogenous MCF, we used a formula based on Eq. (3) as in [15], [135]–[137], which also considers the coupled-power theory [26], leading to Eq. (4) h = 2 · κ2 · R β · D (3) XT = n − n · exp (−(n + 1) · 2 · h · L) 1 + n · exp (−(n + 1) · 2 · h · L) . (4) In Eq. (3), h is the mean crosstalk increase per unit length, calculated by several fiber parameters: κ, β, R, D which is the coupling coefficient, propagation constant, bend radius and core-pitch respectively. Eq. (4) makes use of h from previous equation multiplied by L as the length of the fiber, while n stands for the number of adjacent cores in a hexagonal lattice. For instance, studying the center core of the fiber would require a value of n = 6. For this simulation, we made some as realistic as possible assumptions about the values of these parameters. Thus, κ is ranging from 2 × 10−5 to 3, 5 × 10−3, R is ranging from50to80mm,β is 4 × 106 around the 1550 nm frequency window and D is 45 μm for the 7-core and 35 μmforthe 19-core. While core-pitch is reduced, i.e. D < 35 μm, to investigate theoretical fiber designs, coupling coefficient rises and takesvaluesofκ>10−3 [138]. The worst case of crosstalk will always be that of the center core (or any other core that has the largest number of neighbor cores), when studying hexagonal design MCFs, since it receives unwanted interference from all its adjacent cores. So, the whole SDM link length is often restricted by this core’s crosstalk value. At that point, it is worth mentioning that crosstalk interference takes place only between same frequency slices used in adjacent cores. In our simulation, we assume that the spectrum of each core is fully utilized. As stated in a previous part of this paper, there are spectrum and core allocation algorithms which target in minimizing this crosstalk interference of the MCF link [129]–[131]. In Fig. 18(a) crosstalk values for the two MCFs are plotted regarding the link length from 1 m up to 100 km. It is obvious that for most of the network use-cases or interconnection distances, these fibers’ XT values are quite below the crosstalk threshold, which leaves a lot room for improvement towards packing more and less-spaced cores within a MCF. The target is to have as many cores as possible, with the smallest achievable fiber cross-section area and the minimum XT value for the longest propagation distance that is feasible. Reduction of the fiber core-pitch, immediately raises XT, consequently the link length gets limited. But how long need those SDM links to be? The answer stems from the network itself. It depends on the length of the connection and the nature of the network. For instance, separate network types have unique interconnection distances, such as metro/core and intradatacenter interconnects. In the former case, links need to be in the scale of several hundreds of kilometers (≥100 km), whereas in the latter case, up to 10 meter links could be used for intra-rack interconnection, around 100 meters for inter-rack intra-cluster communication and 1 or very few kilometers for inter-cluster and generally intra-DCN connections. As a result, the tradeoff between the core-pitch and the length of the link needs to be investigated further. In Fig. 18(b), XT for four discrete link distance curves and for various core-pitches is presented.