翻译：In the previous sections, numerous capabilities of SDM technologies and aspects of SDM networking have been presented and thoroughly analyzed. There was also a brief discussion on how metro/core, access and DC networks could potentially benefit from SDM networking. Research gaps, lessons learned and directions of where SDM research should focus in the future are discussed in this part of the paper. In order to provide a more complete and combined vision of SDM technologies and techniques, Fig. 22 summarizes the challenges and potential solutions associated with various kinds of networks (backbone, metro/region and DC/HPC). Regarding long-haul backbone networks, consisting of 1000 km long links, obvious challenges for SDM solutions would be mode coupling and crosstalk as well as amplification. MCFs could be a more reliable solution since a lot of work has been done in minimizing inter-core crosstalk interference. Modemultiplexing, using MMF/FMF or even vortex fiber for OAM multiplexing, in such distances is almost impossible. According to previous studies [146]–[148], optical signals and their OSNR would be strongly affected and degraded by modal dispersion, mode dependent loss and differential mode delay, that even heavy MIMO-DSP in the receiver side would not be enough to recover it properly. However, Few-Mode MCF could be one feasible alternative for long-haul networks that researchers should look upon in the future. As far as SDM amplifiers are concerned, it is clear that their frequent use is absolutely essential in long-haul distances so more efficient solutions need to be produced. SDM transponders and mux/demux in backbone and metro network nodes, do not have any special restriction as far as integration and power consumption is concerned in comparison with datacenter and HPC networks where footprint and energy efficiency are both major and decisive factors. Moreover, regarding backbone and metro networks, novel SDM amplifiers, especially those based on cladding-pumping, are backward compatible with current SMF-based network infrastructure and lead to increased integration, cutting down in resources (i.e. one EDFA instead of multiple) and eventually in great power savings. Of course issues like power and gain balancing are still to be solved. Another important challenge for SDM, is how it can prove itself worthy enough to network vendors, owners and service providers in order for them to introduce its technologies and solutions and possibly integrate them in their present infrastructure. Further studies are required to justify the benefits and establish the merits of SDM correlated with the various network use-cases. A meaningful first step would be the standardization of fibers and other SDM technologies in order to accelerate research to deployment cycles. The ultra-high bandwidth that novel SDM fibers are able to provide, the integration of SDM components (EDFAs, TxRx, mux/demux, etc.) along with the additional level of flexibility in routing and switching the space dimension offers are expected to be among the predominant drivers for future research on this field. Great potential for easy and direct SDM network deployment can be found in newly-built smart cities. These cities are a green field for real life testing of the SDM technologies with realistic traffic flows and bitrates. Smart cities in that way could undoubtedly give researchers a great chance to setup a whole SDM network infrastructure from scratch and observe its behavior while serving a modern’s city’s actual networking needs. Nevertheless, besides the increased capacity and the additional networking features that SDM can offer and are presented in the above sections, the utilization of the space dimension could also open new opportunities and bring potential for optical network virtualization, in example by allocating numerous virtual slices over distinct spatial modes.