We demonstrate differential mode delay (DMD) unmanaged 2500-km 12-core × 3-mode multicore few-mode fiber (MC-FMF) and 6300-km 3-mode FMF transmission. Mode-division multiplexed signals exhibit different transmission behavior depending on excited spatial modes in the presence of mode-relevant physical phenomena including DMD and modedependent loss (MDL). In weakly coupled FMF transmission over long distance where overall DMD and MDL grow almost linearly with fiber length, these phenomena

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... achievable information capacity and/or transmission reach. This paper presents a newly developed transmission scheme and a multiple-input multipleoutput (MIMO) signal processing technique that are designed for achieving long-haul FMF transmission. Permutating spatial channels cyclically at each span induced quasi-strongly coupled transmission regime and hence significantly suppressed DMDinduced pulse broadening. Layered MIMO signal processing comprising multiple stages performing successive signal detection and intermodal interference canceling mitigated MDL impact with more than 2-dB Q-factor improvement. The combined use of these techniques enabled us to achieve the first-ever transoceanic-class long-haul FMF transmission. Index Terms-Differential mode delay (DMD), mode dependent loss (MDL), MIMO equalization, space division multiplexing (SDM), successive interference cancellation (SIC).