Partial-MIMO based Mode-Group transmission and routing in a field-deployed 15-mode network: throughput, DSP resources and network flexibility

Transmission based on mode-division multiplexing can enhance node flexibility in metropolitan networks by exploiting mode-group routing. In fact, each mode group, constituted by degenerate spatial modes, can act as a single modal super-channel: since the inter-group crosstalk remains limited or even negligible among different mode groups for sufficiently short distances, typical of urban scenarios, the mode groups can be optically multiplexed and demultiplexed at the network nodes. Moreover, each group can cover an optical path independent from the propagation of the other groups, and be received by means of a so-called partial-MIMO processing, handling just its own degenerate modes, with a significant reduction of the necessary digital resources. In this paper we experimentally assess the capabilities of such a mode-group approach using a few-mode fiber, supporting 5 mode groups, already deployed in the city of L'Aquila, Italy. Transmission throughput versus DSP resources (in terms of number of taps required to achieved a proper GMI) is analyzed, experimenting different modulation formats (at 50 Gbaud), in case of 75-GHz spaced WDM transmission. The large number of spatial modes supported by the few-mode fiber employed in the field-trial allows to explore several combinations of transmitted mode groups in order to better understand how the inter-group cross-talk affects the capacity performance. Almost 2-Tb/s capacity per WDM channel is demonstrated for 6 WDM channels over 6.1-km few-mode fiber reach, thanks to partial MIMO. Finally, multi-hop routing is also performed in a 4-node scenario, in particular taking into account the impact of the optical mode multiplexer/demultiplexer employed in the field trial on the routed throughput per wavelength.