Capacity and Achievable Rates of Fading Few-mode MIMO IM/DD Optical Fiber Channels

TL;DR

This paper investigates the capacity limits of MIMO optical fiber channels with IM/DD, estimating bounds and proposing an autoencoder-based method for practical transmission optimization under channel variability.

Contribution

It adapts free-space IM/DD MIMO bounds to fiber channels, estimates capacity with the Blahut-Arimoto algorithm, and introduces an autoencoder for robust MIMO transmission optimization.

Findings

Upper bounds from free-space channels are adapted to fiber MIMO channels.

The autoencoder improves practical MIMO transmission robustness.

Pre-coders are effective despite channel variations.

Abstract

The optical fiber multiple-input multiple-output (MIMO) channel with intensity modulation and direct detection (IM/DD) per spatial path is treated. The spatial dimensions represent the multiple modes employed for transmission and the cross-talk between them originates in the multiplexers and demultiplexers, which are polarization dependent and thus timevarying. The upper bounds from free-space IM/DD MIMO channels are adapted to the fiber case, and the constellation constrained capacity is constructively estimated using the Blahut-Arimoto algorithm. An autoencoder is then proposed to optimize a practical MIMO transmission in terms of pre-coder and detector assuming channel distribution knowledge at the transmitter. The pre-coders are shown to be robust to changes in the channel.