Frequency Diversity in Mode-Division Multiplexing Systems

TL;DR

This paper analyzes how frequency-dependent mode gains and losses in mode-division multiplexing systems affect channel capacity, showing that increased bandwidth relative to coherence bandwidth reduces capacity fluctuations.

Contribution

It introduces a statistical model linking mode-dependent gains, losses, and group delays to capacity fluctuations, highlighting the impact of frequency diversity in multimode fibers.

Findings

Capacity fluctuations decrease with increased bandwidth.

Outage capacity approaches average capacity at large bandwidths.

Diversity order is proportional to the ratio of bandwidth to coherence bandwidth.

Abstract

In the regime of strong mode coupling, the modal gains and losses and the modal group delays of a multimode fiber are known to have well-defined statistical properties. In mode-division multiplexing, mode-dependent gains and losses are known to cause fluctuations in the channel capacity, so that the capacity at finite outage probability can be substantially lower than the average capacity. Mode-dependent gains and losses, when frequency-dependent, have a coherence bandwidth that is inversely proportional to the modal group delay spread. When mode-division-multiplexed signals occupy a bandwidth far larger than the coherence bandwidth, the mode-dependent gains and losses are averaged over frequency, causing the outage capacity to approach the average capacity. The difference between the average and outage capacities is found to be inversely proportional to the square-root of a diversity order that is given approximately by the ratio of the signal bandwidth to the coherence bandwidth.