  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.
