Abstract
In this paper, we review linear propagation effects in a multimode fiber (MMF) and their impact on
performance and complexity in long-haul mode-division multiplexing (MDM) systems. We highlight the many similarities
to wireless multi-input multioutput (MIMO) systems. Mode-dependent loss and gain (MDL), analogous to multipath fading,
can reduce average channel capacity and cause outage in narrowband systems. Modal dispersion (MD), analogous to
multipath delay spread, affects the complexity of MIMO equalization, but has no fundamental effect on performance.
Optimal MIMO transmission uses a basis of the Schmidt modes, which may be obtained by a singular value decomposition
of the MIMO channel. In the special case of a unitary channel (no MDL), an optimal basis is the set of principal
modes, which are eigenvectors of a group delay operator, and are free of signal distortion to first order. We present
a concatenation rule for the accumulation of MD along a multisection link. We review mode coupling in MMF, including
physical origins, models, and regimes of weak and strong coupling. Strong mode coupling is a key to overcoming
challenges in MDM systems. Strong coupling reduces the group delay spread from MD, minimizing the complexity of MIMO
signal processing. Likewise, it reduces the variations of loss and gain from MDL, maximizing channel capacity. In the
strong-coupling regime, the statistics of MD and MDL depend only on the number of modes and the variance of
accumulated group delay or loss/gain, and can be derived from the eigenvalue distributions of certain Gaussian random
matrices.
© 2013 IEEE
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