Abstract
Polarization division multiplexed (PDM) quadrature phase-shift keying
(QPSK) coherent optical systems employ blind adaptive linear electronic equalizers
for polarization-mode dispersion (PMD) compensation. In this paper, we compute
the performance of various adaptive, fractionally spaced, feed-forward electronic
equalizers, using the outage probability as a criterion. A parallel programming
implementation of the multicanonical Monte Carlo method is developed, which
automatically performs concurrent loop computation on multicore processors,
for the estimation of the tails of the outage probability distribution. The
constant modulus algorithm (CMA), the decision-directed least mean squares
(DD-LMS), and their combination are applied for the adaptation of electronic
equalizer filter coefficients. In the exclusive presence of PMD, we demonstrate
that half-symbol-period-spaced CMA-based adaptive electronic equalizers perform
slightly better than their DD-LMS counterparts, at links with strong PMD,
whereas the opposite holds true at the weak PMD regime. It is shown that the
successive application of CMA and DD-LMS with 20 complex, half-symbol-period-spaced
taps per finite impulse response filter is adequate to reduce the outage probability
of coherent PDM QPSK systems to less than $10^{-5}$, for a mean differential group delay
of more than twice the symbol period.
© 2011 IEEE
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