  In this paper, we propose efficient importance sampling estimators to
evaluate the outage probability of maximum ratio combining receivers over
turbulence-induced fadings in underwater wireless optical channels. We consider
two fading models: exponential-lognormal, and exponential-generalized Gamma.
The cross-entropy optimization method is used to determine the optimal biased
distribution. We show by simulations that the number of samples required by
importance sampling estimator is much less compared to naive Monte Carlo for
the same accuracy requirement.
