  In this work, we present a unified framework for the performance analysis of
dual-hop underwater wireless optical communication (UWOC) systems with
amplify-and-forward fixed gain relays in the presence of air bubbles and
temperature gradients. Operating under either heterodyne detection or intensity
modulation with direct detection, the UWOC is modeled by the unified mixture
Exponential-Generalized Gamma distribution that we have proposed based on an
experiment conducted in an indoor laboratory setup and has been shown to
provide an excellent fit with the measured data under the considered lab
channel scenarios. More specifically, we derive the cumulative distribution
function (CDF) and the probability density function of the end-to-end
signal-to-noise ratio (SNR) in exact closed-form in terms of the bivariate
Fox's H function. Based on this CDF expression, we present novel results for
the fundamental performance metrics such as the outage probability, the average
bit-error rate (BER) for various modulation schemes, and the ergodic capacity.
Additionally, very tight asymptotic results for the outage probability and the
average BER at high SNR are obtained in terms of simple functions. Furthermore,
we demonstrate that the dual-hop UWOC system can effectively mitigate the short
range and both temperature gradients and air bubbles induced turbulences, as
compared to the single UWOC link. All the results are verified via
computer-based Monte-Carlo simulations.
