Unified performance analysis of vertical underwater wireless optical communications with layered turbulence and pointing errors

This paper provides a comprehensive performance analysis of vertical underwater wireless optical communication (UWOC) systems operating under the combined effects of cascaded oceanic turbulence and pointing errors (PEs). In the vertical UWOC links, oceanic turbulence is inherently inhomogeneous due to depth-dependent temperature variations and salinity gradients. To account for depth-dependent behavior, turbulence is modeled as a cascaded multi-layer fading channel, where each layer is characterized by independent and non-identical mixtures of exponential–generalized gamma (EGG) distributed fading coefficients. In this work, the probability density function (PDF) of a multi-layer EGG turbulence channel is derived using the inverse Mellin transform. Furthermore, the unified PDF and cumulative distribution function (CDF) of the fading coefficients and instantaneous signal-to-noise ratio (SNR) are obtained considering zero- and nonzero-boresight PEs. Unified closed-form expressions are derived for outage probability (OP), ergodic capacity (EC), and average bit error rate (BER) for various modulation schemes, detection techniques, and fading conditions. Finally, numerical results and extensive Monte-Carlo (MC) simulations are presented to validate the analytical expressions. The results demonstrate the accuracy of the proposed analytical framework and highlight its effectiveness in characterizing and optimizing the performance of vertical UWOC systems under realistic oceanic channel conditions.