Performance analysis of spatial diversity vertical UWOC systems over depth-dependent WGG turbulence with approximate...

This paper proposes a unified analytical framework for evaluating the bit error rate (BER) performance of spatial diversity vertical underwater wireless optical communication (UWOC) systems jointly accounting for various system impairments. The attenuation coefficient is characterized using a depth-dependent chlorophyll-based model. The underwater optical turbulence (UOT) is modeled using the mixture Weibull-generalized gamma (WGG) distribution, which serves as a universal form encompassing several turbulence models. To accurately account for asymmetric beam misalignment due to pointing errors (PEs), a mathematically tractable, gamma-approximated Beckmann distribution is employed. The angle-of-arrival (AOA) deviation is modeled as a Rayleigh distribution, where signal outage occurs when the incident angle exceeds the field-of-view (FOV) angle. To the best of our knowledge, for the first time, the probability density function (PDF) of the composite fading coefficient is derived in terms of the generalized Fox's H-function by integrating these models. Furthermore, leveraging this statistical result and the unified conditional BER transformation for various modulation schemes, we analytically derive the accurate average BER expressions for systems employing equal gain combining (EGC) and maximal ratio combining (MRC) diversity reception techniques, utilizing the characteristic function (CHF) and moment generating function (MGF)-based methods, respectively. Numerical results from these expressions are validated through extensive Monte Carlo (MC) simulations, confirming the analytical accuracy and demonstrating the effectiveness of diversity techniques in mitigating impairments. Comprehensive discussions on the impacts of the diversity schemes, diversity order, modulation schemes, different WGG turbulence conditions, and FOV angles on the BER performance are presented. The results provide fundamental theoretical foundations and practical guidelines for the design and optimization of high-reliability UWOC links with diversity reception over composite channels.