Performance evaluation of the vertical UWOC system under jamming over the unified multi-layer turbulence channel

In this study, the impact of jamming on the vertical underwater wireless optical communication (UWOC) link is first investigated over composite cascaded turbulence fading channels, incorporating absorption, scattering, and pointing errors. Specifically, the underwater optical turbulence (UOT)-induced fading on the vertical UWOC link is described by a unified multi-layer model, which integrates both single-lobe and two-lobe statistical characteristics of UOT across the entire range of scintillation indices. Based on this model, exact closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the composite multi-layer channel fading and signal-to-jamming ratio (SJR) are derived. Leveraging these statistical results, average bit error rate (ABER) of the vertical UWOC system with a single-aperture transmitter is derived by assuming that the jamming signal acts as a random noise source subject to similar channel fading conditions as the legitimate signal. To mitigate the effect of jamming on the vertical UWOC link, spatial diversity technology is adopted. The number of apertures on the legitimate transmitter is increased, i.e., a multiple-input single-output (MISO) UWOC system implementation. And then, closed-form ABER expressions of this vertical MISO UWOC system over the unified composite cascaded fading channel are rigorously deduced in the form of multivariate Fox-H functions for both selection combining and equal-gain combining schemes. The validity of these theoretical ABER expressions is finally confirmed through Monte Carlo simulations. Furthermore, a comprehensive performance evaluation of the vertical UWOC system under jamming is conducted for different system and channel parameters. This work could serve as a good guide for the design and research of the jamming-affected vertical UWOC system.