  Post commercial deployment of fifth-generation (5G) technologies, the
consideration of sixth-generation (6G) networks is drawing remarkable attention
from research communities. Researchers suggest that similar to 5G, 6G
technology must be human-centric where high secrecy together with high data
rate will be the key features. These challenges can be easily overcome
utilizing PHY security techniques over high-frequency free-space or underwater
optical wireless communication (UOWC) technologies. But in long-distance
communication, turbulence components drastically affect the optical signals,
leading to the invention of the combination of radio-frequency (RF) links with
optical links. This work deals with the secrecy performance analysis of a mixed
RF-UOWC system where an eavesdropper tries to intercept RF communications. RF
and optical links undergo $\eta-\mu$ and mixture exponential generalized Gamma
distributions, respectively. To keep pace with the high data rate of optical
technologies, we exploit the antenna selection scheme at the source and maximal
ratio combining diversity at the relay and eavesdropper, while the eavesdropper
is unaware of the antenna selection scheme. We derive closed-form expressions
of average secrecy capacity, secrecy outage probability, and strictly positive
secrecy capacity to demonstrate the impacts of the system parameters on the
secrecy behavior. Finally, the expressions are corroborated via Monte-Carlo
simulations.
