  With the completion of standardization of fifth-generation (5G) networks, the
researchers have begun visioning sixth-generation (6G) networks that are
predicted to be human-centric. Hence, similar to 5G networks, besides high data
rate, providing secrecy and privacy will be the center of attention by the
wireless research community. To support the visions beyond 5G (B5G) and 6G, in
this paper we propose a secure radio frequency (RF)-free-space optical (FSO)
mixed framework under the attempt of wiretapping by an eavesdropper at the RF
hop. We assume the RF links undergo alpha-eta-mu fading whereas the FSO link
exhibits a unified Malaga turbulence model with pointing error. The secrecy
performance is evaluated by deducing expressions for three secrecy metrics i.e.
average secrecy capacity, secure outage probability, and probability of
non-zero secrecy capacity in terms of univariate and bivariate Meijer's G and
Fox's H functions. We further capitalize on these expressions to demonstrate
the impacts of fading, atmospheric turbulence, and pointing errors and show a
comparison between two detection techniques (i.e. heterodyne detection (HD) and
intensity modulation with direct detection (IM/DD)) that clearly reveals better
secrecy can be achieved with HD technique relative to the IM/DD method. The
inclusion of generalized fading models at the RF and FSO hops offers
unification of several classical scenarios as special cases thereby exhibiting
a more generic nature relative to the existing literature. Finally, all the
analytical results are corroborated via Monte-Carlo simulations.
