  This paper presents a unified analysis of the mixed radio-frequency
(RF)/free-space optics (FSO) relaying system, with multiple variable-gain
amplify-and-forward relays. The partial relay selection (PRS) is employed to
select the active relay for further re-transmission. Due to fast fading
statistics of the first RF hop, it is assumed that the channel state
information of the RF link is outdated, which is used for both the relay gain
adjustment and the PRS procedure. The RF hops are subject to the Rayleigh
fading, while the FSO hop is affected by the atmospheric turbulence and the
pointing errors. The intensity fluctuations of the optical signal caused by
atmospheric turbulence are modeled by the general Malaga (M) distribution,
which takes into account the effect of multiple scattered components. An exact
expression for the outage probability is derived. In addition,
high-signal-to-noise-ratio approximations are provided, which can be used to
efficiently determine the outage probability floor. Numerical results are
validated by Monte Carlo simulations, which are used to examine the effects of
the system and channel parameters on the RF/FSO system performance.
