  Due to recent advances in laser satellite communications technology,
free-space optical (FSO) links are presented as an ideal alternative to the
conventional radio frequency (RF) feeder links of the geostationary satellite
for next generation very high throughput satellite (VHTS) systems. In this
paper, we investigate the performance of multibeam VHTS systems that account
for nonlinear high power amplifiers at the transparent fixed gain satellite
transponder. Specifically, we consider the forward link of such systems, where
the RF user link is assumed to follow the shadowed Rician model and the FSO
feeder link is modeled by the Gamma-Gamma distribution in the presence of beam
wander and pointing errors where it operates under either the intensity
modulation with direct detection or the heterodyne detection. Moreover,
zero-forcing precoder is employed to mitigate the effect of inter-beam
interference caused by the aggressive frequency reuse in the user link. The
performance of the system under study is evaluated in terms of the outage
probability, the average bit-error rate (BER), and the ergodic capacity that
are derived in exact closed-forms in terms of the bivariate Meijer's G
function. Simple asymptotic results for the outage probability and the average
BER are also obtained at high signal-to-noise ratio.
