  In this paper, we consider an overlay satellite-terrestrial network (OSTN)
where an opportunistically selected terrestrial internet-of-things (IoT)
network assists the primary satellite communications as well as accesses the
spectrum for its own communications under hybrid interference received from
extra-terrestrial sources (ETSs) and terrestrial sources (TSs). Herein, the IoT
network adopts power-domain multiplexing to amplify-and-forward the superposed
satellite and IoT signals. Considering a unified analytical framework for
shadowed-Rician fading with integer/non-integer Nakagami-\emph{m} parameter for
satellite and interfering ETSs links along with the integer/non-integer
Nakagami-\emph{m} fading for terrestrial IoT and interfering TSs links, we
derive the outage probability (OP) of both satellite and IoT networks. Further,
we derive the respective asymptotic OP expressions to reveal the diversity
order of both satellite and IoT networks under the two conditions, namely when
the transmit power of interferers: $(a)$ remains fixed; and $(b)$ varies
proportional to the transmit powers of main satellite and IoT users. We show
that the proposed OSTN with adaptive power-splitting factor benefits the IoT
network while guaranteeing certain quality-of-service (QoS) of satellite
network. We verify the numerical results by simulations.
