  In this paper, we consider a relay-assisted uplink non-orthogonal multiple
access (NOMA) system. In this system, two radio frequency (RF) users are
grouped for simultaneous transmissions, over each resource block, to an
intermediate relay. The relay then forwards the amplified version of the users'
aggregated signals, in the presence of multiuser interference, to a relatively
far destination. In order to cope with the users' ever-increasing desire for
higher data rates, a high-throughput free-space optics (FSO) link is employed
as the relay-destination backhaul link. It is assumed that the FSO backhaul
link is subject to Gamma-Gamma turbulence with pointing error. Also, a Rayleigh
fading model is considered for the user-relay access links. Under these
assumptions, we derive closed-form expressions for the outage probability and
tractable forms, involving only one-dimensional integrals, for the ergodic
capacity. Moreover, the outage probability and ergodic capacity analysis are
extended to the conventional RF-backhauled systems in the presence of multiuser
interference to both relay and destination nodes, and Rician fading for the
relay-destination RF link. Our results reveal the superiority of FSO
backhauling for high-throughput and high-reliability NOMA systems compared to
RF backhauling. This work can be considered as a general analysis of dual-hop
uplink NOMA systems as well as the first attempt to incorporate power-domain
NOMA in mixed RF-FSO systems.
