  Non-orthogonal multiple access (NOMA) and spectrum sharing are two potential
technologies for providing massive connectivity in beyond fifth-generation
(B5G) networks. In this paper, we present the performance analysis of a
multi-antenna-assisted two-user downlink NOMA system in an underlay spectrum
sharing system. We derive closed-form expressions for the average achievable
sum-rate and outage probability of the secondary network under a peak
interference constraint and/or peak power constraint, depending on the
availability of channel state information (CSI) of the interference link
between secondary transmitter (ST) and primary receiver (PR). For the case
where the ST has a fixed power budget, we show that performance can be divided
into two specific regimes, where either the interference constraint or the
power constraint primarily dictates the performance. Our results confirm that
the NOMA-based underlay spectrum sharing system significantly outperforms its
orthogonal multiple access (OMA) based counterpart, by achieving higher average
sum-rate and lower outage probability. We also show the effect of information
loss at the ST in terms of CSI of the link between the ST and PR on the system
performance. Moreover, we also present closed-form expressions for the optimal
power allocation coefficient that minimizes the outage probability of the NOMA
system for the special case where the secondary users are each equipped with a
single antenna. A close agreement between the simulation and analytical results
confirms the correctness of the presented analysis.
