  Non-orthogonal multiple access (NOMA) and spectrum sharing (SS) are two
emerging multiple access technologies for efficient spectrum utilization in
future wireless communications standards. In this paper, we present the
performance analysis of a NOMA-based cooperative relaying system (CRS) in an
underlay spectrum sharing scenario, considering a peak interference constraint
(PIC), where the peak interference inflicted by the secondary (unlicensed)
network on the primary-user (licensed) receiver (PU-Rx) should be less than a
predetermined threshold. In the proposed system the relay and the
secondary-user receiver (SU-Rx) are equipped with multiple receive antennas and
apply selection combining (SC), where the antenna with highest instantaneous
signal-to-noise ratio (SNR) is selected, and maximal-ratio combining (MRC), for
signal reception. Closed-form expressions are derived for the average
achievable rate and outage probabilities for SS-based CRS-NOMA. These results
show that for large values of peak interference power, the SS-based CRS-NOMA
outperforms the CRS with conventional orthogonal multiple access (OMA) in terms
of spectral efficiency. The effect of the interference channel on the system
performance is also discussed, and in particular, it is shown that the
interference channel between the secondary-user transmitter (SU-Tx) and the
PU-Rx has a more severe effect on the average achievable rate as compared to
that between the relay and the PU-Rx. A close agreement between the analytical
and numerical results confirm the correctness of our rate and outage analysis.
