  Non-orthogonal multiple access (NOMA) is being widely considered as a
potential candidate to enhance the spectrum utilization in beyond
fifth-generation (B5G) communications. In this paper, we derive closed-form
expressions for the ergodic rate and outage probability of a
multiple-antenna-assisted NOMA-based cooperative relaying system (CRS-NOMA). We
present the performance analysis of the system for two different receive
diversity schemes - selection combining (SC) and maximal-ratio combining (MRC),
in Nakagami-m fading. We also evaluate the asymptotic behavior of the CRS-NOMA
to determine the slope of the ergodic rate and diversity order. Our results
show that in contrast to the existing CRS-NOMA systems, the CRS-NOMA with
receive diversity outperforms its orthogonal multiple access (OMA) based
counterpart even in the low-SNR regime, by achieving higher ergodic rate.
Diversity analysis confirms that the CRS-NOMA achieves full diversity order
using both SC and MRC schemes, and this diversity order depends on both the
shape parameter m and the number of receive antennas. We also discuss the
problem of optimal power allocation for the minimization of the outage
probability of the system, and subsequently use this optimal value to obtain
the ergodic rate. An excellent match is observed between the numerical and the
analytical results, confirming the correctness of the derived analytical
expressions.
