Non-orthogonal multiple access (NOMA) has come to the fore as a
spectral-efficient technique for fifth-generation and beyond communication
networks. We consider the downlink of a NOMA system with untrusted users. In
order to consider a more realistic scenario, imperfect successive interference
cancellation is assumed at the receivers during the decoding process. Since
pair outage probability (POP) ensures a minimum rate guarantee to each user, it
behaves as a measure of the quality of service for the pair of users. With the
objective of designing a reliable communication protocol, we derive the
closed-form expression of POP. Further, we find the optimal power allocation
that minimizes the POP. Lastly, numerical results have been presented which
validate the exactness of the analysis, and reveal the effect of various key
parameters on achieved pair outage performance. In addition, we benchmark
optimal power allocation against equal and fixed power allocations with respect
to POP. The results indicate that optimal power allocation results in improved
communication reliability.Comment: 6 pages, 5 figures, WCNC 202