  The combination of non-orthogonal multiple access(NOMA) and cooperative
communications can be a suitable solution for fifth generation (5G) and beyond
5G (B5G) wireless systems with massive connectivity, because it can provide
higher spectral efficiency, lower energy consumption, and improved fairness
compared to the non-cooperative NOMA. However,the receiver complexity in the
conventional cooperative NOMA increases with increasing number of users owing
to successive interference cancellation (SIC) at each user. Space time block
code-aided cooperative NOMA (STBC-CNOMA) offers less numbers of SIC as compared
to that of conventional cooperative NOMA. In this paper, we evaluate the
performance of STBC-CNOMA under practical challenges such as imperfect SIC,
imperfect timing synchronization between distributed cooperating users, and
imperfect channel state information (CSI). We derive closed-form expressions of
the received signals in the presence of such realistic impairments and then use
them to evaluate outage probability. Further, we provide intuitive insights
into the impact of each impairment on the outage performance through asymptotic
analysis at high transmit signal-to-noise ratio. We also compare the complexity
of STBC-CNOMA with existing cooperative NOMA protocols for a given number of
users. In addition, through analysis and simulation, we observe that the impact
of the imperfect SIC on the outage performance of STBC-CNOMA is more
significant compared to the other two imperfections. Therefore, considering the
smaller number of SIC in STBC-CNOMA compared to the other cooperative NOMA
protocols, STBC-CNOMA is an effective solution to achieve high reliability for
the same SIC imperfection condition.
