This paper explores the performance gains achieved by space time block coding (STBC)-based non orthogonal multiple access (NOMA) scheme versus regular cooperative NOMA under the influence of imperfect successive interference cancellation (SIC). Imperfection in the SIC process of NOMA causes performance degradation which is proportional to the number of SIC operations involved. It is demonstrated through outage and sum-rate performance that STBC-NOMA maintains a considerable performance margin from conventional cooperative NOMA even under adverse imperfections in SIC. Furthermore, closed-form expressions of signal to interference ratio (SIR) at user equipment (UE) and outage probability for STBC-NOMA under imperfect SIC are also derived and validated.