This article investigates a cooperative-based non-orthogonal multiple access (NOMA) system using its performance metrics over Rayleigh channel. The system under study consists of two users, one closer to base station (BS) and a far end user away from BS. In this work, the decode and forward relay at the near user which will send the signal from the BS to the far user is considered. Then, using techniques called selection combining (SC) and maximal ratio combining (MRC), the signal at the far user from the two-time intervals is combined together in two different cases. For both MRC and SC, the analytical equations of bit error rate (BER) and outage probability are derived. Considering the suggested NOMA systems performance, whenever it comes to bit error rate and outage, the MRC technique outperforms the SC technique as it is observed that outage of MRC reaches a value of probability lower than SC. Similarly, BER value of MRC goes beyond 10−6 compared to SC at 30 dB indicating its performance to be better.