  Non-orthogonal multiple access (NOMA) has attracted great recent attention
due to its high spectral efficiency. Besides, NOMA can be easily implemented in
physical layer, thus its interplays with other psychical layer techniques have
been analyzed widely by researches. Cooperative NOMA schemes are the most
investigated among these interplays. However, these studies mostly analyze
cooperative NOMA schemes in terms of achievable rate and outage probability
whereas bit error probability (BEP) for those systems has not been studied well
although it is one of the most important performance metrics. In this paper, we
investigate the error performance of cooperative NOMA schemes where a
decode-forward relay helps NOMA users. We derive exact BEP expressions in
closed-form over Nakagami-m fading channels. All derived expressions are
validated via computer simulations.
