  Non-orthogonal multiple access (NOMA)-based cooperative relaying systems
(CRS) are very promising to overcome spectral inefficiency of conventional
cooperative communications. Although NOMA-CRS have great recent attention,
almost all studies investigate NOMA-CRS only in terms of capacity and outage
probability. Error performances of NOMA-CRS have not been well-studied. In this
paper, we analyze error performance of NOMA-based diamond relaying network
(NOMA-DRN) with imperfect successive interference canceler (SIC) as a NOMA-CRS
scheme. We derive exact bit error probability (BEP) for NOMA-DRN and provide a
tight approximated BEP in the closed-form. In addition, high-SNR analysis is
conducted to present that NOMA-DRN has an error floor. Moreover, it is proved
that NOMA-DRN turns out to be a non-equiprobable communication system and we
derive priori probabilities of symbols. All derived expressions are validated
via computer simulations.
