  Non-orthogonal multiple access (NOMA) technique has sparked a growing
research interest due to its ability to enhance the overall spectral efficiency
of wireless systems. In this paper, we investigate the pairwise error
probability (PEP) performance of conventional NOMA systems, where an exact
closed form expression for the PEP is derived for different users, to give some
insight about the reliability of the far and near users. Through the derivation
of PEP expressions, we demonstrate that the maximum achievable diversity order
is proportional to the user's order. The obtained error probability expressions
are used to formulate an optimization problem that minimizes the overall bit
error rate (BER) under power and error rate threshold constrains. The derived
analytical results, corroborated by Monte Carlo simulations, are presented to
show the diversity order and error rate performance of each individual user.
