  In this paper, the performance of a promising technology for the next
generation wireless communications, non-orthogonal multiple access (NOMA), is
investigated. In particular, the bit error rate (BER) performance of downlink
NOMA systems over Nakagami-m flat fading channels, is presented. Under various
conditions and scenarios, the exact BER of downlink NOMA systems considering
successive interference cancellation (SIC) is derived. The transmitted signals
are randomly generated from quadrature phase shift keying (QPSK) and two NOMA
systems are considered; two users' and three users' systems. The obtained BER
expressions are then used to evaluate the optimal power allocation for two
different objectives, achieving fairness and minimizing average BER. The two
objectives can be used in a variety of applications such as satellite
applications with constrained transmitted power. Numerical results and Monte
Carlo simulations perfectly match with the derived BER analytical results and
provide valuable insight into the advantages of optimal power allocation which
show the full potential of downlink NOMA systems.
