  The average achievable rate of an Opportunistic Non-Orthogonal Multiple
Access (O-NOMA) based Cooperative Relaying System (CRS) with Channel State
Information (CSI) known at the transmitter end is analyzed under Rician fading
channels. For opportunistic transmission of data signals, CSI is used for the
source-to-relay and source-to-destination links, which helps the transmitter to
select the best out of the cooperative NOMA transmission and the direct
transmission of signals, instantaneously. The average achievable rate of the
O-NOMA and conventional NOMA (C-NOMA) based CRSs is considered for the
performance comparison and the exact and asymptotic analytical expressions of
the achievable rates has been provided. Asymptotic results are verified through
Monte Carlo simulations for various channel powers and power allocation
coefficients. Numerical results verify that the derived analytical results are
matched well with the Monte Carlo simulations and shows that O-NOMA-based CRS
achieves better rate performance than C-NOMA-based CRS with the increasing
power allocation coefficients, transmit Signal-to-Noise Ratios (SNRs) and
channel powers.
