  To improve the sum-rate (SR) of the dual-hop relay system, a novel two-stage
power allocation scheme with non-orthogonal multiple access (NOMA) is proposed.
In this scheme, after the reception of the superposition coded symbol with a
power allocation from the source, the relay node forwards a new superposition
coded symbol with another power allocation to the destination. By employing the
maximum ratio combination (MRC), the destination jointly decodes the
information symbols from the source and the relay. Assuming Rayleigh fading
channels, closed-form solution of the ergodic SR at high signal-to-noise ratio
(SNR) is derived and a practical power allocation is also designed for the
proposed NOMA scheme. Through numerical results, it is shown that the
performance of the proposed scheme is significantly improved compared with the
existing work.
