Two-Stage Superposed Transmission for Cooperative NOMA Systems

In this paper, a two-stage superposed transmission scheme for cooperative nonorthogonal multiple access (NOMA) systems is proposed. During the first N time slots, the source simultaneously transmits the superposition coded symbols to the relay and destination, both of which, instead of decoding, keep the receptions in reserve. At the last time slot, the relay decodes and forwards a new superposition coded symbol with corresponding power allocation factors to the destination. The destination jointly decodes the received signals during the total N + 1 time slots by employing maximum ratio combining. Assuming Rayleigh fading channels, the ergodic sum rate (SR), outage probability, and outage capacity of the system are investigated considering the high transmit signal-to-noise ratio cases. An approximate expression for the ergodic SR is also derived at the expense of a negligible performance loss. By means of numerical results, it is shown that the transmission rate and ergodic SR of the proposed scheme overwhelm that of the time-division multiple access and conventional NOMA schemes.