Outage Probability Analysis of Cooperative NOMA With Successive Refinement

This paper proposes a broadcasting system with cooperative non-orthogonal multiple access (CO-NOMA) and successive refinement (SR) coding. Specifically, signals containing the basic description of the source and the refinement are overlapped at the transmitter, and broadcast to user equipment (UE) having different qualities-of-service (QoS) requirements. Although the far UEs may only be capable of decoding the basic description allocated with higher transmit power, some of them may still demand a high QoS like the near UE. To address this issue, this work utilizes the near UE to establish a relay transmission, thereby the information recovered at the far UE can be refined. Considering three different relaying schemes, the outage probabilities of the proposed system are derived in closed-form, assuming all channels suffer from block Rayleigh fading. Based on the optimal power allocations, the best scheme yielding the lowest outage probabilities is found, and the advantages over down-link NOMA with SR (DN-SR) and conventional CO-NOMA are also demonstrated.