Outage Probability of NOMA with Partial HARQ Over Time-Correlated Fading Channels

This paper investigates the outage performance of downlink non-orthogonal multiple access (NOMA) system over time-correlated Rayleigh fading channels, where the users have heterogeneous quality of service requirements, e.g., a latency-critical user with a low target rate and a delay-tolerant user with a large target rate. In order to meet the different requirements of the users, a partial hybrid automatic repeat request (HARQ) scheme is proposed. The closed-form expressions of outage probabilities for NOMA without and with re-transmission are derived. With the developed outage probabilities, a condition on the superiority of NOMA to orthogonal multiple access (OMA) is obtained. Simulation results demonstrate the accuracy of developed analytical results. It is shown that the performance of NOMA is superior to orthogonal multiple access if the derived condition is satisfied and HARQ-CC can enhance the performance of NOMA over time-correlated fading channels.