LAUSR

In this letter, we propose a dynamic re-transmission strategy for the two-user uplink non-orthogonal multiple access scenario (NOMA) with hybrid automatic repeat request (HARQ) and more than one allowed re-transmissions. In the proposed scheme, whenever both users simultaneously receive negative acknowledgment (NACK) from the base station (BS), the high-power user is asked to re-transmit its previous packet, while the low-power user is requested to send a new packet in the next time slot. The low-power user is allowed to retransmit its previous packets when the other user stops the re-transmission. We propose a Markov model to analyze the dynamics of the proposed scheme and the conventional scheme, where the users always send the re-transmission when receiving a NACK from the BS. We find the optimal power splitting ratios to minimize the maximum packet error rate (PER) amongst users. Simulation results show that the proposed dynamic approach significantly improves the PER of the low-power user at the expense of a negligible increase in the PER of the high-power user.