Investigation on hybrid automatic repeat request (HARQ) design for NOMA with SU-MIMO

Non-orthogonal multiple access (NOMA) utilizing power domain and advanced receiver has been considered as a very promising candidate radio access technology for future wireless communication systems. Hybrid automatic repeat request (HARQ), as an indispensable part of link adaptation, plays a very important role in reliable high speed packet data service through exploiting retransmission diversity and channel coding gain. In this paper, we investigate HARQ design for downlink NOMA with single user-multiple input multiple output (SU-MIMO). The impacts of user pairing, precoding matrix adaptation and transmission power assignment (TPA) ratio adaptation in retransmission on NOMA performance are investigated. An enhanced HARQ algorithm is proposed, where opportunistically HARQ combining is applied and the TPA of retransmission considers HARQ combining gain of the retransmission user. It is found that HARQ probability of NOMA is much higher than that of OMA and flexible retransmission strategies could increase performance gain of NOMA. Furthermore, it found the (wrong, correct) is the dominate case in NOMA HARQ and thus is of most importance for enhanced NOMA HARQ design. Simulation results show the proposed HARQ algorithm is effective and can achieve better performance than the existing algorithm.