HARQ Using Hierarchical Tree-Structured Physical Channel Identifiers in NOMA-Based Random Access for Multi-Packet Reception

This paper proposes an efficient hybrid automatic repeat request (HARQ) method that simultaneously achieves packet combining and resolution of the collisions of physical channel identifiers (PCIDs) during retransmission in a non-orthogonal multiple access (NOMA)-based random access system for multi-packet reception. Here, the PCID functions as a separator for simultaneously received packets that use the same channel in NOMA. An example of this is a scrambling code used in 4G and 5G systems. Since users independently select the PCID, the decoding of received packets fails when multiple users select the same PCID. Random PCID reselection by each user when attempting retransmission can resolve the PCID collision; however, packet combining between the previous and retransmitted packets is not possible in this case because the base station receiver does not know the relationship between the PCID of the previously transmitted packet and that of the retransmitted packet. To address this problem, we propose employing novel hierarchical tree-structured PCID groups in which the PCID for the previous packet transmission has a one-to-one relationship to the set of PCIDs for retransmission. The proposed method resolves PCID collision at retransmission by randomly reselecting for each user a PCID from the dedicated PCID set from the previous transmission. Since the relationship between the PCIDs at the previous transmission and retransmission is known at the base station, packet combining is achieved simultaneously. Computer simulation results show the effectiveness of the proposed method.