Improving Spectral Efficiency in URLLC via NOMA-Based Retransmissions

The requirement to accommodate ultra-reliable low latency communication (URLLC) is one of the most attractive, yet challenging, new features of upcoming 5G systems. A common way to achieve reliability is retransmission; however, the applicability of this mechanism is hindered by the strict latency requirements. Furthermore, the bandwidth is often limited and shared by multiple connections, which may put the packet into a retransmission queue, leading to even larger latency. We address this problem in an uplink setting by introducing the concept of non-orthogonal multiple access hybrid automatic repeat request (NOMA-HARQ). In essence, NOMA-HARQ allows newly incoming packets to share non-orthogonally the same resource with retransmitted packets. The reliability guarantees are preserved by designing a power optimization procedure that takes into account past transmission attempts as well as the time remaining until the deadline.