Towards 6G xURLLC with Non-orthogonal HARQ

The demand for the neXt generation Ultra-Reliable and Low-Latency Communications (xURLLC) poses unprecedented challenges in the design of 6G wireless networks. Hybrid automatic repeat request (HARQ) is the most popular error-control method to enhance the system’s reliability at the cost of increased latency due to the retransmission of erroneous packets. To meet xURLLC requirements, this paper presents an enhanced non-orthogonal HARQ (N-HARQ) scheme inspired by non-orthogonal multiple access (NOMA) technology. Notably, the NHARQ scheme enables simultaneous redundancy retransmission and new packet transmission within a shared time slot in a non-orthogonal fashion. Moreover, unlike conventional approaches, the proposed N-HARQ scheme incorporates a "waiting" state to effectively utilize system resources during severe fading channel conditions. Different performance metrics, including achievable throughput, average packet delay, and system efficiency, are analytically derived based on a Markov model. Numerical results highlight the effectiveness of our proposed N-HARQ over the state-of-the-art. Also, the correctness of the analytical model is verified by Monte-Carlo simulations.