Multi-Process Overlapping HARQ Durations for Periodic Time-Critical Communications Under Nonzero ACK Delay
In time-critical periodic communication, hybrid automatic repeat request (HARQ) ensures stringent reliability and low latency. However, conventional fixed HARQ durations for periodic traffic can lead to resource over-provisioning and limited adaptation to deep fading. To improve resource efficiency, the Overlapping HARQ Durations (OHD) approach-employing opportunistic transmissions within the overlapped durations of adjacent packets - has been proposed for orthogonal multiple access (OMA) systems. Nevertheless, prior OHD studies assumed an idealistic single-process architecture with zero acknowledgment (ACK) delay limiting practical applicability. This letter extends the OHD concept to a practical multi-process HARQ architecture within an OMA framework featuring nonzero ACK delays and fixed slot lengths. Two schemes are proposed: Dedicated Mapping-based OHD (DM-OHD) and Flexible Mapping-based OHD (FM-OHD). Using a novel stochastic vacancy probability analysis, we derive the HARQ durations to satisfy latency and reliability constraints. Numerical results show that both OHD schemes significantly outperform conventional non-OHD schemes in capacity, while FM-OHD achieves lower packet failure rates.