Retransmission Policy with Frequency Hopping for Ultra-Reliable and Low-Latency Communications

In this work, we study the benefit of retransmission to ultra-reliable and low-latency communications (URLLC). We consider a retransmission policy that employs frequency hopping to improve the retransmission success probability for the packets suffering deep fading. We investigate how to satisfy the quality-of-service (QoS) with retransmission by taking downlink transmission as an example. End-to-end (E2E) delay components, including transmission delay, queueing delay and backhaul latency, and packet loss components, including transmission error probability and E2E delay violation probability are considered. Resource allocation for the retransmission policy is optimized. Numerical results show that the requirement on transmission error probability can be significantly relaxed with retransmission, which does not compromise the E2E delay requirement owing to the short packets in URLLC. Simulation results indicate that more users can be supported with QoS guarantee compared to a counterpart system without retransmission, and the performance gain increases with the times of retransmissions.