Energy-Latency Tradeoff in Ultra-Reliable Low-Latency Communication with Short Packets

We consider an ultra-reliable low-latency communication (URLLC) system with short packets employing hybrid automatic repeat request (HARQ). Depending on the delay of HARQ feedback and retransmissions, the latency constraint can be either violated or fulfilled at the expense of power consumption. We focus on the energy-latency tradeoff and explore whether it is better to do one-shot transmission or use HARQ. We analyze the energy consumption for incremental redundancy (IR) HARQ and compare it with the no HARQ case. The analysis relies on closed-form expressions for the outage probability of IR-HARQ with variables both the blocklength and the power. Our results show that for a wide range of blocklength, when the feedback delay is more than half the latency constraint, it is beneficial in terms of energy to use one-shot transmission (i.e., no HARQ).