  High-fidelity, real-time interactive applications are envisioned with the
emergence of the Internet of Things (IoT) and tactile Internet by means of
ultra-reliable low-latency communications (URLLC). Exploiting time diversity
for fulfilling the URLLC requirements in an energy efficient manner is a
challenging task due to the nontrivial interplay among packet size,
retransmission rounds and delay, and transmit power. In this paper, we study
the fundamental energy-latency tradeoff in URLLC systems employing incremental
redundancy (IR) hybrid automatic repeat request (HARQ). We cast the average
energy minimization problem with a finite blocklength (latency) constraint and
feedback delay, which is non-convex. We propose a dynamic programming algorithm
for energy efficient IR-HARQ optimization in terms of number of
retransmissions, blocklength and power per round. Numerical results show that
our IR-HARQ approach could provide around 25\% energy saving compared to
one-shot transmission (no HARQ).
