Analysis and Optimization of HARQ for URLLC

In this paper, we investigate the effectiveness of the hybrid automatic repeat request (HARQ) technique in providing high-reliability and low-latency in the finite blocklength (FBL) regime in a single user uplink scenario. We characterize the packet error rate (PER), throughput, and delay performance of chase combining HARQ (CC-HARQ) and incremental redundancy HARQ (IR-HARQ) in AWGN and Rayleigh fading channel with $m$ retransmissions. Furthermore, we consider a quasi-static fading channel model, which is more accurate than the over-simplified i.i.d. block fading or same channel assumption over consecutive packets. We use finite state Markov model under the FBL regime to model correlative fading. Numerical results present interesting insight into the reliability-latency trade-off of HARQ. Furthermore, we formulate an optimization problem to maximize the throughput performance of IR-HARQ by reducing excessive retransmission overhead for a target packet error performance under different SNRs, Doppler frequencies, and rate regimes.