Low latency hybrid ARQ method using channel state information before channel decoding

This paper proposes a low latency hybrid automatic repeat request (HARQ) method that uses the channel state information before channel decoding to address the increased transmission latency resulting mainly from the delay time required for the channel decoding in HARQ. We focus on applying the proposed method to ultra-reliable low latency communication. The proposed method reduces the transmission latency by requesting early retransmission before the channel decoding process is completed. We consider two metrics for deciding the early retransmission request: (1) mutual information (MI) calculated from the signal-to-noise ratio of the received packet and (2) the a priori log-likelihood ratio (LLR) of the coded bits obtained from the signal detection results of the received packet. Simulation results show that the proposed method successfully reduces the transmission latency at the cost of a very small loss in throughput. Between these two metrics, the MI-based method achieves a slightly better tradeoff between the throughput and transmission latency than the LLR-based method.