Low Latency HARQ Method Using Early Retransmission Prior to Channel Decoding with Multistage Decision

Abstract

This paper proposes an extension to a previously reported low latency hybrid automatic repeat request (HARQ) method that uses early retransmission prior to channel decoding to perform multistage judgment at early-retransmission decision. The previous method mitigates the increased transmission latency resulting mainly from the delay time required for channel decoding in HARQ by requesting early retransmission before the channel decoding process is completed based on the channel state information (CSI) obtained before channel decoding. However, the early-retransmission decision may include error that results in unnecessary retransmission and consequently throughput loss. To address the potential throughput loss due to the decision error in the early-retransmission request, we use multistage decision in which the number of bits conveyed by the early-retransmission packet is controlled dependent on the measured CSI before channel decoding. By setting the number of retransmitted coded bits to a low value when the observed channel state seems on the verge of performing early retransmission, we reduce the throughput loss when the early retransmission is not necessary since the initial packet is eventually channel decoded correctly. Simulation results show that the proposed extension further improves the achievable tradeoff between the transmission latency and throughput, which contributes to achieving ultra-reliable low latency communications (URLLC).