# An Adaptive Hybrid Automatic Repeat Request (A-HARQ) Scheme Based on Reinforcement Learning
## 2 Related Works  
In practice, packet errors and losses are inevitable due to ubiquitous noise, signal interference, and channel fading. As an incorrectly decoded message does not bring about fresh awareness, these packet errors and losses will result in stale information, leading to uncontrollable residual errors, system instability, and incorrect decisions. HARQ retransmission, as a standard technique for improving transmission reliability, has been adopted in various wireless standards [2]. HARQ is a physical-layer mechanism that employs feedback to transmit at higher target block error rates (BLERs) while achieving robustness of the transmission by providing retransmissions based on the feedback of Acknowledgement or Negative Acknowledgement (ACK/NACK). However, the HARQ procedure poses a bottleneck for achieving the previously mentioned latencies because conventional HARQ allows retransmissions only upon receiving a NACK. The Base Station (BS) requires a few time units for detection when it receives the packet for the first time, and then it issues the feedback. In order to improve the success rate of HARQ retransmission, more HARQ iterations are needed to enhance the overall HARQ RTT. Hence, optimizing HARQ to improve the accuracy of information transmission and reduce transmission delay becomes a critical issue. A decoder is responsible for a minimum of 60% of the time required for the user equipment (UE) to receive and process data. Therefore, to fulfill the communication requirements of time-sensitive applications in V2X, it is feasible to predict the decoding result before the successful decoding process by the decoder. In their study, the authors in [3] proposed an Early HARQ (E-HARQ) technique based on decoder result prediction. The accuracy of ACK/NACK feedback for predicting the uncoded bit error rate and indicating block errors was estimated, based on the likelihood ratio of information bits, as approximately 90%. Additionally, the delay is reduced by approximately 50%. However, early feedback errors can significantly affect the interrupt rates and latency. Authors in [4] also conducted both early transmission feedback and regular feedback in an attempt to correct the early incorrect prediction, reducing the influence caused by the inaccurate estimation. In [5], the authors proposed a new spatially coupled code, which is formed by sending a low-density parity block code (LDPC-BC) through block Markov superposition transmission (BMST) to form the BMST-LDPC code. The BMST-LDPC scheme is integrated with the Hybrid Automatic Retransmission Request (HARQ) over the block-fading channel, improving the throughput performance of up to 10% compared with the conventional HARQ scheme. Because the probability of retransmission increases with the increase in the number of receiving ends, it is challenging to apply the traditional HARQ technique. Authors in [6] proposed an external-code-based HARQ, in which the receiver only provides feedback on the number of destroyed code blocks (CBs), while the transmitter utilizes an external code to generate parity CBs and then employs an internal code to transmit these parity CBs for retransmission. This scheme can improve the reliability and resource efficiency of the fading channel and effectively reduce the retransmission rate. Authors in [7] proposed a dynamic Hybrid Automatic Repeat Request scheme that is different from the traditional HARQ. This scheme allows for the dynamic adjustment of the maximum retransmission times based on the last situation. In delay-sensitive applications where channel state information is unavailable to the sender, the scheme demonstrates improved better performance in terms of packet error rate and throughput. In order to achieve the requirement of \(10^{-5}\) BLER as defined by the Ultra-Reliable Low-Latency Communication (URLLC) specification, the authors in [8] proposed a sub-code-based Early HARQ (SC EHARQ) scheme based on the Low-Density Parity Check (LDPC) subcode.