# An Adaptive Hybrid Automatic Repeat Request (A-HARQ) Scheme Based on Reinforcement Learning
###### Abstract  
V2X communication is susceptible to attenuation and fading caused by external interference. This interference often leads to bit error and poor quality and stability of the wireless link, and it can easily disrupt packet transmission. In order to enhance communication reliability, the 3 rd Generation Partnership Project (3 GPP) introduced the Hybrid Automatic Repeat Request (HARQ) technology for both 4G and 5G systems. Nevertheless, it can be improved for poor communication conditions (e.g., heavy traffic flow, long-distance transmission), especially in advanced or cooperative driving scenarios. In this paper, we propose an Adaptive Hybrid Automatic Repeat Request (A-HARQ) scheme that can reduce the average block error rate, the average number of retransmissions, and the round-trip time (RTT). It adapts the Q-learning model to select the timing and frequency of retransmission to enhance the transmission reliability. We also design some transmission schemes--_K_-repetition, _T_-delay and [T, K]-overlap--which are used to shorten latency and avoid packet collision. Compared with the conventional 5G HARQ, our simulation results show that the proposed A-HARQ scheme decreases the system's average BLER, the number of retransmissions, and the RTT to 5.55%, 1.55 ms, and 0.97 ms, respectively.  
hybrid automatic repeat request; reinforcement learning; block error rate; 5G new radio; vehicle-to-everything +
Footnote †: journal: _electronics_  
Automatic Repeat Request (HARQ) technology for both 4G and 5G systems. Although the 5G HARQ technique guarantees its reliability and meets the delay requirements for V2X communication, it does not easily maintain high communication quality in poor conditions (e.g., heavy traffic flow, long-distance communication). This is particularly true in advanced driving or cooperative driving for vehicle platooning information exchange, cooperative collision avoidance, emergency trajectory alignment, cooperative lane change, sensor information sharing, information exchange between vehicles, etc.  
Therefore, it is necessary to explore a more efficient retransmission mechanism for vehicular transmission in 5G NR-enhanced V2X scenarios. This will improve the reliability of information transmission and reduce the retransmission delay. Considering transmission conditions, this study focuses on the retransmission optimization based on the 5G NR HARQ. In this paper, an Adaptive Hybrid Automatic Repeat Request (A-HARQ) scheme is proposed to improve the communication quality, which can reduce the average block error rate, average number of retransmissions, and round-trip time (RTT). The A-HARQ scheme includes not only the 5G HARQ retransmission mechanism, but also the \(K\)-repetition mechanism, which can shorten the RTT and improve the transmission success rate.  
To illustrate our research, we have organized the rest of this paper as follows: In this section, we provide a brief introduction to the background and motivation. In Section 2, we discuss the functioning of HARQ and survey some related works. In Section 3, we state some problems and provide an overview of the system. The proposed Adaptive Hybrid Automatic Repeat Request (A-HARQ) scheme is studied in Section 4. We evaluate its performance through simulation and provide detailed descriptions in Section 5. Finally, we summarize the work presented in this article and discuss future research directions related to HARQ in the Section 6.