# An Adaptive Hybrid Automatic Repeat Request (A-HARQ) Scheme Based on Reinforcement Learning
## 3 Problem Statement and System Framework  
### Problem Statement  
The Stop-and-Wait Protocol of HARQ (SW_ARQ) was used to send data as an error correction technique in the 3 GPP LTE protocol standard. In the SW_ARQ, the transmitter (TX) sends a data frame and stops to wait for the acknowledgment, and then the receiver uses the 1-bit information for ACK or NACK acknowledgment of the data frame. However, the stopping method for confirming each transmission results in low throughput. Hence, the HARQ has been improved in the 5G system to achieve efficient and high-performance error correction. The MAC layer makes up for the waste of air interface resources of a single SW_ARQ process by using multiple SW_ARQ processes, without affecting its timelines [13]. The base station assigns the HARQ process number to the indoor terminal to determine the buffer at the physical layer. The physical layer combines the last decoded failed bit stream with the current retransmitted bit stream through the soft-combining technique to enhance the demodulation gain, offering benefits such as reduced latency and improved the decoding and demodulation gain. However, the disadvantage is that the physical layer of the receiving terminal requires a relatively large cache to store the bit information that is not properly decoded. The complete HARQ procedure requires the MAC layer and the physical layer to work together. The MAC layer implements the SW_ARQ protocol. If MAC frames need to be retransmitted as called by the feedback mechanism, it resends the MAC frames to the receiver (RX). The retransmitted MAC frames will be encoded according to the redundant version number when encoding at the physical layer. The physical layer stores the original bits of the empty interface that are not correctly decoded in the buffer. After the retransmitted data frame is received, it is integrated with the original bit stored in the buffer and decoded it. The introduction of soft-merge technology effectively reduces the proportion of error bits caused by channel interference, so that the correct rate of decoding is improved. The overall probability of decoding and the anti-interference ability are enhanced. Figure 1a and Figure 1b respectively depict the downlink transmission process of a retransmission and double retransmission in a normal HARQ. Assuming the transmission time interval (TTI) is 0.2 ms, the RTT through the 5G HARQ to complete a retransmission and double retransmission is 2.4 ms and 3.6 ms, respectively. Note that here, the UE is sent with a negative time offset relative to the base station (BS) based on the final timing-advance (TA) settings. After the TX sends the data, physical forward error correction (FEC) reduces the number of retransmissions by adding redundant information so that the RX can correct some of the errors. For errors that cannot be corrected by forward error correction, the RX requests the sender to resend the data through the SW_ARQ mechanism. The RX employs an error detection code, typically a cyclic redundancy check (CRC), to detect whether the received packet is wrong. If there is no error, the RX will send a positive ACK to the sender, and the TX will process this and send the next packet upon receiving the ACK. If an error occurs, the RX drops the packet and sends a NACK to the TX. Subsequently, the TX retransmits the same data in turn after receiving the NACK. In addition, the 5G standard incorporates an asynchronous HARQ for both uplink and downlink transmissions. This feature enhances the flexibility of scheduling the timing and resource allocation, particularly in the context of the time-division duplexing (TDD) mode. Multiple parallel HARQ processes are allowed in the 5G standard, and while one HARQ process is awaiting an acknowledgment, the sender simultaneously carries out another HARQ process to transmit data. These aforementioned processes of HARQ form a HARQ entity, which integrate the stop protocol together.