# A Novel QoS-Aware ARQ Scheme for Multi-User Transmissions in IEEE802.11 ax WLANs
## 1 Introduction  
Recently, the wireless local area network (WLAN) has experienced rapid development and has become a popular wireless network access technique. With the vigorous development of Internet technology, constantly emerging applications and scenarios, such as cloud computing, industrial automation, smart healthcare, and ultra-high-definition live broadcast, necessitate stricter quality of service (QoS) requirements, such as higher throughput, lower latency, and less packet loss. The latest IEEE 802.11 ax protocol was officially launched to conquer these bottlenecks [1; 2]. In IEEE802.11 ax WLANs, the transmission efficiency of the media access control (MAC) layer and the QoS performance are enhanced via the multi-user (MU) transmission technology, i.e., Orthogonal Frequency Division Multiple Access (OFDMA) and multi-user multi-input multi-output (MU-MIMO). The MU transmission technology enables the access point (AP) to distribute frames to multiple stations simultaneously (for downlink) and supports the concurrent transmission of frames from different stations to the AP (for uplink). When scheduling the MU transmissions, the IEEE802.11 ax WLANs adopt the aggregate MAC protocol data unit (A-MPDU) aggregation and block acknowledgment (BlockACK) for each user toreduce the MAC layer overhead. Under the error-prone channels, some sub-frames in the A-MPDU frame may be lost due to channel noise. Different automatic repeat request (ARQ) schemes ensure the reliability of transmission for aggregation-enabled WLANs. However, the MU transmissions face two drawbacks which degrade the performances of throughput, latency, and packet loss rate when the existing ARQ schemes schedule the retransmission of the lost packets indicated by the BlockACK/ACK frame:  
(1) Since the WLANs maintain that packets are submitted to the upper layer in order, the lost packets can block the sliding of the aggregation window as described in [3]. That is, those successfully transmitted packets whose sequence numbers are greater than the smallest one among the lost packets should wait in the reordering buffer. Moreover, the queued packets cannot be served until their sequence numbers fall within the aggregation window. Because the originator has to wait for a new transmission opportunity to retransmit the lost packets and multiple retransmissions may be required when the channel quality is poor, the overall latency is increased. (2) The lengths of A-MPDU frames transmitted by different users can be varied. One reason for this phenomenon is that the number and position of the lost packets in each A-MPDU frame of the MU transmission are random when the channel noise is independent between different users. In IEEE802.11 ax WLANs, all users in the same MU transmission should have an identical transmission duration. When the transmission time requested by a user is less than the scheduled duration of the MU transmission, the user should add dummy bits called padding bits to the frame to make the transmission time equal to others. The transmission of the padding bits causes the loss of channel resources [4]. Consequently, the efficiency of the MU transmission decreases. In this article, a novel backup padding ARQ (BP-ARQ) scheme for MU transmissions in IEEE802.11 ax WLANs is presented to address the above problems. The key aspect of the proposed scheme is to utilize the remaining duration to transmit backups of selected packets from the A-MPDU when the scheduled duration is longer than the transmission time of the user. In the proposed scheme, an optimization problem is derived from the sliding of the aggregation window to determine the selected set of backups. A greedy algorithm is recommended to solve the optimization problem by searching for a sub-optimal solution considering the computational complexity and the processing time. The results in Section 4 illustrate that the performances of throughput, latency, and packet loss rate are all improved under the error-prone channels when the proposed scheme is adopted.