# A Novel QoS-Aware ARQ Scheme for Multi-User Transmissions in IEEE802.11 ax WLANs
## 2 Background and Related Works
### Related Works  
A large number of published papers have shown that MU transmission performance has been enhanced via user scheduling and resource allocation. [PERSON] et al. [10] minimized the average upload time for unsaturated and UL scenarios by developing an algorithm to search for the best way to allocate the RUs. In [11], a set of the classic user schedulers was extended for UL OFDMA transmissions in IEEE802.11 ax by adopting a general scheduling algorithm so that the performances of average upload time and throughput were improved. The control-aware low-latency scheduling algorithm in [12] supported more time-sensitive devices, with the latency constrained by allocating the radio resource based on the stability-inducing Lyapunov function. An adaptive-learning-based approach for a joint MU-MIMO user group and dynamic link configuration parameter selection was introduced to improve network performance in [13]. [PERSON] et al. [14] analyzed the influence of distributions of RA-RU and SA-RU on throughput and BSR delivery rate and maximized the throughput by optimally allocating the two kinds of RUs. In [15], a scheduler was proposed to increase RU allocation efficiency based on the information of QoS and buffered data. [PERSON] et al. [16] proposed an adaptive grouping scheme to achieve the optimal efficiency of BSR delivery and improved the throughput when the Uplink OFDMA Random Access (UORA) mechanism was used. Figure 4: An example of the A-MPDU transmission using the block Acknowledgement window automatic repeat request (BAW-ARQ) scheme where W = 64. Figure 3: An example of the A-MPDU transmission using the aggregation selective repeat automatic repeat request (ASR-ARQ) scheme where W = 64. Several works in the literature have been dedicated to the padding problem of MU transmissions. Some of them researched the duration allocation to improve the padding problem. In [17], two algorithms that determined the scheduled duration for the UL transmission were introduced. One algorithm fixed the scheduled duration, and the other considered padding overhead, energy, and fairness based on the queue sizes and channel conditions of users when the scheduled duration was allocated. [PERSON] et al. [18] proposed a novel method that decided the frame aggregation size in MU-MIMO data to improve channel utilization at the cost of a certain delay performance. In [4], a novel data frame construction scheme for MU transmissions was proposed to maximize the transmission efficiency by calculating the optimal length according to the status of buffers and transmission rates. Others broke the rules of MU transmissions to enhance transmission efficiency. [PERSON] et al. modified the structure of DL MU-MIMO and increased the efficiency by splicing frames from different users in one stream to stuff the channel time, which is called frame padding [19; 20]. [PERSON] et al. [21] characterized the frame padding problem as a multi-stream knapsack model and derived the optimal padding scheduler by investigating the inter-stream interference based on the model. Some existing papers focused on the ARQ schemes of the aggregation-enabled WLANs. In [5] and [6], the ASR-ARQ scheme and the aggregated hybrid ARQ were proposed and the models of service time distribution were derived for them. In [7], the impacts of the ASR-ARQ scheme on MAC access delay and throughput were analyzed based on the proposed model for unsaturated networks. [PERSON] et al. [8] expounded a mathematical model to investigate the saturated throughput using the ASR-ARQ scheme. [PERSON] et al. [22] enhanced the ASR-ARQ scheme via a method that aggregated the lost packets and their duplicates in the retransmitted A-MPDU when the number of lost packets was small. A throughput model of single-user transmissions was introduced to determine the execution threshold and the general duplicate times. [PERSON] et al.