# Transmission Delay-Based Uplink Multi-User Scheduling in IEEE 802.11 ax Networks +
## 3 System Model
Second, the AP transmits trigger frames that contain scheduling information. The scheduled stations transmit uplink data streams to the AP. After the reception of uplink data transmissions from the scheduled stations, the AP is supposed to transmit a block ACK to stations as an efficient and simple response mechanism in IEE 802.11 ax. Note that the RTS/CTS mechanism is determined by the length of the transmitted data frame [22]. If the data size is larger than the threshold, data transmission is processed by the RTS and CTS mechanism. The uplink multi-user transmission considered in this paper follows the RTS/CTS mechanism in IEEE 802.11 networks. However, as shown in Figure 1, inappropriate station scheduling for uplink transmissions results in low channel use. The stations may transmit different sizes of data with different transmission rates, i.e., there are differences in the transmission delays of the scheduled stations for uplink transmissions. Although frame aggregation or fragmentation may mitigate the problem of low channel use, the transmission delay difference of scheduled stations is still important to increase channel use in uplink multi-user transmissions, especially for the scenario with multi-user beamforming on a single radio frequency band. Note that the feasibility of frame aggregation depends on the amount of backlogged data, and stations with a small amount of aggregated data cannot be aligned with stations with a large amount of data. On the other hand, the uplink data of stations with a large amount of data can be split into multiple fragments to align transmission delays with stations with a small amount of uplink data. However, in this case, stations performing fragmentation eventually require more transmission opportunities to transmit multiple fragmented data. The network overhead also increases compared to the non-fragmented transmissions; thus, performing fragmentation without considering transmission delay-based station selection is inefficient for a dense network. In addition, as the transmission delay difference among scheduled stations becomes larger, the network allocation vector (NAV) lengthens, and non-scheduled stations have less transmission opportunity even when channels are not occupied by stations. In dense IEEE 802.11 ax networks where a large number of stations is deployed for uplink transmissions, an increase in NAV with underutilized channels for uplink multi-user transmissions may significantly degrade the throughput performance while decreasing the transmission opportunities of non-scheduled stations. To improve channel use for uplink multi-user scenarios in dense IEEE 802.11 ax networks, the proposed method follows clustering-based scheduling so that stations with similar expected transmission delays are scheduled to  
Figure 1: An example of uplink multi-user transmissions in IEEE 802.11 ax networks. gether. Stations are grouped into multiple clusters, and the proposed method selects clusters for uplink multi-user transmissions. The cluster selection is based on the proportional fair approach, which is widely used for schedulers considering both network throughput and fairness performance. The proposed transmission delay-based scheduling approach for uplink multi-user transmissions improves channel use and network throughput performance in IEEE 802.11 ax. The proposed transmission delay-based scheduling method consists of two steps, and each step is designed to improve channel use and the fairness of stations for uplink transmission, separately. First, stations having uplink transmission data are clustered on the basis of their transmission delay so that the channel use improves [21]. The AP calculates the transmission delay of stations and forms clusters so that stations with similar transmission delays are clustered together. Second, the cluster for uplink multi-user transmission is selected following proportional fair-based scheduling. The proportional fairness is calculated for each cluster, and the AP decides the cluster for uplink multi-user transmission.