# Transmission Delay-Based Uplink Multi-User Scheduling in IEEE 802.11 ax Networks +
## 2 Related Work  
### Multi-User Transmissions in IEEE 802.11 ax  
As the amount of traffic increases, improving the spectral efficiency of IEEE 802.11-based network is becoming increasingly important. One of the solutions for improving spectral efficiency is to use a multi-user multiple-input and multiple-output (MU-MIMO) technique and a multi-user orthogonal frequency division multiple access (MU-OFDMA) technique. These techniques are commonly used in conventional wireless systems requiring high network throughput performance. The gain in OFDMA for IEEE 802.11 ax networks has been studied [5; 6]. IEEE 802.11 networks also provide multiple wireless connection services based on MU-MIMO and MU-OFDMA with the help of an efficient channel-sounding process such as an appropriate channel-sounding interval decision with low complexity [7]. IEEE 802.11 ax also specifies the multi-user transmission scenarios, and various studies of IEEE 802.11 ax have been conducted. [PERSON] et al. analyzed the performance of uplink MU-OFDMA in IEEE 802.11 ax networks [8]. In IEEE 802.11 ax networks, a buffer status report (BSR) is transmitted to notify an AP of the buffer status of the stations. However, because information such as BSRs should be shared, there is a trade-off between the overall network throughput performance and the capability to support new stations. The authors investigated this trade-off issue in densely deployed IEEE 802.11 ax networks. [PERSON] studied a channel sounding process for downlink and uplink multi-user transmissions in IEEE 802.11 ax [9]. The author presented the comprehensive simulation results of the channel sounding process for the downlink and uplink multi-user transmissions, and analyzed the results regarding the channel sounding compression modes with phase noise, in-phase and quadrature (IQ) imbalance, or carrier frequency offset (CFO) for the performance improvement of multi-user transmissions. [PERSON] et al. studied the effects of symbol timing synchronization in uplink multi-user transmissions in IEEE 802.11 ax [10]. Asynchronous symbol timing in uplink multi-user transmission may lead to severe performance degradation. To synchronize symbol timing and improve the performance of uplink multi-user transmissions, the authors proposed a two-step synchronization method consisting of coarse symbol timing using a legacy short training field (L-STF) and fine timing synchronization using cross-correlation of a legacy long training field (L-LTF). [PERSON] et al. studied multi-user transmissions in two different scenarios: a random access scenario and a scheduled access scenario [11]. The authors showed that the packet arrival rate and the ratio of downlink and uplink traffic affect the throughput performance of IEEE 802.11 ax networks, and balancing random access stations and scheduled access stations is important to improve throughput performance. [PERSON] et al. studied a random access scenario in IEEE 802.11 ax networks [12]. The trigger frame from the AP initiates the multi-user transmissions, and non-scheduled stations perform the backoff process for channel access. The authors proposed a hybrid random access method that uses carrier sensing to reduce collisions. They presented an analytical model, and then showed that the hybrid random access method can achieve good performance with enough sensing duration. [PERSON] studied a user selection method that efficiently utilizes channel state information (CSI) in IEEE 802.11 ax networks [13]. The CSI estimated from uplink OFDMA frames were directly used to find the next downlink MU-MIMO users with a good channel quality,which maximized the system utility. In order to improve the efficiency of utilizing CSI from uplink OFDMA for selecting MU-MIMO users, the new OFDMA frame structure allocates all sub-channels to each user sequentially for the last few slots to estimate the CSI of the users. [PERSON] and [PERSON] also studied AP-initiated multi-user transmissions in IEEE 802.11 ax networks [14].