# A Novel QoS-Aware ARQ Scheme for Multi-User Transmissions in IEEE802.11 ax WLANs
## 4 Performance Evaluation
### Priority-Based Network Performance  
Finally, the performance of the priority-based network was evaluated. Because we confirmed that the ASR-ARQ is inferior to the BAW-ARQ for the multi-user transmissions in Sections 4.1 and 4.2, the BP-ARQ was compared with the BAW-ARQ in this subsection. The number of stations in the network was 10. There were three streams whose access categories were AC_VO (voice), AC_VI (video), and AC_BE (best effort) transmitting between the AP and each station. The data rate of each stream was 10 Mbps. We researched the end-to-end delay and packet loss rate for the AC_VO and AC_VI streams and the throughput for the AC_BE. The selection of served streams was conducted by the virtual back-off procedure based on the parameters in Table 2.  
Figure 14 shows the QoS performances in the downlink. The packet loss rates of the BP-ARQ and the BAW-ARQ are all around 0.04% for the AC_VO and AC_VI, while the BP-ARQ outperforms the BAW-ARQ in terms of the end-to-end delay. The reduction of the end-to-end delay for the AC_VI is up to 4 ms, and that for the AC_VO is up to 30 ms. The reduction for the AC_VO is greater because the streams with a lower priority wait longer to obtain a new transmission opportunity. The aggregation window slides more slowly for the lower-priority stream in the BAW-ARQ because the retransmission is scheduled in a new transmission opportunity, but the BP-ARQ improves the circumstance. For the AC_BE, the BP-ARQ has a greater throughput than the BAW-ARQ. In the BP-ARQ, low-priority streams have more chances to transmit because high-priority streams have a lower probability of contending for a new transmission opportunity in the case of retransmitted packets. On the other hand, lower-priority streams have fewer transmission opportunities, which causes the arrived packets to accumulate in the buffer. The BP-ARQ enhances the transmission efficiency for the AC_BE by including more new packets in the aggregation window. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline maximum back-off stage for AC\_VO & 2 \\ minimum contention window (CW) size for AC\_VO & 4 \\ maximum back-off stage for AC\_VI & 2 \\ minimum contention window (CW) size for AC\_VI & 8 \\ maximum back-off stage for AC\_BE & 7 \\ minimum contention window (CW) size for AC\_BE & 16 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters for the virtual back-off procedure. In Figure 15, the QoS performances for the uplink are illustrated. In the uplink, streams with different access categories tend to be transmitted in the same MU PPDU because the access category of the stream transmitted in each RU is determined by the station. Therefore, the higher-priority stream has more remaining duration that should be padded. The remaining duration reduces the transmission capacity in the BAW-ARQ, while it loosens the constraints to improve the sliding of the aggregation window in the BP-ARQ. Consequently, the end-to-end delay of the BP-ARQ is reduced by up to 90% compared to that of the BAW-ARQ for the AC_VO. Although the BAW-ARQ or the BP-ARQ cannot guarantee the low latency and packet loss rate for the AC_VI when the BER exceeds a threshold, the BP-ARQ shows a stronger tolerance of channel noise with a higher threshold. In terms of the throughput for the AC_BE, the difference between the BP-ARQ and BAW-ARQ is enlarged when the BER is less than \(2\times 10^{-5}\) and shrinks as the BER is increased. When the BER is increased, the retransmissions of high-priority streams are more frequent in the BP-ARQ and squeeze the transmission opportunities of low-priority streams. Consequently, the BP-ARQ improved the performance of the end-to-end delay and packet loss rate for the AC_VO and AC_VI streams and enhanced the throughput for the AV_BE compared to the BAW-ARQ. Figure 14: The QoS performances vs. the BER in the downlink. (**a**) The end-to-end delay of the AC_VO (voice) and AC_VI (video) traffics. (**b**) The packet loss rates of the AC_VO and AC_VI traffics.