# A Novel QoS-Aware ARQ Scheme for Multi-User Transmissions in IEEE802.11 ax WLANs
## 4 Performance Evaluation
### The Latency and the Packet Loss Rate  
We then studied the end-to-end delay and the packet loss rate under the error-prone channels when the data rate of the UDP stream was 20 Mbps. The number of stations was 20, and the access category of the streams was identical. Figure 13 illustrates the end-to-end delay and the packet loss rate in the uplink and downlink when the BER is varied. Due to the large range of the experimental results, the ordinate is plotted logarithmically in Figure 13.  
Figure 12: The throughput of each station vs. the bit error rate (BER). (**a**) downlink. (**b**) uplink.  
According to Figure 13, the performances of the end-to-end delay and the packet loss rate can be classified into two stages: the stable stage and the overload stage. At the stable stage, when the BER increases, the end-to-end delay follows a steadily increasing tendency with the degradation of the transmission capacity and the packet loss rarely occurs. At the overload stage, the transmission capacity is inferior to the arrival rate of the offered stream, so more and more packets accumulate in the AC buffer and the queuing delay is dramatically increased. Because the packets that exceed the lifetime limitation in the WLAN are dropped, the end-to-end delay is around 500 ms, while the explosion occurs in the packet loss rate at this stage.  
In Figures 13a,b, when the BER is \(10^{-6}\), the end-to-end delay of the three schemes is around 1 ms and the packet loss rates of them are around 0.05%. However, they follow different tendencies with the increase in the BER. The delay of the ASR-ARQ grows most rapidly when the BER is between \(10^{-5}\) and \(3\times 10^{-5}\). The phenomenon is caused by the lowest transmission capacity because the padding problem is severe in the retransmitted A-MPDUs and the queued packets have to wait until the retransmissions of previous packets are finished before they can be transmitted in the ASR-ARQ. Due to it having the lowest transmission capacity, the ASR-ARQ enters the overload stage at a relatively small BER, \(3\times 10^{-5}\) and the packet loss rate of the ASR-ARQ is the highest among the three. As for the BAW-ARQ and the BP-ARQ, both experience the stable stage at a larger range of the BER because they can aggregate the retransmitted packets and the queued packets into an A-MPDU to enhance the transmission efficiency and reduce the queuing delay. When the BER is less than \(5\times 10^{-5}\), the BP-ARQ outperforms the BAW-ARQ by up to 50% because the BP-ARQ accelerates the sliding of the aggregation window to  
Figure 13: The end-to-end delay and packet loss rate vs. the BER. (**a**) The end-to-end delay in the downlink. (**b**) The packet loss rate in the downlink. (**c**) The end-to-end delay in the uplink. (**d**) The packet loss rate in the uplink.  
reduce the waiting time in the reordering buffer. When the BER is further increased, the delay of BAW-ARQ also enters the overload stage, while that of the BP-ARQ is still stable and under 11 ms. Moreover, the packet loss rate of the BAW-ARQ also faces a tremendous increase to 4.2%, while that of the BP-ARQ is still around 0.05%. The reason for the differences is that accelerating the sliding of the aggregation window also enhances the transmission capacity of the BP-ARQ.  
The tendencies in Figure 13c,d are similar to those in the downlink. However, the performances in the uplink are inferior to those in the downlink because more scheduled overhead reduces the transmission capacity in the uplink. As a result, the ASR-ARQ and the BAW-ARQ enter the overload stage at thresholds, which are lower than those in the downlink. Their packet loss rates are also much higher than those in the downlink. In contrast, the BP-ARQ can still maintain the stable stage for given BERs in the uplink.  
Consequently, compared to the ASR-ARQ and the BAW-ARQ, the BP-ARQ shows the strongest tolerance of channel noise and provides stable end-to-end delay and a low packet loss rate for any given BER in the experiments.