## 3 Results and Analysis
### Simulation Results  
In conventional EDCA cases, nine devices generated their UL frames containing motion-sensing information every 1 ms. For conventional EDCA cases, the packet loss rate and \(T_{transfer\_UL}\) are shown in Figure 7. Because VR systems require real-time processing, the number of maximum retransmissions did not need to be set to a large number. If it was set to 2, as shown in Figure 7a, it would lower the PLR for a short time, but then increase to 0.4 PLR shortly afterwards. From the start, the PLR observed in conventional EDCA cases was not usable for consumer VR devices. In the no-retransmission case, as shown in Figure 7b, even though there was a large PLR and relatively small delay, not only was the PLR destructive, but the delay also could not be used. After 8 s, the delay value increased over 1 s for the motion-sensing data delivery. From the results of Figure 7, we can observe that conventional EDCA procedures cannot afford wireless multi-user VR systems. At a minimum, wireless VR systems require ms-level delays and a very low PLR for reasonable user experience. The proposed VR AP disables the conventional EDCA procedure, which can cause tragic delays and PLR results as shown in Figure 7. If there are no interference devices, wh  
Figure 7: Delay and packet loss rate (PLR) for proposed WLAN structure with conventional enhanced distributed channel access (EDCA). (**a**) 9 VR devices with max number of retx = 2; (**b**) 9 VR devices without retx. OBSS devices, VR AP is the only wireless device that initiates the channel access procedure. In this case, a VR AP always obtained transmission opportunity without channel contention and back-off procedure, because the channel was idle during every channel access time period. This meant that the VR AP did not require an additional contention delay that is caused by the back-off procedure. Although the proposed architecture and schemes work very well, as shown in Figure 8, the channel condition could be worse, owing to interference devices in the 5-GHz frequency band. As opposed to 60 GHz band radio, the 5-GHz band radio has a relatively long transmission range, interference range in this case, and 5-GHz frequency band devices are widely used by ordinary consumers. This means that a VR AP should consider the influence of interference. In order to consider and measure the effect of channel interference, in this paper we considered nine additional IEEE 802.11 devices that utilized the 5-GHz band. Each device generated its data frame with an air time of 10 ms every 1 s. The result is shown in Figure 9, and as opposed to the no-interference case, these results showed relatively large delays. Most frame-delivery delays were under 10 ms, but a large delay jitter was observed. If we set the maximum number of retransmission parameters to 0, the delay jitter would be decreased, as shown in Figure 9b. Figure 8: Delay and PLR for proposed WLAN structure with proposed schemes in a no-interference environment. Figure 9: _Cont._  
Even though the 10-ms delay performance was relatively good for multi-user VR, in the severe-interference case, the delay jitter still needed to be minimized. The proposed algorithm to control the trigger frame delivery rate in Figure 6 could handle this severe interference case. Because the proposed VR system could realize variable-motion sensing information delivery rates, owing to its VR video interpolation module in VR headsets, its sensing data report rate could be adjusted to a small value. In this study, we assume a 100 Hz to 1000 Hz sensing data report rate. If the channel conditions are very poor, the VR AP should set its trigger frame delivery rate to 100 Hz. In order to measure the effect of controlling the trigger frame, we simulated the case with a 100-Hz trigger frame delivery rate in the severe-interference scenario. The result is shown in Figure 10, and it showed that in the no-retransmission case, all of the frames were delivered in a 10-ms delay.