## 2 Architecture and Protocol
### IEEE 802.11-Based Wireless VR System Architecture
The generation rate of the VR video frame is restricted by the VRCL, and it prevents the VR application layer from generating meaningless video frames. For VR controllers and VR headsets, motion-tracking information measured by sensors in VR devices is accommodated and utilized in this VR application layer. VR devices, especially VR headsets, would generate interpolation frames in this layer. The interpolation frame is the frame that needs to be displayed between the received real video frames delivered by a VR AP. These interpolated frames could be generated by performing many effective interpolation algorithms [11; 12; 13; 14; 15; 16]. In this paper, VR video interpolation frames need to be based on motion-tracking information that is measured by VR sensors in VR devices [20; 21; 22]. Because this situation was not previously defined, further optimized interpolation methods should be studied. The convergence layer described in Figure 3 is a protocol layer that enables multiple network standard convergence, as well as some special information for the VR application layer. VR videos that are generated by a VR PC are delivered to a VR AP, and the convergence layer in the VR AP determines the transmission interval of video frames based on network conditions, and reports the rate to the VR application layer of the VR PC. If VR video frames are transmitted without these considerations for network conditions, users would suffer poor UX, owing to the large delay time. Similar to the DL VR video frame transmission, the convergence layer also controls the UL frame delivery rate, based on its network condition. This would reduce network congestion or the required network performance of VR devices. As a result, the convergence layer prevents these catastrophic situations by controlling the frame delivery rate. Although some motion-tracking information cannot be delivered, depending on the decision of the convergence layer, the convergence layer still provides motion-tracking information to the VR application layer in the VR headset, to generate interpolation frames. These interpolation frames should be generated considering motion-tracking information, in order to prevent VR sickness. The number of interpolation frames that need to be generated before the next frame may be predicted by the frame arrival rate information obtained from the convergence layer. The station management entity (SME) is used for the accommodation and delivery of parameters for each network layer [9]. In some cases, frame off-loading could be performed by controlling the frame-delivery interval based on PHY and MAC layer parameters. The packet loss rate information and frame interval information are key sets of information delivered by the SME. Each MAC and PHY layer follows its own standards. The convergence layer controls and schedules all frames into those multiple MAC layers properly. For example, DL data frames can be scheduled in the IEEE 802.11 ad/ay MAC and UL data frames can be scheduled in the IEEE 802.11 ax MAC. Because IEEE 802.11 ax is a 5-GHz standard with multi-user support, it is a suitable protocol for the UL transmission protocol in multi-user VR systems. Figure 4 shows how IEEE 802.11 ax could accommodate multiple frames, using orthogonal frequency-division multiple access (OFDMA). The trigger frame is transmitted by an AP, to instruct stations that have UL frames when and where to transmit their UL frames. In usual IEEE 802.11 ax scenarios, the trigger frame would contend with other UL frames to guarantee the opportunity for all stations to access the channel. However, a VR AP requires a very tight delay property, and its traffic pattern is very regular--it is a special-purpose AP for VR devices. This means that for associated devices, there is no need for channel contention to guarantee opportunities for channel access. In other words, in order to fully utilize the UL OFDMA of IEEE 802.11 ax for multi-user VR services, single-user UL transmission should be regulated.