## 2 Architecture and Protocol  
### IEEE 802.11-Based Wireless VR System Architecture  
The proposed multi-user VR systems with wireless interfaces consist of multiple IEEE 802.11 medium access control (MAC) layers and physical (PHY) layers. In order to accommodate multiple  
Figure 2: Motion sensing based video interpolation in VR headsets. IEEE 802.11 protocols, we now propose a novel VR convergence layer (VRCL) and its interworking scheme with a station management entity (SME). The network architecture for multi-user VR systems needs to meet the requirements of very high throughput and low latency. In order to satisfy the high data throughput requirement of high-resolution video images, we can utilize 60-GHz standards, i.e., IEEE 802.11 ad/ay. These amendment standards are designed for wireless high-resolution image devices. Because immersive VR UX could be achieved by these high-resolution video images, the use of 60-GHz standards is inevitable in multi-user VR scenarios. However, although these high-throughput wireless standards are utilized, with a combination of UL and DL transmission in multi-user scenarios, the effective throughput and delay performance would deteriorate. This means that intra-basic service set (BSS) channel contention should be controlled by wireless VR protocols. Without such control algorithms and additional channel resources, VR experience cannot be guaranteed in wireless network environments. If there are only DL video frames transmitted by a VR AP, network degradation may not occur. Multiple VR devices connected to an AP should transmit their motion tracking data and control data very frequently, i.e., 1000 Hz per device. Those uplink motion tracking and control data are problematic, because frequent motion tracking and control data can cause large channel access delays and throughput degradation. In order to resolve such an uplink data contention problem, multiuser control channels conforming to the wireless standard should be utilized for multi-user VR networks. IEEE 802.11 ax is the most representative standard that supports multi-user network in the 5-GHz frequency band [19]. This means that because the 5 GHz channels of the IEEE 802.11 ax standard do not interfere with 60-GHz frequency channels, a VR AP can accommodate multi-user uplink traffic very efficiently. As a result, VR devices including VR APs need to have special multi-standard protocol architecture, described in Figure 3, to support VR connections based on WLANs. IEEE 802.11 ax is the amendment standard for highly efficient WLAN in multiple-device scenarios. IEEE 802.11 ax defines a trigger frame to accommodate multiple uplink frames from multiple devices simultaneously [19]. In many scenarios, it may be utilized in parallel with a conventional single-frame transmission. If the trigger frame requests stations to transmit its UL data, each station transmits its data without additional channel access delays. This trigger frame is able to substantially reduce the contention delay of WLAN systems. The IEEE 802.11 ad amendment standard is designed to utilize the 60-GHz frequency band, which provides wide bandwidth and high throughput. IEEE 802.11 ay is an enhanced version of IEEE 802.11 ad,  
Figure 3: Proposed protocol architecture of a wireless, multi-user VR system. and provides four times the bandwidth using channel bonding and additional spatial streams [5]. Because of the wide bandwidth, the 60-GHz standard is able to provide very high data rates over short distances. Therefore, in order to adequately utilize the high data rate to support multi-user VR, the channel inefficiency caused by channel contention should be minimized by separating the UL transmission and DL transmission. The VR application layer described in Figure 3 is a protocol layer that provides VR images and control information on VR devices. VR video frames are generated using the frame rate that is reported by VRCL, and a detailed explanation on VRCL is provided in Section 2.2.