## 1 Introduction
Moving its latest frame to opposite vector of sensor data is the easiest method of generating interpolation frames. In order to assist the interpolation frame generation, some network characteristics, e.g., the video frame arrival rate and control frame delivery rate, need to be provided to VR devices. When there is a mismatch between the visual and vestibular systems, VR sickness can result. This means that if the VR vision in VR displays cannot reflect the real movement of users, the user experience may be degraded. In order to prevent VR sickness, accurate VR frame interpolation operations are required. Therefore, in the network-condition-based VR frame delivery proposed in this paper, the video frame interpolation procedures are very important, in order to reduce VR sickness. The relationship between the received video frames and interpolated video frames is shown in Figure 2. In this paper, interpolated video frames do not refer to the video frames generated by the graphic processor of a VR PC or a VR console. Here, only frames that are generated by VR headsets after receiving video frames from a VR access point (AP) are referred to as interpolated video frames, which can be generatedusing received frames and motion track information. As shown in Figure 2, the processing unit in a VR headset shall move the last video frame to the reverse direction of user's motion vector, to generate interpolated video frames. The interpolated video frames provide immediate visual responses that solve the mismatch between visual and vestibular systems. In wireless multi-user VR systems, since wireless links with inefficient multi-user channel access performance are bottlenecks, which cannot provide a sufficiently high data rate for a high video frame rate, many interpolated frames are generated. Because of such problems, next-generation wireless multi-user VR systems should be designed to be tightly coupled with wireless systems. The next-generation, wireless, multi-user VR systems should optimize their VR video image and motion tracking rate, considering the wireless link status. Based on the above observation, in order to design a high-quality multi-user VR system over WLANs, both wireless link optimization, which enhances the wireless channel access efficiency, and tight-coupled VR optimization with a wireless system, which prevents unnecessary resource wastage, should be considered. In this study, to provide high-quality VR UX in a multi-user WLAN VR service, we consider both the multi-user wireless link efficiency enhancement and VR optimization tightly coupled with a wireless system. This paper is an extension of one that proposed delay-oriented VR mode that could be utilized by a VR AP [18]. The delay-oriented VR mode is included in this paper as a trigger-based channel access method. This paper proposes a novel wireless multi-user VR protocol structure, as well as specific channel access and system control schemes, to support multi-user VR systems over WLAN, including the delay-oriented VR mode. In addition to the novel structure and the enhanced channel access and control schemes, in this paper, we propose connection-recovery algorithms for a seamless VR UX. The rest of this paper is organized as follows. In Section 2, we explain the proposed system architecture and protocol design, including the connection-recovery algorithm. In Section 3, we investigate the system performance of the proposed VR architecture and multi-user VR schemes, by performing extensive simulations. We also examine the delay and packet loss rate (PLR) performances in various simulation scenarios. Finally, Section 4 explains the reason why conventional EDCA, which is utilized in WLAN systems, cannot handle wireless multi-user VR applications, but the proposed system can handle them.