# Impact of Latency on QoE, Performance, and Collaboration in Interactive Multi-User Virtual Reality
## 1 Introduction  
Multi-user, collaborative Virtual Reality (VR) has the potential to shape the next generation of collaborative applications. Due to its immersive and interactive nature, it is already applied in a number of societal and economical sectors, such as gaming andentertainment, industry [1], (mental) healthcare [2], and VR training [3]. These collaborative immersive experiences typically incorporate elements of skill acquisition, team building, exploration, entertainment, etc. Moreover, due to its virtual essence, it can be easily employed in cases where the real-life counterpart is either too dangerous, too expensive, too time-consuming, or too difficult to realize. In its most common configuration, a collaborative multi-user VR experience consists of a minimum of two users wearing a pair of head-mounted displays (HMDs) connected to a remote (virtual) environment. There they can collaborate to perform a certain task. The content is streamed and/or synchronized over the network. This means that the performance and satisfaction of the participants will be highly dependent on the quality of the network they are connected to. In fact, the dynamicity of the network can result in distortions, such as delay, stalling, visual artifacts, and desynchronizations, which may affect the end-user's experience, interactivity, performance, and collaboration in unexpected ways [4; 5]. However, the impact of networks on the performance and experience of users of a collaborative VR setting remains largely unexplored [6]. Related work on collaborative VR has, in most cases, focused on pure server-client architectures (e.g., Furion [7], Flare [8]), where all content is rendered on the server-side, after which the resulting video stream is sent over the network to the client. These often result in stringent network requirements (Table 1), such as downlink throughputs of 400 to 600 Mbps [9], latency in the 5-20 ms range [9; 10], and below 15 ms round-trip times (RTTs) [11]. Other architectural solutions exist though, in which the server is used more as a mediator for the transportation of synchronization messages between virtual environments being rendered locally at the client, therefore putting much lower pressure on the network [12]. In addition, the motivation for these reported requirements in relation to the end-user's perception, performance, and well-being is often limited. The existing literature that tries to address this problem is mainly focused on standard, uniform latency (i.e., delaying every network packet with a certain amount of time) [4; 5; 13; 14; 15; 16]. However, burst latency (in which the communication channel becomes saturated and is, therefore, blocked for a given time interval) is at least as common as traditional latency when it comes to typical multi-user VR traffic. Nevertheless, it is rarely reported in the literature, especially in the context of multi-user applications. This context is important though, given existing research on user perception in multi-user VR. This seems to point to the fact that multi-user environments tend to induce a higher latency threshold when it comes to user acceptance than is the case for single-user systems. The latter seems to be especially true when a clear gamification and/or collaborative aspect is tied to the experience. Therefore, it would be a valuable addition to the literature to extend these studies beyond uniform latency as well as to broaden subjective evaluations to other aspects than only user perception and acceptance, such as user performance, collaboration, subjective time perception, and cybersickness.