# Impact of Latency on QoE, Performance, and Collaboration in Interactive Multi-User Virtual Reality
## 2 Related Work
### Multi-User Experiences  
[PERSON] et al. [4] presented a user study on the effects of network latency on user experience (QoE, willingness to continue, and performance) in a first person shooter VR multiplayer game. Their results showed that user experience for the chosen game began to suffer in cases of latency greater than approx. 100 ms (round trip time between client and server). [PERSON] et al. [5] investigated how different levels of delay influenced overall QoE in VR multiplayer 'exergames' (exercising and gaming). Their experimental setup consisted of a VR application coupled with a rowing ergometer, allowing races between the user and an artificially created opponent that followed the player with a similar speed, keeping the race tight. To investigate the influence of the delay on both the user's and opponent's side, three levels of network delay were introduced (30 ms, 100 ms, and 500 ms), mixed fully with the different conditions. After each session, participants rated the perceived flow, sense of presence, and the degree to which they noticed the delay in their or the opponent's system. Interestingly, the results showed different perceptions of the delay and QoE depending on the user's own delay. The participants perceived the opponent's player as being delayed even if only the player itself had a network delay, along with a significantly lower rating of QoE only when their delay was high. Regarding the perception of their own delay, only for the 500 ms case was a strong impact on QoE recorded. [PERSON] et al. [13] presented a 3 DTI tele-rehabilitation system in which two collaborators needed to keep a balancing bar as horizontal as possible while doing bicep curls with the hand holding it. They used this environment, among other things, to perform a user study to evaluate the effect of induced latency on the task completion time, the number of corrections, and the consistency between the two user environments. Their results showed that each of the three measurements increased with increasing latency. They identified the operating range of the system, i.e., the range of latency for which the system offered acceptable QoE, for the visual aspect to be 0-130 ms.  
[PERSON] et al. [14] presented a haptic-enabled two-user networked air hockey game. A subjective user study was performed to assess the influence of network latency on perceived interactivity and quality. Their results showed that both deteriorated more seriously with increasing latency. Nevertheless, the impact on quality was observed to be more severe with their corresponding MOS, dropping below 3 for a 100 ms delay, even in the most optimal scenario. [PERSON] et al. [15] carry out a QoE assessment of operability and fairness between players in a two-player networked real-time balloon-bursting game. To this end, latency was varied from 0 to 500 ms for both players. Their results showed that the operability mainly depended on the level of network delay, with subjective scores dropping below 3 for all cases for a latency of 300 ms and beyond. It is important to note, however, that the context of the game, i.e., soft vs. hard and small vs. standard balloons, played an important role in this. The differences between the best- and worst-appreciated balloon type often exceeded a full point on the 1-5 MOS scale. [PERSON] et al. [24] compared the true end-to-end latency across an immersive virtual environment and a video conference link. This was realized by filming the movements of a participant and their remote representation through synchronised cameras. They recordeda mean end-to-end delay of 605 ms from the contemporary to the traditional immersive display and a delay of 414 ms the other way around. [PERSON] et al. [16] investigated the negative effects of network latency in immersive collaborative environments by conducting a user study to assess its impact on user performance. To this end, a cooperative game was designed in which users had to place bicolored cubes into their specific destinations.