## 5 Quality of Experience (QoE) Assessment
### Subjective QoE Assessment  
Visual attention-enhanced models have been introduced to overcome the limitation of objective QoE assessment. Users in an immersive virtual reality environment can only be perceived from their FOV, and usually only focus on certain objects, i.e., buildings which grab their attention. Thus, distortions in various parts of the 360-degree video have distinct effects on QoE. In estimating the PSNR based on the viewer's visual attention distribution, [PERSON], [PERSON] [104] employed the usual PSNR metric to apply weights to the pixel-wise distortion. VQA-OV [57] employs a similar strategy. To induce visual attention, a VR headset's inertial sensors and eye tracker monitor the viewer's head and eye movements. In [105], they created a subject's field of view (FoV) and saliency map to aid VQA evaluation. Although these models incorporate visual information, it is mostly influenced by the user's visual attention. Lately, several studies have recently begun to include human factors in 360-degree video QoE evaluations. A link between eye-based signals and human perceptions has been acknowledged by [106; 107]. For instance, Ref. [80] employs physiological features of viewers' ocular motions such as eye gazing, fixations, saccades, pupillometry, and different types of eye-opening and closing events to infer how contented they are with 360-degree videos. However, the results in [86] found that adding eye-tracking metrics in the model did not further explain individual variation in subjective assessment. The proportion of the total viewing area examined had no impact on the quality of experience. Participants become less sensitive to quality distortions as they look at more moving things, and the effect of freezing becomes rather more adverse. The effects discovered are minor, and the variation on a participant-by-participant basis remains substantial. Visual attention implementation based on these findings is most likely insufficient to enhance objective QoE measures. In VR applications, biosensors may also be utilized to estimate subjective quality. When people watch 360\({}^{\circ}\) videos, for example, [PERSON], [PERSON] [108] record their heart rate and electrodermal activity. The relationship between these two measurements and subjective scores is examined and discussed. Their findings show that HMDs provide higher subjective quality than 2D displays. [PERSON], [PERSON] [109] perform a user study, particularly on tiled 360\({}^{\circ}\) video streaming. They consider the effects of many aspects including bandwidth, latency, and resolution. They also assess perceived quality and cybersickness in response to various forms of latency, such as tile switching and network delays. Their findings suggest that a network delay of 47 milliseconds is acceptable and does not impact quality ratings. [PERSON], [PERSON] [110] calculate the QoE of tiled-based streaming with Mean Opinion Score (MOS) and Individual Score (IS) under uniform-bitrate templates. MOS stands for the average quality scores of a group of subjects assessing an experience of watching tiled 360\({}^{\circ}\) films with HMDs,whereas IS stands for the quality ratings provided by each subject on his or her personal experience. The findings show that the content variables such as video complexity in Temporal Information (TI) and Spatial Information (SI), as well as video quality, dominate the factor categories for overall QoE and most QoE features. [PERSON], [PERSON] [86] investigate the impacts of quality degradations, freezing, and content on QoE, as well as evaluate visual attention as a factor in QoE. Quality degradation did not affect QoE until the threshold. In every circumstance, freezing occurrences lower the QoE below an acceptable threshold. Additional compression is favored over freezing when network resources are limited. Furthermore, the findings of this paper reveal that perceptual and cognitive load (PCL) and cybersickness do not have a significant impact on QoE and are unaffected by manipulations.