I. Introduction
The concept of Virtual Reality (VR) emerged in the 1960s with the development of early VR devices and systems. Until nowadays, the VR paradigm remained the same - immersing the users in artificial digital environment capable to simulate real-world scenarios for various purposes: entertainment, tourism, e-commerce, education and training etc. [1]. The significant advances of semiconductors, networking, wireless communications, haptic devices, 3D tracking (hand and eye), wearables etc., enhanced the VR technology, especially in the past decade. These advances created developmental field for related technologies such Augmented Reality (AR) and Mixed Reality (MR), all-together falling in a paradigm nowadays called as Extended Reality (XR). All of these immersive technologies share similar characteristics in terms of programming, content, and tracking, and share the same vision of co-existence in standalone systems and/or within global visions such as Metaverse [2]–[3].