## 1 Introduction
### Related Work  
#### 1.1.1 Wireless Technologies toward Ultra-Reliable Low Latency Communications  
In response to emerging new applications such as 8K video, VR, gaming, and cloud computing, the IEEE 802.11 Task Group BE started to work on the IEEE 802.11 be standard (Wi-Fi 7) enabling extremely high throughput at low latency [3]. IEEE 802.11 be is anticipated to operate in the 2.4 GHz, 5 GHz, and 6 GHz bands offering 320 MHz channel bandwidth. Higher order modulation constellations up to 4096-ary quadrature amplitude modulation (4096-QAM) will be supported while the number of spatial streams will be increased up to 16 through the use of 16 \(\times\) 16 multi-user multiple-input multiple-output (MU-MIMO) systems. The high signal-to-noise ratio (SNR) required for reliable operation of 4096-QAM shall be achieved by beamforming. IEEE 802.11 be aims to provide at least one operation mode that supports data rates great than 30 Gbps. It is envisaged that a final version of the IEEE 802.11 be standard will be available by early 2024.  
Further, the WiGig specifications produced under the umbrella of the Wireless Gigabit Alliance provides a set of wireless network protocols for operation in the 60 GHz band [4]. The WiGig system, also known as 60 GHz Wi-Fi, includes IEEE 802.11 ad [5] and IEEE 802.11 ay [6], and supports data rates of up to 7 Gbps. It resorts on beamforming which allows to explore the signal reflections induced by the propagation environment, e.g., surfaces, ceilings, and floors. Specifically, the phased array antenna beamforming used in the WiGig system enables robust communication with transmitter-receiver distances of up to 10 m.  
The WirelessHD or UltraGig proprietary standard [7] targets wireless transmission in the 60 GHz band for high-definition (HD) video content. The high date rate physical layer of WirelessHD uses quadrature phase shift keying (QPSK), 16-QAM, and 64-QAM together with equal and unequal error protection, and provides the option for spatial multiplexing. WirelessHD supports data rates of up to 28 Gbps, uses beamforming at the transmitter and receiver, and supports a communication range of up to 10 m. The WirelessHD technology is used in the TPCAST wireless VR system that was developed for the HTC Vive HMD (see Section 2).  
Further, VR and augmented reality (AR) applications and services are foreseen to be delivered over broadband cellular networks such as the currently rolled out 5G networks and to be expanded to cater for digital realities offering immersive experiences in B5G and 6G mobile networks [8]. Regarding HMD technologies for such networks, the Qualcomm Snapdragon XR2 5G reference design, for instance, aims at next-generation 5G-enabled HMDs for untethered VR, AR, and mixed reality (MR). Network technologies considered to deal with the stringent performance indicators of ultra-reliable low latency communications include network slicing and edge-cloud processing architectures such as cloud VR.  
Irrespective of whether a dedicated private network or a network slice over a public network is used for the delivery of untethered immersive media, the underlaying infrastructure must offer: (1) High bandwidth to accommodate the required high data rates, (2) low latency to keep within the MTP budget, (3) high reliability to avoid spatial and temporal artifacts being inflicted to the immersive media. The challenge of providing sufficiently high bandwidth has been tackled by moving toward higher frequencies, e.g., 60 GHz millimeter waves (mmWaves), and the related wider frequency bands. The requirement of low latency is being addressed by finding a balance between advanced on-device processing and moving processing to the edge. Beamforming at the transmitter and receiver along with adaptive coding and modulation schemes are increasingly engaged at the physical layer to provide ultra-reliable wireless links.