# From 5G to 6G--Challenges, Technologies, and Applications
## 3 Sixth-Generation Network's Aspects
### Network Requirements
#### 3.2.2 Technologies  
New, unprecedented, high data rates, low latencies, and high reliability metrics are expected to be achieved to enable many of the previously stated services. It is anticipated that optical wireless communication (OWC) will be a key component of 6G networks. OWC is projected to offer extremely high data rates at short ranges in indoor environments due to the massive availability of the unlicensed spectrum. Furthermore, OWC has a few benefits over regular RF communication, such as zero electromagnetic interference and a high frequency-reuse factor, due to the confinement of light within a room or a closed space [28]. Stepping beyond the millimeter waves' range to a new smaller wavelength range is also expected to unlock a new tier of data rates crossing the 1 Tbps mark, namely the _terahertz_-frequency range, as described earlier in the paper. However, this is expected to  
Figure 5: Some of the 6G system’s applications. come at a price. Mainly, atmospheric absorption increases at higher frequencies due to the shorter wavelengths of the transmitted signals [17]. This raises concerns about how wireless channels should be modeled for tasks such as synchronization and channel estimation, especially when the atmospheric conditions are unstable and variable. Moreover, by looking at extreme link reliability as a key 6G requirement, OWC use will be limited to indoor environments due to its vulnerability in outdoor scenarios, especially in dynamic scenarios. Additionally, the use of extremely high frequencies and massive MIMO constellations complicates the beam management task, which is essential for many network operations, such as mobile user handover, not to mention the difficulty of designing transceivers able to operate at very high frequencies up to a few \(THz\) to offer ultra-high data rates, which are required at the network backbone to process the massive amounts of data generated by the end devices [55]. Another key technology that is expected to be deeply integrated into 6G systems is _intelligent reflective surfaces_ (IRSs). The incentive behind using IRSs is that the wireless channel is de facto the least-controllable part in a given network; thus, it is a priority in next-gen systems to find a means of enhancing the overall performance of wireless systems by working on the channel part. We can look at IRSs as passive signal-scattering elements that are installed between the communicating nodes in a network. There exist variants of this technology such as the ones that are software-defined and coupled with AI to conform with the network requirements, as well as the active surfaces that consume power and perform signal reflections based on a pre-determined angle to enhance the overall received signal strength. IRSs possess multiple benefits for different use scenarios; for instance, they can be considered as a complimentary element for ultra-massive MIMO. Moreover, IRSs can be installed in walls to create smart indoor radio propagation environments that perform frequency-selective signal energy penetration insulation [56] or implement passive beamforming, which can significantly enhance the efficiency of wireless power transfer [57]. However, there are multiple points that need to be addressed under IRSs' implementation such as accurate channel estimation for interference cancellation and alignment with the line-of-sight (LoS) path for optimal system performance. Moreover, how very large numbers of IRSs can work together, especially in heterogeneous and highly dynamic networks such as smart manufacturing plants and V2V/V2X networks, needs to be investigated. This also opens the front for research on the optimal number of IRSs to deploy in each scenario in beyond 5G or 6G networks. Smart cities and self-sustaining networks are closely related to AI; therefore, AI has to undergo heavy developments and enhancements for 6G networks [14].