## 3 Challenges and Related Recommendation
### Future Research Challenges
#### 3.2.1 Hybrid RF-Optical Networks  
One exciting new direction for 6G is the integration of systems operating on many frequency channels--including RF and the entire optical wireless bands--to create diverse and mixed networks capable of supporting all of these bands. This will help bring about the entire spectrum vision of 6G. Indoor, outdoor V2V, open space, and submarine settings are only a few examples of the many uses for RF-optical diverse mixed networks [72; 73]. However, there are a lot of obstacles to overcome on the road to RF-optical mixed networks and systems. First, RF-optical mixed-signal networks share many of the same difficulties as other types of mixed networks, such as those that span space, air, ground, and sea. These include but are not limited to the following: the development of transfer protocols for the network, traffic balancing [74], synchronization of mixed networks, distribution of resources and enhancement of energy efficiency [75], distribution of spectrum [76], and distribution of power and connection access points [77]. Because of its low energy usage and high attainable data speeds, white LEDs have increased the energy efficiency of data transmission and are now used for both lighting and communications. Consequently, LiFi approaches have a wide range of possible uses, including as anti-jamming in military settings, vulnerable civilian contexts like hospitals and airplanes, and broadband indoor communication. One way to increase the system's performance is by allowing wireless networks with various technologies to collaborate, which is called a mixed wireless network. The variations in fading channels, transmission losses, and accessible resources among networks allow for these improvements. Such  
Figure 11: Auto-generation of hardware architectures for target polar encoders [68]. improvements, however, are contingent upon successfully completing the enormous task of creating resource distribution algorithms that distribute bandwidth and power among diverse networks in order to meet varying operation demands. One way to improve the energy efficiency of RF communication systems is to use numerous radio gateways. 6G networks can be utilized for the support of smart buildings together with other technologies. Hybrid LiFi/WiFi networks have been used in smart buildings to assist social distancing [78; 79], where vehicle access is managed and controlled after data exchange at the entrance at the gate of the building using the mixed wireless network. The presented approach can be assisted and improved using 6G networks, especially when using other services such as cloud servers [52], and other solutions. However, the challenge will be providing the proper infrastructure and power for big buildings and high-density environments. When designing RF and optical communication networks, it is important to think about transceivers with unique characteristics. For example, users face significant difficulties with frequent handovers (HOs) [80; 81; 82; 83] due to the diversity of access points when they roam. Several studies have proposed different methods for HO for hybrid LiFi networks. However, since LiFi networks used to provide higher data rates than WiFi, when mixing 6G networks with other networks such as LiFi or any optical network, new challenges will occur including the mobility and access point assignment due to the high data rates not only from the LiFi network but also from the 6G networks, which makes the criteria of data rates, capacity, or network speed are not the main aspects of allocating the network to the user. This remains unknown since LiFi and 6G networks are still under development and are not yet popular for the end-user. Additionally, uplink OWCs still have significant limits owing to the low energy of terminal equipment, the significant impact of mobility on them, and the interference that these uplink OWCs cause with lighting. Lastly, it is impossible to overlook the mixed systems' impact on system security [84].