## 4 OWC Technologies for the 5G, 6G, and IoT Solutions
### Fulfilling the Network and Infrastructure Characteristics  
_Network densification using highly dense heterogeneous networks:_ Three primary means can be used to add capacity to a network--densifying the network, making the spectrum more efficient, and using more frequency spectra. Network densification is defined as the adding of more cell sites to increase capacity. Network densification includes the dense deployment of small cells and the increase of frequency utilization. Cell sites are placed in capacity-stressed areas to add more capacity and offload traffic from the surrounding sites. Densely populated areas, where a huge traffic volume is generated, are considered for network densification. The high system capacity and the high per-user data rates in the 5G/6G communication systems necessitate the densification of access networks  
Figure 3: High-speed connectivity using different OWC technologies. and/or the deployment of additional network infrastructures. The traffic volume can be increased by increasing the number of small cells. Moreover, shortening the access network to the user distance improves the achievable data rate. Hence, network densification in terms of the dense deployment of small cells is a must to meet the requirements of 5G/6G paradigms. Along with macrocells and other wide-area networks, different indoor and outdoor optical or RF small cells will be the networks in this dense deployment. Each and every indoor can contain many optical small cells (e.g., VLC, LiFi, and OCC networks) along with RF small cells. Moreover, many outdoor applications, such as vehicular networks and street lighting, will also use many optical small cells for communication. Therefore, the dense deployment of the OWC networks meets this network densification criterion. The facility of high-capacity FSO backhaul connectivity also ensures the backhaul densification. Figure 4 indicates that the OWC-based small-cell networks along with the RF small cells create highly dense network deployment. _Multi-tier architecture and convergence of heterogeneous networks:_ To meet the demands of future communication, networks will exploit a multi-tier architecture of larger coverage satellite and/or macrocell networks underlying small cells containing RF small cells and optical VLC, LiFi, and OCC networks. The VLC and LiFi even create a tier under RF small cells. Figure 4 presents an example of a multi-tier architecture consisting of macrocells, RF small cells, or optical small cells. The presence of optical small cells, such as VLC and LiFi, creates an opportunity to add additional high capacity in multi-tier wireless heterogeneous networks. As a result of the multi-tier architecture, load will be offloaded from expensive satellite or macrocell networks to small-cell networks. A large number of users can be served by indoor OWC systems. Consequently, outdoor expensive and comparatively low-capacity macrocell and satellite networks can provide better services for outdoor users. Moreover, the limitations of RF-based wireless communication systems are overcome using the OWC technologies in the multi-tier heterogeneous networks. Optical and RF signals do not interfere with each other; hence, the multi-tier networks consisting of RF and optical wireless networks can effectively reduce the interference effect [45]. In other words, the OWC technologies will play a vital role in multi-tier heterogonous networks in 5G, 6G, and more advanced communication systems. _Provision of hybrid network connectivity:_ Each of the individual RF and optical wireless technologies has limitations and advantages. The coexistence of heterogeneous networks (i.e., hybrid systems consisting of both RF and OWC technologies) can effectively overcome the limitations. The presence of two systems improves the link reliability and provides an opportunity for load balancing. Moreover,  
Figure 4: Scenario of heterogeneous multi-tier networks containing an RF macrocell, many RF small cells, and a large number of optical small cells.