## 5 Challenges of the OWC in the 5G/6G and IoT Solutions  
A number of challenging issues must be proficiently addressed to deploy the OWC technologies for the 5G/6G and IoT solutions. A few important challenging issues are briefly discussed below:  
_Frequent handover_: Future communication systems will consist of heterogeneous small dense networks that will create very frequent handovers. A handover will be between optical networks and between optical and RF networks. Optical cells are very small and may trigger many unnecessary handovers. Avoiding an unnecessary handover and the ping-pong effect is also an important issue. The properties of the physical and data-link layers differ in the optical and RF-based wireless networks, thereby bringing about a great challenge for the mobility support in RF/optical hybrid systems. _Inter-cell interference_: Managing the inter-cell optical interference is a serious issue in the deployment of optical VLC and LiFi networks. The dense deployment of LEDs for the OWC technologies may create high interference in the 5G/6G and IoT networks. Therefore, the inter-cell optical interference is a challenging issue. _Atmospheric loss_: The performance of the OWC technologies is affected by scattering, refraction, air absorption, free space loss, and scintillation of the atmosphere. In an outdoor environment, fog and dust obstruct the optical signal from the transmitter to the receiver. The communication link quality in the FSO is degraded because of bad atmospheric conditions. Hence, the atmospheric loss mitigation is challenging with regard to reaching the goal of the 5 GB networks, especially in the outdoor condition. _Limited uplink communication using OWC technologies_: Most of the user equipment is designed with low-power LEDs to reduce the drainage of power. Because of the low-power LEDs, VLC and the LiFi cannot perform well in uplink communication. Moreover, most of the LEDs of the user equipment produce diffused lights with low power that are easily affected by downlink high-power lights and, hence, limit the uplink communication. In addition, a little deflection or movement of the receiver of a user equipment can easily hamper the uplink communication link. Hence, this is an important issue to be solved in future to efficiently support uplink communication using VLC and the LiFi systems. _Low data rate of the OCC system_: One of the most important drawbacks of the existing OCC system is the low data rate. It is challenging to provide a high data rate because of its low-frame rate cameras. The most recently achieved data rate in the OCC system is only 55 Mbps [27]. This data rate should be increased to fulfill the demands of services in the 5G/6G and IoT networks. _Flickering avoidance_: Flickering is defined as the fluctuations in the brightness of a light that can be noticed by humans. This is an important issue in the OWC systems. Different modulation schemes on the OWC systems may cause flickering that has a harmful effect on human health. The modulation of LEDs should be done in such a manner that flickering is avoided. Such is a challenging issue. _Data rate improvement of the FSO backhaul system_: The backhaul systems in the 5G/6G systems have to handle an enormous volume of data traffic to support high-data-rate services at the user level; otherwise, a bottleneck problem will arise. Hence, increasing the FSO backhaul capacity considering the growth of traffic volume is a challenging task. _Machine learning for OWC_: Learning-based networking system will be the key requirement in future 6G communication networks. The ever-increasing complex network structure and requirementsdemand artificial controlling and decision-making in challenging environments. We can use supervised learning for several OWC-based applications such as smart healthcare [83], smart home lighting [84], and OWC data mining. OWC data-based analysis, such as correlating, ranking, spatial and temporal analysis, and flow prediction, can be performed more efficiently by unsupervised methods of machine learning.