# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 4 Challenges and the Road Ahead
### Handover Techniques in FSO-Based NTN  
Achieving a seamless handover in FSO-based NTN requires addressing several critical factors. Ensuring precise alignment between transmitters and receivers is crucial in FSO connections [PERSON] et al. (2019) (Section 11.2.2), especially when dealing with airborne nodes, such as HAPSs and UAVs. For instance, it is discussed in [PERSON] et al. (2019) that determining the number of laser sources per airborne node should not solely rely on the number of ground-based stations served by the node, but should also consider handover requirements. Furthermore, the same study elaborated that the precise positioning of the airborne node in relation to the ground base station is critical. Continuous tracking mechanisms play a pivotal role in maintaining this alignment [PERSON] et al. (2019). Additionally, network redundancy, involving the provision of multiple backup links, can ensure uninterrupted connectivity, particularly in challenging weather conditions. In the context of seamless handover, continuous monitoring of signal strength and link quality is vital. A handover process should be initiated when the signal strength falls below an acceptable threshold or when the link's quality drops below acceptable levels. This proactive approach ensures uninterrupted connectivity by searching for a stronger signal or an improved link quality, provided an alternative link is available.  
The availability of FSO links for seamless handover depends on careful network planning, deployment, and coordination with the movement of HAPSs and UAVs. Utilizing optimization algorithms, predictive models, and network management tools can maximize the availability of FSO links for handover scenarios. In certain situations, indirect links may become necessary, especially when adverse weather conditions, like thick fog, obstruct direct connections between HAPSs and ground stations, gateways, or lower-altitude UAVs. Strategies for ensuring indirect links, e.g., multi-hop relaying or RIS-assisted transmission, should be considered, to maintain seamless communication.  
In addition to network considerations, security measures such as robust encryption and authentication should be implemented to protect data during handovers [PERSON] et al. (2019). Low latency and seamless integration with other communication systems, such as terrestrial networks or satellite links, are other crucial aspects to consider. The integration of handover mechanisms into the network infrastructure is essential to ensure the timely initiation of the handover process. These mechanisms often involve the deployment of specialized hardware and software components that facilitate real-time monitoring and decision-making.  
Intelligent algorithms assess the quality of the available links, predict future conditions, and trigger handovers when necessary. These mechanisms are typically integrated into the broader network architecture, allowing for seamless communication handover as users move between nodes.  
Practical implementation of handover mechanisms may also involve the use of predictive models and Artificial Intelligence (AI) to anticipate user movements and network conditions, enabling proactive handover decisions. The integration of these mechanisms into network management tools streamlines the process and ensures that handovers are initiated promptly, to maintain uninterrupted connectivity. There may still be cases of dropped connections or significant disruptions during the handover process. These disruptions can be caused by various factors, including sudden changes in weather conditions, unexpected interference, or hardware failures. To address such issues, hybrid RF/FSO systems can be considered, so that a lower speed RF link can be used in case a higher speed FSO link is temporary not available. Additionally, redundancy in the network infrastructure and rapid reconfiguration capabilities can help mitigate disruptions and minimize their impact on user experience.