# Mobility Management of Unmanned Aerial Vehicles in Ultra-Dense Heterogeneous Networks
## 4 Research Challenges
### UAV to Ground Channel  
Creating coexisting mechanisms between terrestrial and airborne users is one of the most complex design challenges for developing cellular-connected UAVs. To achieve this coexistence, UAV-ground interference management must be installed. Unlike the ground BS to ground UE communication link, the ground BS to UAV communication link has very different interruption patterns. UAVs may establish LoS communications that are more dependable than those with terrestrial users since they fly higher than BSs. They also make use of significant macro-diversity gains provided by many BSs. Ground users, on the other hand, generate more UL/DL interference than the dominant LoS connections, making Inter-Cell Interference Coordination (ICIC) extremely difficult. Fading, shadowing, and route loss are also important considerations. Traditional ICIC solutions may be adequate for existing cellular designs; however, they fall short when it comes to UAV interference control which involves a large number of BSs, imposing limitations due to its complexity will emerge. As a result, effective interference management strategies are required for the coexistence of ground users and UAVs. Several books on the subject of downlink and UL up-link interference are available [49; 77; 78; 79].  
The most common types of links in the communication channel are Ground-to-UAV (G2U) and UAV-to-Ground (U2G). The G2U link provides downlink control and command for suitable UAV operations in cellular-connected UAVs, while the U2G link provides UL payload communication. Rayleigh fading is the most frequently used small-scale fading model for terrestrial channels, however, Nakagamim and Rician small-scale fading are more common for U2G channels due to the presence of LoS propagation characteristics. Large-scale fading is altered due to the 3D coverage area and the varying heights of UAVs. A free-space channel model, an altitude/angle-dependent channel model, or probabilistic LoS models can be used as large-scale fading models, as follows:  
1. In the free-space channel model, fading and shadowing have little effect and interference is low. This method is most effective in areas where the LoS assumption holds true between high-altitude UAV s and ground stations. Low-altitude UAVs may encounter non-LoS connections in urban environments, necessitating the use of additional methods to accurately assess the propagation environment.
2. In altitude/angle-dependent channel models, channel characteristics, such as shadowing and path loss exponents, are affected by the UAV's elevation or angle. Depending on the deployment, these varieties can be used in residential or sub-residential settings. Altitude-dependent models may not be appropriate if the height does not change or if UAVs fly horizontally. In analytical research, models based on elevation angles are commonly applied, but there is insufficient literature on the subject.
3. Due to buildings, obstructions, or bottlenecks, approaches based on probabilistic LoS models are frequently allowed for residential scenarios where the LoS and NLoS links between UAVs and the ground are recognized. The LoS and NLoS components are separately displayed according to their likelihood of occurrence in a home environment. Their characteristics propagation are statistically determined by the nature of the residential environment in terms of building height and density.