# IoT and UAV Integration in 5G Hybrid Terrestrial-Satellite Networks
## 6 Future Challenges  
The aspects that strengthen the architecture in Figures 2 and 3 open new challenges:  
* Looking at the UAV protocol stack, UAVs could be equipped with one or multiple interfaces towards the IoT devices. In this way, they would act as collectors of data generated by devices based on a specific or multiple IoT technologies and protocols at the same time, contributing to the integration among the plethora of IoT solutions. * UAVs could be equipped with an additional interface to allow them direct access to a radio base station 5G-gNB. In this way, when UAVs are located inside the coverage area of a terrestrial 5G cell, they can choose to send the collected data to the satellite or the radio base station. Data could be sent alternatively or simultaneously through these two links if useful, exploiting multi-path solutions (e.g., MP-TCP). * Moving the 5G-gNB closer or farther to the 5G-UE may have controversial benefits whose impact is still to be investigated; for example, the communication delay, which is higher between 5G-UE and 5G-gNBs and lower between 5G-gNBs and 5G-CN in the architecture in Figure 2 and vice versa in the architecture in Figure 3 and the possible employment of MEC technology if the computational and storage capabilities are closer to the IoT devices. The hardware and software complexity on-board the UAV would increase by considering these aspects, as well as the energy consumption, an issue for these flying objects, even if several technological solutions could relieve this drawback. For example, the complexity could be distributed among more UAVs flying in a swarm where all UAVs are linked together through short-range wireless interfaces (e.g., WiFi). One UAV is in charge of sending/receiving data to/from the satellites, and each of the other UAVs is equipped with one LPWA technology different from the others, to take care of a specific access technology. Additional research challenges and technological issues need to be managed, concerning:  
* Routing: 5G-UE comprises the user endpoints of 5G communications, i.e., the 5G CN is not able to see the multiple IoT devices linked to a UAV. A proper mechanism has to be implemented on-board UAVs to allow them to distribute data destined to IoT devices (e.g., commands, requests, etc.) correctly. The necessary solution may be similar to the one developed by the Network Address Translation (NAT) for IP. This problem is more challenging if more devices based on different IoT solutions are all linked to the same UAV through different interfaces because each solution uses its own format to address the devices, and in some cases, it is proprietary. * Handover: UAVs may change their position by following a predefined path, and also, satellites do not always cover a fixed area if they are non-geostationary satellites. This situation implies that an IoT device cannot always be linked to the same UAV, and a UAV is not always linked to the satellite. Proper handover mechanisms are required to manage ongoing communications during these different kinds of handover events. * Long-term storage: UAVs may be temporarily disconnected from the 5G CN due to predictable or unpredictable faults of the satellite links, both between UAVs and satellites or between satellites and satellite gateways. Communications can suffer from obstacles between UAVs and satellites, which increase the attenuation due to scattering. High fading attenuation affects the satellite communications especially at high-frequency bands: rain attenuation is not negligible in the Ka or higher frequency bands, and outage events may happen during heavy rain at the satellite gateway's site. The duration of link disruptions can range from a few ms up to a few minutes depending on the outage reason (slow or fast fading, obstacle persistence, etc.). Allowing UAVs to store data on-board for medium/long periods may avoid wasting data, bandwidth, and contact opportunities for IoT devices.