In the dynamic landscape of modern technology, satellite utilization has become fundamental for various critical applications. Communication infrastructure based on satellite technology has emerged as a cost-effective solution, serving not only isolated regions where the establishment of terrestrial networks proves infeasible but also providing a viable alternative in emergency scenarios. In addition, the versatility of satellite services is exceptional [1], encompassing a diverse array of applications including military purposes, remote sensing, Earth observation, space exploration, maritime operations, agriculture, and communication provision over the Internet [2,3].

The existing literature has extensively explored the influence of space design parameters on satellites [4,5], including implementations in the Objective Modular Network Testbed in C++ (OMNeT++) [6] and protocol evaluation in space environments [7]. While certain literature studies focus on simulating and analyzing space-terrestrial communication scenarios with OMNeT++, they exhibit a limited emphasis on the incorporation of real-world testbed data into the simulation environment. This distinction is essential to ensure the accuracy of simulation results, as real-world measurements provide valuable insights into the practical performance of communication systems. In the context of Internet of Things (IoT) Case Study scenarios, the evaluation of various parameters impacting the reliability of space-terrestrial communications is crucial. Therefore, the integration of real-world measurements into simulations and the thorough examination of relevant parameters can enhance the relevance and applicability of the findings to practical scenarios.

The evaluation of space infrastructure has gained increasing attention, due to the significant role of satellites, Unmanned Aerial Vehicles (UAVs), drones, and ad hoc networks in facilitating the communication among IoT devices within the Edge–Cloud environment. 

Assuming that communication is feasible, wherein these devices can establish connectivity with the satellite infrastructure, satellites can prove beneficial in diverse scenarios, including environmental emergencies or situations requiring enhanced security measures. In particular, LEO satellite constellations can play a significant role in the smart city and IoT scenarios, which are characterized by intermittent or disrupted communications. These constellations can serve as a primary communication channel in remote areas where terrestrial infrastructures are impractical or can act as backup links in situations involving compromised or damaged equipment. 

The extension of satellite or -in general- space utilization opens up numerous novel research areas. Ships, drones, UAVs, and any ``smart'' devices with suitable characteristics can play vital role in various scenarios. For instance, the deployment of micro/nano satellites or drones with a designated purpose can influence the communication resilience and security in smart-city and IoT scenarios. These devices can serve as relays, contributing to communication continuity or enhancing security in situations where communication infrastructures are jammed, compromised, or under attack. Equipped with specific and secure instructions, these satellites or drones can act as connection bridges, offering temporary solutions to potential issues, such as addressing a security breach in the communication infrastructure.

3. Scenarios

Our research focused on the communication of IoT devices via satellite constellations, utilizing the OMNeT++ and a version of the OS3 framework [8][9]. We aimed to evaluate the reliability and effectiveness of satellite constellations across diverse IoT scenarios with regard to the Round-Trip Time (RTT) and data loss. We investigated the potential use of space infrastructure, in particular, Low-Earth Orbit (LEO) satellite constellations positioned at a 600 km altitude, as a viable substitute for communication among ground stations or, more broadly, among any real-time sensitive ``smart'' device capable of generating and transmitting data. 

We demonstrated two experiment scenarios. The former describes an IoT experiment where deployed sensors communicate between the EU and the U.S. The second scenario entails simulating communication among real sensors positioned in Smart Santander and existing testbeds in the EU and the U.S. The actual geographical location of these sensors was incorporated in the configuration file of OMNeT++ to guarantee the precision of the results. In both scenarios and their respective experiments, we captured snapshots and illustrated diagrams of the RTT over time and ping loss over time while we evaluated the effect of inter-satellite links.

4. Results

Throughout our experiments, we investigated the impact of the number of satellites in a constellation on the resulting RTT and ping loss. While the topology of each experiment, particularly the location of the sender(s) and the receiver(s) on the map compared to the satellite constellation (planes, satellites, orbits, etc.), plays a crucial role, in the majority of our experiments, increasing the number of satellites typically led to a reduction in the generated RTT and ping loss. Therefore, an inversely proportional relationship was detected. This observation aligns with the notion that larger constellations of satellites may more efficiently utilize available routes and paths between the sender and receiver. Moreover, in experiments where the intersatelliteLink parameter was enabled, an additional reduction in the RTT and ping loss was noticed. This indicates that the utilization of inter-satellite links contributes to further optimizing communication performance in the simulated scenarios.

Regarding the effect of inter-satellite links, our findings revealed consistently compressed range values for the RTT across all experiments involving active inter-satellite links. In fact, the minimum, maximum and mean RTT values exhibited reduced values when compared to scenarios without the intersatelliteLink parameter. This outcome is attributed to the interconnection of satellites within the same plane, resulting in shorter propagation delays, as opposed to higher propagation delays due to the more frequent connections of space and ground. Inter-satellite links proved beneficial, serving as the sole alternative in scenarios where no ground station existed between a distant sender and receiver. 

35. Conclusions

This paper investigated the application of LEO satellite constellations at an altitude of 600km for the communication among smart devices in IoT scenarios. Our experiments validated the initial hypothesis, indicating the existence of an inversely proportional relationship between the number of satellites and the resulting RTT and ping loss. The introduction of inter-satellite links further contributed to the reduction of mean and range RTT, as well as ping loss. Nevertheless, there were instances where the placement of ground stations and satellites emerged as a dominant factor, leading to improved communication even with a lower number of satellites in the constellation. Additionally, cases were observed where an occupied communication channel resulted in a temporary and substantial increase in the RTT, affecting the overall performance of the communication.