Radio Resource Management techniques with Multi User MIMO for VLEO-NTN Systems

Non-Terrestrial Networks (NTNs) have gained increasing attention in the evolution of 5G and future 6G systems due to their ability to provide global connectivity, especially in under-served or hard-to-reach areas. In this context, Very Low Earth Orbit (VLEO) satellite constellations that operate at altitudes of 300-500 kilometers, are of specific interest as a result of decreased propagation delay and increased link budgets. On the other hand, such advantages come with challenges, as the high speed of VLEO satellites means strong Doppler shifts and rapidly varying link conditions. This calls for effective design of Radio Resource Management (RRM) schemes, which are critical to ensure proper service to the users. This thesis investigates user scheduling strategies for NTNs, focusing on both location-based and Channel State Information (CSI)-based algorithms. Modifications to existing schemes are introduced to improve fairness and guarantee that all users are effectively scheduled. The algorithms are first evaluated under a baseline scenario with uniformly distributed users. Simulation results show that, while D-MADOC achieves the highest peak sum-rate capacity, this is not exploited in all clusters. Instead, Heuristic RRM (H-RRM) and D-MCC-DSatur produce narrower cluster size distributions, leading to better exploitation of the resources. The study then extends to a more realistic dense-urban setting, where users are concentrated in populated areas, to capture the impact of non-uniform deployments. With users now closer to each other, it is more challenging for the beamformer to spatially separate them, leading to a reduced algorithm performance. Furthermore, the influence of varying satellite transmit power, expressed in terms of Equivalent Isotropically Radiated Power (EIRP), is analyzed. Performance is assessed using key performance indicators (KPIs) such as the cluster size, Signal-to-Noise Ratio, throughput and outage.